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Abstract On February 18, 2021 NASA's Perseverance rover landed in Jezero crater, located at the
northwestern edge of the Isidis basin on Mars. The uppermost surface of the present-day crater floor is
dominated by a distinct geologic assemblage previously referred to as the dark-toned floor. It consists of
a smooth, dark-toned unit overlying and variably covering light-toned, roughly eroded deposits showing
evidence of discrete layers. In this study, we investigated the stratigraphic relations between materials
that comprise this assemblage, the main western delta deposit, as well as isolated mesas located east
of the main delta body that potentially represent delta remnants. A more detailed classification and
differentiation of crater floor units in Jezero and determination of their relative ages is vital for the
understanding of the geologic evolution of the crater system, and determination of the potential timeline
and environments of habitability. We have investigated unit contacts using topographic profiles and
DEMs as well as the distribution of small craters and fractures on the youngest portions of the crater floor.
Our results indicate that at least some of the deltaic deposition in Jezero postdates emplacement of the
uppermost surface of the crater floor assemblage. The inferred age of the floor assemblage can therefore
help to constrain the timing of the Jezero fluviolacustrine system, wherein at least some lake activity
postdates the age of the uppermost crater floor. We present hypotheses that can be tested by Perseverance
and can be used to advance our knowledge of the geologic evolution of the area.
Plain Language Summary On February 18, 2021 NASA's Perseverance rover landed in
Jezero crater on Mars. In the past, the crater was filled with water, forming a lake, and in the western
part of the crater an ancient delta is preserved. Part of the present-day crater floor has been interpreted
to represent a lava flow that was deposited after the lake dried out, meaning that the floor unit would be
younger than the western delta. In order to understand how the Jezero crater lake has developed over
time, including the potential timeline and environments of habitability, it is necessary to work out the
relations between the geologic units in Jezero crater. In this work, we have analyzed orbital images of
Jezero crater and reveal how the crater floor and delta deposits relate to each other in time. Our results
show that at least some of the deltaic deposits in Jezero overlie the youngest crater floor unit(s). It is
therefore possible to learn broadly when fluvial activity in the crater has been effective from the age of the
crater floor. Our work presents hypotheses that can be tested by Perseverance to advance our knowledge
of how the area has evolved geologically over time.
HOLM-ALWMARK ET AL.
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Stratigraphic Relationships in Jezero Crater, Mars:
Constraints on the Timing of Fluvial-Lacustrine Activity
From Orbital Observations
S. Holm-Alwmark1,2,3 , K. M. Kinch1 , M. D. Hansen1, S. Shahrzad4, K. Svennevig5 ,
W. J. Abbey6, R. B. Anderson7 , F. J. Calef III6 , S. Gupta8, E. Hauber9 ,
B. H. N. Horgan10 , L. C. Kah11 , J. Knade9, N. B. Miklusicak11, K. M. Stack6, V. Z. Sun6 ,
J. D. Tarnas6, and C. Quantin-Nataf12
1Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark, 2Natural History Museum Denmark,
University of Copenhagen, Copenhagen, Denmark, 3Department of Geology, Lund University, Lund, Sweden,
4School of Earth and Environment, University of Leeds, Leeds, UK, 5Geological Survey of Denmark and Greenland,
Copenhagen, Denmark, 6Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA, 7U.S.
Geological Survey, Astrogeology Center, Flagstaff, AZ, USA, 8Department of Earth Science and Engineering,
Imperial College London, London, UK, 9German Aerospace Center, Institute of Planetary Research, Berlin,
Germany, 10Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA,
11Department of Earth and Planetary Sciences, University of Tennessee-Knoxville, Knoxville, TN, USA, 12Laboratoire de
Géologie de Lyon -Terre, Planètes, Environnement, University of Lyon, Lyon, France
Key Points:
• We have studied stratigraphic
relations between geologic units in
Jezero crater for determination of
relative age relations in the crater
• Topographic profiles and digital
elevation models indicate that
the western delta is on top of the
youngest crater floor unit(s)
• We thus place constraints on the
timeline of fluvial-lacustrine activity
in Jezero crater
Correspondence to:
S. Holm-Alwmark,
sanna.alwmark@nbi.ku.dk
Citation:
Holm-Alwmark, S., Kinch, K. M.,
Hansen, M. D., Shahrzad, S.,
Svennevig, K., Abbey, W. J., etal. (2021).
Stratigraphic relationships in Jezero
crater, Mars: Constraints on the timing
of fluvial-lacustrine activity from orbital
observations. Journal of Geophysical
Research: Planets, 126, e2021JE006840.
https://doi.org/10.1029/2021JE006840
Received 23 JAN 2021
Accepted 8 JUN 2021
10.1029/2021JE006840
RESEARCH ARTICLE
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Journal of Geophysical Research: Planets
1. Introduction
Jezero crater is the landing site for the Mars 2020 Perseverance rover mission. The Perseverance rover,
which landed on Mars on February 18, represents the first step in a multimission program to collect, cache,
and return samples from Mars to Earth (Farley etal.,2020). The crater is ∼45km in diameter and located
at the northwestern edge of the Isidis basin, in the Nili Fossae region of Mars (at 18.4°N, 77.7°E; Figure1;
e.g., Bramble etal.,2017; Ehlmann, Mustard, Fassett, et al.,2008;Ehlmann, Mustard, Murchie, et al.,2008;
Fassett & Head,2005; Goudge etal.,2015,2017; Scheller & Ehlmann,2020; Stack etal.,2020). The presence
of an inlet valley entering the crater and an outlet valley exiting the crater, together with the preservation
of a fluvial deltaic sedimentary body (the western delta) at the mouth of the inlet valley implies a pro-
longed history of lacustrine activity within the crater (Figures1c and2; e.g., Fassett & Head,2005; Goudge
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Figure 1. (a) Regional overview map with MOLA elevation data (elevation in meters) of the area where Jezero crater is located (credit USGS Astrogeology
Science Center, Goddard Space Flight Center, NASA; https://planetarymaps.usgs.gov/mosaic/Mars_MGS_MOLA_ClrShade_merge_global_463m.tif). Elevation
in meters. (b) Color-coded elevation image of Jezero crater based on HRSC DEM and corresponding orthoimages. All elevations (in m) below Mars's areoid. (c)
HiRISE image mosaic close-up of the study area (Fergason, Galuszka, et al.,2020) with the Perseverance landing site marked with a star.
Journal of Geophysical Research: Planets
etal.,2015,2017; Mangold etal.,2020; Salese, Kleinhans, etal.,2020; Schon etal.,2012; Stack etal.,2020).
Additional sedimentary fan deposits present in the northern part of the crater are highly eroded and it is
presently not clear whether those deposits are alluvial or deltaic in origin, to what extent deposits are linked
to the western delta, or to what extent they may represent a discrete delta linked to a northern river inlet
(e.g., Goudge etal.,2015; Horgan etal.,2020; Salese, Kleinhans, etal.,2020; Schon etal.,2012).
Understanding the timing of deposition of sedimentary units in Jezero crater is vital to the understanding
of both hydrological and geologic history of the region, with broader implications for the entire planet.
Jezero crater contains a series of geomorphic/geologic units, including discrete, identifiable subunits, that
are distributed across the original crater floor (Table1 and Figure2; Stack etal.,2020). Investigation of
contacts relations and the relative elevation of these discrete subunits permits a first-order hypothesis of
the organization of the stratigraphy—and therefore the relative age—of the subunits (Stack etal.,2020).
Identified units include three morphologically distinct rough fractured units [crater floor fractured 1 (Cf-f-
1), crater floor fractured 2 (Cf-f-2), and crater floor fractured rough (Cf-fr)], and an undifferentiated smooth
unit (Us; Figures2 and 3). Us is a surficial unit that is interpreted to mantle bedrock in the map area of
Stack etal.(2020), e.g., on the crater rim, within and on the western delta in Jezero, and on the crater floor.
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Figure 2. (a) Photogeologic map showing bedrock units and surficial cover of the undifferentiated smooth (Us) unit on the crater floor fractured rough unit
in the study area in Jezero crater. Note that the Us unit is a surficial unit that mantles bedrock in the map area of Stack etal.(2020), e.g., on the crater rim, on
the western delta, and on the crater floor but that this map only shows the extent of the most prominent cover of Us on the Cf-fr unit, as this is the focus of this
study. Note the higher-standing blue features, possible delta remnants, here referred to as delta-associated remnant deposits, to the east of the main delta body.
Units discussed in this paper are Cf-f-1: crater floor fractured 1, Cf-f-2: crater floor fractured 2, Cf-fr: crater floor fractured rough, Us: undifferentiated smooth.
Figure modified after Stack etal.(2020). Us unit extent after Sun and Stack(2020b). (b–d) Surface expression of the Cf-f-1, Cf-fr, and Us units in Jezero crater.
HiRISE image mosaic base (Fergason, Galuszka, etal.,2020). HiRISE, High Resolution Imaging Science Experiment.
Journal of Geophysical Research: Planets
Although Us units mapped by Stack etal.(2020) generally have uniform textures and tones (at map scale),
these deposits were likely not all deposited at the same time, are not comprised of the same material or de-
posited in the same environment (Stack etal.,2020). Relations observed during detailed mapping suggests
that, in all cases, deposition/formation of the Us unit postdates deposition of the rough fractured units (Kah
etal.,2020; Stack etal.,2020; Warner, Schuyler, etal.,2020); the stratigraphic relationships among differen-
tiated rough fractured units, however, remains poorly constrained.
In previous studies, units identified here as Cf-fr and Us had been interpreted as a single, highly crater-re-
taining unit (e.g., “volcanic floor unit,” “dark-toned mafic floor;” Goudge etal.,2015; Shahrzad etal.,2019),
which has allowed previous authors to determine its age through crater counting. Although crater-counting
ages have varied widely, from 3.45Ga (Goudge etal.,2015) to 1.4Ga (Schon etal.,2012), more recent works
(Shahrzad etal.,2019; Warner, Schuyler, et al., 2020) provide strong arguments for an early Amazonian
age [2.6±0.5Ga; Shahrzad etal.,2019, 2.3±0.2Ga; Warner, Schuyler, etal.,2020; see also discussion in
Marchi(2021)]. In light of current mapping, we should remember that the 2.6 and 2.3Ga dates represents
crater counting on a surface composed of two stratigraphically distinct subunits. That is, the Us is inter-
preted as a dark mantling unit that variably drapes the surface of the light-toned Cf-fr unit by Stack etal.
(2020; Figures2 and3). Warner, Schuyler, etal.(2020b) interpreted the smooth upper subunit of the Cf-fr
[corresponding to Us in Stack etal.(2020)] in their study area in the center of the crater as a regolith. Crater
counting on in the central area of Jezero (Warner, Schuyler, etal.,2020) indicate that the surface is not at
equilibrium with surface processes, as 20–70m craters are being eroded faster than they are produced, and
recent work (Quantin-Nataf etal.,2021) highlights spatial variation in the distribution of craters on the
floor unit in Jezero, of the sizes that were counted by Shahrzad etal. (2019). Quantin-Nataf et al.(2021)
showed that the youngest crater floor unit has a complex history of cover and later exhumation, complicat-
ing interpretation of the crater statistics on the surface.
The aim of this study is to characterize the stratigraphic relations between the youngest crater floor units
in Jezero crater (Cf-fr and Us), the western delta stratigraphic units, and a set of possible remnant deposits
forming isolated mesas close to the western delta scarp that may represent lacustrine or deltaic deposits as-
sociated with the main delta (Figure2). According to the interpretation that the dark floor unit (Cf-fr+Us)
represents the youngest deposits (Goudge etal., 2015) inside of the crater, their emplacement age would
also define the timing of when fluvial activity inside Jezero crater ceased. We show how our new observa-
tions can be used to place more robust temporal constraints on geologic events that characterize Jezero cra-
ter. Finally, investigating specific features that modify units that make up the Jezero crater floor (e.g., crater
density distribution and the distribution of fractures), we extend our understanding of the origin of crater
floor units, and present detailed hypotheses that can be tested using the Perseverance rover.
2. Background and Geological Setting
The timing and nature of fluvial activity on Mars have been intensely debated (Kite, 2019; Palumbo
etal.,2020, and references therein). Studies of a variety of Martian valley networks, e.g., Craddock and
Howard(2002), Howard, Craddock and Moore,(2005), Hoke and Hynek(2009), Hynek etal.(2010), Davis
etal.(2016), and Salese, McMahon, etal.(2020) strongly suggest an active hydrologic cycle with prolonged
periods of precipitation, suggesting a different climate on Mars in the past. Although the nature of the
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Unit (this work; after Stack
etal.,2020)
Schon
etal.(2012) Goudge etal.(2015) Shahrzad etal.(2019) Horgan etal.(2020)
Sun and
Stack(2020b)
Warner,
Schuyler,
etal.(2020)
Undifferentiated smooth (US)
Volcanic unit Volcanic floor unit Dark-toned (mafic)
floor Mafic floor
Dark mantle Regolith
Crater floor fractured rough
(Cf-fr)
Jezero floor unit Mafic floor
unit
Crater floor fractured 1 (Cf-f-1) Basin fill Light-toned floor unit Light-toned floor unit Light-toned floor unit Lower etched —
Crater floor fractured 2 (Cf-f-2) Upper etched —
Table 1
Nomenclature of Crater-Fill Units in Jezero Used in This Study and Comparison With That Used in Examples of Older Literature
Journal of Geophysical Research: Planets
ancient Mars climate remains contested, e.g., Carr and Head (2003), and Howard etal.(2005), have pro-
posed that rapid melting of snow or ice could produce discharges consistent with valley network morpholo-
gies in the absence of a more traditional, warm fluvial system. Grau Galofre etal.(2020) recently suggested
that the majority of valley networks are a result of combined surface and subglacial runoff, relaxing the
requirement for warm surface conditions on the wet, ancient Mars. Regardless, such observations require
an atmosphere on Mars capable of maintaining an active hydrologic cycle that included water that was
stable on the surface.
Jezero crater is inferred to have maintained a standing body of water associated with a time of ongoing
fluvial activity on Mars (Fassett & Head,2008b). During this period, fluvial input along the western, and po-
tentially northern, margins of the crater supplied drainage of surficial fluids from the surrounding regions
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Figure 3. Close-up on HiRISE images (Fergason, Galuszka, etal.,2020) showing surface expression of the Us and Cf-fr units in Jezero crater, with decreasing
mantling (or presence of) the Us unit (highlighted by direction of arrows). (a) Smooth texture of the Us unit. (b) Transition from smooth Us-dominated surface
to heavily mantled Cf-fr unit. (c) Mantled. (d) Less mantling of Cf-fr unit with rough texture and crater retaining. (e) Rough texture of Cf-fr and with relatively
poor retention of craters and crater rims. North is up in all images. HiRISE, High Resolution Imaging Science Experiment.
Journal of Geophysical Research: Planets
(e.g., Mangold etal.,2020; Salese, Kleinhans, etal.,2020, and references therein); an outlet channel on the
eastern margin of Jezero may have resulted in episodic draining of the crater lake (Holo & Kite,2017; Salese
etal.,2019; Salese, Kleinhans, et al.,2020). The duration of deltaic deposition and the persistence of lacus-
trine activity in Jezero have been estimated to have occurred over several years to millions of years (Fassett
& Head,2005; Goudge etal.,2015; Lapôtre & Ielpi,2020; Mangold etal.,2020; Salese etal.,2019; Salese,
Kleinhans, et al.,2020; Schon etal.,2012). In terms of the timing of aqueous activity, Mangold etal.(2020)
were unable to definitely determine an age of delta formation, but concluded that the latest delta-related
deposits formed during a period of reactivation of the lower sections of the fluvial valley that fed the west-
ern delta in Jezero, and that this reactivation occurred sometime in the Late Noachian to Hesperian. Flu-
vial activity that carved the older, upper part of the valley likely commenced during the Middle Noachian
(Mangold etal.,2020). Such an interpretation is consistent with observations from Irwin, Craddock, How-
ard(2005), and Howard, Craddock & Moore(2005), Fassett and Head(2008a), Hoke and Hynek(2009), and
Mangold etal.(2020), who hypothesized that the majority of fluvial activity (and therefore paleolacustrine
activity; Fassett & Head,2008b) on Mars peaked around the Noachian-Hesperian boundary, and terminated
soon thereafter. In the time since fluvial and lacustrine activity ended, most paleolake deposits, including
those within Jezero, have experienced significant geological modification by processes that may include vol-
canism (Schon etal.,2012, and references therein), aeolian deposition, and erosion. Prior to the upcoming
in-situ investigation of Jezero crater by the Perseverance rover and potential sample return, critical analysis
of stratigraphic relationships in the surficial units of Jezero crater provides the best mechanism for building
a stratigraphic and temporal framework.
The age of Jezero crater can be bracketed by the formation of the Isidis basin (3.96±0.01Ga; Werner,2008)
and the emplacement of a regional olivine-carbonate unit that is inferred to be younger than Jezero crater
as it overlaps at least the northern rim of Jezero (3.82±0.07Ga; Goudge etal.,2015; Mandon etal.,2020),
which places it in the Noachian period (see also Ehlmann etal.,2009; Goudge et al., 2015). The crater
presently has a shallow depth profile compared to fresh Martian impact craters (Garvin etal.,2003; Schon
etal.,2012). The shallow depth profile suggests substantial crater-fill deposited on top of the original cra-
ter floor, which has an estimated thickness of about 1km (Ehlmann, Mustard, Fassett, etal.,2008; Schon
etal.,2012). These deposits would be expected to include a combination of different materials, including
impact ejecta, volcanic deposits, lacustrine sediments, alluvial sediments, and aeolian deposits that cover
the primary impactites (if not previously removed by erosion) and the entire central uplift of the crater.
Using scaling relationships presented by Whitehead etal.(2010), the height of the central uplift of Jezero
can be estimated to 0.33km (if crater diameter is 45km), although it should be noted that central uplift
height relative to diameter of crater has been shown to vary greatly on Mars [see, e.g., discussion in Calef
etal. (2016)].
2.1. Crater-Fill Units
In this study, we follow the naming scheme defined by Stack etal.(2020; see also Figure2). Table1 sum-
marizes the relevant units with the names used in this study, as well as names employed by a number of
previous authors.
All crater-fill units in Jezero crater may have a more complicated history than would be inferred by con-
tinuous deposition inside of the crater following the law of superposition. Following evidence from other
craters (e.g., Gale crater, Malin & Edgett,2000; Watkins etal.,2016), deposits may have been buried and
then later exhumed (see also Quantin-Nataf etal.,2021; Warner, Schuyler, etal.,2020), and the stratigraphic
succession in the crater is likely to contain a number of local to regional unconformities (Stack etal.,2020).
It is also possible that the units that can be observed from orbit may represent complex lateral interfingering
facies rather than traditional “layer cake” deposition (Stack etal.,2020). The units that are recognized as
exposed Jezero crater floor are described below.
2.1.1. Crater Floor Fractured 1 and 2
Units Cf-f-1 and Cf-f-2 (Figure2) are massive, light-toned units with meter-scale fractures. The contact
between the two is gradational. Stack etal.(2020) distinguishes the topographically higher Cf-f-2 from the
lower Cf-f-1 primarily by elevation, but also by the presence of small bumps and ridges that gives Cf-f-2
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a “pock-marked” texture. Previous publications (Goudge et al., 2015;
Horgan etal.,2020), group the two units together as “light-toned floor”
or “basin fill” (Schon etal., 2012). Cf-f-2 has textural and morphologi-
cal similarities with the Cf-fr (described below), but Cf-f-2 is significant-
ly less crater retaining than Cf-fr, at all crater scales. Spectrally, the Cf-f
units both display olivine and carbonate signatures, and so they are not
separated into two units by Horgan etal.(2020). The Cf-f units have been
interpreted as either a crater-filling material that is unique to Jezero by
Schon etal.(2012), or as part of the regionally extensive carbonate-bear-
ing and olivine-bearing unit that is prevalent throughout the Isidis-Nili
Fossae region (Brown etal.,2020; Goudge etal.,2012; Kremer etal.,2019;
Mandon etal.,2020). Given that spectral signatures of the Cf-f units are
broadly similar to those in the regional olivine-carbonate unit in the Jeze-
ro-area (Horgan etal.,2020), this indicates that there are two possible
scenarios for the units (Horgan etal.,2020): either that they represent re-
worked sediments from the regional olivine-carbonate unit, or that they
are the local expression of this regional unit.
2.1.2. Crater Floor Fractured Rough
Cf-fr is a crater-retaining unit interpreted as bedrock that is more resistant
to erosion, and contains fractures at two distinct scales (fracture lengths
of meters-and of several hundreds of meters; Stack etal.,2020). This unit
has lobate margins and surrounds current and formerly higher-standing
outcrops of Cf-f-1. It covers much of the Jezero crater floor, and is partial-
ly overlain by the Us unit, and appears to be underlain by Cf-f-1 (Stack
etal.,2020). Previous workers have generally interpreted Cf-fr and the
overlying smooth mantle (Us) as the same unit with the names “dark-
toned floor,” “mafic floor,” and “volcanic floor” all employed by different
authors (e.g., Goudge etal.,2015; Horgan etal.,2020; Schon etal.,2012;
Shahrzad etal.,2019). The dark tone that has been associated with this
unit, and led to the name “dark-toned floor, mafic floor” etc. (Table1), is
explained by Stack etal.(2020) as likely reflecting partial cover by a dark-
er mantling unit (Us). Spectrally, the uppermost crater floor units (i.e.,
both Cf-fr and Us) are characterized by a high-calcium pyroxene (HCP)
signal (Figure4a; Horgan etal.,2020). The HCP signal is interpreted to
be derived from the unit itself, in contrast to olivine-enriched streaks of
sediment extending across the unit that are likely derived from the Cf-f
units and low-calcium pyroxene (LCP) signal (Figure4b) associated with
deposition of material eroded from the western delta and delta-associated
remnant deposits (Horgan etal.,2020).
There are a variety of depositional mechanisms proposed for the Cf-fr
unit; igneous (e.g., Goudge etal.,2012,2015; Schon etal.,2012) as well as
fluvial or aeolian sedimentary (Kah etal.,2020; Stack etal.,2020). Several
authors (e.g., Goudge etal.,2015; Horgan etal.,2020; Sun & Stack,2020b)
have suggested a potential correlation between the crater floor unit and a
unit mapped outside of the crater, the Sun and Stack(2020b) Nili plains 2
unit and Bramble etal.'s (2017) mafic capping unit. If the Cf-fr unit is re-
lated to the geographically widespread Nili Planum capping unit (see also
Hundal etal.,2020; Sun & Stack,2020a,2020b), the unit spans too large
a range in elevation for a volcanic flow to be a viable emplacement mechanism, whereas airfall deposition
could explain the range in elevation (Sun & Stack,2020a,2020b).
A volcanic source for the Cf-fr (e.g., Goudge etal.,2012,2015; Schon etal.,2012) was proposed based on the
observations that the unit is topographically smooth and has a low regional slope; has lobate margins, high
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Figure 4. (a) Mafic spectral signatures from primary minerals showing
mineralogical differences between the delta, the Cf-fr+Us, and Cf-f units.
After Horgan etal.(2020). Green=low-calcium pyroxene, blue=high-
calcium pyroxene, red=olivine and Fe-carbonates. Frame is the same
as in Figure2b. (b) Distribution of spectral signals consistent with
low-calcium pyroxene in the western delta, crater floor units, and delta-
associated deposits (blue is low, red is high). (c) THEMIS-derived thermal
inertia map of part of Jezero crater. Note the general low thermal inertia
of the area close to the western delta edge, where the Us unit is most
prominent (see Fergason etal.,2006).
Journal of Geophysical Research: Planets
thermal inertia, a high capacity to retain small craters; and has CRISM spectral signatures are consistent
with volcanic rocks on Mars (Goudge etal.,2015; Horgan etal.,2020). Horgan etal.(2020) hypothesized
that, if the unit is a locally sourced volcanic lava flow or tephra, the source could be either dikes and fissures
in the crater floor that are covered by the unit or an enigmatic conical and cratered edifice located adjacent
to the southeastern rim of Jezero. Crater retention and morphometry on the Cf-fr unit(s) indicate that the
rock is resistant (Warner, Schuyler, etal.,2020). Warner, Schuyler, etal.(2020) compared their observations
from Jezero with the resistant basaltic lava plain at the InSight landing site showing important similarities
in crater morphology and time scales of erosion between the units, with one notable difference, the thick-
ness of the regolith, and suggested that the Cf-fr unit is possibly igneous rock.
Other authors have suggested that a lack of textures expected from volcanic units on Mars such as platy
textures or ridges (Golder, Miklusicak, etal.,2020), a variety of intermixed surface textures, the embayment
of topographically high regions of crater fill, and enhanced mantling in regional topographic lows are more
consistent with a different mode of formation, such as a low viscosity flow or by airfall/aeolian deposition
[Golder, Miklusicak, etal.,2020; Grant & Schultz,1990; Horgan etal.,2020; Kah etal.,2020; see also dis-
cussion in Stack etal.(2020)], and that many of these features may arise from a relation with dark-toned
mantling material (i.e., the Us). It should be noted that the specific comparative textures lacking on the Cf-fr
unit used in the analysis by Golder, Miklusicak, etal.(2020) are associated with Amazonian-aged lava flows
(e.g., Golder, Burr & Kattenhorn,2020; Jaeger etal.,2010; Keszthelyi etal., 2000), and primary volcanic
textures are not expected to be preserved over time scales that are relevant here. In fact, Golder, Miklusicak,
etal.(2020) noted that the textures on the unit were more similar to Hesperian to Noachian aged units in
the Eridania basin. At the InSight landing site only vague flow boundaries, and no primary surface textures
are associated with the basaltic material (the formation age of the smooth terrain covering the InSight land-
ing ellipse is Early Amazonian; Warner, Grant, etal.,2020).
2.1.3. Undifferentiated Smooth Surface Cover
Us is a dark-toned mantling unit that overlies and drapes the Cf-fr unit. The unit is characterized by a rel-
atively low thermal inertia (Figure4c; see also Stack etal.,2020). The Us unit has a smooth upper surface
and a complex relation with Cf-fr. Closer to the western delta deposit, the Us unit is inferred to be thicker,
as the surface has a smoother appearance (Figures2 and3) that lacks lighter-toned rough textures and large
craters (Stack etal.,2020). Stack etal.(2020) hypothesize that the unit may be a residual deposit from ash,
aeolian, or residual accumulation of sand, pebbles, and cobbles left after deflation of the landscape. Accord-
ing to the hypothesis of Warner, Schuyler, etal.(2020), that the Us is a regolith at least in their investigated
area in the center of Jezero, enhanced mantling in topographic lows could result from enhanced preserva-
tion of a smooth regolith in regions that are protected from wind deflation. According to this hypothesis,
the larger-scale transition from smoother to rougher texture is caused by incomplete regolith deflation or
exhumation of the Cf-fr unit. Areas of the Us unit with the smoothest appearance stand out as exhibiting
clear CRISM spectral signatures of HCP (Figure4a). Because HCP signatures are rare on the delta and ab-
sent in the Cf-f units, this may suggest a distinct source for Us materials (Horgan etal.,2020). There is also
a low-calcium pyroxene (LCP) spectral signature on the Us, especially prominent on the area closest to the
delta scarp (Figure4b).
Variations in surface textures on the Cf-fr and Us units are most obvious when it comes to smoothness (Fig-
ures2 and3; see also, e.g., Golder, Miklusicak, etal.,2020; Kah etal.,2020; Stack etal.,2020). The smooth
texture is most prominent close to the edge of the western delta, and the smooth regions grade into regions
that expose progressively more of the underlying unit (Figure3; see, e.g., Golder, Miklusicak, etal.,2020;
Kah etal.,2020). Craters from the underlying unit are argued to be variably draped and show up as “ghost
craters” as the Us gets thicker by Kah etal.(2020). Warner, Schuyler, etal.(2020), on the other hand, de-
scribe these as degraded features resulting from regolith development and aeolian infilling. Increased expo-
sure of Cf-f1/2 unit is shown by CRISM signatures displaying stronger olivine signal and weaker pyroxene
signals (Horgan etal.,2020; see also Figure4a).
In this study, we analyzed features of the Cf-fr and Us units. Although these two units are clearly distinct,
ambiguities regarding the complex relation between these units and the nature of both units hinder broad-
scale interpretation. For discussion purposes, we follow earlier work (e.g., Goudge etal.,2015; Shahrzad
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etal., 2019) and often lump these together. When discussed as a single composite unit, we refer to these
units as “the youngest crater floor units.”
2.2. Western Delta Deposit
The western delta in Jezero crater has been interpreted as a river delta deposit that built out into a lake that
existed in the crater (Ehlmann, Mustard, Fassett, etal.,2008; Fassett & Head,2005; Goudge et al.,2017;
Schon etal.,2012). The deposit has a fan-like plan view geometry (Fassett & Head,2005); its surface displays
pervasive layering and cross-cutting linear ridges that are interpreted as fluvial channel-belt deposits on its
surface (Goudge etal.,2018). The eastern edge of the western delta deposit forms a distinct erosional scarp
(Fassett & Head,2005; Schon etal.,2012) that exposes internal layering within its walls. Schon etal.(2012)
estimate that the outer third of the delta complex, specifically delta front deposition and the prodelta de-
posits that are potentially composed of the highest concentration of clay-size particles, have been largely
removed by aeolian erosion. Lakeward of the present-day delta edge, there are also numerous isolated hills
and mesas that have been interpreted as either remnants of delta deposits or isolated deposits of lacustrine
strata that were deposited at the same time as primary deltaic deposition (Fassett & Head, 2005; Schon
etal., 2012; Stack etal.,2020, Figure2). Given the uncertainty associated with the genesis of these hills,
they are perhaps better described as “delta-associated sedimentary deposits” to leave open the possibility
that they may be lacustrine in origin rather than true delta remnants. Therefore, we will refer to these as
“delta-associated remnant deposits.”
The first detailed studies of Jezero crater proposed that the dark crater floor material (Cf-fr and Us) was
emplaced after deltaic activity (Goudge etal.,2015; Schon etal.,2012; Figure5). Goudge etal.(2015) placed
the Cf-fr and Us units stratigraphically higher than the delta on the basis that the units appear to embay
the western delta and the northern fan in Jezero. More recently, different temporal relationships between
the lithological units in Jezero have been proposed, including hypotheses that crater floor units could pre-
date deltaic deposition (Ruff,2017), or that they predate at least a portion of the delta deposits (Horgan
etal.,2020; Miklusicak etal.,2020; see also Sun & Stack,2020a). Stack etal.(2020) presented four scenarios
for the stratigraphic relations of units within Jezero crater (Figure5). One, their scenario 3, is consistent
with the previously published interpretation (i.e., Goudge etal.,2015), whereas the other three—including
the possibility that the Cf-fr was deposited before the delta, but the Us formed after delta deposition—re-
quire more complex depositional relations, and suggest more complex age interpretations, between the
Jezero delta and other crater-fill units.
3. Data and Methods
We have studied geological features on the delta, Cf-fr, and Us units in Jezero crater on the Mars 2020 Ter-
rain Relative Navigation HiRISE Orthorectified Image Mosaic (Fergason, Galuszka, etal.,2020). The six
HiRISE stereo pairs on this mosaic include: Jezero_E: ESP_048908_1985 and ESP_048842_1985; Jezero_N:
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Figure 5. Hypotheses for the stratigraphic relations between geologic units in Jezero crater presented by Goudge etal.(2015) and Stack etal.(2020). Cf-fr:
crater floor fractured rough unit. Us: undifferentiated smooth unit. Cf-f-1: crater floor fractured 1 unit. Cf-f-2: crater floor fractured 2 unit.
Crater unknown
contact
Cf-f-1+2
Delta
Cf-fr(+Us)
Goudge et al. (2015)
unconformity
Stack et al. (2020) #1
Crater rim
Cf-f-1
Cf-f-2
conformable
contact
Delta
Cf-fr
Crater rim
Stack et al. (2020) #2
Cf-f-1
Cf-f-2
Cf-fr
Delta
Crater rim
Delta
flooding
surface
Cf-f-2 Cf-fr
Cf-f-1
Interfingering
facies
Stack et al. (2020) #4
Relative age
Journal of Geophysical Research: Planets
ESP_037818_1990 and ESP_037330_1990; Jezero_C: ESP_046060_1985 and ESP_045994_1985; Jezero_DL:
PSP_003798_1985 and PSP_002387_1985; Jezero_W: ESP_042315_1985 and ESP_037396_1985; and Jeze-
ro_CR: ESP_037119_1985 and ESP_036618_1985. All HiRISE image observations reported in this study
(except crater counting and fracture mapping, see below) were made on this image mosaic.
3.1. Definitions
In our stratigraphic analysis in Jezero crater, we focused on the nature of the geological contacts. We de-
scribe the absence or presence of marginal depressions, i.e., “erosional moats,” at unit contacts. Erosional
moats are topographic depressions that result from differential erosion of adjacent geologic units. If high-
er-standing relatively less resistant material is eroded, at the contact of an embaying, or surrounding, unit
with higher resistance, a moat appears as the topographically higher material is removed (see, e.g., Chap-
man etal.,2010; Hauber etal.,2008; Ruff,2017). For example, if lava flows up to an island of existing, less
resistant, material, a moat will appear at the unit contact as the older less resistant material is removed.
Ruff(2017) pointed out the similarities between such stratigraphic contacts in Gusev crater and Jezero cra-
ter, with a focus on moats and “kipukas.” Kipukas are islands of higher-standing geologic material embayed
by younger lower-standing unit(s). We are not using the word kipuka in this work, because it implies both
a stratigraphic relationship between adjacent units, and a specific geologic origin (i.e., as a lava) of the em-
baying unit, which in this case would be the youngest crater floor unit(s).
3.2. Topographic Analysis/Profiles
The image data that we have used in this study are from the Context Camera (CTX; Malin etal.,2007) and
High Resolution Imaging Science Experiment (HiRISE; McEwen etal.,2003) instruments on the Mars Re-
connaissance Orbiter. Jezero crater was mapped with CTX images (5m/px) as well as HiRISE visible image
basemap (25cm/px) and derived digital elevation models (DEMs; 1m/px; Fergason, Galuszka, etal.,2020;
Fergason, Hare, et al.,2020) acquired from web-based GIS tool CAMP (for source files for this tool, see Data
availability section below; see also Calef etal.,2019; Stack etal.,2020). The data were processed in a Geo-
graphical Information System (GIS) software (QGIS (http://qgis.osgeo.org) and ArcMap) and cross-section
profiles were constructed across unit boundaries.
3.3. Fracture Mapping
Occurrence of fractures on the Cf-fr and Us units were investigated on HiRISE images. Mapping was per-
formed by marking fractures as polyline features in the GIS program ArcMap. A specific scale (1:3,000)
was used for consistency during mapping of the fractures, which is similar to the 1:5,000 scale used during
the mapping of Jezero crater by Miklusicak etal. (2020). The fractures were mapped on the area broadly
recognized as the “dark-toned floor” of Goudge etal.(2015), here interpreted as the combined Cf-fr and
Us. Mapped fractures represent a suite of through-going fractures, with lengths generally greater than tens
to hundreds of meters (e.g., Miklusicak etal., 2020) Rather than presenting a thorough inventory of all
fractures, mapping here was performed with the purpose of getting an overview of the primary orientations
represented by easily visible fractures.
3.4. Crater Counting
The craters used for crater counting on three selected areas were mapped on HiRISE imagery (image IDs are
034495, area 1; 023168, area 2; 023524, area 3) in conjunction with HiRISE DEMs (McEwen etal.,2003) in
ArcMap. Each crater was marked manually, using the CraterTools (Kneissl etal.,2011) plug-in to ArcMap.
Using this tool, all features that could be safely determined as craters above a minimum diameter of 10m
were counted on the selected areas using three points along the crater rim to produce a circle.
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3.5. Spectral Endmember Calculation
CRISM band parameters from Viviano-Beck etal. (2014) to isolate CRISM pixels used for spectral end-
member extraction. Band parameters were calculated for image HRL000040FF using the CRISM Analysis
Toolkit (CAT). We used the LCPINDEX2 parameter to illustrate the distribution of spectral signals consist-
ent with LCP in the CRISM scene in Figure4b.
4. Results
We analyzed the stratigraphic relations and reconstructed the sequence of events in Jezero crater by a series
of investigations: based (1) on observations from profiles extending from the delta out onto the crater floor,
(2) on crater counts, (3) on fracture patterns on the crater floor. For our discussion, we also use: (4) CRISM
spectral data and thermal inertia and (5) comparative studies of stratigraphic relations in other craters. Our
results are presented below.
4.1. DEM Analysis and Profiles
Boundaries between, and features of, units inside Jezero crater were investigated by cross-section profiles
and analysis of DEMs from HiRISE scenes (Figures6 and7). Since the Us unit is so thin (∼5 m; Golder,
Miklusicak, etal.,2020) elevation profiles likely reflect the topography of the Cf-fr unit, and not topographic
features that are related primarily to the Us unit.
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Figure 6. DEM of the study area in Jezero crater with the western delta in the left side of the image. Delta-associated
remnant deposits are higher-standing features on the crater floor and five of these, labeled A-F with white bold text, are
discussed in this work. Transects A-A′, B-B′, C-C′, and D-D′ represent the location of cross-sections shown in Figure7.
White arrows point to topographic depressions (“moats”) at unit contacts, as discussed in the text. For data used to
produce the DEM, see Malin etal.(2007), Fergason, Hare, etal.(2020), and the Data availability section for more
information. DEM, digital elevation model.
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Figure 7. (a–f) Topographic profiles (see locations in Figure6) from the western delta in Jezero out onto the crater floor. Lower version of profile at 3 × vertical
exaggeration (VE). Colors of the line corresponds to the individual geologic units (see key). Possible past extensions of the delta are shown as blue dashed lines,
and the regional surface slope of the Cf-fr(+Us) unit is shown as beige dashed line.
Journal of Geophysical Research: Planets
The uppermost crater floor unit and the delta scarp are separated by 30–40m in elevation, with the delta
having the higher elevation. The two units are separated by sedimentary cover resulting from erosion of the
steep delta scarp. The steepness of the western delta scarp immediately adjacent to the delta-crater floor
boundary varies between 6° and 27°. The contact between the delta scarp and the youngest crater floor units
includes both smooth and more abrupt transitions, but there is no clear evidence of a localized topographic
low (i.e., a “moat”) at the contact between the delta scarp and the youngest crater floor units.
By contrast, the contact between Cf-f-1 and Cf-fr-Us is in some places characterized by distinct narrow
topographic moats (Figures6,7b and7f). Such moats are more prevalent at the western side of these expo-
sures; the eastern sides are more commonly covered by aeolian sediment, resulting from sediment transport
by a prevailing easterly wind (see Day & Dorn,2019 for discussion on wind directions in Jezero; Figure6).
Although these topographic moats are shallow (on the order of ∼10m deep; compare Figure7a with7b,
and7e with7f), they form a prominent feature of these contacts (Figure6).
Low hillocks east of the delta scarp, here referred to as “delta-associated remnant deposits,” and labeled
A–F, are high-standing topographic features located on the crater floor, both close to the western delta
deposit, and somewhat further to the east (Figures6 and 8). These sedimentary deposits show a range
of morphological expressions. Deposit A displays a cone-shaped cross-section profile and clear internal
stratification (Figure8a), similar to deposit F (Figure8b). Deposits located further to the east, specifically
B–D (Figures8c–8e), also display internal stratification, but have less distinctive overall morphologies. The
deposits trap bedforms along their flank slopes, obscuring the contact to the crater floor. Still, in all instanc-
es, the boundary between delta-associated remnant deposits and the adjacent Cf-fr-Us deposits is marked
by a smooth change in topographic slope on all sides. As with the edge of the main delta body, there is no
clear evidence of topographic moats at the boundary between delta-associated remnant deposits and the
youngest crater floor units.
Our topographic analysis also shows that the youngest crater floor units adjacent to the delta edge slope
away from the delta edge, and the slope is the same as the slope of the present delta surface itself (Figures7c
and7d). The slope is observed along axes that are radial to the delta rather than uniform across the Cf-fr-Us
units. Regionally, the crater floor preferentially slopes north to south, rather than east to west (Figure9).
4.2. Fractures
The crater floor in Jezero is fractured at two different spatial scales (Figures10a and10b). Small-scale po-
lygonal fractures are prevalent across the Cf-fr unit, and through-going linear fractures that intersect both
Cf-fr and regions of the crater floor mantled by Us materials (Stack etal.,2020). In this investigation, we
focus only on these through-going linear fractures. At the mapping scale we employed, the fractures are
typically between 1 and 2m in width and range from a few hundred meter to more than 1km in length.
Fracture intersections are often orthogonal, and show both offset and cross-cutting relationships (cf. Fig-
ures11a and11b), which could indicate multiple fracture generations (Peacock etal.,2018). More rarely,
fractures can terminate at the intersection with another fracture (Figure11a), which may also indicate dif-
ferent fracture generations. Fractures also appear to intersect the ejecta and rims of craters exposed in the
Cf-fr-Us units (cf. Figure11c). These large, linear fractures show distinct differences in their distribution
across the Jezero crater floor. Most notably, large, linear fractures are recorded only within the region of the
crater floor defined by the combined Cf-fr and Us map units (Figure10c). Fractures also appear to be more
concentrated along the delta front, and appear more sparsely to the SE. This distribution, however, does not
necessarily reflect a lack of fractures; fractures are more easily identifiable in regions near the delta front,
which are more heavily mantled by Us, and less easily identifiable in regions dominated by Cf-fr. Across the
crater floor, there is no clear orientation for these larger, linear fractures, although fractures with the longest
exposures favor a NW-SE orientation (Figure10d; see also Miklusicak etal.,2020). In total, more than 1,700
fractures were mapped.
More detailed examination of fractures in the NE region of the Jezero crater floor, where fracturing is most
dense, shows distinct patterns of fracture orientation between the edge of the western delta and the lobate
margins of the Cf-fr-Us units (Figure10c), where fractures appear to parallel the delta scarp (Figures10c
and10d). A similar pattern was observed by Miklusicak etal.(2020), who showed that fracture orientation
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shown in the southern portion of Figure10c insetalso parallel the lobate margin of the Cf-fr-Us unit to the
southeast, and are consistent with a broader pattern of fractures that occur parallel to the lobate margin of
the Cf-fr-Us unit(s). We have not observed such a relationship between fracture orientation and the delta
edge further to the north, suggesting that apparent alignment of fractures to the delta front may simply re-
flect that the delta scarp itself in that area has the same overall shape as the lobate margin of the Cf-fr and
outcrop of Cf-f-1 southeast of the delta scarp (Figures1c and2).
An interesting relationship also occurs between the fractured Cf-fr-Us unit and outcroppings of the delta
and delta-associated remnant deposits. First, fractures typically approach the delta front at high angles,
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Figure 8. Photos of delta-associated remnant deposits (HiRISE image mosaic; Fergason, Galuszka, etal.,2020).
(a) Deposit A, located just east of the western delta scarp. Note prominent cone-shape. Arrows point to internal
stratification and fracture that terminates at the base of the deposit. (b) Deposit F, located southeast of the western delta
scarp. Arrows point to internal stratification and fractures that terminate at the base of the deposit. (c) Deposits B–E,
located further east of the western delta scarp. (d) Close-up of deposit E, with arrow pointing to a fracture that crosses
the deposit. (e) Internal stratification in deposit C. North is up in all images. See Figure1c for locations. HiRISE, High
Resolution Imaging Science Experiment.
Journal of Geophysical Research: Planets
where they either terminate abruptly or are buried by sediment either eroded from the delta edge or depos-
ited along this topographic margin as aeolian dunes (Figure11d; see also Miklusicak etal.,2020). We also
observe that fractures do not cross delta-associated remnant deposits A and F (Figures8a and8b), but either
end at the base or terminate underneath the deposit, although at least some fractures are observed to cross
delta-associated remnant deposits located further to the east (Figure8d).
4.3. Craters
The density of craters on the Cf-fr and Us units varies across the surface, which at least partially is caused
by presence, or extent of, the Us unit on Cf-fr. There is a marked difference apparent to the naked eye espe-
cially near the western delta edge. All craters above a minimum diameter of 10 m were counted on three
different areas, not for quantitative reasons or for establishing an age, but for displaying the differences in
the number of small craters on different areas of the crater floor in Jezero (Figure12). Craters that were
counted on the three selected areas are 10–370m in diameter, and for each area the spread in crater diam-
eters is 10–331m (area 1), 10–370m (area 2), and 16–352m (area 3). Since the area counted is limited in
size (area 1: 15.35km2, 2: 8.58km2, 3: 8.19km2), we are aware that there are limitations to our observations,
but comparisons between the areas is still possible, and we hope to resolve information about the local
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Figure 9. Topographic map of part of Jezero crater showing the slope direction of the crater floor and the landing ellipse of the Mars 2020 mission. Red lines
are 50m contours and colors correspond to topography (blue=low, green to yellow=moderate, red=high). Note that the crater floor preferentially slopes
north to south, rather than east to west.
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Figure 10. (a) Examples of through-going linear fractures on the Cf-fr and Us units in Jezero crater. (b) Smaller-scale polygonal fracturing characterizing the
Cf-fr unit (a and b with HiRISE image mosaic as background; Fergason, Galuszka, etal.,2020). (c) Overview of mapped through-going linear fractures (in
yellow) on the Cf-fr and Us units in Jezero crater shown on CTX image. Mapped fractures are typically ∼1m in width and ∼a few hundred m in length. The red
box in the bottom right corner is a close-up of the area where fractures have different directions than the general NW/SE direction on the unit. Black line after
Goudge etal.(2015), marking the “volcanic floor unit.” (d) Rose diagram showing the trends of the all the mapped fractures from (c). HiRISE, High Resolution
Imaging Science Experiment; CTX, Context Camera.
60
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Figure 11. Fractures on the Cf-fr and Us units on HiRISE image mosaic (Fergason, Galuszka, etal.,2020). (a) Close-up showing fractures with different general
directions. White arrows point to offset relations between possible different fracture generations. Black arrows point to abutting relations between (possible)
different fracture generations. (b) Close-up showing fractures that cross small impact crater on the crater floor. Arrows point to offset relations between
possible different generations of fractures. (c) Fractures crossing small impact craters on the Cf-fr unit. (d) Close-up of fractures at the delta scarp. Note lack of
reorientation before they disappear. See Figure1c for location. HiRISE, High Resolution Imaging Science Experiment.
Journal of Geophysical Research: Planets
processes, or processes that affect only a small area [see discussion in, e.g., Kukkonen and Kostama(2018)].
Closer to the delta scarp, only 154 craters were counted (corresponding to a frequency of ∼19craters/km2;
Figure12), and this is where the Us unit is most prominent. On the two other areas, labeled as 1 and 2, 1,028
(∼67craters/km2) and 1,269 (∼148 craters/km2) craters were counted, corresponding to a difference of a
factor of ∼8, respectively.
Small craters, 10–20m in diameter, on the smooth surface immediately east of the delta scarp exhibit non-
rocky rims and ejecta blankets. At crater sizes of 25–30m, rims and ejecta appear rocky (Figure13; compare
also Warner, Schuyler, etal.,2020).
5. Interpretation and Discussion
5.1. What Can Fractures on Cf-fr and Us Tell us About Stratigraphic Relations in Jezero Crater?
Several observations regarding large, linear fractures observed in the Jezero crater floor may help us in-
terpret stratigraphic relationships. First, these fractures intersect both the combined CF-fr+Us unit and,
critically, the crater rims and ejecta that are exposed within these units (Figure11c). These observations in-
dicate that fracture formation is younger than both the deposition and cratering age of these floor materials.
Second, on the regional scale, there appears to be no clear orientation to these large, linear fractures or any
clear pattern of their intersections. These observations suggest that the origin of these fractures results from
hydrofracture (i.e., the fracture of materials from the overpressuring of subsurface fluids; Gudmundsson
etal.,2002; Mandl,2005; Philipp etal.,2013). Finally, on a more local scale, observation of an apparent re-
lationship between fracture orientation and the lobate margins of the combined Cf-fr+Us unit (Figure10,
see also Figures2 and6), provides critical data on the stress fields at the time of fracturing.
The pattern and orientation of fractures results from both the mechanism of fracture formation and
the mechanical properties of the material being fractured (e.g., Goehring,2013, and references therein;
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Figure 12. Close-up of the northern part of the crater floor in Jezero crater (CTX image). Areas marked with blue are
areas where craters were counted. The areas are of similar size, but note the difference in number of craters counted in
the area to the west compared to the two areas to the east. Total number of counted craters in area 1: 1,028, 2: 1,269, 3:
154. Black line after Goudge etal.(2015), marking the “volcanic floor unit.” CTX, Context Camera.
Journal of Geophysical Research: Planets
Gudmundsson,2011; Mandl,2005). Fractures that result from tensional stresses within a single unit, such
as those that occur with cooling of igneous units, desiccation of water-rich sediment, or thermal expansion
and contraction tend to form discrete polygonal networks with a dominance of 120° intersection angles
(Goehring,2013; Kronyak, Kah, Miklusicak etal.,2019), and tectonic fractures are most easily recognized
by a clear pattern of orientation that relates to the tectonic stress field (e.g., Gudmundsson,2011, and ref-
erences therein). By contrast, hydrofracture events most commonly lack clear orientations, except when af-
fected by stresses associated with the boundaries of a geologic unit (Gudmundsson etal.,2002; Mandl,2005;
Philipp etal.,2013).
Observations that fractures locally appear to be oriented both parallel and perpendicular to the lobate edges
of the Cf-fr+Us unit are consistent with differential stresses along the natural edge of the Cf-fr+Us unit
(cf. Miklusicak etal.,2020). If this is the case, then if the CF-fr+Us unit were younger than the Jezero delta
and delta-associated remnant deposits, we would expect to see a similar fracture pattern at these margins.
Such a fracture pattern, however, is not observed along the entire length of the delta front, suggesting that
the fracturing process (which must occur after primary deposition of the Cf-fr unit) was unaffected by the
presence of deltaic sediments. More commonly, we see fractures abruptly disappear under the bedform and
talus along the boundary of the delta scarp, with no changes in fracture orientation or morphology, even
when fractures can be seen traced between aeolian bedforms otherwise obscuring the contact (Figure11d).
If the western delta was already deposited at the time of the fracturing process, the weight of the sediment
would be expected to create stress fields in the crater floor material leading to fracture reorientation, im-
plying that deltaic sediments had yet to be deposited. We therefore suggest that these observations are most
consistent with a scenario in which the fractures continue underneath the delta and delta-associated rem-
nant deposits, and were simply buried during deltaic sedimentation.
Additionally, we do not observe propagation of fractures upward from the Jezero crater floor into overlying
sedimentary materials of the delta and delta-associated remnant deposits. It is common for the hydrofrac-
tures to be affected by the mechanical properties of the rock units through which they propagate (Weert-
man, 1980). Most commonly, hydrofractures are observed to terminate at contacts where the overlying
material has a strength sufficient to arrest fracture propagation (Gudmundsson & Brenner,2001; Kronyak,
Kah, Edgett, etal.,2019). The inferred fine-grained nature of sedimentary materials associated with the
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Figure 13. Differences in roughness of ejecta and rim blockyness of small craters on the Us unit immediately adjacent
to the western delta scarp in Jezero crater. Note blockyness of ejecta associated with the fresh crater in the center of the
photo. See Figure1c for location. HiRISE image mosaic (Fergason, Galuszka, etal.,2020). HiRISE, High Resolution
Imaging Science Experiment.
Journal of Geophysical Research: Planets
delta front and delta-associated remnant deposits (e.g., Goudge etal.,2017), however, and the extensive ero-
sion of these units, suggest that the strength of these units would be insufficient to cause arrest of fracture
propagation. If, on the other hand, fractures formed after deposition of the delta and delta-associated rem-
nant deposits, the absence of visible fractures in these units would suggest that materials were sufficient-
ly weak that pressures rapidly dissipated via formation of abundant small-scale fractures. Such fractures
are not visible in orbital imaging, and our combined observations are more consistent with a scenario in
which deltaic deposits are wholly younger than the fractured crater floor. The alternative scenario presented
above, however, can be tested by the Perseverance rover when it reaches the delta front.
5.2. What Can We Learn From Stratigraphic Relations in Other Craters?
Investigation of the nature of, and stratigraphic relationships between, rock units in Gale crater by the Cu-
riosity science team (e.g., Grotzinger etal.,2015; Stack etal.,2016) resulted in the recognition of a former
standing body of water and an understanding that the body of water could have persisted for long enough
time to have been relevant for habitability. In Magong crater, Xanthe Terra, a flat, polygonally fractured floor
has been exposed where active erosion of the Sabrina Vallis fan results in a moat between the fan and the
crater floor unit (Knade etal.,2017, Figure14). This moat marks the eroded margin of the fan, at least along
the entire southern and eastern sides (Knade etal.,2017) and it is interpreted that the dark-toned, mafic cra-
ter floor unit is both younger than the fan and more erosionally resistant than the fan. These observations
thus allow constraints of stratigraphic relationships between fan and crater floor units in Magong crater.
We do not observe moats between the delta and youngest crater floor units in Jezero crater, either where
the delta is eroding away, exposing the underlying units, at the western delta scarp, or in association with
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Figure 14. Magong crater, Xanthe Terra. (a) Merged HiRISE DEM showing the Sabrina Vallis fan and the mafic crater floor unit, with the moat separating
the two units. (b and c) Detail of the moat showing the exposed polygonally fractured unit in the moat that forms as the fan sediments erode. HiRISE, High
Resolution Imaging Science Experiment.
Journal of Geophysical Research: Planets
delta-associated remnant deposits (Figure7). The Cf-fr and Us units consistently continue all the way up
to the sediment cover at the base of the western delta scarp, with no underlying unit exposed in between,
either because it is not present or is covered by sediments (see, e.g., Figures8a and8b). If such a moat is
present but obscured by talus and aeolian sediments, it is clear that there was very limited scarp retreat since
emplacement of the youngest crater floor unit (compare the exposed moat in Magong crater; Figure14).
The youngest crater floor unit(s) either formed ∼2–2.5Ga, or has been exposed since that time (Shahrzad
etal.,2019; Warner, Schuyler, etal.,2020), and insignificant scarp retreat since then is not consistent with
erosion rate estimates (e.g., Golombek & Bridges,2000; Golombek etal.,2006; Ramirez etal.,2020), espe-
cially of nonresistant deltaic material which is known to erode relatively fast given the lack of small craters
on the delta (Mangold etal.,2020). Moats at contacts between a young resistant unit overlying an older less
resistant unit in Gusev crater can be compared with features in Jezero (Ruff,2017). Ruff(2017) noted the
presence of moats between kipukas and the crater floor lava flows in Gusev, and that such moats were also
present in Jezero, but only at contacts between the Cf-f unit and the Cf-fr/Us. Ruff(2017) suggested that one
explanation for the lack of moats at delta-associated remnant deposits can be explained by the hypothesis
that the delta is older than the floor if the rate of erosion of the embaying Cf-fr+Us unit keeps pace with or
outpaces that of the delta-associated remnant deposit material, which is inconsistent with observations of
crater retention between the units, and that the Cf-f erodes more easily than the delta-associated remnant
deposits. However, the lack of moats around the western delta and delta-associated remnant deposits in
Jezero (Figure7) is also consistent with the current expression of the delta being younger than the floor
and overlying the floor units, something that was also noted by Ruff(2017). As a comparison, moats can be
easily observed around the western margins of the Cf-f-1 unit in a map showing elevation relationships in
Jezero (Figure6). There are no western margins between the main western delta body and the crater floor
units, but moats are lacking on all sides of the delta-associated remnant deposits.
Combined, the absence of an erosional moat and inferences based on fracture orientation are consistent
with a scenario in which at least some sedimentary rocks of the delta deposit postdate deposition (and po-
tentially fracturing) of the youngest crater floor units.
5.3. Constraining the Relative Timing of Events in Jezero
The relative age of the western delta in Jezero has been estimated by buffered crater counting of river
valleys (Fassett & Head,2008a) and by crater counts on the delta itself (Mangold etal., 2020), but can be
further constrained by the inferred relation between deltaic sediments and the crater floor unit. Fassett and
Head(2008a) concluded based on buffered crater counts of inlet valleys, that fluvial activity ceased about
3.8Ga, and Mangold etal.(2020) presented an age of the delta itself of 3.2–3.6Ga based on crater counting.
These crater count ages would seem to argue that the delta predates emplacement of the youngest crater
floor unit (Cf-fr and Us), although the 3.2–3.6Ga age for the delta is close to the age of the floor, if the full
age within error is considered [2.6±0.5Ga; Shahrzad etal.,2019; see also discussion in Marchi(2021)].
Dating small surface areas such as the Jezero western delta using crater counts involves significant uncer-
tainty (Palucis etal.,2020; Warner etal.,2015; see also Quantin-Nataf etal.,2019).
A late-stage reactivation of the proximal section of the fluvial system feeding sediment into Jezero has been
proposed from differences in valley morphology (Mangold etal.,2020). Reactivation of the valley's lower
sections could lead to renewed progradation of the delta and sediment accumulation on the delta surface,
without significantly affecting the buffered crater counts on the valley system as also argued by Mangold
etal.(2020). Many craters on the delta surface are formed onto the youngest ridge-formed deposits inferred
to be former river channel-belt fill, and thus postdate the last phase of fluvial activity, but there are some
examples of craters on the delta that show possible evidence of modification by fluvial activity and thus may
predate some fluvial activity (Figure15). We note, e.g., that there are channels that appear to cross craters
(Figure15c), though more robust analysis is needed. Horgan etal.(2020) stated that the state of preservation
of the western delta deposit—by comparison to the northern crater-edge sedimentary package (northern
fan)—could potentially be explained by the uppermost exposures being significantly younger than strati-
graphically lower deposits, or that they could be potentially older delta deposits preserved in the northern
part of the crater. This is consistent with the understanding that sporadic fluvial activity on Mars potentially
extended into the Hesperian/Amazonian (e.g., Goddard etal.,2014; Grotzinger etal.,2013,2015; Hauber
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etal.,2013; Kite etal.,2019; Salese, Di Achille, etal.,2016; see also discussion below). A scenario of multi-
ple episodes of deposition forming the western delta needs validation from observations on the surface in
Jezero, or could even require sample return. Nevertheless, we want to highlight here that discharge fluctu-
ations leading to intermittency is likely a relevant factor if the delta developed over an extended period of
time, as opposed to Jezero lake formation due to a catastrophic event (see discussion in, Salese, Kleinhans,
etal.,2020). Such a scenario may also be consistent with distinct differences in the dominant primary min-
eralogy observed in different delta facies, which could suggest major changes in the watershed geology over
extended periods of time (Horgan etal.,2020).
The derived age of the floor can also be questioned. It is possible that the Cf-fr unit was covered at some
point, and has been gradually exposed for an extended period of time (Quantin-Nataf etal.,2021; Warner,
Schuyler, etal.,2020). This would mean that an overlying protective unit(s) has prevented new craters from
forming on the Cf-fr unit and that the derived model age therefore represents an “exposure age” of the sur-
face rather than continuous crater accumulation since the original deposition of the unit. The crater data
(Shahrzad etal.,2019; Warner, Schuyler, etal.,2020) indicate that the Cf-fr unit must have been mostly ex-
humed and exposed to erosion by 2.0–2.5Ga to explain the crater counts. Erosion of the unit is still ongoing,
explaining the abundance pattern of small craters observed by Warner, Schuyler, etal.(2020). If the Cf-fr
unit was in fact covered and has been exhumed, this leads to important constraints for Mars sample return
concerning potentially using this unit for tying the Martian crater chronology to an absolute age [e.g., Herd
etal.,2021, see also discussion in Shahrzad etal.(2019)].
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Figure 15. Craters on the western delta in Jezero. (a) Overview image of part of the delta. Note the large crater, Belva,
with a relatively well-preserved rim. (b) Examples of craters that potentially could predate a phase of fluvial activity
on the delta (marked with circles). (c) Crater with channel (?) crossing through. The arrow points to a ridge that marks
a potential channel. HiRISE image mosaic base (Fergason, Galuszka, etal.,2020). HiRISE, High Resolution Imaging
Science Experiment.
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In summary, we cannot rule out a scenario where the delta postdates emplacement of the Cf-fr-Us units
based on the crater counts on the delta, or on the floor. We find that it is possible that the delta developed
over a protracted time period, with potentially several reactivation episodes. The last stage could have been
very much later (on the order of 0.5–1Ga) than when delta deposition first started, based on the late-stage
reactivation scenario presented by Mangold etal. (2020) and significant differences in preservation state
between the western delta and northern fan in the crater.
In the stratigraphic scenario proposed here (Figure16), the original morphology of the Jezero impact cra-
ter is obscured by crater-fill deposits. The youngest of these deposits are Cf-f-1 and Cf-f-2. These crater-fill
deposits were exposed at the surface long enough to become both cratered and eroded, forming relief along
the crater floor. The origin of Cf-fr is ambiguous, but the exact origin does not have an effect on the broad
temporal scenario proposed here. For example, Cf-fr could represent an extrusive volcanic deposit formed
during a paused episode of fluvial activity in Jezero, or an additional episode of fluvial or aeolian sedimen-
tary emplacement, erosion, and cratering, wherein Cf-fr is more resistant to erosion than Cf-f-1 and 2, re-
sulting in the formation of moats (e.g., Figures7a,7b,7e and7f). An alternative scenario has been proposed
by Kah etal. (2020) wherein Cf-fr represents the eroded upper surface of Cf-f-1 and Cf-f-2 that has been
made more resistant to erosion by the association with the Us unit. However, this hypothesis may not be
consistent with the spectral properties of the units as Cf-f exhibit strong olivine signatures, which are rare
in both Cf-fr and Us (Horgan etal.,2020).
Regardless, in our scenario, the western delta is deposited on top of at least the Cf-fr unit (possibly along
with the Us unit), which in turn is followed by erosion of all units, including delta deposits, and formation
of delta-associated remnant deposits that can be observed today. We leave open the possibility that the
western delta may have formed through an extended series of depositional events, some predating Cf-fr
and some postdating Cf-fr, that event including the formation of the current expression of the delta, until
there is further understanding of the relationship between fractures and the delta edge. In either case, the
Cf-fr (and possibly Us) units were emplaced during a phase of ongoing (for fluvial sediments) or tempo-
rarily paused (for aeolian sediments) lacustrine/fluvial activity. This scenario is similar to scenarios 2 and
4 described in Stack etal. (2020; Figure5), and thus also similar to scenarios proposed by, e.g., Ruff(2017),
Horgan etal.(2020), and Miklusicak etal.(2020), although we have reinforced the “younger deltaic dep-
osition-scenario” by investigation of stratigraphic relationships. Our scenario thus explains the relatively
well-preserved state of the western delta, the unit contacts observed between crater floor units and crater
floor units and delta deposits, and the varied surface texture of the crater floor in terms of smoothness.
Contradicting surface ages resulting from crater counting could potentially result from capping of the Cf-fr
unit preventing new craters from forming, although we have highlighted the significant uncertainty as-
sociated with derived ages both of the Cf-fr unit and of the delta. Our scenario is also consistent with the
idea that Martian geological history is characterized by complex cycles of erosion of and deposition in
fluvial/lacustrine environments, with significant periods of hiatus (e.g., Edgett etal.,2018; Kite,2019; Kite
etal.,2017,2019; Malin & Edgett,2001; Salese, Ansan, etal.,2016).
It is possible that the delta-associated remnant deposits A–F were deposited in deeper water, profundal lake
environments, even though they are tied to the same depositional system. Deposits B–E (located further
east of the western delta scarp; Figure 8) have somewhat different morphology than deposits A and F.
They have a more hummocky expression, in fact similar to the surface expression of the northern delta in
Jezero. Fractures can also be observed to cross delta-associated remnant deposits in this area further east
of the western delta scarp (Figure8d), whereas fractures terminate at the base of deposits A and F and the
western delta (Figures8a,8b and11d). This could be consistent with different timing of formation, possibly
because they represent basin fill deposits (i.e., profundal lake deposits) rather than “true” delta remnants.
Different timing, and mechanisms, of formation could also mean that the sedimentary rocks constituting
these deposits have different mechanical properties that lead to differences in tendency to fracture, causing
the observations that we have made. Given that these deposits are located further to the east, they are also
likely to have experienced increased weathering and erosion as well, which likely caused variations in mor-
phological expression compared to deposits closer to the delta scarp.
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Figure 16. Overview sketch of our scenario for a sequence of events in Jezero crater. The profile shown in c–g is
roughly along our A-A′ profile (Figure7). Note that this figure does not take into account potential cover and later
exhumation of the Cf-fr unit, but is focused at showing the relation between the delta and Cf-fr. (a) Jezero crater forms
in the Noachian period (see main text). (b) The crater is eroded and postimpact deposits form, covering the central
uplift of the structure [modified after Osinski etal.(2018)]. The youngest crater-fill deposits at this stage are Cf-f-1 and
2. (c) Cf-f-1 and 2 are eroded, forming relief. (d) The crater floor fractured rough (Cf-fr) unit is emplaced. (e) Cf-fr and
Cf-f-1,2 are eroded, and between the three units, Cf-fr is the more resistant unit, leading to formation of topographic
depressions, moats, at the contacts between Cf-fr and Cf-f-1,2. (f) At the latest stage of fluvial/lacustrine activity in
the crater, the western delta extends out into the crater. (g) More erosion, including of the delta. This results in the
formation of delta remnants.
Journal of Geophysical Research: Planets
Elevation profiles presented here show that the delta and the delta-associated remnant deposits line up, so
we have used the deposits to estimate the potential past extent of the delta (Figure7). The past extent of the
delta coincides with the prominent smoothness of the floor (i.e., the mapped extent of the Us surficial cover.
5.4. Genesis of the Cf-fr and Us Crater Floor Units
As stated above, the youngest crater floor units in Jezero crater have previously been grouped together,
and attributed to volcanism (preferably flow-related emplacement resulting in lobate margins; e.g., Goudge
etal.,2015; Ruff,2017). In some more recent studies (e.g., Golder, Miklusicak, etal.,2020; Stack etal.,2020;
Sun & Stack,2020b) other formational histories for the units were suggested. Edgett etal. (2018) pointed
out that crater-retaining sedimentary rock units on Mars can be confused with lava plains. Some important
observations on differences in lithological aspects of the units investigated here can be made from the ability
to retain craters [see discussion in, e.g., Warner, Grant, etal.,2020]. The delta materials are sedimentary, and
small craters do not preserve well on the delta (Mangold etal.,2020) compared to the Cf-fr unit (Warner,
Schuyler, etal.,2020).
HCP in the youngest crater floor unit(s) (Horgan etal.,2020) distinguishes them from the other units in
Jezero crater, including the delta deposits. In fact, HCP-composition is associated with more evolved mag-
ma compositions of Hesperian and Amazonian volcanics (Mangold etal.,2010; Mustard etal.,2005). While
HCP is unique to the Cf-fr in Jezero, it is observed as the main spectral signature of a regionally extensive
thin mantling unit, “the pitted capping unit,” outside of Jezero crater (Goudge etal.,2015). Thus, this sim-
ilarity could either suggest a shared origin for the youngest crater floor units and the regional mantling de-
posit, or that the HCP-bearing materials are sourced from aeolian reworking of these regional units (Horgan
etal.,2020). Alternatively, in a similar scenario, the HCP-bearing materials could be sourced from fluvial
reworking of those regional units.
Our topographic profiles (Figure7) show that the Cf-fr unit has a slope equivalent to that of the upper sur-
face of the western delta. The inclination is along axes that are radial to the delta rather than uniform across
the unit. This slope toward the crater center may represent a primary paleo-depositional inclination of sed-
iments laid down on the lake floor of the ancient delta-lacustrine sedimentary system, which would have
deepened to the east. The inclination direction that we observe is different from the general slope of the
crater floor in Jezero, which is preferentially from north to south rather than from east to west (Figure9).
This is suggestive of a common origin of the Cf-fr and delta, although processes resulting in emplacement
of a material mantling existing topography, such as a volcanic deposit, could also be consistent with the
observation. Topography-mantling erupted material is typically thin tephra fall deposits or ash turbidites
on Earth, although mass flows that thicken into depressions and areally limited, valley-confined lava flows
can modify topography without forming all new relief (Manville etal.,2009). If the Cf-fr was emplaced as a
regionally extensive lava flow this observation is more difficult to explain, unless there has been significant
tectonic activity tilting the area postdeposition. To our knowledge, no such large-scale regional activity that
could cause such tilting has been discussed. Also, large-scale regional tectonic activity causing tilting as
observed by us would be expected to manifest itself as consistent tilt in one direction, rather than tilt radial
to the delta. Aside from these primary formation mechanisms there is also the possibility that differential
weathering and/or erosion has influenced the topography of the landscape, creating the observed regional
slope.
Small craters are significantly less common on the areas that have more prominent mantling of the Us unit
(Figure12), and these areas are preferentially located just east of the western delta edge. The material that
constitutes the Us has clear and distinct HCP signatures away from the delta, but near the delta edge the
signature is a mix of HCP and LCP (Figures4a and4b; see also Horgan etal.,2020), meaning that the Us is
delta-like in CRISM-signature close to the delta scarp. There is no HCP anywhere else in the crater, mean-
ing that it is not likely that the material has an external source and was recently deposited as, e.g., aeolian
sediment. Given that the material has a spectral signature that is different than the delta itself (Horgan
etal.,2020), an origin as wind-blown material that exclusively originates from the delta is unlikely. This is
also inconsistent with predominant wind directions in Jezero, which are east to west (Day & Dorn,2019).
The observations are suggestive of a scenario where the Us is an in-place unit (e.g., consistent with the
hypothesis of Warner, Schuyler, etal.(2020) where the unit is interpreted as regolith), although it could be
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locally deflated. The HCP signal could be sourced from the primary mineral composition of the Cf-fr unit.
The unit is unlikely to be sourced from deflation of the Cf-f units because deflation of this surface produces
sediments with strong olivine signatures (Horgan etal.,2020), so it must be a distinct unit. Given the distri-
bution of pixels with absorptions consistent with more LCP-rich material, which are concentrated near the
western delta front and also around the delta-associated remnant deposits, it is likely that variations in LCP
modal abundance are changing the CRISM absorptions, with LCP-rich material derived from or preserved
by the eroding delta having higher abundance near the deltaic material. The LCP/HCP signatures of the
broader crater floor region (Horgan etal.,2020) correspond to a clearly arcuate region of low thermal inertia
(Figure4c) that appears to possibly correspond with Us and, based on the arcuate shape, to the past extent
of the youngest delta deposits. The nonblocky nature of rims and ejecta of small craters formed onto the Us
just off the delta scarp (Figure13) is constant with the thermal inertia observation of the Us unit.
We find that erosion of the delta scarp could expose the Us unit which is stratigraphically higher than the
Cf-fr unit, or alternatively that the units are genetically related and that erosion of the delta edge leaves the
Us behind. Given the decreased mantling of craters toward the east by the Us (Kah etal.,2020), it is possible
that the Us has had a previously more extensive distribution but has eroded to expose the Cf-fr unit. A sur-
face exposed on Mars for 2.5Gyr (i.e., the age of the “dark-toned floor unit” in Jezero (Shahrzad etal.,2019),
should have a significant regolith cover (e.g., Hartmann etal., 2001; Warner, Grant, et al., 2020), unless
surface processes have removed the regolith or the surface was protected [e.g., covered; see discussion in
Rogers etal.(2018)]. Regardless of whether the hypothesis of Warner, Schuyler, etal.(2020), that Us is that
regolith, is correct or not, regolith is very thin or largely missing on the Cf-fr unit in the investigated area,
except for just off the delta scarp. This indicates that Cf-fr (and Us?) was either buried and later exhumed
(preventing a regolith from forming; see Quantin-Nataf etal.,2021) or that the unit has been stripped of its
regolith (see Warner, Schuyler, etal.,2020). These scenarios are both also consistent with observations of
crater distribution, as pointed out by us, with significant difference in crater density close to the delta and
also discussed by Warner, Schuyler, etal.(2020). A relation between the Us unit and the delta (or alterna-
tively former cover by the delta) is strengthened by the clear association with the thickness of the smooth
unit increasing with proximity to delta, and the fact that undifferentiated smooth material also occurs on/
in the delta (Stack etal.,2020). A possible scenario is that Us could be formed by deposition of relatively
larger-sized particles after reactivations of the fluvial system after periods of dormancy [see discussion in
Mangold etal.(2020)]. In fact, the proximal part of the inlet channel has a spectral signature consistent with
presence of HCP (Horgan etal.,2020).
5.5. Implications for the Aqueous History of Jezero
Fluvial activity on Mars did not end with the demise of the late Noachian fluvial stage. In addition to large
outflow channels (Head,2007, and references therein), evidence for at least episodic fluvial activity has
also been presented, e.g., in the form of reactivation of Noachian valleys (Baker & Partridge,1986), im-
pact-induced melting of ice (Morgan & Head,2009), morphology of alluvial fans (Goddard etal.,2014; Kite
etal.,2017) and deltas around Chryse Planitia and the Aeolis region (Hauber etal.,2013), fluvial landforms
on fresh impact ejecta (Mangold,2012), and estimations of runoff production over time (Kite etal.,2019),
as well as evidence for lacustrine activity in Gale crater extending into the Hesperian era (e.g., Grotzinger
etal., 2013,2015). Late-stage reactivation of the downstream part of the fluvial system feeding sediment
into Jezero crater has also been proposed by Mangold etal.(2020). In the near-Jezero region, there is con-
siderable evidence for fluvial activity extending well into the Hesperian illustrated, for instance, by the pres-
ence of an alluvial fan inside of Hargraves crater (which is Hesperian in age; Goudge etal.,2015; Mangold
etal.,2007), and observations of fluvial landforms in the Syrtis Major area [Mangold etal.,2008; see also
discussion in Mangold etal.(2020)]. We suggest that at least one episode of fluvial reactivation happened
after deposition of the youngest lithified crater floor unit, here taken as the Cf-fr unit, because we show that
the youngest deltaic sediments clearly occur higher in the overall stratigraphy than the floor units.
As discussed, the ages obtained from crater counting on the floor and western delta are not necessari-
ly straight-forward to transfer to an estimate of the timing of fluvial activity in Jezero. Therefore, we are
limited to reporting that there was fluvial activity in the area after deposition of the youngest crater floor
units. Further support for late-stage fluvial deposition, even if short-lived and limited in terms of sediment
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volume delivery to the lake, is the general slope of the crater floor, which is more prominent from north
to south than from west to east (Figure9). One possible explanation for this observation is that deposition
from the north had a greater impact on the crater floor geomorphology than deposition from the west that
resulted in the current expression of the western delta. Temporary late-stage fluvial activity on Mars has
been attributed to numerous processes, such as climate variations (Kite etal.,2017,2019), volcanic activity
(Gulick,1998; Gulick & Baker,1990), and impact cratering (Goddard etal.,2014; Mangold,2012; Salese, Di
Achille, etal.,2016; Segura etal.,2008), although the process inducing late fluvial activity in Jezero crater
specifically, is not presently known (Mangold etal.,2020). Given that late-stage reactivation of the fluvial
system might not have happened during a time of more broadly habitable conditions, but rather during a
short-lived temporary humid episode (see references above), our observations are relevant for searching for
signs of past life in Jezero crater. Our observations also improve the understanding of crater floor materials
and their stratigraphy in Jezero, something that is essential for sample collection strategies (e.g., Farley
etal.,2020).
We have presented hypotheses for the stratigraphic relations between and origin of units in Jezero crater
that ultimately will require in-situ investigation, or even sample return, for testing. Analysis of high-resolu-
tion images, geochemical and spectral data, and ground-penetrating radar data acquired by instruments on
Perseverance will provide critical insights into the complex geologic history of Jezero crater.
6. Conclusions
We present new observations of the stratigraphic relationship between crater-fill units in and near the land-
ing area for the Mars 2020 mission in Jezero crater. Analysis of topographic profiles and DEMs across the
western delta and crater floor units reveal that the present expression of the delta, i.e., the present delta
scarp and delta-associated remnant deposits, is younger than at least the youngest significant crater floor
unit (Cf-fr), and possibly also the thin mantling unit that is on top of it (Us). We present the following ob-
servations in support of that hypothesis:
1. The boundary between the Cf-fr and Cf-f-1 units is characterized by shallow depressions, or “moats.”
Moats are lacking between delta scarp/associated remnant deposits and the Cf-fr unit. At the contact
between the youngest crater floor units and delta-associated remnant deposits, the floor goes right up to
the deposits without exposing another unit.
2. In addition, we find the youngest crater floor unit(s) slope away from the delta, and this inclination is
along axes that are radial to the delta rather than uniform across the unit. This may be consistent with a
shared origin, although, e.g., an overlying mantling unit draping existing topography cannot be excluded.
3. Fractures on the youngest crater floor units that are oriented roughly perpendicular to the delta scarp
abruptly end at the scarp, and we find that one possible explanation for this observation is that the frac-
ture-forming event took place before the western delta was deposited.
4. Small craters are sparse on areas of the crater floor where the Us unit is most prominent (proximal to the
delta), indicating recent coverage of the Cf-fr unit close to the delta by some overlying unit.
5. CRISM data indicate that the smoothest areas of the youngest crater floor units stand out as having clear
signatures of high-calcium pyroxene (HCP). These areas that are proximal to the delta scarp are also are
delta-like in CRISM low-calcium pyroxene (LCP) spectral signature.
6. Compared to surrounding crater floor terrain, LCP absorptions are deeper in Us material near the west-
ern delta front and surrounding the eastern delta remnants. Similarly strong LCP absorptions are also
present on the western delta surface. This observed distribution is consistent with LCP-bearing Us mate-
rial in the crater floor assemblage being derived from or preserved by overlying deltaic material.
7. The HCP signatures of the broader crater floor region correspond to a clearly arcuate region of low
thermal inertia that appears to possibly correspond with Us and, based on the arcuate shape, to the past
extent of the youngest delta deposits.
8. Finally, we have also estimated the past minimum extent of the western delta in Jezero to have extended
further out compared to the present extent. This area corresponds to, at least, the smoothest area of the
Us, indicating that the scarcity of impact craters on the underlying unit near the delta is due to recent
coverage.
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A scenario where a portion or all of the western delta postdates deposition of the youngest crater floor
unit(s) cannot be ruled out on the basis of results of crater counting on the delta and the floor because of the
limited area of the delta. In addition, the Cf-fr unit may have been protected from impact cratering during
period(s) since its formation.
Crater-fill deposits in Jezero were deposited during a complex history with multiple fluvial episodes, and
the rocks do not only have great potential to shed light on the geologic evolution of Mars, but also hold key
information on the evolution of the early Martian climate, surface environments, and habitability.
Data Availability Statement
Maps of fractures and small craters reported in this work are available through Holm-Alwmark etal.(2021).
For HiRISE mosaic of Jezero crater, see Fergason, Galuszka, etal.(2020). For data used to produce the
DEM (Figure6), see Fergason, Hare, etal.(2020), Malin etal.(2007); https://astrogeology.usgs.gov/search/
map/Mars/Mars2020/JEZ_ctx_B_soc_008_DTM_MOLAtopography_DeltaGeoid_20m_Eqc_latTs0_lon0.
CAMP source files “Web-based Spatial Data Infrastructure for Planetary Science Operations” are available
on GitHub at https://github.com/NASA-AMMOS/MMGIS.
References
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Acknowledgments
Danish participation in the Mars
2020 project was supported by the
Carlsberg Foundation grant CF19-0023.
S. Holm-Alwmark is supported by an
International Postdoc grant from the
Swedish Research Council (Grant 2017-
06388). J. D. Tarnas was supported by
a NASA Postdoctoral Fellowship. The
authors want to thank H. Dypvik and S.
Ruff for insightful discussions. We also
want to thank D. Tirsch for help with
the preparation of Figure1b. Finally,
the authors acknowledge thorough and
constructive reviews by N. Warner, F.
Salese, and one anonymous reviewer, as
well as professional editorial handling
by D. Rogers.
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