Unraveling the Water Sources in Comet 103P/Hartley 2 from Deep Impact Flyby Observations

Abstract

The Deep Impact eXtended Mission flyby of comet 103P/Hartley 2 offered a rare opportunity to study a hyperactive comet with high-cadence and high-resolution observations from a fixed vantage point. Using the high signal-to-noise data from the HRI-IR instrument, the H 2 O and CO 2 distributions in the comet’s coma are mapped and quantified over one complete rotation period, revealing two major sources of water: direct sublimation from the nucleus, and sublimation of icy grains in the coma in the antisunward direction. These icy grains contributed 25%–40% of the total water production during the 2010 perihelion passage, quantifying the source of Hartley 2’s hyperactivity. In addition, sublimation from slower-moving icy grains redeposited at the comet’s gravitational low is detectable within 5.2 km of the nucleus, accounting for a few percent of the water production. CO 2 production distinctly tracks with the nucleus’s small lobe, which is seen to be active throughout Hartley 2’s rotation even when not illuminated and thus is less dependent on instantaneous solar illumination than water. Differences in CO 2 and H 2 O sources lead to spatially resolved CO 2 /H 2 O ratios ranging from 5% to 21% sampled at various times and locations in the coma throughout a single rotation, while the global abundance ratio varies by a factor of ∼2 throughout a single rotation (6%–12%). These observations highlight the complex interaction between solar insolation, comet rotation, and volatile outgassing and suggest that the lobes of Hartley 2 may have different formational or evolutionary origins, implying large-scale mixing in the protoplanetary disk.

Full text

Unraveling the Water Sources in Comet 103P/Hartley 2 from Deep Impact Flyby
Observations
L. M. Feaga
1
and J. M. Sunshine
1,2
1
Department of Astronomy, University of Maryland, College Park, MD 20742, USA; lfeaga@umd.edu
2
Department of Geology, University of Maryland, College Park, MD 20742, USA
Received 2024 September 27; revised 2025 February 28; accepted 2025 March 12; published 2025 April 17
Abstract
The Deep Impact eXtended Mission flyby of comet 103P/Hartley 2 offered a rare opportunity to study a
hyperactive comet with high-cadence and high-resolution observations from a fixed vantage point. Using the high
signal-to-noise data from the HRI-IR instrument, the H
2
O and CO
2
distributions in the comet’s coma are mapped
and quantified over one complete rotation period, revealing two major sources of water: direct sublimation from the
nucleus, and sublimation of icy grains in the coma in the antisunward direction. These icy grains contributed 25%–
40% of the total water production during the 2010 perihelion passage, quantifying the source of Hartley 2’s
hyperactivity. In addition, sublimation from slower-moving icy grains redeposited at the comet’s gravitational low
is detectable within 5.2 km of the nucleus, accounting for a few percent of the water production. CO
2
production
distinctly tracks with the nucleus’s small lobe, which is seen to be active throughout Hartley 2’s rotation even when
not illuminated and thus is less dependent on instantaneous solar illumination than water. Differences in CO
2
and
H
2
O sources lead to spatially resolved CO
2
/H
2
O ratios ranging from 5% to 21% sampled at various times and
locations in the coma throughout a single rotation, while the global abundance ratio varies by a factor of ∼2
throughout a single rotation (6%–12%). These observations highlight the complex interaction between solar
insolation, comet rotation, and volatile outgassing and suggest that the lobes of Hartley 2 may have different
formational or evolutionary origins, implying large-scale mixing in the protoplanetary disk.
Unified Astronomy Thesaurus concepts: Comets (280);Comae (271);Comet volatiles (2162);Spectroscopy
(1558);Infrared astronomy (786);Flyby missions (545);Short period comets (1452)
1. Introduction
After the successful prime mission and impact experiment at
comet 9P/Tempel 1 in 2005 (M. F. A’Hearn et al. 2005),
NASA approved the Deep Impact eXtended Investigation
(DIXI)to explore the diversity of Jupiter-family comets (JFCs;
M. F. A’Hearn 2008). The DIXI flyby of comet 103P/Hartley
2(see Table 1), a smaller (2.3 km in its longest dimension;
P. C. Thomas et al. 2013), bilobed, and much more active
comet than Tempel 1, occurred on 2010 November 4
(M. F. A’Hearn et al. 2011). Observations of Hartley 2 from
the Deep Impact Flyby (DIF)spacecraft followed a specifically
designed, multimonth approach and departure imaging and
mapping campaign. In particular, the rich data set afforded by
the Deep Impact High Resolution Instrument Infrared Spectro-
meter (HRI-IR),a1–5μm instrument capable of detecting H
2
O
(2.66 μm)and CO
2
(4.26 μm)simultaneously (D. L. Hampton
et al. 2005), included spectral maps of the nucleus and
innermost coma acquired at an unparalleled sampling cadence
throughout the flyby. These detailed observations of Hartley 2
enable unique temporal and spatial investigations of the
comet’s outgassing activity and primary coma composition
neither achievable from the ground nor soon to be replicated for
another comet.
Hartley 2 is considered a hyperactive comet because its
water production rate during previous perihelion passages
implied that nearly 100% of its surface is active (O. Groussin
et al. 2004). Upon arrival at Hartley 2, it was clear in the
images and spectral maps that a large population of icy grains
existed in the coma and contributed significantly to the comet’s
water production rate (Figure 1;M.F.A’Hearn et al. 2011);
however, a question remained: how much water vapor stems
from direct sublimation from the nucleus versus sublimation
from icy grains? It was also evident that the activity of the two
lobes and waist of the bilobed comet each behaved differently,
with heterogeneous distributions of H
2
O and CO
2
vapor. The
small lobe was enriched in CO
2
, which escaped from the
nucleus, dragging subsurface water ice with it into the coma,
driving the comet’s hyperactivity, while the large lobe supplied
the coma with much less material overall and was not enhanced
in CO
2
(Figure 1;M.F.A’Hearn et al. 2011). It was
hypothesized that fallback of icy grains accumulated on the
waist of the nucleus, a gravitational low connecting the two
lobes, creating the smooth surface region seen in images
(M. F. A’Hearn et al. 2011). The fallback material then
sublimated, producing the near-nucleus coma enhancement of
H
2
O above the waist seen in the spectral maps (Figure 1;
M. F. A’Hearn et al. 2011). The localized relative abundance of
CO
2
compared to H
2
O around the nucleus was relatively high
compared to other comets (M. F. A’Hearn et al. 2011;
T. Ootsubo et al. 2012;M.F.A’Hearn et al. 2012)and varied
by a factor of 2 throughout Hartley 2’s rotation (M. F. A’Hearn
et al. 2011).M.F.A’Hearn et al. (2011)reasoned that the
heterogeneity seen in the outgassing activity and near-nucleus
coma composition is primordial rather than evolutionary
because the complex rotation state of the comet at the time
of the flyby (M. J. S. Belton et al. 2013)suggested that the
entire surface of Hartley 2 was illuminated by the Sun and
heated every full rotational period (∼55 hr).
The Planetary Science Journal, 6:95 (18pp), 2025 April https://doi.org/10.3847/PSJ/adc094
© 2025. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms
of the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s)and the title
of the work, journal citation and DOI.
1

In this paper, we analyze the HRI-IR data in more depth to
better understand the distribution and abundance of the primary
volatiles in the coma, investigate their relationship with each
other and with the comet’s volatile reservoirs, and quantify the
contribution to the water production rate from two distinct
sources, the nucleus and icy grains.
2. Observations and Data Reduction of the Spectra
The double prism 1–5μm HRI-IR imaging spectrometer
(D. L. Hampton et al. 2005)collected data throughout the DIXI
mission. On approach, the DIF spacecraft orientation limited
downlink opportunities and thus data storage, restricting the
sampling frequency for IR scans to every 1–2 hr. The scan
frequency increased dramatically within an hour of closest
approach and, due to continuous downlink geometries,
remained at one IR scan every 30 minutes during departure
from the comet. To study Hartley 2 at highest resolution over
its 18.4 hr primary rotation period (M. J. S. Belton et al. 2013),
data spanning one rotation before and after closest approach are
examined here (from 2010 November 3 to 5, closest approach
was 13:59:47 UTC on 2010 November 4; M. F. A’Hearn et al.
2011). The DIF observed Hartley 2 at 1.06 au at a solar phase
angle of 86°on approach and 93°phase on departure. The
calibrated radiance data (in units of W/(m
2*
sr
*
μm);
K. P. Klaasen et al. 2013)are archived in the NASA Planetary
Data System (http://pds.nasa.gov; S. A. McLaughlin et al.
2014; see Table 2for details).
D. L. Hampton et al. (2005)describe the HRI-IR instrument
design. All the data presented here were acquired in binned
full-frame modes of the instrument, where each frame consists
of pixels with an instantaneous field of view of 10 μrad, with
256 spatial pixels along the slit, aligned with the projected
Sun–anti-Sun line, and 512 spectral channels ranging from 1.05
to 4.83 μm. The double-prism design results in a variable
resolving power (λ/δλ)that had a minimum of ∼200 near
2.6 μm. Two different instrument modes, binned full-frame
(BINFF)and alternating binned full-frame (ALTFF), were used
in these observations of Hartley 2. These modes are identical,
except under ALTFF the signal integration time was halved
owing to an alternating readout method, which is advantageous
in reducing possible saturation near the warm nucleus. To build
a two-dimensional spatial image, the HRI-IR slit was scanned
via spacecraft slews perpendicular to the flight direction at a
rate of one (10 μrad)or two (20 μrad)slit widths per exposure.
Spectral image cubes were constructed by considering the scan
rate and stacking the individual frames from each scan
sequence together in an orientation that always places the
projected direction of the Sun to the right (Figure 1). For the
scans that were acquired at a rate of two slit widths per
exposure (20 μrad), each frame was replicated when added to
the image stack to produce pixels with equal area. To reduce
the effects of saturation, especially of a warm cometary nucleus
at perihelion, the HRI-IR included a spectral attenuator
(antisaturation filter (ASF)) covering the middle third of the
slit, greatly suppressing the strong thermal contribution above
4μm(D. L. Hampton et al. 2005; K. P. Klaasen et al.
2008,2013). However, for all the data analyzed here, the
nucleus was positioned to fall outside the attenuated region.
Any coma observed under the ASF will have reduced signal at
wavelengths >2.7 μm. The best spatial resolution achieved in
our subset of data is ∼45 m pixel
−1
(7 minutes after closest
approach), while the data at both ends of the 18 hr window
have spatial resolution as large as ∼8000 m pixel
−1
.
In cases where the nucleus was not resolved in the spatial
scans (i.e., the nucleus was subpixel), the position of the warm
(300–360 K; O. Groussin et al. 2013)nucleus was identified
via its thermal emission. For this calculation, the thermal signal
was integrated from 4.65 to 4.80 μm, a region free of strong
gas emission for Hartley 2, which has negligible CO emission
near 4.67 μm(<0.5% CO/H
2
O; H. A. Weaver et al. 2011).
Locations of the nucleus determined from thermal signatures
were validated in cases in which context images from the
visible Medium Resolution Instrument (MRI)were acquired
during the HRI-IR scans. Based on the position of the nucleus
in the MRI image, its expected location in the IR scan was
estimated using the boresight offset between the MRI and HRI-
Table 1
103P/Hartley 2 Characteristics during the DIXI Flyby on 2010 November 4
(M. F. A’Hearn et al. 2011)
103P/Hartley 2 Characteristics
Dynamical Class Periodic; Jupiter-family Comet
Orbital period 6.47 yr
Perihelion distance 1.059 au
Encounter date 4-Nov-2010 13:59:47.31 UTC
Time post-perihelion 7 days
Heliocentric distance at flyby 1.064 au
Closest-approach flyby distance 694 km
Flyby speed 12.3 km s
−1
Nucleus size 0.69 ×2.33 km
(smallest ×largest diameter)
Primary rotation period 18.34 hr (on 2010 November 4)
Figure 1. Spatial distribution maps of volatile species in Hartley 2 derived from an HRI-IR spectral scan 7 minutes after closest approach. While H
2
O and CO
2
are
ubiquitous in the scene, enhancements in their abundances are uncorrelated. However, the distribution of H
2
O ice appears to be correlated with CO
2
. Figure modified
from M. F. A’Hearn et al. (2011)(PDS data set dif-c-hrii-3_4-epoxi-hartley2-v3.0; DOY 308, Exposure ID hi5006000, scan start time 2010-11-04T14:05:49; spatial
resolution 52 m pixel
−1
; Sun to the right; S. A. McLaughlin et al. 2014).
2
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

IR instruments (D. L. Hampton et al. 2005), the timing of the
context image relative to the closest IR frame, and the
spacecraft motion during a scan. The nucleus locations derived
from both methods were found to match to 1-pixel accuracy in
both spatial directions. The MRI context images have also been
used in more distant observations when the nucleus thermal
emission was too small to detect in the HRI-IR data alone
(L. M. Feaga et al. 2014).
To isolate gaseous emission bands of H
2
O, organics, and
CO
2,
a model continuum was fit to each pixel following the
Table 2
HRI-IR Observation Parameters for the DIF Hartley 2 Encounter ±18.5 hr
Encounter
Timing ObsID Mode Scan Rate (pixels frame
–1
)
Scan
Direction
No. of
Frames Exposure Time (s frame
–1
)
Pixel Scale
(m pixel
−1
)
E–18.0 hr 4000001 BINFF 1 N →S 30 7.00 7978
E–16.0 hr 4000003 BINFF 1 N →S 30 7.00 7091
E–14.0 hr 4000200 BINFF 2 N →S 16 7.00 6204
E–12.0 hr 4000202 BINFF 1 N →S 30 7.00 5317
E–10.0 hr 4000500 BINFF 2 N →S 16 5.00 4431
E–08.0 hr 4000502 BINFF 1 N →S 30 4.00 3544
E–06.0 hr 4000600 BINFF 1 N →S 30 3.00 2658
E–04.0 hr 4000602 BINFF 1 N →S 30 3.00 1771
E–03.0 hr 4000604 ALTFF 1 N →S 30 2.50 1326
E–02.0 hr 5000000 ALTFF 1 N →S 30 2.50 883
E–01.0 hr 5000002 ALTFF 1 N →S 30 1.44 441
E–48.0 minutes 5000004 ALTFF 1 N →S 30 1.44 357
E–36.0 minutes 5000006 ALTFF 1 N →S 30 1.44 268
E–25.0 minutes 5000008 ALTFF 1 N →S 30 1.44 176
E–14.0 minutes 5001000 ALTFF 1 N →S 116 1.44 130
E+07.0 minutes 5006000 ALTFF 1 S →N 56 1.44 45
E+14.0 minutes 5007000 ALTFF 1 S →N 116 1.44 91
E+25.0 minutes 5007002 ALTFF 1 S →N 30 1.44 168
E+38.0 minutes 5007004 ALTFF 1 S →N 30 1.44 285
E+52.0 minutes 5007006 ALTFF 1 S →N 30 1.44 383
E+01.0 hr 5008000 ALTFF 1 S →N 30 1.44 433
E+01.5 hr 5008002 ALTFF 2 S →N 32 2.00 655
E+02.0 hr 4000700 ALTFF 2 S →N 32 2.25 877
E+02.5 hr 4000800 ALTFF 2 S →N 32 2.50 1099
E+03.0 hr 4000900 ALTFF 2 S →N 32 2.75 1321
E+03.5 hr 4001000 ALTFF 2 S →N 32 3.00 1543
E+04.0 hr 4001100 BINFF 2 S →N 32 3.00 1763
E+04.5 hr 4001200 BINFF 2 S →N 32 3.00 1985
E+05.0 hr 4001300 BINFF 2 S →N 32 3.00 2207
E+05.5 hr 4001400 BINFF 2 S →N 32 3.00 2428
E+06.0 hr 4001500 BINFF 2 S →N 32 3.00 2650
E+06.5 hr 4001600 BINFF 2 S →N 32 3.00 2872
E+07.0 hr 4001700 BINFF 2 S →N 32 3.50 3094
E+07.5 hr 4001800 BINFF 2 S →N 32 3.50 3316
E+08.0 hr 4001900 BINFF 2 S →N 32 4.00 3538
E+08.5 hr 4002000 BINFF 2 S →N 32 4.25 3760
E+09.0 hr 4002100 BINFF 2 S →N 32 4.50 3982
E+09.5 hr 4002200 BINFF 2 S →N 32 4.75 4203
E+10.0 hr 4002300 BINFF 2 S →N 32 5.00 4425
E+10.5 hr 4002400 BINFF 2 S →N 32 5.00 4647
E+11.0 hr 4002500 BINFF 2 S →N 32 5.50 4869
E+11.5 hr 4002600 BINFF 2 S →N 32 5.50 5091
E+12.0 hr 4002900 BINFF 2 S →N 32 6.00 5313
E+12.5 hr 4003000 BINFF 2 S →N 32 6.00 5535
E+13.0 hr 4003100 BINFF 2 S →N 32 7.00 5757
E+13.5 hr 4003200 BINFF 2 S →N 32 7.00 5979
E+14.0 hr 4003300 BINFF 2 S →N 32 7.00 6200
E+14.5 hr 4003400 BINFF 2 S →N 32 7.00 6422
E+15.0 hr 4003500 BINFF 2 S →N 32 7.00 6644
E+15.5 hr 4003600 BINFF 2 S →N 32 7.00 6866
E+16.0 hr 4003700 BINFF 2 S →N 32 7.00 7087
E+16.5 hr 4003800 BINFF 2 S →N 32 7.00 7309
E+17.0 hr 4003900 BINFF 2 S →N 32 7.00 7531
E+17.5 hr 4004000 BINFF 2 S →N 32 7.00 7753
E+18.0 hr 4004100 BINFF 2 S →N 32 7.00 7974
E+18.5 hr 4004200 BINFF 2 S →N 32 7.00 8196
3
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

methods of S. Protopapa et al. (2014). The model continuum
includes reflected sunlight, thermal emission, and H
2
O-ice
absorption features at 1.5, 2, and 3 μm. These components
were simultaneously fit and the free parameters constrained
(e.g., blackbody temperature, emissivity)to avoid solutions
with physically unrealistic values. After continuum subtraction,
a 5-pixel resistant mean with a 2.5σcutoff was then applied to
the spectral dimension of each scan to remove outliers and
flagged bad pixels.
Where the bright nucleus is resolved, within ∼1 hr of closest
approach, thermal light scattered in the instrument and off the
edges of the ASF, manifesting as bright and dark rings around
the nucleus in the data (at a level of 5% of the measured
thermal continuum). These rings are most evident at wave-
lengths >3μm. The ringing is present in the raw data and not
an artifact of saturation, calibration, or processing, and it is
much more pronounced once the background sky and strong
thermal continuum from the comet and coma are removed. To
reduce contamination in the analysis, especially at CO
2
wavelengths, a secondary continuum removal was applied
based on a three-segment linear fit between the short-
wavelength (2.40–3.05 μm), intermediate-wavelength
(3.02–3.98 μm), and long-wavelength (3.99–4.54 μm)flanks
of the emission bands (Figure 2).
Finally, distribution maps of H
2
O and CO
2
were derived by
integrating the continuum-removed radiance across the spectral
extent of each species’emission band, 2.56–2.82 μm for H
2
O
and 4.18–4.37 μm for CO
2
. These distribution maps were
spatially smoothed using a 3 ×3 pixel resistant mean with a 2σ
cutoff for outliers. Representative maps are displayed in
Figures 3–5.
3. Volatile Distribution
3.1. Distribution Maps
The CO
2
and H
2
O maps range in size and field of view
depending on the executed observing sequence and the
spacecraft-to-comet distance (e.g., Figures 3–5). The maps
are very informative, revealing asymmetries in the gas
distributions and whether behavior is persistent or variable.
For select comparisons below, we apply an azimuthal
averaging technique to better highlight the regions where the
volatile distribution is above or below the mean activity level
(N. H. Samarasinha et al. 2013). The technique is applied to
individual distribution maps by computing a radially dependent
azimuthal average of the image centered on the nucleus and
then dividing the average from the image. By definition, it is
important to note that removing an azimuthal average results in
regions that apparently lack gas; however, in reality, CO
2
and
H
2
O are present in every pixel of the fields of view
presented here.
3.1.1. CO
2
Maps
A source region around the endcap of the small lobe, at one
end of the long axis of the nucleus (+Z-direction in the body-
fixed frame), dominates the CO
2
distribution maps (Figures 3,
5, and 6). The expelled CO
2
gas was found to entrain micron-
sized H
2
O ice, organics, and dust but little H
2
O gas
(M. F. A’Hearn et al. 2011, L. M. Feaga et al. 2012, S. Proto-
papa et al. 2014, L. M. Feaga et al. 2023), suggesting that the
small lobe has a subsurface reservoir of CO
2
ice (M. F. A’He-
arn et al. 2011). Because the CO
2
sublimation temperature
is much colder than that of H
2
O(80 K vs. 180 K),H
2
O-ice
grains would remain solid as they are propelled out of the
nucleus with the escaping CO
2
gas (J. M. Sunshine &
L. M. Feaga 2021). The rotational analysis here shows that
CO
2
emission from the small lobe is sustained over the rotation
period even when the small lobe is not illuminated, i.e., the
small lobe never ceases activity. The position of the CO
2
jet
region is seen to track with the 18.4 hr primary rotation of the
nucleus, with the enhancement returning to the encounter
geometry (E+38 minutes)seen from a large distance at E +
18.5 hr (Figure 5). The large lobe, on the other hand, lacks any
CO
2
enhancement at any orientation, whether illuminated or
not. This suggests that the lobes are heterogeneous and have
different accessible volatile reservoirs. To accentuate the CO
2
correlation with the small lobe, we divide out an azimuthal
average centered on the nucleus for each CO
2
map.
Representative times are shown in Figure 6and emphasize
the directionality of the CO
2
distribution and that it follows the
projected direction of the small lobe. In cases where the long
axis of the nucleus is significantly pointed toward or away from
the spacecraft, rather than mostly perpendicular to the line of
sight, the projection of the conical CO
2
distribution emitted
from the small lobe appears more hemispherical but is still
centered on the projected direction of the small lobe.
To better quantify the distribution of CO
2
around the
nucleus, we plot the normalized radiance around the nucleus in
consecutive 15°sectors at a snapshot in time at various
distances from the nucleus (E+6 hr; Figure 7). The amount of
CO
2
varies by a factor of ∼3 from the maximum to minimum
brightness around the nucleus. The distribution shows no
dependence on radial distance but a clear correlation with the
location of the small lobe. Further, we derive a light curve from
CO
2
emission from the small lobe throughout a nucleus
rotation. These light-curve data (Figure 8)sample a projected
distance of 20 km from the small lobe in a 30°opening angle
centered on the positive rotation pole. The peak in this plot
occurs around 4 hr after closest approach while the small lobe
is pointed in the sunward hemisphere and is illuminated. As the
comet rotates and the small lobe is no longer pointed toward
the Sun, the illumination conditions change, and the production
of CO
2
decreases but does not cease. A minimum in CO
2
Figure 2. Representative continuum-removed spectrum for a single pixel in the
E+5 hr scan (dif-c-hrii-3_4-epoxi-hartley2-v3.0; DOY 308, Exposure ID
hi4001300, scan start time 2010-11-04T18:58:19; S. A. McLaughlin
et al. 2014). Emissions due to H
2
O, CO
2
, and a weak organic blend are present
in this spectrum. The regions used for final continuum removal flanking each
emission are indicated with thick red lines.
4
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

occurs around 14 hr after closest approach, with the small lobe
having been in darkness for ∼10 hr, approximately half of the
primary period of the complex rotation of the nucleus. Similar
to the difference in the distribution of CO
2
around the nucleus,
the CO
2
brightness over time varies by a factor of ∼3 over the
rotation of the nucleus. However, at all times, the small-lobe jet
region is seen to contribute to Hartley 2’sCO
2
production.
3.1.2. H
2
O Maps
While the CO
2
maps are dominated by a single cometesimal-
scale source (the small lobe), the H
2
O vapor distribution maps
reveal several large-scale source regions from the nucleus and
the coma. Sublimation directly from the nucleus is best studied
in maps where the spatial resolution is better than
∼2 km pixel
−1
, roughly the length of the longest dimension
of the nucleus, i.e., in scans within 4 hr of closest approach.
Figure 4displays the near-nucleus H
2
O distribution pre-
encounter, which is asymmetric with an underlying enhance-
ment in the sunward hemisphere and a more discrete
enhancement emanating perpendicular to the sunlit waist.
These two distributions of H
2
O are a result of direct
sublimation of H
2
O ice from the nucleus. As H
2
O ice is
detected in only a few specific regions on the nucleus
(J. M. Sunshine et al. 2012), the sunward hemispherical
distribution suggests a global near-surface source of H
2
O ice
that undergoes sublimation due to direct solar insolation. The
localized enhancement of H
2
O vapor near the smooth waist of
the nucleus can be explained by sublimation of fallback
material originating from the small-lobe CO
2
jet region, which
settles in the gravitational low between the two lobes of the
nucleus (P. C. Thomas et al. 2013; J. E. Richardson &
T. J. Bowling 2014)and is composed of thermally processed
dust and H
2
O ice (M. F. A’Hearn et al. 2011; J. M. Sunshine &
L. M. Feaga 2021). A more detailed analysis of the water
released from the waist is discussed in Section 4.1 below.
In larger fields of view, e.g., Figures 5and 9, individual
near-nucleus sources of H
2
O are not readily discernible. The
waist contribution is no longer apparent, although Figure 10
illustrates that within ∼10 km in the sunward direction there is
evidence of solar-insolation-induced H
2
O sublimation from the
sunward hemisphere of the nucleus. In contrast, a very
pronounced antisunward distribution of H
2
O vapor is clear in
data acquired after E +4 hr. As with the CO
2
distribution,
enhanced H
2
O maps are also created by dividing out the
azimuthal average centered on the nucleus. At radial distances
beyond 20–40 km, an antisunward enhancement persists with
time, which is generally consistent with the behavior and
distribution of the particles at distances greater than 4 km from
the nucleus in visible data (M. S. Kelley et al. 2013;
M. S. P. Kelley et al 2015). However, previous studies have
been highly uncertain about the derived particle properties and
may not be sampling the same small sublimating icy grain
population.
The distribution maps can be further probed by quantita-
tively analyzing H
2
O production from different H
2
O sources as
a function of the distance from the nucleus (Figure 11). As was
done for the CO
2
, the H
2
O distribution was examined at
various distances from the nucleus (2, 3.8, and 5.2 km). Here
Figure 3. CO
2
vapor distribution maps 4 hr, 3 hr, 2 hr, 1 hr, and 14 minutes prior to closest approach of Hartley 2. CO
2
is ubiquitous in the near-nucleus coma but is
enhanced by a factor of ∼2 compared to the mean in the direction extending from the small lobe of the comet (+Z-axis indicated by black arrows). The small insets are
images of the nucleus simulated from the Hartley 2 shape model (T. L. Farnham & P. C. Thomas 2013; P. C. Thomas et al. 2013)at the orientation and solar
illumination conditions as seen by the spacecraft incorporating the rotation rate of M. J. S. Belton et al. (2013). The nucleus undergoes ∼20% of its primary axis
rotation during the 4 hr time span. The location of the unresolved nucleus is marked with a black plus sign. In the E –14-minute map, the saturated nucleus is overlaid
with the shape model simulation. Each image is normalized to the mean of its radiance map. The Sun is to the right in all the images, and Ecliptic North is up.
5
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

five observations within 1 hr after closest approach, where the
geometry of the observations was similar, were averaged to
achieve higher signal-to-noise ratio. The resulting values are
plotted in Figure 11. At all three distances, the peak in the H
2
O
distribution is clearly dependent on both the direction of the
waist and the solar illumination direction. For the closest
distance, 2 km, the peak is dominated by the H
2
O released
directly from the illuminated waist, while by 3.8 km the waist
H
2
O has diffused and the peak has shifted to the sunward
direction as the subsolar component of the nucleus outgasses.
The peak remains in the subsolar direction at 5.2 km, while
evidence of the waist is still present. The antisunward
enhancement cannot be discerned this close to the nucleus; in
fact, the antisunward direction has the lowest abundance of
H
2
O at these distances. The data are modeled with three
components, one each for the waist, subsolar region, and
underlying ambient outgassing (Figure 11). These model fits
are used in Section 4.1 to determine the contribution of each
source to the total H
2
O vapor output of Hartley 2.
At later times (E+6 hr through E +15 hr), larger distances
(10 km)were sampled in a similar manner. As with CO
2
,
radial distance is not a factor in the location of the peak in H
2
O
production at E +6 hr. From 10 to 60 km radially from the
nucleus, the peak in H
2
O occurs in the antisunward direction in
the coma. Coincidentally, this is the same hemisphere as the
small lobe (+Zpole), but in contrast to the CO
2
, the H
2
O
distribution is broad and not narrowly peaked in the projected
small-lobe direction. Azimuthal extractions from representative
scans during about half of Hartley 2’s rotation (Figure 12)more
fully demonstrate that the antisunward enhancement is stable at
these distances. Data for E +6 hr, E +9 hr, E +12 hr, and E
+15 hr at a radial distance of 60 km are plotted. In contrast to
the near-nucleus distribution, at these larger distances the peak
of the H
2
O distribution remains constant in the antisunward
direction and is about a factor of 2 larger than in the sunward
direction.
3.2. Radial Profiles
We next examine the CO
2
and H
2
O distributions with
respect to their radial behavior, providing a more quantitative
measure of their column densities and relative contributions to
the coma. To accomplish this, we select a direction and angular
width of interest and consider a wedge of data centered on the
nucleus. We determine the projected radial distance from the
nucleus for each pixel in the wedge, and for each distance bin
we calculate a mean volatile abundance and compare the
results. Like the maps, the bin size for the different radial
profiles is dependent on the time-varying pixel scale and the
amount of data that was combined. When opacity effects are
negligible, line-of-sight column densities can be derived from
the radiance values, i.e., the number of CO
2
and H
2
O
molecules per unit area along the line of sight. We use the
following equation to convert from radiance to column density:
()( )( )( )( )
()
///NS lhcRg4,
1
hspecies band pix FOV band 2band
pWW=
where N
species
is the column density of either CO
2
or H
2
O
[m
−2
],S
band
is the integrated band radiance [Wm
−2
Sr
−1
],Ω
pix
is the solid angle of a pixel [sr],Ω
FOV
is the solid angle of the
field of view for the measurement (in this case the same as a
Figure 4. H
2
O vapor distribution maps at E –4 hr, E –3 hr, E –2 hr, E –1 hr, and E –14 minutes prior to closest approach. H
2
O is ubiquitous in the scene but
asymmetric and is especially enhanced in the sunward direction and above the smooth waist in these fields of view and spatial resolution. The annotations are similar
to those in Figure 3; however, here the red arrows are the projected normal to the illuminated waist area of the nucleus. Each map is normalized to its mean, and the
Sun is to the right, with Ecliptic North toward the top.
6
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

pixel; [sr]),λ
band
is the central wavelength of the emission band
[m],his Planck’s constant, cis the speed of light, R
h
is the
heliocentric distance of the comet (1.06 au for these data; [au]),
and g
band
is the optically thin fluorescent g-factor of the band at
R
h
=1au(2.8 ×10
−3
s
−1
for CO
2
and 3.1 ×10
−4
s
−1
for
H
2
O; A. M. Gersch 2014). Finally, production rates are derived
from the column densities using a simplified Haser equation,
Q
species
=(2vr)(N
species
), with the assumption that at 1 au the
outflow velocity, v, is a constant 780 m s
−1
out to a distance r
from the nucleus (from v=800 R
h
−0.5
), which matches actual
gas velocity measurements for Hartley 2 during this apparition
(J. Crovisier et al. 2013).
Within ∼100 km from Hartley 2’s nucleus, it is likely that
the conditions for complete fluorescent equilibrium in the coma
are not met, and within ∼10 km of the nucleus the outflowing
gas may still be accelerating (M. R. Combi et al. 2004). Using a
spherical adaptation of the coupled escape probability whereby
the effects of opacity and coma morphology are addressed,
A. M. Gersch (2014)showed that between ∼10 and 100 km
from the nucleus the effective g-factors are ∼70%–90% of the
equilibrium g-factors, i.e., 70%–90% of the optically thin
values, in the column density regimes we measure for Hartley
2. We employ a Q-curve analysis (Figure 13), often used to
interpret infrared spectroscopy of comets (e.g., N. Dello Russo
et al. 1998; M. A. DiSanti et al. 2001; B. P. Bonev 2005),to
show that Hartley 2’sH
2
O and CO
2
production rates become
asymptotic to a terminal value typically between 20 and 40 km
from the nucleus. Furthermore, we derive our absolute
production rates based on data >10 km from the nucleus and
using azimuthal averages similar to the method used in U. Fink
et al. (2016), reducing the influence of a variable velocity in the
simplified Haser model, and show that the absolute results we
report are not dependent on the radial distance for H
2
OorCO
2
(Figures 7and 12). Given the inverse proportionality between
the g-factor and column density and the direct proportionality
between the production rate, column density, and outflow
velocity, the competing over- or underestimation, respectively,
of production rates offset each other, and therefore our
measurements are robust with at most 30% uncertainty.
For data acquired outside of closest approach but where the
nucleus is still resolved (E+25 minutes, E +38 minutes, and
E+52 minutes), the nucleus orientation with respect to the
Sun and the spacecraft does not change significantly, as
the elapsed time is only ∼3% of the primary rotation period of
the nucleus. In our radial profile analysis in the following
sections, we therefore combine these three scans to increase the
sampling and signal-to-noise ratio. Similarly, we combine the
data half a rotation later, E +9 hr and E +9.5 hr, to increase
the sampling and signal-to-noise ratio for that orientation,
which is opposite the orientation near closest approach, with
the end of the large lobe now in sunlight. Additionally, since
Figure 5. H
2
O(blue color scale images)and CO
2
(green color scale)distribution maps, normalized to their mean, at 3 hr increments post-encounter showing coma
enhancements in each species and their time variability. Illuminated shape model images (not to scale)of the simulated orientation and solar illumination conditions of
Hartley 2 as seen by the spacecraft are included for each time step. In the H
2
O maps, a 90°wedge is overplotted in gray, indicating the antisunward quadrant where
water concentrates after closest approach. In the CO
2
maps, a black arrow is drawn in the projected direction of the small lobe of the nucleus (the +Z-axis). As in the
approach data (Figure 3), the CO
2
distribution is strongly correlated with the small lobe, rotating with it throughout the 18 hr period. The unresolved nucleus (black
plus sign)is located at the center of each of the maps. The Sun is to the right, and Ecliptic North is down.
7
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

the projected directions of the lobes are similar in the combined
scans (within ∼40°), radial profiles were extracted along the
projected comet-Sun/anti-Sun line (along the slit)rather than
the averaged projected lobe directions. Evaluation of radial
profiles extracted from the maps using a wedge with a full-
width opening angle of 60°validated these proxies, which
afford a slightly longer radial baseline than the foreshortened
projected lobe direction.
3.2.1. CO
2
Profiles
For CO
2
, we compare the volatile production from the small
and large lobes and how they vary with time and illumination
to assess the compositional differences of the lobes and aid in
our understanding of Hartley 2’s formational history using
maps separated in time by 8.5 hr, approximately half of the
primary rotation period, and oriented such that the majority of
one of the lobes is illuminated by the Sun while the other is not.
Specifically, we use a compilation of the E +25-minute, E +
38-minute, and E +52-minute maps to explore the behavior
when the small lobe is sunward and the E +9 hr and E +9.5 hr
maps to explore the behavior when the small lobe is pointing
antisunward and the large lobe is pointing sunward.
Figure 13 clearly shows that the highest CO
2
production
occurs when the small lobe is illuminated. Furthermore, the
CO
2
activity of the small lobe is always greater than or equal to
the large lobe at any given time, even when the small lobe is in
darkness. As can be seen in all four examples, the CO
2
column
density profiles are well behaved and follow an r
−1
falloff
beyond 20 km from the nucleus. Opacity effects and the
acceleration region of the coma close to the nucleus cause the
data to deviate from the r
−1
profiles within 20 km. Associated
production rates are also evaluated and confirm these results
(Figure 13, bottom panel). The small lobe is fully illuminated
Figure 6. Enhanced maps of CO
2
vapor in Hartley 2’s coma observed after closest approach, revealing that in all fields of view the enhanced CO
2
distribution is
correlated with the small lobe and rotates with the nucleus. Simulated nucleus orientations (left column, not to scale)and CO
2
distribution maps (middle column)are
as in Figure 5. The rightmost column displays normalized distribution maps after an azimuthal average was divided out. The Sun is to the right, and Ecliptic North
is down.
8
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

and more productive in CO
2
by a factor of two than the
nonilluminated large lobe during the time frame from E +25
minutes through E +52 minutes, 8.7 ×10
26
molecules s
−1
compared to 4.3 ×10
26
molecules s
−1
. From E +9 hr through
E+9.5 hr, the large lobe is pointing in the sunward direction
and is in full sunlight, while the small lobe is oriented in the
antisunward direction and is not illuminated, yet the small lobe
is still more productive than the large lobe by nearly a factor of
two, 4.3 ×10
26
molecules s
−1
compared to 2.3 ×10
26
molecules s
−1
. In addition, a steady production rate is reached
for both lobes beyond the distances within which opacity or gas
flow acceleration is a key factor in calculating accurate
production rates.
Assuming that Hartley 2 has been in complex rotation for
several apparitions, as was observed during the 2010 apparition
(M. J. S. Belton et al. 2013), then the entire surface has been
repeatedly exposed to solar insolation and recently evolved at
similar rates. If both lobes had a homogeneous composition and
a similarly accessible subsurface CO
2
reservoir, one would
expect the activity from the large lobe to be comparable to that
of the small lobe and, because of its larger size, to have the
largest production rate when fully illuminated. Thus, these data
imply that the two lobes of Hartley 2 must fundamentally differ
in composition or have differing pre-accretion evolution
histories.
3.2.2. H
2
O Profiles
For H
2
O, we determine and compare the volatile production
from two discrete sources, the nucleus and icy grains, and their
dependence on rotational phase and illumination to assess the
relative contribution of each source to Hartley 2’s overall
hyperactive H
2
O production rate. To begin, we combine and
examine the same subset of data as we did for CO
2
in
Section 3.2.1, where the small lobe is generally oriented in the
projected Sun direction in the E +25-minute through E +52-
minute data and opposite the Sun in the E +9 hr through E +
9.5 hr data.
For the time period closest to encounter, including the data
from E +25 minutes through E +52 minutes, the sunward
direction dominates the H
2
O vapor production in the maps
(Section 3.1.2)with a distinguishable contribution from the
waist, which is also oriented in the sunward hemisphere.
However, when examining radial profiles out to distances of
200 km created with 60°opening angles (Figure 13), the H
2
O
vapor emanating from the waist is not obvious when super-
imposed on the sunward coma produced by ambient sublima-
tion from the nucleus and thus is now a minimal contribution to
the overall coma. Although the field of view of the observations
precludes analysis of the antisunward large lobe at distances
greater than 30 km from E +25 minutes through E +52
minutes, continuing the smooth slope of the antisunward data
suggests that by ∼40 km the abundance of H
2
O in the
antisunward direction may be equivalent to or larger than the
sunward distribution. As with the CO
2
profiles, the H
2
O
column density is optically thick at distances less than ∼20 km,
but beyond that it generally follows an r
−1
falloff from the
nucleus in the sunward coma, indicative of a steady nucleus
sublimation source outside the acceleration region. In contrast,
upon examining the E +9 hr through E +9.5 hr time frame
when the small lobe points opposite the Sun, the antisunward
H
2
O vapor radial profile has a completely different slope than
the sunward profile and is more abundant than an r
−1
profile,
again supporting the argument for a secondary H
2
O source.
Associated H
2
O production rates are also derived and plotted in
Figure 13, confirming that a steady state is reached beyond
distances of 20 km only in the sunward coma.
Given the production rate curves in Figure 13, the derived H
2
O
production rate in the optically thin regime of the sunward coma
nearest in time to encounter is 6.55 ×10
27
molecules s
−1
,whilethe
antisunward coma achieves a production rate of 5.4 ×10
27
molecules s
−1
but has not yet plateaued when reaching the edge of
the fieldofviewat30km.Halfarotationlater,inlargerfields of
view, the antisunward coma is enriched in H
2
O by a factor of ∼3
over the sunward direction at 130 km and results in a production
rate of 7.8 ×10
27
molecules s
−1
as compared to 2.65 ×10
27
molecules s
−1
.TheseH
2
O production rates are consistent with
contemporaneous measurements made with Herschel/PACS,
Figure 7. Azimuthal distribution of CO
2
around the nucleus measured at E +6
hr from extracted radiance in adjacent 15°sectors at radial distances of 10, 26,
40, and 60 km from the nucleus and normalized to the peak radiance for each
distance. The peak in the distribution of CO
2
around the nucleus is clearly not
dependent on either radial distance or solar direction, but rather is directly
correlated to the direction of the small lobe (+Zpole). As in Figures 6and 8,
this result demonstrates the nonuniform release of CO
2
from the nucleus, with a
maximum at the small lobe that is a factor of ∼3 larger than its minimum.
Figure 8. Small-lobe CO
2
production decreases, but does not cease, when
unilluminated. The peak in CO
2
production at ∼4 hr and minimum at ∼14 hr
are approximately separated in time by half of the primary rotation of the
nucleus’s complex rotation period of ∼18 hr, corresponding to when the small
lobe is pointed in the antisunward hemisphere. The CO
2
production of the
small lobe never completely stops and increases when illuminated again with a
maximum production that is ∼3 larger than its minimum. The CO
2
is measured
20 km off the small lobe of the nucleus in a 30°opening angle.
9
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

SOHO/SWAN, and Keck/NIRSPEC giving total H
2
O production
rates of 1.2 ×10
28
s
−1
,(8.5 ±0.6)×10
27
s
–1
,and(8.8 ±1)×
10
27
s
−1
to (1.4 ±0.2)×10
28
s
−1
, respectively (K. J. Meech et al.
2011;M.R.Combietal.2011; N. Dello Russo et al. 2011),
providing an important cross-reference.
A representative subset of all the post-encounter H
2
O
observations (Figure 14)shows that the H
2
Ocomainthe
antisunward direction over a full primary rotation of Hartley 2 is
denser than the coma in the sunward direction for all the data
except those nearest to closest approach (e.g., <E+2hr)when
the small lobe was oriented in and expelling icy grains in the
sunward direction and the field of view was smaller. In addition,
even when the small lobe is oriented toward the Sun, beyond 40
km the antisunward H
2
O vapor coma is just as abundant but
started with less initial production from the comet nucleus. This
provides further evidence that H
2
O-ice-containing grains are
sublimating after being pushed by solar radiation pressure in the
antisunward direction and contribute to the overall H
2
Ovapor
production rate of the comet, inflating it beyond that expected
from sublimation from the nucleus alone (M. F. A’Hearn et al.
2011, J. M. Sunshine & L. M. Feaga 2021). Both the sunward
and antisunward profiles converge to an asymptotic value
between 100 and 200 km. The antisunward H
2
O column density
of ∼3.2 ×10
15
cm
−2
is ∼3 times larger than the sunward value
of ∼1.0 ×10
15
cm
−2
, again indicative of an antisunward
extended source of water. The general sunward–antisunward
asymmetries described here have been previously reported in
larger-scale ground-based Hartley 2 observations from the 2010
apparition (K. J. Meech et al. 2011; M. J. Mumma et al. 2011;
N. Dello Russo et al. 2013; H. Kawakita et al. 2013;B.P.Bonev
et al. 2013; M. M. Knight & D. G. Schleicher 2013; F. La Forgia
et al. 2017). Hartley 2’s coma distribution contrasts with several
other JFC coma distributions, which likely lack substantial icy
grains and where an asymmetric enhancement in the H
2
Ovapor
Figure 9. Enhanced maps of H
2
O vapor in Hartley 2’s coma (analogous to Figure 6). The resulting distribution maps emphasize that in small fields of view (top)the
water vapor is more symmetric about the nucleus, but in the later, larger fields of view an antisunward enhancement of a factor of ∼3 is very pronounced. The
antisunward asymmetry persists with time and is independent of how the nucleus is oriented, consistent with a water vapor contribution from icy grains pushed
antisunward by solar radiation pressure. The Sun is to the right, and Ecliptic North is down.
10
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

is persistent toward the sunward direction (e.g., 9P/Tempel 1,
2P/Encke, and 67P/Churyumov-Gerasimenko; L. M. Feaga
et al. 2007; C. A. Ihalawela et al. 2011;U.Finketal.2016).
Notably, Hartley 2’s asymmetry in H
2
O production is similar to
what was observed around comet 46P/Wirtanen, which is
thought to be a hyperactive comet like Hartley 2 and likely also
contains icy grains in the coma, even though no direct detections
of H
2
O ice were made during its 2018 apparition (B. P. Bonev
et al. 2021;M.M.Knightetal.2021; S. Protopapa et al. 2021;
N. X. Roth et al. 2021; T. Kareta et al. 2023;Y.Khanetal.
2023).
4. Sources of H
2
O
4.1. Contributions from Nucleus and Coma Sources
Several lines of evidence have been presented that support
the fact that multiple sources contribute to the overall water
production rate of Hartley 2. Consistent with the categorization
from past apparitions that Hartley 2 is a hyperactive comet
(O. Groussin et al. 2004), the comet must experience a process
by which more than surface sublimation contributes to its
gaseous water coma. Additionally, like all other comets studied
via spacecraft encounters, images of Hartley 2’s nucleus
showed only minor patches of surface ice (J. M. Sunshine et al.
2012); thus, sublimation from a large fraction of its surface is
ruled out. Finally, there is a large assortment of published work
from the 2010 apparition showing asymmetric antisunward
distributions of water and its photodissociation by-products
(K. J. Meech et al. 2011; M. J. Mumma et al. 2011; N. Dello
Russo et al. 2013; H. Kawakita et al. 2013; B. P. Bonev et al.
2013; M. M. Knight & D. G. Schleicher 2013; F. La Forgia
et al. 2017). Here we have been able to distinguish multiple
enhancements in the water distribution of Hartley 2’s coma that
are not spatially resolved in any other data set and correlate
them with specific regions on the nucleus and with an icy grain
population directly detected by the DIF instruments.
Figures 10–12 suggest that discrete nuclear sources from the
waist and subsolar point dominate the water production rate in
apertures 10 km in size, while an antisunward source
dominates the production rate at larger distances (40 km).
The individual contribution of each source is calculated from
the fits in Figure 11, integration of the antisunward hemi-
spherical contribution above the ambient level in Figure 12,
and the steady state production rates in Figure 13. The direct
sublimation of water from the waist accounts for 2%–3% of the
sunward hemisphere outgassing, while emission from the
subsolar point accounts for 2%–5%. Within 5 km from the
nucleus, the sunward hemisphere contributes 60% to the water
production, and the antisunward hemisphere contributes 40%.
In comparison, the steady-state production rates reached
beyond 40 km and attributed to the antisunward icy grains,
i.e., nonnucleus sources, account for 25%–40% of the overall
water production in Hartley 2.
4.2. Modeling the Sources
Although not a detailed physical model of the sublimation
from each of the sources, a simplified model representative of
the H
2
O sublimation occurring at Hartley 2 is compared to the
observations from E +6.5 hr (Figure 15). The major individual
sources, nucleus and icy grain population, are assumed to exhibit
an isotropic r
−1
falloff in production in the model, with the
contribution from the grains a multiplicative scaling factor
smaller than the central nucleus source. First, a single source
is placed at the center of the map, representing isotropic
sublimation from the nucleus. As expected, the resulting
symmetric distribution from this scenario does not match the
observations. Second, individual sources representing the grains
are then added within the first 4 pixels (within 12 km of the
nucleus)around the nucleus in all directions. Third, individual
sources were added from 4 to 20 pixels (∼60 km)with a conical
distribution in the antisunward half of the map. The scaling
factor applied to these hypothetical grains is 20% for the
symmetric grain population near the nucleus, 15%–7.5% for the
antisunward icy grains decreasing radially from 12 to 30 km, and
7.5% for the grains 30–60 km from the nucleus. A smoothing
function (5 spatial pixels)and random noise (0%–25%)are
applied to the simulated source map (Figure 15(b)).
The simulated map with grains is analyzed in the same way
as the observations, generating enhanced maps with an
azimuthal average removed and radial profiles with 60°
opening angles in the sunward and antisunward directions.
This simulated distribution reproduces most properties of the
observations. Figures 15 and 16 show the resulting model maps
and extracted radial profiles compared to actual observations
from E +6.5 hr. The morphology of the observations, as well
as relative column densities around the nucleus, is well
replicated by the model. The radial profiles in Figure 16 show
more quantitatively that the model matches the observations
from E +6 hr and E +6.5 hr well in the sunward direction, the
direction without an enhancement from icy grains. In the
antisunward direction, the radial profiles from the observations
themselves have a bit more variation; however, the model fits
nicely between the two time steps separated by 30 minutes. It is
also clear that the inclusion of sublimation from antisunward
icy grains in the model mimics the asymmetric radial profiles
from the observations, with the antisunward column densities
Figure 10. H
2
O distribution map with contours taken at E +6.5 hr showing a
strong antisunward enhancement, which out to ∼10 km is superimposed on the
nucleus contribution in the sunward direction. The pixel containing the nucleus
is circled in black, and the Sun is to the right, with Ecliptic North down.
11
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

twice as large as those in the sunward direction. From this
simple simulation and analysis, the DI observations of Hartley
2 are qualitatively and quantitatively consistent with icy grain
sublimation from the coma.
Following from the fact that the H
2
O enhancement in the
coma is evident beyond 20 km and out to the radial limits of
our analysis (i.e., ∼120–200 km), and if small icy grain
aggregates move on the order of 0.5–5ms
−1
(J. K. Harmon
et al. 2011; B. Hermalyn et al. 2013; S. Protopapa et al. 2014;
M. S. P. Kelley et al. 2015), the hypothesized particles must
have lifetimes on the order of or greater than hours to days to
reach these distances from the nucleus before sublimating.
These lifetimes are physically realistic for pure H
2
O ice and
consistent with the pure ice previously measured in the near-
nucleus coma of Hartley 2 by S. Protopapa et al. (2014)and the
pure ice that was measured in the ejecta of 9P/Tempel 1
expelled from that comet’s interior during the impact experi-
ment (J. M. Sunshine et al. 2007). Thus, the hypothesis that
pure icy grains released with the CO
2
from the small lobe are
responsible for the antisunward enhancement observed in the
HRI-IR data is plausible.
N. Fougere et al. (2013)and Y. Shou et al. (2025)combined
radiative transfer modeling of the HRI-IR Hartley 2 spectral
maps with two-dimensional axisymmetric Direct Simulation
Monte Carlo modeling and inversion techniques, respectively,
to determine active area sites on the nucleus and gas production
rates. N. Fougere et al. (2013)deduced that Hartley 2’s waist
contributes 16% of the H
2
O production, 7% is produced by the
rest of the nucleus, and 77% originates from icy grains. Of
similar scale, Y. Shou et al. (2025)concluded that only 15% of
the H
2
O production comes from the nucleus and 85% is
produced by an antisunward extended source. The models
deduce that the icy grains contribute twice as much H
2
O to the
coma than what we conclude here (80% vs. 40%). Our detailed
analysis of all the HRI-IR data spanning a full rotation argues
that there are opportunities for improvements to be incorpo-
rated in radiative transfer models to more accurately represent
Hartley 2’s actual activity. Furthermore, modeling activity at
Figure 11. Near-nucleus distribution of H
2
O measured from the extracted radiance in azimuthally adjacent 15°sectors at radial distances of 2, 3.8, and 5.2 km from
the nucleus and normalized to the peak radiance for each distance (top left). The peak in the distribution is dependent on the distance from the nucleus and the solar
illumination. For the closest distance, 2 km, the peak is due to the H
2
O released directly from the illuminated waist, while by 3.8 km the waist H
2
O has diffused and
the peak shifts to the sunward direction and is due to subsolar nucleus outgassing. The peak remains in the subsolar direction at 5.2 km. The antisunward enhancement
cannot be discerned this close to the nucleus. The plots for 2 and 5.2 km are offset for clarity by +0.1 and −0.1, respectively. Bottom left: the azimuthal radiance plots
are fit with three parabolic components, accounting for contribution from the waist (orange dotted), the subsolar point (orange dashed), and an ambient sublimation
background (orange dashed–dotted)in the example fit offset and plotted for the 5.2 km distance. The corresponding total fit is overplotted as a thick solid line for each
radial distance. The minimum occurs in the antisunward direction. Right: the annotated E +52-minute H
2
O distribution map is representative of this time period and
geometry, with the nucleus at the center of the image, 90°at the top, and the azimuthal angle increasing in the clockwise direction. The brighter pixels correspond to
higher abundance, and the scale is ∼8 km across.
12
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

other comets without comprehensive coverage as that provided
by DIXI may be problematic.
5. Discussion
The complex spatial asymmetries, multiple origins, and
variability in production rates of both CO
2
and H
2
O with
rotation documented in these unique observations of Hartley 2
cast into doubt any simple interpretation of the typical global
relative abundances measured telescopically. However, an
examination of the range of ratios and what conditions of
Hartley 2’s coma the global ratios correspond to can still be
informative. In addition to the spatially distinct distributions
between the two species, azimuthal light curves indicate peaks
when the species do not correlate, even when the small lobe is
not illuminated, and explain the existence of wide ranges of the
abundance ratio for a single comet. In Figure 17, for example,
CO
2
peaks in the projected direction of the small lobe, while
H
2
O peaks in the antisunward direction in observations
collected at E +6 hr. In this example, the CO
2
/H
2
O
abundance ratio is 21% at 60 km from the nucleus in the
small-lobe direction (azimuthal angle of 0°), while in the
antisunward direction (azimuthal angle of 270°)the ratio is
7.9%. In contrast, the E +6 hr global average CO
2
/H
2
Ois
12%. Alternatively, deriving a mixing ratio for conditions
nearest closest approach and at closer range to the nucleus
(when the small lobe is illuminated, and therefore overall CO
2
production is highest, and the sunward-facing waist and
subsolar point contribute to a resolved and enhanced sunward
H
2
O abundance)leads to a relative abundance of CO
2
/H
2
Oof
13% in the sunward direction. Half a rotation later, when
instead the large lobe is illuminated, the sunward abundance
ratio drops to 8.7%, while in the antisunward grain-enriched
region of the coma the abundance drops to 5.5% owing to the
much larger column densities of H
2
O there. Overall, the range
of CO
2
/H
2
O values (5%–21%)is consistent with measure-
ments made during previous apparitions (L. Colangeli et al.
1999; J. Crovisier et al. 1999)and places Hartley 2 in the
typical to high range for comets within 2 au from the Sun
(T. Ootsubo et al. 2012;M.F.A’Hearn et al. 2012; O. Harri-
ngton Pinto et al. 2023; M. R. Huffman et al. 2024), but it
shows that an individual measurement of the mixing ratio in
time and space is not necessarily representative of the whole
comet. Although the number of direct CO
2
-to-H
2
O measure-
ments in comets is still sparse because direct CO
2
measure-
ments cannot be made from the ground, the availability of such
data, including spatial correlations, is expected to dramatically
increase over the next decade with observations made by the
James Webb Space Telescope.
Additionally, as has been pointed out previously, e.g., by
B. P. Bonev et al. (2020), due to the heliocentric distance
dependence of sublimation, heterogeneous spatial distributions
with extended sources, and temporal variation, H
2
O is not
necessarily the best volatile baseline for abundance ratios. For
Hartley 2, documented secular decreases in H
2
O production
(M. R. Combi et al. 2011; M. M. Knight & D. G. Schleic-
her 2013; E. Jehin et al. 2023)will also have implications for
the interpretation of abundance ratios from one apparition to
the next. In Hartley 2’s case, it is interesting to consider what
may be causing this secular decrease. The reduction in H
2
O
production may be an indication of fewer icy grains in the
coma, removing much of the contribution to the H
2
O
production from the grains. However, it is not clear whether
the icy grains are being depleted because the CO
2
reservoir is
being depleted and is not available to deliver as much H
2
O ice
to the coma or because the comet’s rotational state has changed
and the illumination history and thermal wave propagation in
the small lobe have changed.
Although Rosetta was in orbit around comet 67P/Churyu-
mov-Gerasimenko (C-G)for 2 yr during its 2015 perihelion
and was able to monitor the comet’s activity over a longer time
frame, its infrared imaging spectrometer (VIRTIS-M)did not
map the coma at high cadence or with the field of view and
constant viewing geometry afforded by the Hartley 2 observa-
tions (U. Fink et al. 2016; A. Migliorini et al. 2016). After
VIRTIS-M ceased acquiring data owing to a cryocooler failure
Figure 12. Azimuthal distribution of H
2
O measured from extracted radiance in azimuthally adjacent 15°sectors at a radial distance of 60 km from the nucleus and
normalized to the peak radiance for each time step representing over half of Hartley 2’s rotation (E+6 hr through E +15 hr). The thick black curve is the average of
the plotted data. At these larger distances, the relative production of H
2
O is stable over time, and thus the comet’s rotation, and shows peak production from icy grains
in the antisunward direction by a factor of ∼2 over the minimum.
13
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

when the comet was at 1.9 au pre-perihelion, VIRTIS-H, a
point spectrometer, continued observing C-G throughout the
comet’s perihelion. However, the instrument was unable to
map a single rotation of the nucleus at high cadence (D. Boc-
kelee-Morvan et al. 2016). Nonetheless, there are noteworthy
comparisons between the primary volatile (H
2
O and CO
2
)
distributions and abundances in Hartley 2 and C-G near their
respective perihelia. Both comets exhibit a similar wide range
of CO
2
/H
2
O abundance ratios with localized enhancements,
Hartley 2 from 5% to 21% (this paper)and C-G from 1% to
32% (e.g., U. Fink et al. 2016; A. Migliorini et al. 2016;
D. Bockelee-Morvan et al. 2016). Additionally, CO
2
is seen to
Figure 13. Radial profiles and production rates of CO
2
(left column)and H
2
O(right)showing where the data deviate from an r
−1
falloff. The top panels are the
combined E +25-minute through E +52-minute data, and the middle panels are the combined E +9 hr through E +9.5 hr data, representative of the period in the
comet’s rotation where each of the lobes is pointed either sunward (red)or antisunward (black). The CO
2
and H
2
O profiles are inversely proportional to radial distance
beyond 20 km, inside of which the profiles suffer from varying degrees of opacity and accelerating gas flow, for both the small (diamonds)and large (squares)lobes of
the comet. The exception is the H
2
O profile for the antisunward small lobe, where the deviation from r
−1
between 10 and 150 km is attributed to sublimation from an
icy grain population. Associated production rates are displayed in the bottom panels. The H
2
O production in the antisunward-facing small-lobe plot stands out because
it never levels off to a steady rate. These data integrate a 60°wedge with vertex at the nucleus. The gaps in the small-lobe sunward data from ∼16 to 28 km in CO
2
and
∼160–195 km antisunward in the H
2
O data are due to poorly calibrated data with the HRI-IR ASF.
14
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

be released from both comets even when the active areas are
not illuminated and both comets have enhanced H
2
O activity
above their waist/neck regions, with little accompanying CO
2
release. Intense seasonal effects dominate C-G’s current
activity and result in an abundance of material transport
between hemispheres (e.g., D. Bockelee-Morvan et al. 2016;
M. Lauter et al. 2019; H. U. Keller et al. 2015)because of its
high obliquity and rotation state (L. Jorda et al. 2016). The
numerous seasonal influences on C-G cause a hemispherical
dichotomy in activity and composition as measured in the
coma, making it difficult to disentangle these short-term
evolutionary effects from possible formational composition
differences across the nucleus of C-G, although there is little
indication noted of compositional differences between the
lobes (I. R. H. G. Schroeder et al. 2019; M. Lauter et al. 2020).
In contrast, due to Hartley 2’s orientation and complex rotation
state, the entire surface is exposed to sunlight over its ∼55 hr
period (M. J. S. Belton et al. 2013), and there is an absence of
noticeable seasonal effects on Hartley 2. Therefore, the
cometesimal-scale activity and compositional heterogeneity
between the two distinct lobes can be reasonably attributed to
formational or evolutionary differences of the lobes prior to
their accretion.
6. Conclusions
The DIXI flyby of Hartley 2 presented a unique opportunity
to study a hyperactive comet, nearly evenly illuminated owing
to its complex rotation, at 1.06 au from the Sun. With an
intentionally designed observing campaign with a near-
constant vantage point at high spatial and temporal resolution,
the well-calibrated HRI-IR provided repeated views of Hartley
2 and its innermost coma with excellent signal-to-noise ratio
over several rotations. This type of in-depth analysis of a
comet’s primary volatiles will not be easily replicated,
especially since there are no similarly dedicated comet missions
with comparable extended observation potential in the near
future and Earth-based telescopes typically cannot dedicate
large amounts of uninterrupted observing time to one target.
The H
2
O distribution in Hartley 2’s coma exhibits behaviors
documented in many other JFCs, namely a sunward enhance-
ment from direct solar insolation of the nucleus and a
secondary source antisunward of the nucleus from the
sublimation of icy grains. For the first time, the spatially
resolved data of the innermost coma presented here allow for
the detailed separation of two major sources of water (direct
nucleus sublimation and a coma source)and a robust
calculation of the contribution of each to Hartley 2’s total
H
2
O production rate, with the icy grains in the coma
accounting for 25%–40% of the water during the comet’s
2010 apparition. A simple model was employed and matches
the data well, showing the plausibility of the antisunward coma
enhancement being supplied by H
2
O-ice grains sublimating
∼20–200 km away from the nucleus. In addition, the
observations and analyses provide clear evidence that the
process of fallback of icy material onto a comet is real, but in
the case of Hartley 2’s waist, it contributes minimally to the
total H
2
O production rate (a few percent).
Hartley 2 has been argued to be a hyperactive comet end-
member along a continuum of cometary composition and
activity (J. M. Sunshine & L. M. Feaga 2021), with its
detectable H
2
O-ice grains driven out of the nucleus by CO
2
.
Thus, the result that Hartley 2’s icy grains account for
25%–40% of the H
2
O coma implies that icy grains of various
volatile compositions can be the origin of a secondary,
nonnucleus source region of coma gas for any comet and
contribute anywhere from ∼0% to 40% to the overall comet
production rate. Additionally, hyperactive comets, which likely
have a larger proportion of activity from icy gains in the coma
Figure 14. Radial profiles of H
2
O column density for a representative sample of data covering a full primary rotation of Hartley 2. An r
−1
falloff is overplotted (solid
black line)for reference. The antisunward coma has more H
2
O than predicted from a nucleus sublimation source. Both the sunward and antisunward profiles converge
to an asymptotic value at 100–200 km. The antisunward value (∼3.2 ×10
15
cm
−2
)is larger than the sunward value (∼1.0 ×10
15
cm
−2
)by a factor of ∼3, indicative
of an antisunward extended source of water.
15
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

(J. M. Sunshine & L. M. Feaga 2021), have D/H ratios closer
to terrestrial values (D. C. Lis et al. 2019), suggesting that
water from grains that originate from the cometary interior may
be less fractionated than water derived from near the comet’s
surface, which has been more thermally processed (e.g., the
fallback material). Thus, this balance of activity from coma and
nuclear sources will complicate measures of fractionation.
Furthermore, with 25%–40% of the water production from
grains in the coma, one must be very careful when comparing
abundance ratios of comets, making sure to constrain the
parameter space as much as possible. It is important to
recognize that in addition to heliocentric variations in volatile
sublimation, there is likely a factor of a few variation in
outgassing for each comet depending on where the measure-
ment was taken spatially and with respect to the comet’s
rotation and secular variability from one apparition to the next.
No matter the root cause of the secular decrease in Hartley 2’s
water production, its classification as a hyperactive comet may
no longer be applicable, making the Deep Impact observations
even more valuable in untangling the individual sources that
contribute to a comet’sH
2
O production.
Finally, while the values derived here for the H
2
O and CO
2
coma distribution and production rates are for Hartley 2’s 2010
apparition, they give insight into all comets and suggest caution
in interpreting production rates and abundance ratios of
volatiles derived from lower-resolution data of other comets.
Synthesizing the observations from a full rotation of the
nucleus, the small lobe is always as or more productive in CO
2
than the large lobe regardless of illumination conditions and
rotational phase, emphasizing that the small lobe dominates
Hartley 2’sCO
2
production and is fundamentally different
from the large lobe. Under the current conditions of Hartley 2’s
rotation state and inferred solar insolation, the compositional
heterogeneity of the two lobes of the comet implies that the
Figure 15. Simple icy grain source sublimation model described in Section 4.2 compared to H
2
O column density distribution maps from Hartley 2: (a)model with
implanted r
−1
sources; (b)smoothed model; (c)random noise added to the model; (d)observed map of the H
2
O distribution of Hartley 2 at E +6.5 hr; (e)azimuthal
average divided out of the model to accentuate the antisunward distribution enhancement; (f)azimuthal average divided out of the observations to accentuate the
antisunward distribution enhancement in the H
2
O distribution of Hartley 2 at E +6.5 hr. This simple model replicates the observations in both morphology and
relative scale to within ∼10%, increasing the plausibility that H
2
O-ice grains are responsible for Hartley 2’s extended H
2
O and are producing the observed asymmetry
in the coma. Model plots are in units of 10
16
cm
−2
, and E +6.5 hr data are in cm
−2
. The Sun is to the right, and the nucleus location is indicated by the black circle.
16
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

lobes had different formational or evolutionary histories before
they came together as one body. With a higher CO
2
production
rate, the small lobe may have originated in a colder, more
distant environment that preserved CO
2
ice more efficiently
than the large lobe, which suggests large-scale mixing of
cometesimals in the protosolar disk.
Acknowledgments
We dedicate this paper to M. F. A’Hearn, who asked us the
Big Question (“How much does each source contribute to the
overall water production rate in Hartley 2?”), and to Michael J.
S. Belton, who lovingly and patiently prodded us to examine
the CO
2
and H
2
O light curves. We also acknowledge and thank
T. Farnham for providing ancillary SPICE data and shape
model context and for scientific discussions, S. Protopapa for
modeling the continuum in the data, and S. Besse and F. Merlin
for their calibration and preliminary analysis work during the
EPOXI mission. Finally, we appreciate the suggestions from
the anonymous reviewers, which have improved our
manuscript.
This work was supported by NASA through two Discovery
Data Analysis Programs: 80NSSC18K1041 and 80NSSC18
K1280. Calibrated HRI-IR data are located in the Planetary
Data System. EPOXI was an extension of the Deep Impact
mission funded by NASA’s Discovery Program contract
NNM07AA99C to the University of Maryland and task order
NMO711002 to the Jet Propulsion Laboratory.
ORCID iDs
L. M. Feaga https://orcid.org/0000-0002-4230-6759
J. M. Sunshine https://orcid.org/0000-0002-9413-8785
References
A’Hearn, M. F. 2008, SSRv,138, 237
A’Hearn, M. F., Belton, M. J. S., Delamere, W. A., et al. 2005, Sci,310, 258
A’Hearn, M. F., Belton, M. J. S., Delamere, W. A., et al. 2011, Sci,332, 1396
A’Hearn, M. F., Feaga, L. M., Keller, H. U., et al. 2012, ApJ,758, 29
Belton, M. J. S., Thomas, P., Li, J.-Y., et al. 2013, Icar,222, 595
Bockelee-Morvan, D., Crovisier, J., Erard, S., et al. 2016, MNRAS,462, S170
Bonev, B. P. 2005, PhDT, Univ. Toledo
Bonev, B. P., Dello Russo, N., DiSanti, M. A., et al. 2020, DPS, 52, 316.02
Bonev, B. P., Dello Russo, N., DiSanti, M. A., et al. 2021, PSJ,2, 45
Bonev, B. P., Villanueva, G. L., Paganini, L., et al. 2013, Icar,222, 740
Colangeli, L., Epifani, E., Brucato, J. R., et al. 1999, A&A, 343, L87
Combi, M. R., Bertaux, J.-L., Quémerais, E., et al. 2011, ApJL,734, L6
Combi, M. R., Harris, W. M., & Smyth, W. H. 2004, in Comets II, ed.
M. C. Festou et al. (Tucson, AZ: Univ. Arizona Press),523
Crovisier, J., Encrenaz, Th., Lellouch, E., et al. 1999, in The Universe as Seen
by ISO, ESA-SP 427, ed. P. Cox & M. F. Kessler (Paris: ESA),161
Crovisier, J., Colom, P., Biver, N., et al. 2013, Icar,222, 679
Dello Russo, N., DiSanti, M. A., Mumma, M. J., et al. 1998, Icar,135, 377
Dello Russo, N., Vervack, R. J., Lisse, C. M., et al. 2011, ApJL,734, L8
Dello Russo, N., Vervack, R. J., Weaver, H. A., et al. 2013, Icar,222, 707
DiSanti, M. A., Mumma, M. J., Dello Russo, N., & Magee-Sauer, K. 2001,
Icar,153, 361
Farnham, T. L., & Thomas, P. C. 2013, PLATE SHAPE MODEL OF COMET
103P/HARTLEY 2 V1.0, DIF-C-HRIV/MRI-5-HARTLEY2-SHAPE-V1.0,
NASA Planetary Data System
Feaga, L. M., A’Hearn, M. F., Farnham, T. L., et al. 2014, AJ,147, 24
Feaga, L. M., A’Hearn, M. F., Sunshine, J. M., et al. 2007, Icar,190, 345
Feaga, L. M., Protopapa, S., Besse, S., et al. 2012, LPICo, 1667, 6441
Feaga, L. M., Sunshine, J. M., Bonev, B. P., et al. 2023, LPICo, 2851, 2476
Fink, U., Doose, L., Rinaldi, G., et al. 2016, Icar,277, 78
Fougere, N., Combi, M. R., Rubin, M., & Tenishev, V. 2013, Icar,225, 688
Gersch, A. M. 2014, PhD Thesis, Univ Maryland
Groussin, O., Lamy, P., Jorda, L., & Toth, I. 2004, A&A,419, 375
Groussin, O., Sunshine, J. M., Feaga, L. M., et al. 2013, Icar,222, 580
Hampton, D. L., Baer, J. W., Huisjen, M. A., et al. 2005, SSRv,117, 43
Harmon, J. K., Nolan, M. C., Howell, E. S., et al. 2011, ApJL,734, L2
Harrington Pinto, O., Kelley, M. S. P., Villanueva, G. L., et al. 2023, PSJ,4, 208
Hermalyn, B., Farnham, T. L., Collins, S. M., et al. 2013, Icar,222, 625
Huffman, M. R., McKay, A. J., & Cochran, A. L. 2024, PSJ,5, 39
Ihalawela, C. A., Pierce, D. M., Dorman, G. R., et al. 2011, ApJ,741, 89
Jehin, E., Vander Donckt, M., Hmiddouch, S., et al. 2023, ATel, 16275, 1
Jorda, L., Gaskell, R., Capanna, C., et al. 2016, Icar,277, 257
Kareta, T., Noonan, J. W., Harris, W. M., et al. 2023, PSJ,4, 85
Kawakita, H., Kobayashi, H., Dello Russo, N., et al. 2013, Icar,222, 723
Keller, H. U., Mottola, S., Davidsson, B., et al. 2015, A&A,583, A34
Kelley, M. S., Lindler, D. J., Bodewits, D., et al. 2013, Icar,222, 634
Kelley, M. S. P., Lindler, D. J., Bodewits, D., et al. 2015, Icar,262, 187
Khan, Y., Gibb, E. L., Roth, N. X., et al. 2023, AJ,165, 231
Klaasen, K. P., A’Hearn, M., Besse, S., et al. 2013, Icar,225, 643
Klaasen, K. P., A’Hearn, M. F., Baca, M., et al. 2008, RScI,79, 091301
Knight, M. M., & Schleicher, D. G. 2013, Icar,222, 691
Knight, M. M., Schleicher, D. G., & Farnham, T. L. 2021, PSJ,2, 104
La Forgia, F., Bodewits, D., A’Hearn, M. F., et al. 2017, AJ,154, 185
Lauter, M., Kramer, T., Rubin, M., & Altwegg, K. 2019, MNRAS, 483, 852
Figure 16. Sunward (+)and antisunward (–)radial profiles from the model
(black solid line)described in Section 4.2 compared to radial profiles extracted
from H
2
O distribution maps of Hartley 2 at E +6.0 hr (blue dashed line)and E
+6.5 hr (red solid line). Quantitatively, the model represents the observations
well, agreeing with the r
−1
nucleus sublimation radial falloff in the sunward
direction and the asymmetric factor of 2 larger column density in the
antisunward direction due to a secondary H
2
O source of ice gains in the coma.
Figure 17. The average H
2
O distribution (blue)at 60 km from Figure 12
compared with the CO
2
distribution (orange)at the same distance at E +6 hr.
The peaks in the two species differ as expected, with the H
2
O peak near the
anti-Sun direction and the CO
2
peak near the projected small-lobe direction.
17
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine

Lauter, M., Kramer, T., Rubin, M., & Altwegg, K. 2020, MNRAS,498, 3995
Lis, D. C., Bockelée-Morvan, D., Güsten, R., et al. 2019, A&A,625, L5
McLaughlin, S. A., Carcich, B., Sackett, S. E., et al. 2014, DIF-C-HRII-3/4-
EPOXI-HARTLEY2-V3.0, NASA Planetary Data System
Meech, K. J., A’Hearn, M. F., Adams, J. A., et al. 2011, ApJL,734, L1
Migliorini, A., Piccioni, G., Capaccioni, F., et al. 2016, A&A,589, A45
Mumma, M. J., Bonev, B. P., Villanueva, G. L., et al. 2011, ApJL,734, L7
Ootsubo, T., Kawakita, H., Hamada, S., et al. 2012, ApJ,752, 15
Protopapa, S., Kelley, M. S. P., Woodward, C. E., et al. 2021, PSJ,2, 176
Protopapa, S., Sunshine, J. M., Feaga, L. M., et al. 2014, Icar,238, 191
Richardson, J. E., & Bowling, T. J. 2014, Icar,234, 53
Roth, N. X., Bonev, B. P., DiSanti, M. A., et al. 2021, PSJ,2, 54
Samarasinha, N. H., Martin, M. P., & Larson, S. M. 2013, Cometary Coma
Image Enhancement Facility, http://cie.psi.edu
Schroeder, I. R. H. G., Altwegg, K., Balsiger, H., et al. 2019, MNRAS,
489, 4734
Shou, Y., Combi, M., Feaga, L., et al. 2025, Icar,435, 116557
Sunshine, J. M., & Feaga, L. M. 2021, PSJ,2, 92
Sunshine, J. M., Feaga, L. M., Groussin, O., et al. 2012, LPICo, 1667, 6438
Sunshine, J. M., Groussin, O., Schultz, P. H., et al. 2007, Icar,190, 284
Thomas, P. C., A’Hearn, M. F., Veverka, J., et al. 2013, Icar,222, 550
Weaver, H. A., Feldman, P. D., A’Hearn, M. F., et al. 2011, ApJL,734, L5
18
The Planetary Science Journal, 6:95 (18pp), 2025 April Feaga & Sunshine