S. Marchi

Southwest Research Institute, San Antonio, Texas, United States

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Publications (253)959.59 Total impact

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    ABSTRACT: One of the main aims of the ESA Rosetta mission is to study the origin of the solar system by exploring comet 67P/Churyumov-Gerasimenko at close range. In this paper we discuss the origin and evolution of comet 67P/Churyumov-Gerasimenko in relation to that of comets in general and in the framework of current solar system formation models. We use data from the OSIRIS scientific cameras as basic constraints. In particular, we discuss the overall bi-lobate shape and the presence of key geological features, such as layers and fractures. We also treat the problem of collisional evolution of comet nuclei by a particle-in-a-box calculation for an estimate of the probability of survival for 67P/Churyumov-Gerasimenko during the early epochs of the solar system. We argue that the two lobes of the 67P/Churyumov-Gerasimenko nucleus are derived from two distinct objects that have formed a contact binary via a gentle merger. The lobes are separate bodies, though sufficiently similar to have formed in the same environment. An estimate of the collisional rate in the primordial, trans-planetary disk shows that most comets of similar size to 67P/Churyumov-Gerasimenko are likely collisional fragments, although survival of primordial planetesimals cannot be excluded. A collisional origin of the contact binary is suggested, and the low bulk density of the aggregate and abundance of volatile species show that a very gentle merger must have occurred. We thus consider two main scenarios: the primordial accretion of planetesimals, and the re-accretion of fragments after an energetic impact onto a larger parent body. We point to the primordial signatures exhibited by 67P/Churyumov-Gerasimenko and other comet nuclei as critical tests of the collisional evolution.
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    ABSTRACT: The inner solar system's biggest and most recent known collision was the Moon-forming giant impact between a large protoplanet and proto-Earth. Not only did it create a disk near Earth that formed the Moon, it also ejected several percent of an Earth mass out of the Earth-Moon system. Here, we argue that numerous kilometer-sized ejecta fragments from that event struck main-belt asteroids at velocities exceeding 10 kilometers per second, enough to heat and degas target rock. Such impacts produce ~1000 times more highly heated material by volume than do typical main belt collisions at ~5 kilometers per second. By modeling their temporal evolution, and fitting the results to ancient impact heating signatures in stony meteorites, we infer that the Moon formed ~4.47 billion years ago, which is in agreement with previous estimates. Copyright © 2015, American Association for the Advancement of Science.
    Science 04/2015; 348(6232):321-3. DOI:10.1126/science.aaa0602 · 31.48 Impact Factor
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    ABSTRACT: At a global scale, Mercury is dominated by contractional features manifested as lobate scarps, wrinkle ridges and high-relief ridges. Here, we show that some of these features are associated with strike-slip kinematic indicators, which we identified using flyby and orbital Mercury Dual Imaging System (MDIS) data and digital terrain models. We recognize oblique-shear kinematics along lobate scarps and high-relief ridges by means of (1) map geometries of fault patterns (frontal thrusts bordered by lateral ramps, strike-slip duplexes, restraining bends); (2) structural morphologies indicating lateral shearing (en echelon folding, pop-ups, pull-aparts); and (3) estimates of offsets based on displaced crater rims and differences in elevation between pop-up structures and pull-apart basins and their surroundings. Transpressional faults, documented across a wide range of latitudes, are found associated with reactivated rims of ancient buried basins and, in most cases, linked to frontal thrusts as lateral ramps hundreds of kilometres long. This latter observation suggests stable directions of tectonic transport over wide regions of Mercury's surface. In contrast, global cooling would imply an overall isotropic contraction with limited processes of lateral shearing induced by pre-existent lithospheric heterogeneities. Mantle convection therefore may have played an important role during the early tectonic evolution of Mercury. Estimating absolute model ages for compressional features and comparing it from what it is envisaged with thermal modeling based on cooling alone can be useful in determining if other processes could have been responsible for lobate scarps nucleation. In particular, ages more ancient that the one predicted by the models would imply other kind of tectonic processes ongoing during the early evolution of Mercury. For this reason, we date an extended thrust system, which we term the Blossom Thrust System, located between 80 • E and 100 • E, and 30 • N and 15 • S, and which consists of several individual lobate scarps exhibiting a north–south orientation and a westward vergence. The age of the system was determined using several different methods. Traditional stratigraphic analysis was accompanied by crater counting of units that overlap the thrust system and by using the buffered crater-counting technique, allowing us to determine an absolute model age for the tectonic feature. These complementary methods give consistent results, implying that activity on the thrust ended between 3.5 and 3.7 Ga, depending on the adopted absolute-age model. Since these ages imply an unexpected early nucleation and end of a major lobate scarp system, the data, if confirmed elsewhere on Mercury surface, would have major implications for models of the thermal evolution and/or tectonic processes of the planet as a whole.
    EGU 2015, Vienna; 04/2015
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    ABSTRACT: Asteroid (16) Psyche is likely a metallic planetesimal core, stripped by hit-and-run impacts. It offers a unique window into core and dynamo formation.
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    ABSTRACT: The young Marcia crater on Vesta displays several interesting features, including pitted and smooth terrains, exposure of relatively bright and dark material, and enrichments of hydrated material in the ejecta. Several questions arise about the origin of Marcia and of the dark material (exogenic material vs volcanic or impact melts) and the smooth and pitted terrains. Here we describe the results of the spectral and thermal analysis of the Marcia crater, with a particular effort to assess the composition of the different units, identifying the presence of OH and its correlation with dark material. Detailed studies of the Marcia crater wall, smooth and floor units reveal a compositional rich terrain with small areas enriched in diogenites with respect to the general eucritic regolith dominating the equatorial region of Vesta. The signature of OH is particularly clear in the pitted floor, dark material, smooth unit, and ejecta. The pitted terrains, beside their appearance, also show thermal anomalies, being colder with respect to the surrounding terrains. The presence of OH, concentrated in darker layers, and the pitted crater floor indicate that the area where the Marcia impact event occurred was rich in volatiles. The results show how the relatively young impact events have modified the surface of Vesta, disrupting a layer of dark material once present on Vesta's equatorial terrain and exposing fresh, bright material rich in pyroxene.
    Icarus 02/2015; 248. DOI:10.1016/j.icarus.2014.10.051 · 2.84 Impact Factor
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    ABSTRACT: Rheasilvia formed not so long ago/Ceres got hit by more than one blow/Look to Dione but of craters/Only Ceres will know.
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    ABSTRACT: Using our E-belt model that matches Earth/Moon/asteroid belt constraints, we argue Mars’ observed D 〉 150 km craters were all made by the LHB 2.6─4.1 G.y. ago.
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    ABSTRACT: Images of comet 67P/Churyumov-Gerasimenko acquired by the OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) imaging system onboard the European Space Agency's Rosetta spacecraft at scales of better than 0.8 meter per pixel show a wide variety of different structures and textures. The data show the importance of airfall, surface dust transport, mass wasting, and insolation weathering for cometary surface evolution, and they offer some support for subsurface fluidization models and mass loss through the ejection of large chunks of material. Copyright © 2015, American Association for the Advancement of Science.
    Science 01/2015; 347(6220):aaa0440. DOI:10.1126/science.aaa0440 · 31.48 Impact Factor
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    ABSTRACT: Images from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activity at a distance from the Sun of >3 astronomical units is predominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 billion years ago, or a single body where a gap has evolved via mass loss.
    Science 01/2015; 347(6620). DOI:10.1126/science.aaa1044 · 31.48 Impact Factor
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    ABSTRACT: Abstract In this paper we present a time-stratigraphic scheme and geologic time scale for the protoplanet Vesta, based on global geologic mapping and other analyses of {NASA} Dawn spacecraft data, complemented by insights gained from laboratory studies of howardite–eucrite–diogenite (HED) meteorites and geophysical modeling. On the basis of prominent impact structures and their associated deposits, we propose a time scale for Vesta that consists of four geologic time periods: Pre-Veneneian, Veneneian, Rheasilvian, and Marcian. The Pre-Veneneian Period covers the time from the formation of Vesta up to the Veneneia impact event, from 4.6 Ga to >2.1 Ga (using the asteroid flux-derived chronology system) or from 4.6 Ga to 3.7 Ga (under the lunar-derived chronology system). The Veneneian Period covers the time span between the Veneneia and Rheasilvia impact events, from >2.1 to 1 Ga (asteroid flux-derived chronology) or from 3.7 to 3.5 Ga (lunar-derived chronology), respectively. The Rheasilvian Period covers the time span between the Rheasilvia and Marcia impact events, and the Marcian Period covers the time between the Marcia impact event until the present. The age of the Marcia impact is still uncertain, but our current best estimates from crater counts of the ejecta blanket suggest an age between ∼120 and 390 Ma, depending upon choice of chronology system used. Regardless, the Marcia impact represents the youngest major geologic event on Vesta. Our proposed four-period geologic time scale for Vesta is, to a first order, comparable to those developed for other airless terrestrial bodies.
    Icarus 12/2014; 244(2014):158 - 165. DOI:10.1016/j.icarus.2014.06.027 · 2.84 Impact Factor
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    ABSTRACT: The ROSETTA mission has reached the nucleus of comet 67P/Churyumov-Gerasimenko in early August, allowing a detailed mapping of its surface with the onboard OSIRIS imaging system, up to resolutions of 50 cm or even better in some areas. Shape reconstruction techniques have been used since July 2014 to build a very detailed 3D model of the comet surface and to retrieve highly accurate rotational parameters. The attached Figure shows an early 3D shape of the comet. The most striking property of the global shape is the presence of two clearly separated components. We use a combination of images and 3D models to quantitatively characterize their bulk and surface properties: relative position, volume, topography and roughness. The position of the center of mass and the direction of the principal axis of rotation are used to constrain the internal mass distribution. Digital terrain models, slope, gravity, and geo-referenced images of the most interesting features observed at the surface of the comet are presented and consequences for their formation are discussed. If they are detected, topographic changes which could have occured between August and December 2014 are also presented.
    AGU Fall Meeting 2014; 12/2014
  • 77th Annual Meeting of the Meteoritical-Society; 09/2014
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    Geological Society London Special Publications 08/2014; 401(1). DOI:10.1144/SP401.21 · 2.58 Impact Factor
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    ABSTRACT: The history of the Hadean Earth (∼4.0-4.5 billion years ago) is poorly understood because few known rocks are older than ∼3.8 billion years old. The main constraints from this era come from ancient submillimetre zircon grains. Some of these zircons date back to ∼4.4 billion years ago when the Moon, and presumably the Earth, was being pummelled by an enormous flux of extraterrestrial bodies. The magnitude and exact timing of these early terrestrial impacts, and their effects on crustal growth and evolution, are unknown. Here we provide a new bombardment model of the Hadean Earth that has been calibrated using existing lunar and terrestrial data. We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt. This model may explain the age distribution of Hadean zircons and the absence of early terrestrial rocks. Existing oceans would have repeatedly boiled away into steam atmospheres as a result of large collisions as late as about 4 billion years ago.
    Nature 07/2014; 511(7511):578-582. DOI:10.1038/nature13539 · 42.35 Impact Factor
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    ABSTRACT: This paper reviews the observations of crater-like features on cometary nuclei. We compare potential crater sizes and morphologies, we discuss the probability of impacts between small asteroids in the Main Belt an a comet crossing this region of the Solar System. Finally, we investigate the fate of the impactor and its chances of survival on the nucleus. We find that comets do undergo impacts although the rapid evolution of the surface erases most of the features and make craters difficult to detect. In the case of a collision between a rocky body and a highly porous cometary nucleus, two specific crater morphologies can be formed: a central pit surrounded by a shallow depression, or a pit, deeper than typical craters observed on rocky surfaces. After the impact, it is likely that a significant fraction of the projectile will remain in the crater. During its two years long escort of comet 67P/Churyumov-Gerasimenko, ESA's mission Rosetta should be able to detect specific silicates signatures at the bottom of craters or crater-like features, as evidence of this contamination. For large craters, structural changes in the impacted region, in particular compaction of material, will affect the local activity. The increase of tensile strength can extinct the activity by preventing the gas from lifting up dust grains. On the other hand, material compaction can help the heat flux to travel deeper in the nucleus, potentially reaching unexposed pockets of volatiles, and therefore increasing the activity. Ground truth data from Rosetta will help us infer the relative importance of those two effects.
    Planetary and Space Science 07/2014; 107. DOI:10.1016/j.pss.2014.06.008 · 1.63 Impact Factor
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    ABSTRACT: Vesta has a complex cratering history, with ancient terrains as well as recent large impacts that have led to regional resurfacing. Crater counts can help constrain the relative ages of different units on Vesta's surface, but converting those crater counts to absolute ages requires a chronology function. We present a cratering chronology based on the best current models for the dynamical evolution of asteroid belt, and calibrate it to Vesta using the record of large craters on its surface. While uncertainties remain, our chronology function is broadly consistent with an ancient surface of Vesta as well as other constraints such as the bombardment history of the rest of the inner Solar System and the Ar-Ar age distribution of howardite, eucrite and diogenite (HED) meteorites from Vesta.
    Planetary and Space Science 06/2014; DOI:10.1016/j.pss.2014.05.013 · 1.63 Impact Factor
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    ABSTRACT: Space missions and thermal infrared observations have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact. Laboratory experiments and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids.
    Nature 04/2014; 508(7495). DOI:10.1038/nature13153 · 42.35 Impact Factor
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    ABSTRACT: Small morphologically fresh impact craters (<10 km in diameter) on Vesta's surface with a photometrically distinct ejecta blanket are expected to represent fresh surface material, and thus provide the opportunity to study the composition of the unweathered surface. Dawn-FC and VIR data reveal impact craters with bright, dark, as well as mixed, i.e. partly bright and dark, ejecta existing on Vesta's surface, which not only differ in the visible albedo from their surroundings but also in their composition. Differences in the composition are related to the visible albedo and/or the geographic location of the impact craters. Bright ejecta, only seen in the southern Vestan hemisphere, are dominated by howardite/eucrite-like material as expected for Vesta's upper crust. Dark ejecta associated with dark impact craters are dominated by a strongly absorbing, spectrally neutral compound, supporting an origin from carbon-rich impactors. Few impact craters of intermediate albedo in Vesta's southern hemisphere contain material resembling diogenites, which are expected to exist in the deeper parts of Vesta's interior. The geological settings suggest that the diogenite-like material represents a part of a layer of diogenitic material surrounding the Rheasilvia basin or local concentrations of diogenitic material as part of the ejecta excavated during the latter stage of the Rheasilvia impact event. The spectral differences between eucrite- and diogenite-dominated materials also could be verified due to spin-forbidden absorptions in the visible spectral range, which are known from laboratory spectra of pyroxenes, but, which have been identified in the VIR spectra of Vesta for the first time.
    The Journal of Geophysical Research Planets 04/2014; 119(4). DOI:10.1002/2013JE004388 · 3.44 Impact Factor
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    ABSTRACT: We propose to visit an iron core by sending a mission to (16) Psyche, by far the largest exposed iron metal body in the solar system.
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    ABSTRACT: Scattered ejecta from the Moon-forming impact hit asteroids and made ancient Ar-Ar reset ages. They suggest the Moon formed 100 ± 30 Ma after CAIs.

Publication Stats

2k Citations
959.59 Total Impact Points


  • 2012–2015
    • Southwest Research Institute
      San Antonio, Texas, United States
    • Brown University
      • Department of Geological Sciences
      Providence, RI, United States
  • 2012–2014
    • NASA
      Вашингтон, West Virginia, United States
  • 2012–2013
    • Planetary Science Institute
      Houston, Texas, United States
  • 2011–2013
    • Observatoire de la Côte d'Azur
      Grasse, Provence-Alpes-Côte d'Azur, France
    • Collège de France
      Lutetia Parisorum, Île-de-France, France
    • National Institute of Astrophysics
      • Institute for Space Astrophysics and Planetology IAPS
      Roma, Latium, Italy
  • 2008–2013
    • It-Robotics
      Vicenza, Veneto, Italy
  • 1998–2013
    • University of Padova
      • Department of Geosciences
      Padua, Veneto, Italy
  • 2010–2012
    • University of Nice-Sophia Antipolis
      Nice, Provence-Alpes-Côte d'Azur, France
  • 2009–2012
    • German Aerospace Center (DLR)
      • Institute of Planetary Research
      Köln, North Rhine-Westphalia, Germany
  • 2002–2007
    • Università di Pisa
      Pisa, Tuscany, Italy