O. Groussin

Aix-Marseille Université, Marsiglia, Provence-Alpes-Côte d'Azur, France

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Publications (208)578.64 Total impact

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    ABSTRACT: Spacecraft observations of atmosphereless Solar System bodies, combined with thermophysical modeling, provide important information about the thermal inertia and degree of surface roughness of these bodies. The thermophysical models rely on various methods of generating topography, the most common being the concave spherical segment. We here compare the properties of thermal emission for a number of different topographies – concave spherical segments, random Gaussians, fractals and parallel sinusoidal trenches – for various illumination and viewing geometries, degrees of surface roughness and wavelengths. We find that the thermal emission is strongly dependent on roughness type, even when the degrees of roughness are identical, for certain illumination and viewing geometries. The systematic usage of any single topography model may therefore bias determinations of thermal inertia and level of roughness. We outline strategies that may be employed during spacecraft observations to disentangle thermal inertia, level of roughness and type of topography. We also compare the numerically complex and time consuming full-scale thermophysical models with a simplified statistical approach, which is fairly easy to implement and quick to run. We conclude that the simplified statistical approach is similar to thermophysical models for cases tested here, which enables the user to analyze huge amounts of spectral data at a low numerical cost.
    Icarus 05/2015; 252. DOI:10.1016/j.icarus.2014.12.029 · 2.84 Impact Factor
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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: Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency's Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10(-10) to 10(-7) kilograms, and 48 grains of mass 10(-5) to 10(-2) kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails. Copyright © 2015, American Association for the Advancement of Science.
    Science 01/2015; 347(6220):aaa3905. DOI:10.1126/science.aaa3905 · 31.48 Impact Factor
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    Astronomy and Astrophysics 01/2015; 573(A62). DOI:10.1051/0004-6361/201424735 · 4.48 Impact Factor
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    ABSTRACT: We developed a new 3D shape reconstruction method which combines stereo, photoclinometry and the deformation of a triangular mesh describing the surface of the object. The method deforms the mesh - initially a sphere - until the set of synthetic images, created from the mesh (Jorda et al., SPIE 2010) match the observed one. Stereo control points can be used as a constraint in the deformation of the mesh, but it is not required at low resolutions. This new technique has been applied to images of the nucleus of comet 67P/Churyumov-Gerasimenko acquired by the OSIRIS instrument aboard the Rosetta spacecraft. The technique allowed to reconstruct the shape of the nucleus and to retrieve its rotational parameters from low-resolution images obtained with the narrow-angle camera of OSIRIS in mid-July 2014, when stereo-based techniques were still inapplicable. This model called "SHAP1" has been delivered to ESA and to the lander team in July. The technique has also been applied to higher-resolution images of the nucleus later on, using the stereo information as a constraint. A comparison of the reconstructed global and local models with those retrieved with other techniques, such as stereophotoclinometry (Gaskell et al., MPS 2008) and stereophotogrammetry (Preusker et al., PSS 2012) will be presented.
    AGU Fall Meeting 2014; 12/2014
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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
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    ABSTRACT: Aims. Approach observations with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) experiment onboard Rosetta are used to determine the rotation period, the direction of the spin axis, and the state of rotation of comet 67P's nucleus. Methods. Photometric time series of 67P have been acquired by OSIRIS since the post wake-up commissioning of the payload in March 2014. Fourier analysis and convex shape inversion methods have been applied to the Rosetta data as well to the available ground-based observations. Results. Evidence is found that the rotation rate of 67P has significantly changed near the time of its 2009 perihelion passage, probably due to sublimation-induced torque. We find that the sidereal rotation periods P 1 = 12.76129 ± 0.00005 h and P 2 = 12.4043 ± 0.0007 h for the apparitions before and after the 2009 perihelion, respectively, provide the best fit to the observations. No signs of multiple periodicity are found in the light curves down to the noise level, which implies that the comet is presently in a simple rotation state around its axis of largest moment of inertia. We derive a prograde rotation model with spin vector J2000 ecliptic coordinates λ = 65 • ± 15 • , β = +59 • ± 15 • , corresponding to equatorial coordinates RA = 22 • , Dec = +76 • . However, we find that the mirror solution, also prograde, at λ = 275 • ± 15 • , β = +50 • ± 15 • (or RA = 274 • , Dec = +27 •), is also possible at the same confidence level, due to the intrinsic ambiguity of the photometric problem for observations performed close to the ecliptic plane.
    Astronomy and Astrophysics 09/2014; 569(L2). DOI:10.1051/0004-6361/201424590 · 4.48 Impact Factor
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    ABSTRACT: On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI) successfully encountered comet 103P/Hartley 2, when it was at a heliocentric distance of 1.06 AU. Spatially resolved near-IR spectra of comet Hartley 2 were acquired in the 1.05–4.83 μm wavelength range using the HRI-IR spectrometer. We present spectral maps of the inner ∼∼10 km of the coma collected 7 min and 23 min after closest approach. The extracted reflectance spectra include well-defined absorption bands near 1.5, 2.0, and 3.0 μm consistent in position, bandwidth, and shape with the presence of water ice grains. Using Hapke’s radiative transfer model, we characterize the type of mixing (areal vs. intimate), relative abundance, grain size, and spatial distribution of water ice and refractories. Our modeling suggests that the dust, which dominates the innermost coma of Hartley 2 and is at a temperature of 300 K, is thermally and physically decoupled from the fine-grained water ice particles, which are on the order of 1 μm in size. The strong correlation between the water ice, dust, and CO2 spatial distribution supports the concept that CO2 gas drags the water ice and dust grains from the nucleus. Once in the coma, the water ice begins subliming while the dust is in a constant outflow. The derived water ice scale-length is compatible with the lifetimes expected for 1-μm pure water ice grains at 1 AU, if velocities are near 0.5 m/s. Such velocities, about three order of magnitudes lower than the expansion velocities expected for isolated 1-μm water ice particles ( and ), suggest that the observed water ice grains are likely aggregates.
    Icarus 08/2014; 238:191–204. DOI:10.1016/j.icarus.2014.04.008 · 2.84 Impact Factor
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    ABSTRACT: We report on the physical properties of the craters of Achaia region of the main-belt Asteroid (21) Lutetia, based on images obtained with the OSIRIS instrument during the Rosetta flyby that took place on 10 July 2010. Images of the surface were acquired with its Narrow Angle Camera, from which Digital Terrain Models (DTM) of the surface were constructed. These DTMs give access to the geometrical properties of the craters of the asteroid. On a complex asteroid shape, slopes and depth-to-diameter ratios (d/D) of craters should be carefully measured taking into account the local topography to obtain a value that is physically related to the work of forces resisting to mass displacement (associated with gravity and/or material strength) occurring in either excavation or degradation processes. We present new measurements of d/D and internal slopes of impact craters of the Achaia region, which offers optimal conditions of observations and a large population of craters. We find that d/D values for Achaia craters differ from previous works on Lutetia and are consistent with the values found on other asteroids such as (243) Ida or (951) Gaspra. The Achaia region may be divided into three units based on geomorphological analysis. The mean d/D values of the three units are different, revealing differences in resurfacing history by impact-related ejecta blanketing and seismic shaking. Some of these geological events may be recent compared to the age of the region since several lineaments intersect most craters of one of the three units. Independent evidence for ejecta blanket have been given for the unit associated with low d/D values confirming the contribution of this process to crater modification. Moreover, we suggest that displacements along faults identified as surface lineaments may have been responsible for the erasure of small craters. Our results are finally integrated into a chronology sequence of events explaining the present characteristics of the Achaia region.
    Icarus 01/2014; doi. DOI:10.1016/j.icarus.2014.10.014 · 2.84 Impact Factor
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    ABSTRACT: Comet 103P/Hartley 2 made a close approach to the Earth in October 2010. It was the target of an extensive observing campaign and was visited by the Deep Impact spacecraft (mission EPOXI). We present observations of HCN and CH3OH emission lines conducted with the IRAM Plateau de Bure interferometer on 22-23, 28 October and 4, 5 November 2010 at 1.1, 1.9 and 3.4 mm wavelengths. The thermal emission from the dust coma and nucleus is detected simultaneously. Interferometric images with unprecedented spatial resolution are obtained. A sine-wave variation of the thermal continuum is observed in the 23 October data, that we associate with the nucleus thermal light curve. The nucleus contributes up to 30-55 % of the observed continuum. The large dust-to-gas ratio (in the range 2-6) can be explained by the unusual activity of the comet for its size, which allows decimeter size particles and large boulders to be entrained by the gas. The rotational temperature of CH3OH is measured. We attribute the increase from 35 to 46 K with increasing beam size (from 150 to 1500 km) to radiative processes. The HCN production rate displays strong rotation-induced variations. The HCN production curve, as well as those of CO2 and H2O measured by EPOXI, are interpreted with a geometric model which takes into account the rotation and the shape of the comet. The HCN and H2O production curves are in phase, showing common sources. The 1.7h delay, in average, of HCN and H2O with respect to the CO2 production curve suggests that HCN and H2O are mainly produced by subliming icy grains. The scale length of production of HCN is determined to be on the order of 500-1000 km, implying a mean velocity of 100-200 m/s for the icy grains producing HCN. The modulation of the CO2 prouction and of the velocity offset of the HCN lines are interpreted in terms of localized sources of gas on the nucleus surface.
    Icarus 10/2013; 228. DOI:10.1016/j.icarus.2013.10.010 · 2.84 Impact Factor
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    ABSTRACT: Deep Impact acquired a unique rotational data set during its close flyby of comet 103/P Hartley 2 (DIXI mission, November 4, 2010). The HRI-IR spectrometer (1.05-4.85 microns) monitored the coma throughout the encounter acquiring infrared scans every 2 hr over the 18 hr period prior to closest approach and every 30 min for 2 days after closest approach. Water vapor (2.7 microns), carbon dioxide (4.3 microns), and bulk organics 3.4 microns) were the dominant emission bands detected in these spectra and their distribution was found to be highly asymmetric and variable. In particular, the distribution maps from the 8 hrs following closest approach half of the dominant 18.4 hr rotation period) are unique with spatial resolutions ranging from 0.2-3.5 km/pixel. These data allow us to explore correlations among the volatiles and the role of extended coma sources. These data will also help to quantify the heterogeneity of the outgassing and better locate specific source regions on the nucleus of Hartley 2.
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    ABSTRACT: THERMAP is a mid-infrared (8-16 μm) spectroimager, selected by the European Space Agency (ESA) in February 2013 for the scientific payload of the Marco Polo R M-class mission. We present in this paper the instrument and its scientific objectives.
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    ABSTRACT: Aims. Trans-Neptunian objects (TNOs) are bodies populating the Kuiper belt and they are believed to retain the most pristine and least altered material of the solar system. The Herschel open time key programme entitled “TNOs are Cool: A survey of the trans-Neptunian region” has been awarded 373 h to investigate the albedo, size distribution and thermal properties of TNOs and Centaurs. Here we focus on the brightest targets observed by both the PACS and SPIRE multiband photometers: the dwarf planet Haumea, six TNOs (Huya, Orcus, Quaoar, Salacia, 2002 UX25, and 2002 TC302), and two Centaurs (Chiron and Chariklo). Methods. Flux densities are derived from PACS and SPIRE instruments using optimised data reduction methods. The spectral energy distribution obtained with the Herschel PACS and SPIRE instruments over 6 bands (centred at 70, 100, 160, 250, 350, and 500 μm), with Spitzer-MIPS at 23.7 and 71.4 μm, and with WISE at 11.6 and 22.1 μm in the case of 10199 Chariklo, has been modelled with the NEATM thermal model in order to derive the albedo, diameter, and beaming factor. For the Centaurs Chiron and Chariklo and for the 1000 km sized Orcus and Quaoar, a thermophysical model was also run to better constrain their thermal properties. Results. We derive the size, albedo, and thermal properties, including thermal inertia and surface emissivity, for the 9 TNOs and Centaurs. Several targets show a significant decrease in their spectral emissivity longwards of ∼300 μm and especially at 500 μm. Using our size estimations and the mass values available in the literature, we also derive the bulk densities for the binaries Quaoar/Weywot (2.18 (+0.43;−0.36)g/cm3), Orcus/Vanth (1.53 (+0.15; −0.13) g/cm3), and Salacia/Actea (1.29 (+0.29; −0.23) g/cm3). Quaoar’s density is similar to that of the other dwarf planets Pluto and Haumea, and its value implies high contents of refractory materials mixed with ices.
    Astronomy and Astrophysics 09/2013; DOI:10.1051/0004-6361/201322047 · 4.48 Impact Factor
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    ABSTRACT: Little is known about the physical properties of the nuclei of Oort cloud comets. Measuring the thermal emission of a nucleus is one of the few means for deriving its size and constraining some of its thermal properties. We attempted to measure the nucleus size of the Oort cloud comet C/2009 P1 (Garradd). We used the Plateau de Bure Interferometer to measure the millimetric thermal emission of this comet at 157 GHz (1.9 mm) and 266 GHz (1.1 mm). Whereas the observations at 266 GHz were not usable due to bad atmospheric conditions, we derived a 3-sigma upper limit on the comet continuum emission of 0.41 mJy at 157 GHz. Using a thermal model for a spherical nucleus with standard thermal parameters, we found an upper limit of 5.6 km for the radius. The dust contribution to our signal is estimated to be negligible. Given the water production rates measured for this comet and our upper limit, we estimated that Garradd was very active, with an active fraction of its nucleus larger than 50%.
    Astronomy and Astrophysics 07/2013; 557. DOI:10.1051/0004-6361/201321372 · 4.48 Impact Factor
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    ABSTRACT: We present results from SEPPCoN, an on-going Survey of the Ensemble Physical Properties of Cometary Nuclei. In this report we discuss mid-infrared measurements of the thermal emission from 89 nuclei of Jupiter-family comets (JFCs). All data were obtained in 2006 and 2007 with the Spitzer Space Telescope. For all 89 comets, we present new effective radii, and for 57 comets we present beaming parameters. Thus our survey provides the largest compilation of radiometrically-derived physical properties of nuclei to date. We conclude the following. (a) The average beaming parameter of the JFC population is 1.03+/-0.11, consistent with unity, and indicating low thermal inertia. (b) The known JFC population is not complete even at 3 km radius, and even for comets with perihelia near ~2 AU. (c) We find that the JFC nuclear cumulative size distribution (CSD) has a power-law slope of around -1.9. (d) This power-law is close to that derived from visible-wavelength observations, suggesting that there is no strong dependence of geometric albedo with radius. (e) The observed CSD shows a hint of structure with an excess of comets with radii 3 to 6 km. (f) Our CSD is consistent with an intrinsic distribution that lacks many sub-kilometer objects.
    Icarus 07/2013; 226(1). DOI:10.1016/j.icarus.2013.07.021 · 2.84 Impact Factor
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    ABSTRACT: NASA’s EPOXI mission used the Deep Impact (DI) Flyby spacecraft to deliver a payload of three scientific instruments, two visible cameras and an IR spectrometer, to a close flyby of Comet 103P/Hartley 2 in November 2010. Interpretation of the scientific measurements made using these instruments depends on accurate calibration of the instruments’ performance. Updates to the instrument calibrations achieved during the Deep Impact primary mission and results of continued monitoring of their performance during EPOXI are reported here. The instruments’ performance has remained remarkably stable over the nearly 7 years of flight. Significant improvements in the understanding and calibration of the IR spectrometer response non-linearity, time-varying background level, flat field, wavelength map, and absolute spectral response have been achieved. Techniques for reducing some semi-coherent horizontal noise stripes in the visible cameras’ readouts were developed, and some adjustments have been made to their absolute radiometric conversion constants. The data processing pipeline has been updated to incorporate the improvements in the instrument calibrations.
    Icarus 07/2013; 225(1):643–680. DOI:10.1016/j.icarus.2013.03.024 · 2.84 Impact Factor
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    ABSTRACT: Re-calibrated near-infrared spectroscopy of the resolved nucleus of Comet 9P/Tempel 1 acquired by the Deep Impact spacecraft has been analyzed by utilizing the post-Stardust-NExT nucleus shape model and spin pole solution, as well as a novel thermophysical model that explicitly accounts for small-scale surface roughness and thermal inertia. We find that the thermal inertia varies measurably across the surface, and that thermal emission from certain regions only can be reproduced satisfactory if surface roughness is accounted for. Particularly, a scarped/pitted terrain that experienced morning sunrise during the flyby is measurably rough (Hapke mean slope angle ∼45°) and has a thermal inertia of at most 50 J m−2 K−1 s−1/2, but probably much lower. However, thick layered terrain and thin layered terrain experiencing local noon during the flyby have a substantially larger thermal inertia, reaching 150 J m−2 K−1 s−1/2 if the surface is as rough as the scarped/pitted terrain, but 200 J m−2 K−1 s−1/2 if the terrain is considered locally flat. Furthermore, the reddening of the nucleus near-infrared 1.5–2.2 μm spectrum varies between morphological units, being reddest for thick layered terrain (median value 3.4% kÅ−1) and most neutral for the smooth terrain known to contain surface water ice (median value 3.1% kÅ−1). Thus, Comet 9P/Tempel 1 is heterogeneous in terms of both thermophysical and optical properties, due to formation conditions and/or post-formation processing.
    Icarus 05/2013; 224(1):154–171. DOI:10.1016/j.icarus.2013.02.008 · 2.84 Impact Factor
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    ABSTRACT: We present an analysis of comet activity based on the Spitzer Space Telescope component of the Survey of the Ensemble Physical Properties of Cometary Nuclei. We show that the survey is well suited to measuring the activity of Jupiter-family comets at 3-7 AU from the Sun. Dust was detected in 33 of 89 targets (37 +/- 6%), and we conclude that 21 comets (24 +/- 5%) have morphologies that suggest ongoing or recent cometary activity. Our dust detections are sensitivity limited, therefore our measured activity rate is necessarily a lower limit. All comets with small perihelion distances (q < 1.8 AU) are inactive in our survey, and the active comets in our sample are strongly biased to post-perihelion epochs. We introduce the quantity epsilon-f-rho, intended to be a thermal emission counterpart to the often reported A-f-rho, and find that the comets with large perihelion distances likely have greater dust production rates than other comets in our survey at 3-7 AU from the Sun, indicating a bias in the discovered Jupiter-family comet population. By examining the orbital history of our survey sample, we suggest that comets perturbed to smaller perihelion distances in the past 150 yr are more likely to be active, but more study on this effect is needed.
    Icarus 04/2013; 225(1). DOI:10.1016/j.icarus.2013.04.012 · 2.84 Impact Factor
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    ABSTRACT: a b s t r a c t The nucleus of comet Tempel 1 has been investigated at close range during two spacecraft missions sep-arated by one comet orbit of the Sun, 5½ years. The combined imaging covers $70% of the surface of this object which has a mean radius of 2.83 ± 0.1 km. The surface can be divided into two terrain types: rough, pitted terrain and smoother regions of varying local topography. The rough surface has round depressions from resolution limits ($10 m/pixel) up to $1 km across, spanning forms from crisp steep-walled pits, to subtle albedo rings, to topographic rings, with all ranges of morphologic gradation. Three gravitationally low regions of the comet have smoother terrain, parts of which appear to be deposits from minimally modified flows, with other parts likely to be heavily eroded portions of multiple layer piles. Changes observed between the two missions are primarily due to backwasting of scarps bounding one of these probable flow deposits. This style of erosion is also suggested by remnant mesa forms in other areas of smoother terrain. The two distinct terrains suggest either an evolutionary change in processes, topo-graphically-controlled processes, or a continuing interaction of erosion and deposition.
    Icarus 02/2013; 222:453-466. DOI:10.1016/j.icarus.2012.02.037 · 2.84 Impact Factor

Publication Stats

2k Citations
578.64 Total Impact Points

Institutions

  • 2012–2015
    • Aix-Marseille Université
      • Laboratory of Astrophysics of Marseille (UMR 7326 LAM)
      Marsiglia, Provence-Alpes-Côte d'Azur, France
  • 2008–2013
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 2011
    • Thales Group
      Cannes, Provence-Alpes-Côte d'Azur, France
    • Observatoire Astrophysique de Marseille Provence
      • Laboratory of Astrophysics of Marseille
      Marsiglia, Provence-Alpes-Côte d'Azur, France
  • 2010
    • Durham University
      Durham, England, United Kingdom
  • 2006–2008
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 2005
    • Loyola University Maryland
      Baltimore, Maryland, United States
    • University of Hawaiʻi at Mānoa
      • Institute of Astronomy
      Honolulu, HI, United States
  • 2004
    • Konkoly Observatory
      Budapeŝto, Budapest, Hungary
  • 1999
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States