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An asteroid's history is determined in large part by its strength against collisions with other objects.sup.1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites.sup.3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale.sup.4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples.sup.6 and images of the asteroid (25143) Itokawa.sup.7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu's metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75 [plus or minus] 0.75 million years. Analysis of the size and depth of craters on boulders on the asteroid (101955) Bennu indicates that Bennu has been in near-Earth space for 1.75 [plus or minus] 0.75 million years., Author(s): R.-L. Ballouz [sup.1] , K. J. Walsh [sup.2] , O. S. Barnouin [sup.3] , D. N. DellaGiustina [sup.1] , M. Al Asad [sup.4] , E. R. Jawin [sup.5] , [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.06.016 Byline: Ryo Ishimaru (a), Hiroki Senshu (a), Seiji Sugita (b), Takafumi Matsui (a) Keywords: Comets; Impact processes; Jupiter, Satellites; Organic chemistry; Satellites, Surfaces Abstract: The synthesis of organic molecules via chemical reactions within impact vapor plumes has been proposed as a mechanism to supply organics on a planet. However, the kinetics of chemical reactions within a rapidly expanding vapor plume or quenching process of the reactions has not been studied extensively. In this study, we constructed a new numerical model that calculates kinetics of the entire chemical reactions within an impact vapor plume. Numerical results revealed that the semi-analytical models proposed so far, in which the final amount of a chemical species was given by the equilibrium abundance at the quenching temperature of the fastest reaction path involving the species, underestimates the yield of organic molecules, such as HCN, by up to a factor of 10. This is because the previously used assumption that a species can achieve equilibrium with the rest of the reaction system via the fastest reaction path involving the species is not necessarily valid. Our analysis of the high-temperature H/C/N/O reaction system suggests that the quenching of slow reactions divides the reaction network into smaller reaction sub-systems isolated from the rest of the reaction system. Then, the fastest reaction path cannot equilibrate an isolated reaction sub-system with the rest of the reaction system. Simulation of this actual disequilibrium mechanism requires a simultaneous numerical calculation of the entire reaction network, which is equivalent to conducting a full kinetic model calculation, such as our model. Our numerical code makes it possible to discuss quantitatively the impact chemistry for various situations, such as the Galilean satellites. In this study, our numerical model is applied to the delivery of organic molecules via cometary impact on the Galilean satellites. Our numerical results indicate that small-particle impacts would produce HCN efficiently. Resulting HCN may freeze out immediately and be deposited on satellite surfaces, where it may be eventually converted into complex organics via irradiation of charged particle. On the other hand, large-size impacts may form transient CH.sub.4-N.sub.2 atmospheres, in which complex organics (tholin) may be formed via energy deposition of UV and/or charged particle. Resulting complex organics may subsequently precipitate on the satellite surfaces without clear correlation with the locations of impact craters. Such distribution of complex organics created by chemical reactions within vapor plumes due to cometary impacts may explain an absorption (4.57[mu]m) on Galilean satellites nonassociated with observable (moderate- and large-size) impact craters. Author Affiliation: (a) Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino City, Chiba 275-0016, Japan (b) Department of Complexity Science and Engineering, Graduate School of Frontier Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan Article History: Received 3 February 2010; Revised 10 May 2010; Accepted 12 June 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.05.023 Byline: Gwendolyn D. Bart (a), H.J. Melosh (b) Keywords: Moon, Surface; Cratering; Impact processes Abstract: We investigate the spatial distributions of boulders ejected from 18 lunar impact craters that are hundreds of meters in diameter. To accomplish this goal, we measured the diameters of 13,955 ejected boulders and the distance of each boulder from the crater center. Using the boulder distances, we calculated ejection velocities for the boulders. We compare these data with previously published data on larger craters and use this information to determine how boulder ejection velocity scales with crater diameter. We also measured regolith depths in the areas surrounding many of the craters, for comparison with the boulder distributions. These results contribute to understanding boulder ejection velocities, to determining whether there is a relationship between the quantity of ejected boulders and lunar regolith depths, and to understanding the distributions of secondary craters in the Solar System. Understanding distributions of blocky ejecta is an important consideration for landing site selection on both the Moon and Mars. Author Affiliation: (a) University of Idaho, Department of Physics, Campus Box 440903, Moscow, ID 83844-0903, USA (b) Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN 47907, USA Article History: Received 15 June 2009; Revised 24 May 2010; Accepted 25 May 2010

Observations are analyzed to explain an unusual feature of the nighttime atmospheric structure inside Arizona's idealized, basin-shaped Meteor Crater. The upper 75%-80% of the crater's atmosphere, which overlies an intense surface-based inversion on the crater's floor, maintains a near-isothermal lapse rate during the entire night, even while continuing to cool. Evidence is presented to show that this near-isothermal layer is produced by cold-air intrusions that come over the crater's rim. The intrusions are driven by a regional-scale drainage flow that develops over the surrounding inclined Colorado Plateau. Cold air from the drainage flow builds up on the upwind side of the crater and splits around the crater at low levels. A shallow layer of cold air, however, spills over the 30-60-m-high rim and descends partway down the crater's upwind inner sidewall until reaching its buoyancy equilibrium level. Detrainment of cold air during its katabatic descent and compensatory rising motions in the crater atmosphere destabilize the basin atmosphere, producing the observed near-isothermal lapse rate. A conceptual model of this phenomenon is presented. DOI: 10.1175/2010JAMC2470.1

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.02.019 Byline: Gareth A. Morgan (a), James W. Head (a), Francois Forget (b), Jean-Baptiste Madeleine (b), Aymeric Spiga (b) Keywords: Mars; Climate; Planetary dynamics; Geological processes Abstract: The unusual 80km diameter Noachian-aged Asimov crater in Noachis Terra (46[degrees]S, 5[degrees]E) is characterized by extensive Noachian-Hesperian crater fill and a younger superposed annulus of valleys encircling the margins of the crater floor. These valleys provide an opportunity to study the relationships of gully geomorphology as a function of changing slope orientation relative to solar insolation. We found that the level of development of gullies was highly correlated with slope orientation and solar insolation. The largest and most complex gully systems, with the most well-developed fluvial landforms, are restricted to pole-facing slopes. In contrast, gullies on equator-facing slopes are smaller, more poorly developed and integrated, more highly degraded, and contain more impact craters. We used a 1D version of the Laboratoire de Meteorologie Dynamique GCM, and slope geometries (orientation and angle), driven by predicted spin-axis/orbital parameter history, to assess the distribution and history of surface temperatures in these valleys during recent geological history. Surface temperatures on pole-facing slopes preferential for water ice accumulation and subsequent melting are predicted to occur as recently as 0.5-2.1Ma, which is consistent with age estimates of gully activity elsewhere on Mars. In contrast, the 1D model predicts that water ice cannot accumulate on equator-facing slopes until obliquities exceed 45[degrees], suggesting they are unlikely to have been active over the last 5Ma. The correlation of the temperature predictions and the geological evidence for age differences suggests that there were two phases of gully formation in the last few million years: an older phase in which top-down melting occurred on equator-facing slopes and a younger more robust phase on pole-facing slopes. The similarities of small-scale fluvial erosion features seen in the gullies on Mars and those observed in gullies cut by seasonal and perennial snowmelt in the Antarctic Dry Valleys supports a top-down melting origin for these gullies on Mars. Author Affiliation: (a) Department of Geological Sciences, Brown University, Providence, RI 02912, USA (b) Laboratoire de Meteorologie Dynamique, Universite Jussieu Paris VI, Case postale 99, 4, Place Jussieu, F 75252 Paris Cedex 05, France Article History: Received 5 November 2009; Revised 25 February 2010; Accepted 26 February 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.10.023 Byline: Hiroyuki Sato (a), Kei Kurita (b), David Baratoux (a) Keywords: Mars, Surface; Geological processes; Impact processes Abstract: Floor-fractured craters (FFC) are a peculiar form of degradation of impact craters defined by the presence of crevice networks and mesas affecting crater floors. They are preferentially distributed near chaotic terrains and outflow channels. The scope of this paper is to present a detailed systematic analysis of FFC at Xanthe Terra. FFC morphologies in this region are classified into five types making a picture of different stages of the same degradation process. FFC are geographically intermixed with un-fractured normal craters (non-FFC). Young craters are less prone to show this type of degradation, as suggested by fresh ejecta layer with preserved crater floor. Size distributions of FFC and non-FFC indicate that larger craters are preferentially fractured. Careful examinations of the crater floor elevations reveal that the crevices often extend deeper than the original crater cavity. Furthermore, an onset depth for the formation of FFC is evidenced from the difference of spatial distributions between FFC and non-FFC. Roof-collapsed depressions observed in the same region have been also documented and their characteristics suggest the removal of subsurface material at depth from about 1200 to 4000m. These observations taken together suggest a subsurface zone of volume deficit at depth from 1 to 2km down to several kilometers responsible for FFC formation. Then a scenario of FFC formations is presented in the context of groundwater discharge events at the late Hesperian. This scenario involves two key processes, Earth fissuring and piping erosion, known to occur with rapid groundwater migrations on Earth. Author Affiliation: (a) Observatoire Midi-Pyrenees, UMR 5562 CNRS, Laboratoire Dynamique Terrestre et Planetaire, 14, Avenue Edouard Belin, 31400 Toulouse, France (b) Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, Japan Article History: Received 18 December 2006; Revised 29 July 2009; Accepted 4 October 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.08.021 Byline: Charles A. Wood (a)(b), Ralph Lorenz (c), Randy Kirk (d), Rosaly Lopes (e), Karl Mitchell (e), Ellen Stofan (f) Keywords: Saturn, Satellites; Titan; Cratering; Geological processes Abstract: Five certain impact craters and 44 additional nearly certain and probable ones have been identified on the 22% of Titan's surface imaged by Cassini's high-resolution radar through December 2007. The certain craters have morphologies similar to impact craters on rocky planets, as well as two with radar bright, jagged rims. The less certain craters often appear to be eroded versions of the certain ones. Titan's craters are modified by a variety of processes including fluvial erosion, mass wasting, burial by dunes and submergence in seas, but there is no compelling evidence of isostatic adjustments as on other icy moons, nor draping by thick atmospheric deposits. The paucity of craters implies that Titan's surface is quite young, but the modeled age depends on which published crater production rate is assumed. Using the model of suggests that craters with diameters smaller than about 35km are younger than 200 million years old, and larger craters are older. Craters are not distributed uniformly; Xanadu has a crater density 2-9 times greater than the rest of Titan, and the density on equatorial dune areas is much lower than average. There is a small excess of craters on the leading hemisphere, and craters are deficient in the north polar region compared to the rest of the world. The youthful age of Titan overall, and the various erosional states of its likely impact craters, demonstrate that dynamic processes have destroyed most of the early history of the moon, and that multiple processes continue to strongly modify its surface. The existence of 24 possible impact craters with diameters less than 20km appears consistent with the model of the effectiveness of Titan's atmosphere in destroying most but not all small projectiles. Author Affiliation: (a) Planetary Science Institute, Tucson, AZ 85721, United States (b) Wheeling Jesuit University, Wheeling, WV 26003, United States (c) Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723, United States (d) US Geological Survey, Branch of Astrogeology, Flagstaff, AZ 86001, United States (e) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, United States (f) Proxemy Research, Bowie, MD 20715, United States Article History: Received 12 March 2009; Revised 26 July 2009; Accepted 28 August 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.08.010 Byline: R.M.C. Lopes (a), E.R. Stofan (b), R. Peckyno (c), J. Radebaugh (d), K.L. Mitchell (a), G. Mitri (a), C.A. Wood (e), R.L. Kirk (f), S.D. Wall (a), J.I. Lunine (g), A. Hayes (h), R. Lorenz (i), T. Farr (a), L. Wye (j), J. Craig (a), R.J. Ollerenshaw (a), M. Janssen (a), A. LeGall (a), F. Paganelli (b), R. West (a), B. Stiles (a), P. Callahan (a), Y. Anderson (a), P. Valora (d), L. Soderblom (f) Keywords: Titan; Geological processes; Radar observations Abstract: The Cassini Titan Radar Mapper is providing an unprecedented view of Titan's surface geology. Here we use Synthetic Aperture Radar (SAR) image swaths (Ta-T30) obtained from October 2004 to December 2007 to infer the geologic processes that have shaped Titan's surface. These SAR swaths cover about 20% of the surface, at a spatial resolution ranging from [approximately equal to]350m to [approximately equal to]2km. The SAR data are distributed over a wide latitudinal and longitudinal range, enabling some conclusions to be drawn about the global distribution of processes. They reveal a geologically complex surface that has been modified by all the major geologic processes seen on Earth - volcanism, tectonism, impact cratering, and erosion and deposition by fluvial and aeolian activity. In this paper, we map geomorphological units from SAR data and analyze their areal distribution and relative ages of modification in order to infer the geologic evolution of Titan's surface. We find that dunes and hummocky and mountainous terrains are more widespread than lakes, putative cryovolcanic features, mottled plains, and craters and crateriform structures that may be due to impact. Undifferentiated plains are the largest areal unit; their origin is uncertain. In terms of latitudinal distribution, dunes and hummocky and mountainous terrains are located mostly at low latitudes (less than 30[degrees]), with no dunes being present above 60[degrees]. Channels formed by fluvial activity are present at all latitudes, but lakes are at high latitudes only. Crateriform structures that may have been formed by impact appear to be uniformly distributed with latitude, but the well-preserved impact craters are all located at low latitudes, possibly indicating that more resurfacing has occurred at higher latitudes. Cryovolcanic features are not ubiquitous, and are mostly located between 30[degrees] and 60[degrees] north. We examine temporal relationships between units wherever possible, and conclude that aeolian and fluvial/pluvial/lacustrine processes are the most recent, while tectonic processes that led to the formation of mountains and Xanadu are likely the most ancient. Author Affiliation: (a) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, United States (b) Proxemy Research, Bowie, MD 20715, United States (c) Oregon State University, Dept of Geosciences, Corvallis, OR, United States (d) Department of Geological Sciences, Brigham Young University, Provo, UT 84602, United States (e) Wheeling Jesuit University, Wheeling, WV 26003, United States (f) US Geological Survey, Branch of Astrogeology, Flagstaff, AZ 86001, United States (g) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, United States (h) California Institute of Technology, Pasadena, CA, United States (i) Johns Hopkins Applied Physics Laboratory, Laurel, MD 20723, United States (j) Department of Geophysics and Electrical Engineering, Stanford University, Stanford, CA 94305, United States Article History: Received 19 March 2009; Revised 30 July 2009; Accepted 17 August 2009

Since 2004, Saturn's moon Iapetus has been observed repeatedly with the Imaging Science Subsystem of the Cassini spacecraft. The images show numerous impact craters down to the resolution limit of ~10 meters per pixel. Small, bright craters within the dark hemisphere indicate a dark blanket thickness on the order of meters or less. Dark, equator-facing and bright, poleward-facing crater wails suggest temperature-driven water-ice sublimation as the process responsible for local albedo patterns. Imaging data also reveal a global color dichotomy, wherein both dark and bright materials on the reading side have a substantially redder color than the respective trailing-side materials. This global pattern indicates an exogenic origin for the redder leading-side parts and suggests that the global color dichotomy initiated the thermal formation of the global albedo dichotomy. 10.1126/science.1177088

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.06.032 Byline: J. Vaucher (a), D. Baratoux (a), N. Mangold (b), P. Pinet (a), K. Kurita (c), M. Gregoire (a) Keywords: Mars; Volcanism Abstract: Central Elysium Planitia (CEP) is located south of Elysium Mons. Back to the era of the Viking orbiters, clues accumulated in favor of recent volcanism in relation with ground water release and the formation of long sub-parallel fissures. Four aqueous flood channel systems emanate from linear fissures. Recent eruptions of low viscosity lavas originate from these fissures and from low shield volcanoes. The objective of this paper is to constrain the volcanic history of this region, and to determine the chronological relationships with fluvial/erosional processes. New observations (e.g., new shield volcanoes and one new fluvial event) are summarized on a context map. Thirty-five surfaces have been dated from the count of about 15,000 impact craters. Ages have been cross-checked with relative stratigraphy when possible. A probabilistic approach has been introduced to compare similar ages and define periods of volcanic activity. Our results confirm that some volcanic features are extremely recent ([approximately equal to]2My). Active periods are found at 2.5-3My, 4.3My, 13.5-16.2My, 19My, 21-32My, 58My, 71My, 85-95My, 134My, 173My and 234My, not excluding the possibility that some of the gaps would be filled with additional crater counts. The volcanic activity thus extended for at least the last 250My. The lava volumes have been estimated from the topographic modeling of the floor of depressions filled up by volcanic products, including the volumes of several large crater cavities buried under lavas (>20% of the total volume). Our new estimation of the total lava volume is 1.5[+ or -]0.2x10.sup.5 km.sup.3. This value corresponds to an average thickness of one hundred meters of lavas for the young volcanic plain. As a consequence, the total eruption rate at CEP, defined as the total volume of lava divided by the time of emplacement 1.4x10.sup.-2-1.8x10.sup.-2 m.sup.3/s is lower than values typically estimated for terrestrial hot spots or large igneous provinces, suggesting longer inactive periods. The concept of mantle plumes responsible for terrestrial flood volcanism may not be applicable to the case of CEP and the mechanism proposed in offers a plausible alternative to explain our observations. Author Affiliation: (a) Laboratoire Dynamique Terrestre et Planetaire, UMR 5562, Observatoire Midi-Pyrenees, Universite de Toulouse and CNRS, Toulouse, France (b) Universite de Nantes, UFR des Sciences et des Techniques, Laboratoire de Planetologie et Geodynamique, 2 rue de la Houssiniere, B.P. 92208, 44322 Nantes Cedex 3, France (c) Earthquake Research Institute, Zip. 113-0032, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, Japan Article History: Received 12 June 2007; Revised 17 May 2009; Accepted 1 June 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.02.007 Byline: L.V. Moroz (a)(b), A.T. Basilevsky (c), T. Hiroi (d), S.S. Rout (a), D. Baither (e), C.H. van der Bogert (a), O.I. Yakovlev (c), A.V. Fisenko (c), L.F. Semjonova (c), V.S. Rusakov (f), D.A. Khramov (c), N.G. Zinovieva (g), G. Arnold (b)(a), C.M. Pieters (d) Keywords: Mars; surface; Spectroscopy; Impact processes; Regoliths Abstract: To simulate the formation of impact glasses on Mars, an analogue of martian bright soil (altered volcanic soil JSC Mars-1) was melted at relevant oxygen fugacities using a pulsed laser and a resistance furnace. Reduction of Fe.sup.3+ to Fe.sup.2+ and in some cases formation of nanophase Fe.sup.0 in the glasses were documented by Mossbauer spectroscopy and TEM studies. Reflectance spectra for several size fractions of the JSC Mars-1 sample and the glasses were acquired between 0.3 and 25 [mu]m. The glasses produced from the JSC Mars-1 soil show significant spectral variability depending on the method of production and the cooling rate. In general, they are dark and less red in the visible compared to the original JSC Mars-1 soil. Their spectra do not have absorption bands due to bound water and structural OH, have positive spectral slopes in the near-infrared range, and show two broad bands centered near 1.05 and 1.9 [mu]m, typical of glasses rich in ferrous iron. The latter bands and low albedo partly mimic the spectral properties of martian dark regions, and may easily be confused with mafic materials containing olivine and low-Ca pyroxene. Due to their disordered structures and vesicular textures, the glasses show relatively weak absorption features from the visible to the thermal infrared. These weak absorption bands may be masked by the stronger bands of mafic minerals. Positive near-infrared spectral slopes typical of fresh iron-bearing impact or volcanic glasses may be masked either by oxide/dust coatings or by aerosols in the Mars' atmosphere. As a result, impact glasses may be present on the surface of Mars in significant quantities that have been either misidentified as other phases or masked by phases with stronger infrared features. Spectrometers with sufficient spatial resolution and wavelength coverage may detect impact glasses at certain locations, e.g., in the vicinity of fresh impact craters. Such dark materials are usually interpreted as accumulations of mafic volcanic sand, but the possibility of an impact melt origin of such materials also should be considered. In addition, our data suggest that high contents of feldspars or zeolites are not necessary to produce the transparency feature at 12.1 [mu]m typical of martian dust spectra. Author Affiliation: (a) Institute of Planetology, University of Munster, Wilhelm-Klemm Str. 10, D-48149 Munster, Germany (b) German Aerospace Center (DLR), Institute of Planetary Research, Rutherfordstr. 2, D-12489 Berlin, Germany (c) Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Kosygin St. 19, 117975 Moscow, Russia (d) Department of Geological Sciences, Brown University, Providence, RI 02912, USA (e) Institute of Materials Physics, University of Munster, Wilhelm-Klemm Str. 10, D-48149 Munster, Germany (f) Department of Physics, Moscow State University, Vorob'evy Gory, 117234 Moscow, Russia (g) Department of Geology, Moscow State University, Vorob'evy Gory, 117234 Moscow, Russia Article History: Received 28 December 2008; Accepted 13 February 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.10.014 Byline: Devon M. Burr (a)(b), Marie-Therese Enga (c), Rebecca M.E. Williams (d), James R. Zimbelman (e), Alan D. Howard (f), Tracy A. Brennand (g) Keywords: Mars; surface; Mars; climate; Geological processes Abstract: A survey of THEMIS visible wavelength images in the Aeolis/Zephyria Plana region over the two western lobes of the equatorial Medusae Fossae Formation (MFF) shows [approximately equal to]150 sinuous ridges having a variety of morphologies and contexts. To systematize investigation, we use a classification scheme including both individual ridge and ridge network types, as well as associations with impact craters and fan-shaped features. The morphology of the ridges, their location downslope from higher topography (e.g., crater rims and scarps), and their association with fan-shaped forms indicate that most sinuous ridges formed through overland aqueous flow. Analysis of observations by individual ridge type leads to interpretation of most of these sinuous ridges as inverted fluvial channels or floodplains and a few as possible eskers, with the origin of the remaining ridges under continuing investigation. About 15% of the sinuous ridges are associated with impact craters, but data analysis does not support a genetic relationship between the craters and the sinuous ridges. Instead, analysis of one sinuous ridge network associated with a crater indicates that the water source for the network was atmospheric in origin, namely, precipitation runoff. The broad areal distribution of these [approximately equal to]150 ridges and the network morphologies, in particular the branched and subparallel types, suggest that an atmospheric water source is generally applicable to the population of sinuous ridges as a whole. This concentration of sinuous ridges is the largest single population of such landforms on Mars and among the youngest. These ridges are situated at a paleoscarp between Cerberus Palus and the Aeolis highlands, suggesting that the precipitation that formed them was orographic in origin. The ages of the equatorial MFF units in which this population of sinuous ridges is found imply that this orographic rain and/or snow fell during some period from the late Hesperian through the middle Amazonian. Author Affiliation: (a) Earth and Planetary Science Department, University of Tennessee Knoxville, 306 EPS Building, 1412 Circle Dr., Knoxville, TN 37996-1410, United States (b) Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 515 N Whisman Rd, Mountain View, CA 94043, USA (c) Department of Physics and Astronomy, Northern Arizona University, Box 6010, Flagstaff, AZ 86011-6010, USA (d) Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA (e) Center for Earth and Planetary Sciences, MRC 315, National Air and Space Museum, Smithsonian Institute, Washington, DC 20013-7012, USA (f) Department of Environmental Sciences, University of Virginia, P.O. Box 400123, Charlottesville, VA 22904-4123, USA (g) Department of Geography, Simon Fraser University, 8888 University Drive, Burnaby, BC, V5A 1S6 Canada Article History: Received 4 June 2008; Revised 19 October 2008; Accepted 24 October 2008

Analyses of spaceborne imaging radar-C (SIR-C) data and field data from the northwestern Yucatan Peninsula, Mexico, demonstrate that spaceborne multifrequency polarimetric radars are excellent tools for characterizing patterns of wetland flooding. Seasonal flooding can be detected in most types of forest and marsh in the radar backscatter magnitude and phase data of both L and C band. Field observations made in the wet and dry seasons concurrent with the space missions and chemical analyses of floodwaters confirm that flooding is the product of discharge from the Yucatan aquifer, which consists of a fresh-water lens floating on seawater. This discharge controls the distribution of wetlands. Therefore, vegetation and flooding patterns, mapped with SIR-C imagery, provide valuable information on the hydrogeology of the region. Radar-image maps of wetlands and flooding indicate that there are three major zones of groundwater discharge that correlate with structures of the buried Chicxulub crater--zone 1 with the peak ring, zone 2 with the crater rim, and zone 3 with the exterior ring. Zone 1 has sulfate-poor discharge, unlike the sulfate-rich discharge in zones 2 and 3. The highest discharge is in zone 3, where the buried crater is closest to the surface. This groundwater-discharge pattern can be explained by tidal pumping of fresh water to the surface through high permeability zones developed in the Tertiary carbonates overlying crater faults and escarpments. Keywords: Chicxulub crater, coastal environment, groundwater, radar, remote sensing, wetlands.

The characteristics of polar frosts located in spherical bowl-shaped craters on the Moon, Mercury and Mars were studied analytically. The results showed that the temperature in the shadows in craters in both Mercury and the Moon were below 102 K. However, the latitudinal extent of the cold zone of the Moon's coild zone was greater than Mercury's. Illuminated water frosts on Mars grow rougher due to the effects of tempearture.

The topographical characteristics of impact craters located in the 'softened' terrain of Mars were studied by using photoclinometry on images obtained by the Viking orbiter spacecraft. The results indicated that the impact craters were topographically distinct. Softened craters featured rounded rims and convex crater wall slopes. In addition, they were shallower than unsoftened craters.

Crater chains an Callisto due to tidal split will be formed on the side facing Jupiter and the formation will be visible from Ganymede. A model was constructed to analyze future possibilities of crater formation on Callisto and Ganymede due to tidal spliting. Tensional axis, parallel to the radius vector, causes tidal stress in ellipsoidal and spherical bodies. A distinct line, unique to this model, was formed by the pieces of the split comet. In a model where the comet bursts due to rotational movement and in which the total stress immediately splits the rotating comet, the pieces form a long loop.

BILLED AS A 'NEW experiment in volunteer science,' the Mars Clickworkers study at NASA's Ames Research Center wants to see if regular folks, with no scientific training or background, can [...]

Hydrocode modeling is a fundamental tool for the study of melt production in planetary impact events. Until recently, however, numerical modeling of impacts for melt production studies has been limited to vertical impacts. We present the first results of the investigation of melt production in oblique impacts. Simulations were carried out using Sandia's three-dimensional hydrocode CTH, coupled to the SESAME equation of state. While keeping other impact parameters constant, the calculations span impact angles (measured from the surface) from 90 [degrees] (vertical impact) to 15 [degrees]. The results show that impact angle affects the strength and distribution of the shock wave generated in the impact. As a result, both the isobaric core and the regions of melting in the target appear asymmetric and concentrated in the downrange, shallower portion of the target. The use of a pressure-decay power law (which describes pressure as function of linear distance from the impact point) to reconstruct the region of melting and vaporization is therefore complicated by the asymmetry of the shock wave. As an analog to the pressure decay versus distance from the impact point, we used a 'volumetric pressure decay,' where the pressure decay is modeled as a function of volume of target material shocked at or above the given shock pressure. We find that the volumetric pressure decay exponent is almost constant for impact angles from 90 [degrees] to 30 [degrees], dropping by about a factor of two for a 15 [degrees] impact. In the range of shock pressures at which most materials of geologic interest melt or begin to vaporize, we find that the volume of impact melt decreases by at most 20% for impacts from 90 [degrees] down to 45 [degrees]. Below 45 [degrees], however, the amount of melt in the target decreases rapidly with impact angle. Compared to the vertical case, the reduction in volume of melt is about 50% for impacts at 30 [degrees] and more than 90% for a 15 [degrees] impact. These estimates do not include possible melting due to shear heating, which can contribute to the amount of melt production especially in very oblique impacts. Studies of melt production in vertical impacts suggest an energy scaling law in agreement with the point source limit. An energy scaling law, however, does not seem to hold for oblique impacts, even when the impact velocity is substituted by its vertical component. However, we find that for impact angles between about 30 [degrees] and 90 [degrees] (a range that includes 75% of impact events on planetary surfaces) the volume of melt is directly proportional to the volume of the transient crater generated by the impact. Key Words: impact processes; cratering; surfaces, planets.

At first, I mistook the skittering footfalls outside my tent for thunder. I jammed my pillow over my ears and returned my attention to the geosurvey data on my phone. [...]

Analysis of a previously unpublished well log for the Yucatan No. 6 well shows the Chicxulub structure on the Yucatan peninsula as a volcanic sequence of Late Cretaceous age and not characteristic of a Cretaceous-Tertiary asteriod impact. The well log does not show the chemical homogeneity and simple succession seen in impact melt sheets. Also, the form and angular distribution of the lamellae observed demonstrate a tectonic or volcanic origin.