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By successively orbiting both 4 Vesta and I Ceres the Dawn mission directly addresses the long-standing goals of understanding the origin and evolution of the solar system. Ceres and Vesta are two complementary terrestrial protoplanets (one apparently "wet" and the other "dry"), whose accretion was probably terminated by the formation of Jupiter. They provide a bridge in our understanding between the rocky bodies of the inner solar system and the icy bodies of the outer solar system. Ceres appears to be undifferentiated while Vesta has experienced significant heating and likely differentiation. Both formed very early in the history of the solar system and while suffering many impacts have remained intact, thereby retaining a record of events and processes from the time of planet formation. Detailed study of the geophysics and geochemistry of these two bodies provides critical benchmarks for early solar system conditions and processes that shaped its subsequent evolution. Dawn provides the missing context for both primitive and evolved meteoritic data, thus playing a central role in understanding terrestrial planet formation and the evolution of the asteroid belt. Dawn is to be launched in May 2006 arriving at Vesta in 2010 and Ceres in 2014, stopping at each to make I I months of orbital measurements. The spacecraft uses solar electric propulsion, both in cruise and in orbit, to make most efficient use of its xenon propellant. The spacecraft carries a framing camera, visible and infrared mapping spectrometer, gamma ray/neutron spectrometer, magnetometer, and radio science. (C) 2003 Elsevier Ltd. All rights reserved.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.018 Byline: D. Loizeau (a)(b), N. Mangold (c), F. Poulet (a), V. Ansan (c), E. Hauber (d), J.-P. Bibring (a), B. Gondet (a), Y. Langevin (a), P. Masson (b), G. Neukum (e) Keywords: Mars, Surface; Mineralogy; Spectroscopy; Infrared observations Abstract: The Mawrth Vallis region contains an extensive (at least 300kmx400km) and thick ([greater than or equal to]300m), finely layered (at meter scale), clay-rich unit detected by OMEGA. We use OMEGA, HRSC DTMs derived from stereoscopic imagery, HRSC color imagery and high resolution imagery such as MOC, CTX and HiRISE to characterize the geometry and the composition of the clay-rich unit at the regional scale. Our results show that the clay-bearing unit can be divided into sub-units on the basis of differences in color and composition. In false-color visible imagery, alternating white/bluish and orange/red colored units correspond to a compositional succession of, respectively, Al- and Fe- or Mg-phyllosilicate rich material. Geological cross-sections are presented along the principal outcrops of the region in order to define the stratigraphy of these sub-units. This method shows that the dips of the sub-units are frequently close to the slopes of the present topography, except for scarps visible at the dichotomy boundary, inside impact craters walls, and outcrops inside Mawrth Vallis. In addition to the Al- and Fe- or Mg-phyllosilicate rich sub-units, an altered surface is identified as the lower basement unit. We propose two possible end-member scenarios to explain the derived stratigraphy: (1) alteration of volcaniclastic, aeolian or aqueous layered deposits of various compositions by groundwater, resulting in distinct altered rocks; or (2) Alteration coeval with the deposition of sediments under varying chemical conditions, in wet pedodiagenetic environment. Author Affiliation: (a) IAS-UMR8617, Universite Paris-Sud XI, Bat. 121, 91405 Orsay cedex, France (b) IDES-UMR8148, Universite Paris-Sud XI, Bat. 509, 91405 Orsay cedex, France (c) LPGN-UMR6112, Universite de Nantes, 2 rue de la Houssiniere, BP 92208, 44322 Nantes cedex 3, France (d) Institute for Planetary Exploration, DLR, Rutherfordstr. 2, 12489 Berlin, Germany (e) Freie Universitaet Berlin, FR Planetologie und Fernerkundung, Malteserstr. 74-100, 12249 Berlin, Germany Article History: Received 19 August 2008; Revised 10 April 2009; Accepted 14 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.10.001 Byline: M. Grott (a), E. Hauber (a), S.C. Werner (c), P. Kronberg (b), G. Neukum (c) Keywords: Mars; surface; Mars; interior; Tectonics Abstract: Insight into the state of the early martian lithosphere is gained by modeling the topography above surface breaking thrust faults in the southern Thaumasia region. Crater counts of key surface units associated with the faulting indicate a scarp emplacement in the late Noachian-early Hesperian periods between 4.0 and 3.7 Gyr. The seismogenic layer thickness at the time of faulting is constrained to 27-35 km and 21-28 km for the two scarps investigated, implying paleo geothermal gradients of 12-18 and 15-23 Kakm.sup.-1, corresponding to heat flows of 24-36 and 30-46 mWam.sup.-2. The heat flow values obtained in this study are considerably lower than those derived from rift flank uplift at the close-by Coracis Fossae for a similar time period, indicating that surface heat flow is a strong function of regional setting. If viewed as representative for magmatically active and inactive regions, the thermal gradients at rifts and scarps span the range of admissible global mean values. This implies dT/dz=17-32Kkm.sup.-1, with the true value probably being closer to the lower bound. Author Affiliation: (a) Institute of Planetary Research, German Aerospace Center (DLR), RutherfordstraAe 2, 12489 Berlin, Germany (b) Institute of Geology, Technical University of Clausthal, LeibnizstraAe 10, 38678 Clausthal-Zellerfeld, Germany (c) Institute of Geological Sciences, Freie Universitat Berlin, Malteser StraAe 74-100, 12249 Berlin, Germany Article History: Received 2 June 2006; Revised 25 September 2006

Slope streaks are gravity-driven albedo features observed on martian slopes since the Viking missions. The debated mechanism of formation could involve alternatively dry granular flow or wet mass wasting. A systematic mapping of slope streaks from the High Resolution Stereo Camera is presented in this paper. Two regions known for their slope streaks activity have been studied, the first one is located close to Cerberus lava flow, and the second one is inside the Olympus Mons Aureole. The statistics of slope streaks shapes measured from orthorectified images confirm previous results from Mars Orbiter Camera surveys. Preferential orientations of slope streaks are reported. Slope streaks occur preferentially on west facing slopes at latitudes lower than 30[degrees] N for Olympus and on south west facing slopes for Cerberus. Wind directions derived from a General Circulation Model during the dusty season correlate with these orientations. Furthermore, west facing slopes at Olympus have a thicker dust cover. These observations indicate that slope streaks are dust avalanches controlled by the preferential accumulation of dust in the downstream side of the wind flow. The paucity of slope streaks at high latitudes and their preferential orientation on south facing slopes have been presented as tin evidence for a potential role of [H.sub.2]O phase transition in triggering or flow. The potential role of [H.sub.2]O cannot be ruled out from our observations but the dust avalanche model together with the atmospheric circulation could potentially explain all observations. The role of [H.sub.2]O might be limited to at stabilizing effect of dust deposits on northward facing slopes at intermediate latitudes (30[degrees] N-33[degrees] N) and on all slopes further north. Keywords: Mars, surface; Geological processes

Two narrow, broadly arcuate, low ridges extend for 600-700 km in western Terra Tyrrhena, Mars, crosscut ancient Noachian terrain, and are associated with Early Hesperian plains, which cover -30 % of Mars. Geological relationships suggest that the ridges represent near-surface erosional remnants of subsurface dikes, solidified magma-filled cracks that were responsible for the volcanic emplacement of the plains. Ridge width and geometry are consistent with very high-effusion-rate flood basalt eruptions, emplacement events that would form smooth featureless plains and input significant volcanic gas into the atmosphere. Geological relationships suggest that the ridges were exposed by erosion (fluvial, sublimation, eolian) and partial removal of a regional volatile-rich dust layer. Keywords: dike, Noachian, Hesperian, magma, Mars, eruptions.

Author(s): J. W. Head (corresponding author) [1]; G. Neukum [2]; R. Jaumann [3]; H. Hiesinger [1]; E. Hauber [3]; M. Carr [4]; P. Masson [5]; B. Foing [6]; H. Hoffmann [...]

Images acquired of Saturn's rings and small moons by the Cassini Imaging Science Subsystem (ISS) during the first 9 months of Cassini operations at Saturn have produced many new findings. These include new saturnian moons; refined orbits of new and previously known moons; narrow diffuse rings in the F-ring region and embedded in gaps within the main rings; exceptionally fine-scale ring structure in moderate- to high-optical depth regions; new estimates for the masses of ring-region moons, as well as ring particle properties in the Cassini division, derived from the analysis of linear density waves; ring particle albedos in select ring regions; and never-before-seen phenomena within the rings., Major objectives of the Cassini Imaging Science investigation were the search for new satellites of Saturn within and external to the rings and determination of their orbits; refinement of the [...]

The Cassini Imaging Science Subsystem acquired high-resolution imaging data on the outer Saturnian moon, Phoebe, during Cassini's close flyby on 11 June 2004 and on Iapetus during a flyby on 31 December 2004. Phoebe has a heavily cratered and ancient surface, shows evidence of ice near the surface, has distinct layering of different materials, and has a mean density that is indicative of an ice-rock mixture. Iapetus's dark leading side (Cassini Regio) is ancient, heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief. Local albedo variations within and bordering Cassini Regio suggest mass wasting of ballistically deposited material, the origin of which remains unknown., Phoebe orbits Saturn at a distance of 12.9 x [10.sup.6] km every 550 days in a retrograde orbit, which suggests that it was captured early in solar system history (1). [...]

The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of ~375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning., During the Voyager flybys of the giant planets in the 1980s, Saturn was probably the windiest planet in the solar system (1). Its equatorial jet flowed eastward at 470 m [...]

Lightcurve observations of 16 near-Earth objects (NEOs) and 2 Mars-crossers in 1996-1999 from three observational sites are presented. Eight objects were observed shortly after their discovery within the follow-up program of NEO observations. We were able to determine rotation periods for 14 asteroids. For 8 objects (5626, 5732, 1998 FM5, 1998 FX2, 1998 UT18, 1998 VO33, 1999 RQ36, 1999 US3) our determinations constitute new results while in the remaining 6 cases (3200, 4341, 7025, 7822, 11066, 1992 QN) we could improve or confirm previously published periods. We also put constraints on the spin vector of 3200 Phaethon. Key Words: asteroids; rotation; photometry.

It is shown that the size-frequency distribution (SFD) of a time-averaged projectile population derived from the lunar crater SFD of Neukum and Ivanov (in Hazards Due to Comets and Asteroids (T. Gehrels, Ed.), 1994, pp. 359-416, Univ. of Arizona Press, Tucson) provides a convincing fit to the SFD of the current near-Earth asteroid (NEA) population, as deduced from the results of asteroid search programs. Our results suggest that the shape of the SFD of the impactor flux has remained in a steady state since the late heavy bombardment, so that the current NEA population can be viewed as a snapshot of the flux of impactors on the Moon. The number of bodies in the projectile population with diameters of 1 km or more is 700 [+ or -] 130, which is in good agreement with recent estimates of the total number of NEAs in this size range. Our results imply that the contribution to the projectile flux from comets is small for diameters below 10 km. Key Words: near-Earth asteroid; cratering statistics; population estimate.