Stephanie C. Werner¹, Anouck Ody², François Poulet³

¹The Centre for Earth Evolution and Dynamics, University of Oslo, Sem Sælandsvei 24, 0371 Oslo, Norway.
²Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, Université de Lyon 1 (CNRS, ENS-Lyon, Université de Lyon), rue Raphaël Dubois 2, 69622 Villeurbanne, France.
³Institut d’Astrophysique Spatiale, Université Paris Sud 11, Bâtiment 121, 91405 Orsay, France.

Absolute ages for planetary surfaces are often inferred by crater densities and only indirectly constrained by the ages of meteorites. We show that the <5 million-year-old and 55-km-wide Mojave Crater on Mars is the ejection source for the meteorites classified as shergottites. Shergottites and this crater are linked by their coinciding meteorite ejection ages and the crater formation age and by mineralogical constraints. Because Mojave formed on 4.3 billion–year-old terrain, the original crystallization ages of shergottites are old, as inferred by Pb-Pb isotope ratios, and the much-quoted shergottite ages of <600 million years are due to resetting. Thus, the cratering-based age determination method for Mars is now calibrated in situ, and it shifts the absolute age of the oldest terrains on Mars backward by 200 million years.

Reference
Werner SC, Ody A and Poulet F (2014) The Source Crater of Martian Shergottite Meteorites. Science 343:1343-1346.
[doi:10.1126/science.1247282]
Reprinted with permission from AAAS

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Krohn^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aInstitute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany

The Quadrangles Av-11 and Av-12 on Vesta are located at the northern rim of the giant Rheasilvia south polar impact basin. The primary geologic units in Av-11 and Av-12 include material from the Rheasilvia impact basin formation, smooth material and different types of impact crater structures (such as bimodal craters, dark and bright crater ray material and dark ejecta material). Av-11 and Av-12 exhibit almost the full range of mass wasting features observed on Vesta, such as slump blocks, spur-and-gully morphologies and landslides within craters. Processes of collapse, slope instability and seismically triggered events force material to slump down crater walls or scarps and produce landslides or rotational slump blocks. The spur-and-gully morphology that is known to form on Mars is also observed on Vesta; however, on Vesta this morphology formed under dry conditions.

Reference
Krohn et al. (in press) Mass movement on Vesta at steep scarps and crater rims. Icarus
[doi:10.1016/j.icarus.2014.03.013]
Copyright Elsevier

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Blewett^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aPlanetary Exploration Group, Space Department, Johns Hopkins University Applied Physics Laboratory, MS 200-W230, 11100 Johns Hopkins Road, Laurel, Maryland 20723 USA

As part of systematic global mapping of Vesta using data returned by the Dawn spacecraft, we have produced a geologic map of the north pole quadrangle, Av-1 Albana. Extensive seasonal shadows were present in the north polar region at the time of the Dawn observations, limiting the ability to map morphological features and employ color or spectral data for determination of composition. The major recognizable units present include ancient cratered highlands and younger crater-related units (undivided ejecta, and mass-wasting material on crater floors). The antipode of Vesta’s large southern impact basins, Rheasilvia and Veneneia, lie within or near the Av-1 quadrangle. Therefore it is of particular interest to search for evidence of features of the kind that are found at basin antipodes on other planetary bodies. Albedo markings known as lunar swirls are correlated with basin antipodes and the presence of crustal magnetic anomalies on the Moon, but lighting conditions preclude recognition of such albedo features in images of the antipode of Vesta’s Rheasilvia basin. “Hilly and lineated terrain,” found at the antipodes of large basins on the Moon and Mercury, is not present at the Rheasilvia or Veneneia antipodes. We have identified small-scale linear depressions that may be related to increased fracturing in the Rheasilvia and Veneneia antipodal areas, consistent with impact-induced stresses (  and ). The general high elevation of much of the north polar region could, in part, be a result of uplift caused by the Rheasilvia basin-forming impact, as predicted by numerical modeling ( Bowling et al., 2013). However, stratigraphic and crater size-frequency distribution analysis indicate that the elevated terrain predates the two southern basins and hence is likely a remnant of the ancient vestan crust. The lack of large-scale morphological features at the basin antipodes can be attributed to weakened antipodal constructive interference of seismic waves caused by an oblique impact or by Vesta’s non-spherical shape, or by attenuation of seismic waves because of the physical properties of Vesta’s interior. A first-order analysis of the Dawn global digital elevation model for Vesta indicates that areas of permanent shadow are unlikely to be present in the vicinity of the north pole.

Reference
Blewett et al. (in press) Vesta’s North Pole Quadrangle Av-1 (Albana): Geologic Map and the Nature of the South Polar Basin Antipodes. Icarus
[doi:10.1016/j.icarus.2014.03.007]
Copyright Elsevier

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David A. Williams^a, R. Aileen Yingst^b and W. Brent Garry^c

^aSchool of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287-1404.
^bPlanetary Science Institute, Tucson, Arizona
^cNASA Goddard Spaceflight Center, Greenbelt, Maryland

The purpose of this paper is to introduce the Geologic Mapping of Vesta Special Issue/Section of Icarus, which includes several papers containing geologic maps of the surface of Vesta made to support data analysis conducted by the Dawn Science Team during the Vesta Encounter (July 2011-September 2012). In this paper we briefly discuss pre-Dawn knowledge of Vesta, provide the goals of our geologic mapping campaign, discuss the methodologies and materials used for geologic mapping, review the global geologic context of Vesta, discuss the challenges of mapping the geology of Vesta as a small airless body, and describe the content of the papers in this Special Issue/Section. We conclude with a discussion of lessons learned from our quadrangle-based mapping effort and provide recommendations for conducting mapping campaigns as part of planetary spacecraft nominal missions.

Reference
Williams DA, Yingst RA and Garry WB (in press) Introduction: The Geologic Mapping of Vesta. Icarus
[doi:10.1016/j.icarus.2014.03.001]
Copyright Elsevier

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