• Ages of rampart craters in equatorial regions on Mars: Implications for the past and present distribution of ground ice

      Reiss, D.; Van Gasselt, S.; Hauber, E.; Michael, G.; Jaumann, R.; Neukum, G. (The Meteoritical Society, 2006-01-01)
      We are testing the idea of Squyres et al. (1992) that rampart craters on Mars may have formed over a significant time period and therefore the onset diameter (minimum diameter of a rampart crater) only reflects the ground ice depth at a given time. We measured crater size frequencies on the layered ejecta of rampart craters in three equatorial regions to derive absolute model ages and to constrain the regional volatile history. Nearly all rampart craters in the Xanthe Terra region are ~3.8 Gyr old. This corresponds to the Noachian fluvial activity that region. Rampart crater formation declines in the Hesperian, whereas onset diameters (minimum diameter) increase. No new rampart craters formed after the end of the Hesperian (~3 Gyr). This indicates a lowering of the ground ice table with time in the Xanthe Terra region. Most rampart craters in the Valles Marineris region are around 3.6 Gyr old. Only one large, probably Amazonian-aged (~2.5 Gyr), rampart crater exists. These ages indicate a volatile-rich period in the Early Hesperian and a lowering of the ground ice table with time in the Valles Marineris study region. Rampart craters in southern Chryse Planitia,which are partly eroded by fluvial activity, show ages around 3.9 Gyr. Rampart craters superposed on channels have ages between ~1.5 and ~0.6 Gyr. The onset diameter (3 km at ~1.5 Gyr) in this region may indicate a relatively shallow ground ice table. Loss of volatiles due to diffusion and sublimation might have lowered the ground ice table even in the southern Chryse Planitia region afterwards. In general, our study implies a formation of the smallest rampart craters within and/or shortly after periods of fluvial activity and a subsequent lowering of the ground ice table indicated by increasing onset diameter to the present. These results question the method to derive present equatorial ground ice depths from the onset diameter of rampart craters without information about their formation time.
    • Impact craters in the northern hemisphere of Mars: Layered ejecta and central pit characteristics

      Barlow, Nadine G. (The Meteoritical Society, 2006-01-01)
      Mars Global Surveyor (MGS) and Mars Odyssey data are being used to revise the Catalog of Large Martian Impact Craters. Analysis of data in the revised catalog provides new details on the distribution and morphologic details of 6795 impact craters in the northern hemisphere of Mars. This report focuses on the ejecta morphologies and central pit characteristics of these craters. The results indicate that single-layer ejecta (SLE) morphology is most consistent with impact into an ice-rich target. Double-layer ejecta (DLE) and multiple-layer ejecta (MLE) craters also likely form in volatile-rich materials, but the interaction of the ejecta curtain and target-produced vapor with the thin Martian atmosphere may be responsible for the large runout distances of these ejecta. Pancake craters appear to be a modified form of double-layer craters where the thin outer layer has been destroyed or is unobservable at present resolutions. Pedestal craters are proposed to form in an ice-rich mantle deposited during high obliquity periods from which the ice has subsequently sublimated. Central pits likely form by the release of vapor produced by impact into ice-soil mixed targets. Therefore, results from the present study are consistent with target volatiles playing a dominant role in the formation of crater morphologies found in the Martian northern hemisphere.
    • The fracture of water ice Ih: A short overview

      Schulson, Erland M. (The Meteoritical Society, 2006-01-01)
      This paper presents a short overview of the fracture of water ice Ih. Topics include the ductile-to-brittle transition, tensile and compressive strength, compressive failure under multiaxial loading, compressive failure modes, and brittle failure on the geophysical scale (Arctic sea ice cover, Europa's icy crust). Emphasis is placed on the underlying physical mechanisms. Where appropriate, comment is made on the formation of high-latitude impact craters on Mars.