Search

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.12.008 Byline: S.C. Werner Keywords: Mars; Cratering; Mars; surface; Moon; surfaces Abstract: Impact basin formation ages give insight into the early evolution of a planet. The martian basins Hellas, Isidis and Argyre provide an important time-marker for the cessation of the magnetic dynamo and the crustal thickness distribution, both established before 4 Ga ago. No martian surfaces are older than 4.15 Ga based on crater count statistics, and all are younger than the oldest lunar ones. I show that the heavy bombardment period on the Moon and Mars evolved similarly, but endogenic processes have removed the oldest martian basin record. The basin-forming projectile population appears to be different from the impactor population observed today in the inner Solar System. It is yet uncertain whether the heavy bombardment period is cataclysmic or characterized by the decaying flux of planetary formation. Author Affiliation: Geological Survey of Norway (NGU), Leiv Eirikssons vei 39, N-7491 Trondheim, Norway Article History: Received 16 January 2007; Revised 20 November 2007

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

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.