ALBERT

Beschreibung: Mars is the only planet other than Earth in the Solar System that has a preserved nonpolar geological record of glaciation on its surface. Nonpolar ice deposits on Mars have been linked to variations in spin-axis obliquity that cause mobilization of polar ice and redeposition at lower latitudes, forming ice-rich and glacial deposits. Remnant nonpolar glacial deposits are found across the northern mid-latitudes where surface ice is not currently stable, implying that different climatic conditions existed on Mars in the past. Individual glacial deposits are often too small to date reliably using impact crater size-frequency data. We describe a novel approach that allows us to derive new information about when glaciation occurred in broad areas of the northern mid-latitudes. In this region we have classified (1) craters that superpose preexisting glacial deposits and were modified by later accumulation (and therefore formed during an epoch when glaciation was occurring), and (2) craters that are superposed on glacial deposits but are themselves unmodified by ice accumulation (and thus post-date significant glaciation). The sparse population of post-glacial craters reveals that the last period of extensive ice deposition of this type in this latitude band was recent (Late Amazonian). The substantial number of craters formed during the recurring glacial periods implies that northern mid-latitude glaciation was a long-lived recurring process, occurring over a period of at least ~600 m.y. On the basis of Mars atmospheric general circulation models, these results are consistent with higher obliquity being common in the past, with recurring periods of obliquity exceeding the 25° axial tilt of Mars today. These observations support the statistical prediction of J. Laskar and colleagues that the median obliquity during the Amazonian was ~35°–40°.

Beschreibung: Images acquired by NASA’s MESSENGER spacecraft have revealed the morphology of frozen volatiles in Mercury’s permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits. The images show extensive, spatially continuous regions with distinctive reflectance properties. A site within Prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance, indicating that the surficial ice was emplaced after formation of the underlying craters. In areas where water ice is inferred to be present but covered by a thin layer of dark, organic-rich volatile material, regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries. The sharp boundaries indicate that the volatile deposits at Mercury’s poles are geologically young, relative to the time scale for lateral mixing by impacts, and either are restored at the surface through an ongoing process or were delivered to the planet recently.

Beschreibung: Orbital images of Mercury obtained by the MESSENGER spacecraft have revealed families of troughs, interpreted to be graben, on volcanic plains material that largely or completely buried preexisting craters and basins. The graben are partially to fully encircled by rings of contractional wrinkle ridges localized over the rims of the buried impact features to form systems of associated contractional and extensional landforms. Most of the buried craters and basins with graben identified to date are located in the extensive volcanic plains that cover much of Mercury’s northern high latitudes. The distinctive relationship between wrinkle ridges and graben in buried craters and basins on Mercury is interpreted to be the result of a combination of extensional stresses from cooling and thermal contraction of thick lava flow units and compressional stresses from cooling and contraction of the planet’s interior.