No evidence for thick deposits of ice at the lunar south pole.

Shackleton crater at the Moon's south pole has been suggested as a possible site of concentrated deposits of water ice, on the basis of modelling of bi-static radar polarization properties and interpretations of earlier Earth-based radar images. This suggestion, and parallel assumptions about other topographic cold traps, is a significant element in planning for future lunar landings. Hydrogen enhancements have been identified in the polar regions, but these data do not identify the host species or its local distribution. The earlier Earth-based radar data lack the resolution and coverage for detailed studies of the relationship between radar scattering properties, cold traps in permanently shadowed areas, and local terrain features such as the walls and ejecta of small craters. Here we present new 20-m resolution, 13-cm-wavelength radar images that show no evidence for concentrated deposits of water ice in Shackleton crater or elsewhere at the south pole. The polarization properties normally associated with reflections from icy surfaces in the Solar System were found at all the observed latitudes and are strongly correlated with the rock-strewn walls and ejecta of young craters, including the inner wall of Shackleton. There is no correlation between the polarization properties and the degree of solar illumination. If the hydrogen enhancement observed by the Lunar Prospector orbiter indicates the presence of water ice, then our data are consistent with the ice being present only as disseminated grains in the lunar regolith.

The root of anomalously specular reflections from solid surfaces on Saturn's moon Titan.

Saturn's moon Titan has a methane cycle with clouds, rain, rivers, lakes, and seas; it is the only world known to presently have a volatile cycle akin to Earth's tropospheric water cycle. Anomalously specular radar reflections (ASRR) from Titan's tropical region were observed with the Arecibo Observatory (AO) and Green Bank Telescope (GBT) and interpreted as evidence for liquid surfaces. The Cassini spacecraft discovered lakes/seas on Titan, however, it did not observe lakes/seas at the AO/GBT anomalously specular locations. A satisfactory explanation for the ASRR has been elusive for more than a decade. Here we show that the ASRR originate from one terrain unit, likely paleolakes/paleoseas. Titan observations provide ground-truth in the search for oceans on exoearths and an important lesson is that identifying liquid surfaces by specular reflections requires a stringent definition of specular; we propose a definition for this purpose.

Pyroclastic Flow Deposits on Venus as Indicators of Renewed Magmatic Activity.

Radar-bright deposits on Venus that have diffuse margins suggest eruptions that distribute debris over large areas due to ground-hugging flows from plume collapse. We examine deposits in Eastern Eistla, Western Eistla, Phoebe, and Dione Regiones using Magellan data and Earth-based radar maps. The radar-bright units have no marginal lobes or other features consistent with viscous flow. Their morphology, radar echo strength, polarization properties, and microwave emissivity are consistent with mantling deposits comprised of few-cm or larger clasts. This debris traveled downhill up to ~100 km on modest slopes, and blanketed lava flows and tectonic features to depths of tens of cm to a few meters over areas up to 40×103 km2. There is evidence for ongoing removal and exhumation of previously buried terrain. A newly identified occurrence is associated with a ridge belt south of Ushas Mons. We also note radar-bright streaks of coarse material west of Rona Chasma that reflect the last traces of a deposit mobilized by winds from the formation of Mirabeau crater. If the radar-bright units originate by collapse of eruption columns, with coarse fragmental material entrained and fluidized by hot gases, then their extent suggests large erupted volatile (CO2 or H2O) amounts. We propose that these deposits reflect the early stage of renewed magmatic activity, with volatile-rich, disrupted magma escaping through vents in fractured regions of the upper crust. Rapidly eroding under Venus surface conditions, or buried by subsequent eruptions, these markers of recently renewed activity have disappeared from older regions.

Radar evidence for liquid surfaces on Titan.

Arecibo radar observations of Titan at 13-centimeter wavelength indicate that most of the echo power is in a diffusely scattered component but that a small specular component is present for about 75% of the subearth locations observed. These specular echoes have properties consistent with those expected for areas of liquid hydrocarbons. Knowledge of the areal extent and depth of any deposits of liquid hydrocarbons could strongly constrain the history of Titan's atmosphere and surface.