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Seeing Below the Surface Marte Vallis represents the largest of the young outflow channels on Mars. Thought to be the result of an ancient megaflood that occurred during a period that is otherwise known to be cold and dry, this channel system extends more than approximately 1000 kilometers in length and 100 kilometers in width. The channels' depths are unknown, however, because they have been covered by lava flows. By using the SHARAD radar sounder on the Mars Reconnaissance Orbiter, Morgan et al. (p. 607 , published online 7 March) produced a three-dimensional reconstruction of the subsurface channels. The channels are twice as deep as previously thought and the floodwaters came from the Cerberus Fossae extensional fracture system.

Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO(2)) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO(2) volatile release. If released into the atmosphere at times of high obliquity, the CO(2) reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.

Saturn's satellite Iapetus has the largest albedo asymmetry of any natural satellite, with the optical albedo of its trailing hemisphere as much as 10 times that of its leading hemisphere ([ 1 ][1]). The sharp contrast on this synchronously rotating satellite suggests that exogenic materials, such

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.