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Under the surface

The first observations from Mars Express' radar experiment allow scientists to look into the Red Planet's depths — and its past.
November 30, 2005
Mars Express, a European Space Agency (ESA) spacecraft orbiting the Red Planet, used the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) to provide the first three-dimensional views beneath the Red Planet's surface. Using radar, MARSIS probed the planet's subsurface and revealed ancient, buried craters. Such findings will help scientists better understand the planet's geologic evolution.

The data revealed a hidden, 155-mile-diameter (250 kilometers) depression in Chryse Planitia. Although the basin isn't buried deeply, it is invisible from the surface. MARSIS' ground-penetrating radar shows distinctive arcs that correspond to ring structures, and a flat feature nearly 100 miles (160 km) long.
MARSIS
MARSIS’ two 65-foot-long (20 meters) booms emit radio waves and detect their echoes. The radar can penetrate more than a mile below Mars’ surface.
ESA
The MARSIS team interprets the rings as the rims of one or more impact basins, and the flat surface, at roughly 1 to 1.5 miles (1.5 to 2.5 km) deep, as the boundary between the crater's floor and overlying material. This other material might be a water-ice mix — an earlier outflow from nearby Valles Marineris — that partially fills the basin.

"[The detection of buried impact basins] may force us to reconsider our chronology of the formation and evolution of the surface," comments MARSIS' co-principal investigator Jeffrey Plaut of NASA. Previously, the only ways to look into Mars' geologic history were by analyzing exposed layers in crater or valley walls, or drawing cross-sections based on surface maps.
Mars’ Chryse Planitia
Radar reveals parabolic arcs beneath Mars’ Chryse Planitia. These arcs indicate the walls of an ancient, buried crater there.
ESA
MARSIS also detected a water-ice layer roughly a half-mile (1 km) thick at the North Polar Cap. However, it has recorded no evidence of subsurface liquid water anywhere on the planet. Its search for liquid water will begin in earnest next spring; meanwhile, the South Polar Cap and Hellas and Argyre basins are its next targets.

ESA has extended Mars Express' mission for another martian year, roughly 2 Earth years, beginning in December, in part to allow MARSIS to continue searching for water. Mars Express arrived at the Red Planet in December 2003, but concerns about how some instruments might affect the spacecraft delayed MARSIS' use. MARSIS has been collecting data for roughly 5 months in 30-minute intervals three times a day during as many orbits as possible.
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