Friday, December 14, 2007

MARSIS and Subsurface Geology

One of the purposes of the MARSIS instrument is to probe Mars' subsurface geology to a depth of five kilometers. To do this, MARSIS sends low-frequency radio waves down to the surface and records the echoes that have bounced back to Mars Express. In November 2005, the European Space Agency (ESA) reported that the MARSIS team had discovered buried impact craters and hints of the presence of deep underground water ice.

Credit: ASI/NASA/ESA/Univ. of Rome/JPL

First results revealed an almost circular structure, about 250 kilometers in diameter, shallowly buried under the surface of the northern lowlands of Chryse Planitia (see the map below). Scientists have interpreted it as a buried basin of impact origin. Echo structures, as shown in the radargram images above, form a distinctive collection that include parabolic arcs and an additional planar reflecting feature parallel to the ground, 160 km long. The images were taken in two different orbits, spaced about 50 km apart.

Credit: ASI/NASA/ESA/Univ. of Rome/JPL/MOLA

The topographic map, based on Mars Orbiter Laser Altimeter (MOLA) data, shows the MARS Express groundtracks and the arc structures detected by MARSIS that are interpreted to be part of the buried impact basin. The topographic relief represented in the image is 1 km, from low (purple) to high (red). The projected arcs are shown in red for orbit 1892 and white for orbit 1903. There is no obvious feature in the surface topography that corresponds to the buried feature identified with MARSIS data.

The parabolic arcs correspond to ring structures that could be interpreted as the rims of one or more buried impact basins. Other echoes show what may be rim-wall 'slump blocks' or 'peak-ring' features. The planar reflection is consistent with a flat interface that separates the floor of the basin, situated at a depth of about 1.5 to 2.5 km, from a layer of overlying different material. It is possible that this planar feature is a low-density, water-ice-rich material at least partially filling the basin.

Credit: ASI/NASA/ESA/Univ. of Rome/JPL/MOLA Science Team

MARSIS also probed the layered deposits that surround the north pole of Mars, in an area between 10º and 40º East longitude. The interior layers and the base of these deposits are poorly exposed. Prior interpretations could only be based on imaging, topographic measurements and other surface techniques. However, MARSIS results (above) show two strong and distinct echoes coming from the area corresponding to a surface reflection and subsurface interface between two different materials.

The MARSIS radargram image (top) shows data from the subsurface of Mars in the layered deposits that surround the north pole. The lower image shows the position of the ground track on a topographic map of the area based on MOLA data. The total elevation difference shown in the topographic map is about 2 km, between the lowest surface (magenta) and the highest (orange) over an area 458 km wide.

The MARSIS echo trace splits into two traces to the right of center, at the point where the spacecraft's groundtrack crosses from the smooth plains onto the elevated layered deposits on the right. The upper trace is the echo from the surface of the deposits, while the lower trace is interpreted to be the boundary between the lower surface of the deposits and the underlying material, believed to be basaltic regolith. The strength of the lower echo suggests that the intervening material is nearly pure water ice. The time delay between the two echoes reaches a maximum of 21 microseconds at the right of the image, corresponding to a thickness of 1.8 km of ice. This conclusion appears to rule out the hypothesis of a melt zone at the base of the northern layered deposits.

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