Wednesday, December 12, 2007


Credit: ESA

Continuing with our discussion of instruments aboard Mars Express:

MARSIS, the Mars Advanced Radar for Subsurface and Ionosphere Sounding, was developed by the University of Rome, Italy, in partnership with NASA’s Jet Propulsion Laboratory (JPL). It is the first instrument to actually look below the surface of Mars, using low-frequency microwaves reflected by the different layers of matter. Its primary objective is to map the distribution of water, both liquid and solid (ice), in the upper portion of the crust of Mars; the instrument is also designed to probe Mars' subsurface geology and to measure the planet's ionosphere. MARSIS consists of three antennas: two "dipole" booms 20 meters long, and one seven-meter "monopole" boom oriented perpendicular to the first two. The instrument works by sending a coded stream of radio waves towards Mars at night, and analyzing their distinctive echoes. From this, scientists can then make deductions about the surface and subsurface structure. Operations are conducted on both Mars' day-side and night-side. The night-side is for deep subsurface sounding: during the night the ionosphere of Mars does not interfere with the lower-frequency signals needed by the instrument to penetrate the planet's surface, down to a depth of five kilometers. Day-side operations use higher frequency radio waves, which allows MARSIS to conduct shallow probing of the subsurface and atmospheric sounding. The MARSIS operation altitudes are up to 800 kilometers for subsurface sounding and up to 1200 kilometers for studying the ionosphere.

The extension of the three MARSIS booms was originally planned to deploy in April 2004. However, computer simulations pointed to a risk that the booms could lash back and harm the spacecraft and its instruments during deployment. The ESA then delayed deployment until the boom supplier (JPL) and the spacecraft prime contractor (Astrium, France), together with ESA’s experts, had conducted further analyses and simulations of the boom behavior during deployment and the possible impact on the spacecraft. Once the magnitude of the risk involved had been assessed and the relevant mitigation scenarios defined, ESA decided to proceed with releasing the MARSIS antennas in May 2005. Deployment of the first boom was started on May 5, 2005. At first, there was no indication of any problems, but later it was discovered that one segment of the boom did not lock. Using the Sun's heat to expand the segments of the MARSIS antenna, the last segment locked in successfully on May 10th. The second 20-meter boom was successfully deployed on June 14th, and the third boom on June 17th. On June 22nd, the ESA announced that MARSIS was fully operational, and the instrument began science operations on July 4th.

The above drawing is an impression of the completely deployed MARSIS experiment on board ESA's Mars Express orbiter with the two 20-meter and one 7-meter booms sprung out and locked into place.

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