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Planetary News: Mars (2005)

Mars Express: State-of-the-Art Radar Instrument, MARSIS, Ready for Work

By A.J.S. Rayl
June 23, 2005

The first-of-its-kind radar instrument onboard the European Space Agency's Mars Express has been fully deployed and is ready to start operations.

The Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument -- known simply as MARSIS -- is designed to look above and below the surface of Mars by using low frequency microwaves reflected by the different layers of matter.

Planetary scientists and Mars enthusiasts around the world have been particularly anxious for the instrument's debut because it is the first device to be able to detect what lies beneath the surface, as far down in fact as 3 miles / 5 kilometers. The fact that MARSIS will specifically be on the lookout for reservoirs of underground water ice increases its profile and raises the possibility, and the hopes, however tenuous, of discovery of habitats hospitable to Martian life.

Whatever it may or may not find on the waterfront, MARSIS will also be used to characterize terrains underneath layers of sediment, and conduct large-scale altimetry mapping and provide data on the planet's ionosphere.

With MARSIS now deployed, Mars Express -- which reached the Red Planet on Christmas Day 2003 -- has at long last its full complement of instruments available to study the planet's atmosphere, surface, and subsurface. It has been a long time coming for Giovanni Picardi, the instrument's principle investigator, and his team, as well as the orbiter's team at large.

"Overcoming all the technical challenges to operate an instrument like MARSIS, which had never flown in space before this mission, has been made possible thanks to magnificent cooperation between experts on both sides of the Atlantic," said David Southwood, ESA's Science Program Director.

The challenges were multiple, and getting the three booms each safely deployed has been anything but a cakewalk. MARSIS features three antenna booms -- two 'dipole' booms, each 65.6 feet / 20 meters long, and one 23-foot / 7-meter 'monopole' boom, oriented perpendicular to the first two.

The instrument was initially supposed to be deployed in April 2004 toward the end of Mars Express' commissioning phase. New computer simulations, however, indicated a previously unrealized, potential risk that the booms could lash back and harm the spacecraft and/or its instruments during deployment. ESA therefore delayed deployment until experts from JPL, which supplied the boom, and Astrium (France), the spacecraft prime contractor, could meet with ESA's experts to conduct further analyses and simulations of boom behavior during deployment.

Once the magnitude of the risk involved had been assessed and relevant mitigation scenarios defined, ESA decided to proceed with releasing the MARSIS antennas last month. The delicate three-stage phase of radar instrument's three booms began May 2 with deployment of the first boom. When that completed on May 10, performance data showed that one of the outermost segments had deployed but was not locked into position. ESA's ground team at the European Space Operations Centre (ESOC) in Darmstadt, Germany, came up with a plan to roll the spacecraft and warm the hinges with the Sun. It worked. So the team utilized the same 'trick' during the deployment of the second long boom, which was successfully locked into position on June 14. Subsequently, they commanded the non-critical deployment of the third boom on June 17, and that proceeded smoothly, as planned.

Last Sunday, MARSIS's ability to transmit radio waves in space was tried out for the first time when the instrument was switched on and performed a successful transmission test. "The effort," said Southwood, "is indeed worthwhile as, with MARSIS now at work, whatever we find, we are moving into new territory."

The instrument works by sending a coded stream of radio waves towards Mars at night, and analyzing their distinctive echoes or 'signatures' of the elements and materials that are hidden underneath, a technique called ground-penetrating radar. The same methods can also be used by day to probe the structure of the upper atmosphere. It is similar to the radar geologists use on Earth to prospect for water, oil, rock layers, or rock faults underground.

Before starting its scientific observations, MARSIS has to undergo its commissioning phase. This is a routine procedure for any spacecraft instrument, necessary to test its performance in orbit using real targets in situ. In this case, the commissioning will last about 10 days, or 38 spacecraft orbital passes, starting today and ending July 4. During the commissioning phase, MARSIS will be pointed straight down (nadir pointing mode) to look at the Red Planet from those parts of the elliptical orbit where the spacecraft is closest to the surface (around the pericenter).

In this phase, MARSIS will cover the areas of Mars between 15° S and 70° N latitude. This area includes interesting features such as the northern plains and the Tharsis region, an enormous volcanic upland located near Mars' equator, at the western end of Valles Marineris that scientists believe may hold the keys to many of the Red Planet's mysteries, including its large-scale shape and gravity field, and its early climate and water distribution. So there is a small chance of exciting discoveries being made early on.

On July 4, as Deep Impact heads toward its crash landing on comet Tempel 1, and space enthusiasts begin their journey to The Planetary Society's Comet Bash, MARSIS will start its nominal science observations, in survey mode, taking observations of the planet's night-side. This is favorable to deep subsurface sounding, because 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. It could make for quite a night in space exploration.

Through mid-July, MARSIS will be charged with looking at all Martian longitudes between 30° S and 60° N latitude, in nadir pointing mode. This area, which includes the smooth northern plains, may have once contained large amounts of water. The MARSIS operation altitudes are up to 497 miles / 800 kilometers for subsurface sounding and up to about 746 miles / 1200 kilometers for studying the ionosphere.

From mid-July, the orbit's closest approach point will enter the day-side of Mars and stay there until December. In this phase, using higher frequency radio waves, the instrument will continue shallow probing of the subsurface and start atmospheric sounding.

MARSIS, which was developed by the University of Rome, Italy, in partnership with JPL, is one of seven science experiments carried onboard Mars Express.