Mars Express: Takeout Service

Rendezvous in Paris Cafe

To the uninitiated, Mars Express may at first sound like a fast-food restaurant, but this week, its services will be called for takeaway. While operating a surface mission that can look in panoramic splendor up into the sky, the European orbiter, Mars Express will simultaneously be looking down on the Spirit rover with unprecedented clarity and at the orbiter’s closest approach yet.

The experiment was first conceived in a Paris cafe one year ago, but could be finalized only two weeks ago. When combined, data from looking up while looking down can be used to subtract atmospheric distortions, and give scientists an alternative view of the rock history, the site topography, and even the aqueous mineral content around Gusev crater, near the rover’s landing site.

On Friday, Spirit’s science team will take advantage of the special possibilities presented by Mars Express flying almost directly overhead, about 300 kilometers (186 miles) high. Mars Express successfully entered orbit around Mars on Christmas. Spirit will be looking up with its panoramic camera while Mars Express uses three instruments to look down.

Mars Express above the thin Martian upper atmosphere successfully achieved orbital insertion on Christmas
Credit: ESA

"This is an historic opportunity," said Dr. Ray Arvidson of Washington University in St. Louis, deputy principal investigator for the science instruments on Spirit and on its twin Mars Exploration Rover, Opportunity. "The intent is to get observations from above and to get observations from below at the same time to do the best possible job of determining the dynamics of the atmosphere." The Mars Express observations are also expected to supplement earlier information from two NASA Mars orbiters about the surface minerals and landforms in Spirit’s neighborhood within Gusev Crater.

Friday is Sol 13 for the Spirit rover, and in the martian afternoon (at 2 p.m. local time), the obiter overhaed will be looking down with a high-resolution stereo camera, a spectrometer for identifying minerals in infrared and visible wavelengths, and another spectrometer for studying atmospheric circulation and composition. Spirit will be looking up with its panoramic camera and its infrared spectrometer (mini-TES).

"On Mars, where has the water been?" is one question Arvidson and the science team have posed. "Is it bound up in minerals?"

Arvidson says that scientists participating on both NASA and ESA missions joined forces to design Friday’s experiment. While "in the works for over a year," said Arvidson, "the schedule was firmed up only two weeks ago, science team to science team." One reason for the serendipitous opportunity was uncertainties about the fate of the Beagle lander and when exactly the timing might be right for an overpass view of the Spirit rover for Mars Express. "The overpass will happen when it is nice and hot on Mars, in the mid-afternoon. The rocks will be cooking by then, and so infrared imagery will show up well in infrared (0.35-0.5 microns)."

Arvidson said this infrared range is diagnostic for water bound in rocks, or aqueous minerals like "clays, hydrated carbonates, sulfates, iron oxides as water-bearing, even nitrates." He is a participant in orbiter instrument called OMEGA, which is designed to map Mars in infrared.

On Friday, the orbiter will make north-to-south passes over Gusev crater in strips 100 kilometers wide. These maps will be 16 to 32 pixels wide, and each pixel is an infrared spectrum to diagnose the chemical composition remotely.

Arvidson said that simultaneous mapping from above and below is key, if scientists want to remove the strong infrared properties of atmospheric dust and distortion from what future panoramic infrared images may provide for distant objects, like the eastern hills in Gusev two to three kilometers away.

Arvidson recalled that the overpass experiment "began as an informal conversation in a Paris cafe, in early summer [2003]."

Over its multi-year mission life, an onboard camera on Mars Express offers high-resolution stereo views of Mars. Its comprehensive maps will feature 10 meter resolution, but some particularly interesting regions will get a close-up view to 2 meters [about the size of small car, as seen from orbit]. Although hundreds of thousands of images have been part of previous mapping missions, as much as ninety-seven percent of the planet remains unexplored at high resolution.

Image of searched-for Beagle lander moving away from Mars Express at separation.After a joint journey of 250 million miles (400 million km), the British-built Beagle 2 spacecraft and the European Space Agency’s Mars Express orbiter have parted and gone their separate ways. Credit: ESA

As part of its two to four year survey, the Mars Express orbiter passes within 250 kilometers [150 miles] of the surface at closest approach. Employing seven major instruments for its on-the-spot analyses, the orbiting Mars Express will be able to identify signs of water in liquid, solid, or vapor form on Mars. During each orbit, the overhead spacecraft will collect data from Mars between a half-hour to an hour, while spending the rest of its time broadcasting those results back to Earth.

Up and Down Views, Did the Water Flow?

OMEGA, the infrared mapping spectrometer, for example, could determine the mineral composition of the soil around the Spirit rover, allowing some hypotheses of rock origins to be eliminated. PFS, the planetary Fourier spectrometer, could measure the amount of carbon dioxide and water ice present, the temperature of the rocks compared with their surroundings and the pressure of the local atmosphere.

MARSIS, the radar sounder, could determine the thickness of any frozen material not visible on the martian surface and the HRSC, the camera, could take high-resolution, 3D, full-color images of the rover site. The radar uses a large 40 meter antenna to collect the sound waves that bounce off any density pockets below the rusty-red soil. If pockets of water are found to a depth of 2 kilometers [1.2 miles], then theories of active hydrology on Mars will be borne out. Subterranean aquifers are considered one possible way in which liquid water could exist in the frigid, hostile conditions, where the atmosphere is about one percent of the Earth’s pressure.

Ray Arvidson, deputy principal investigator for the MER missions.
Credit: Washington University, St. Louis

In addition to the European mission, two NASA orbiters–the Mars Odyssey and Mars Global Surveyor–are part of the constellation of satellites that help mapping and communication tasks independently and jointly in support of the surface missions for the twin rovers, Spirit and Opportunity.

Once Spirit reaches the surface with six wheels in the soil on Thursday, the science teams hopes to use the Microscopic Imager on its robotic arm to "get focus", said Arvidson, "and move laterally [to get two images of the same spot up close]. This gives a stereo pair", useful for three-dimensional visualization.

"To a geologist in a new place," said Arvidson,"you get a rock, pull out a hand lens, and look at sizes and shapes of rock grains. On Mars, the microscopic imager will take pictures with each pixel equal to thirty microns [half the width of a human hair]. Those shapes and grain sizes can diagnose rounded grains for an aqueous mineral or sharp grains for ejecta fragments [if the rock was part of crater debris]."

Arvidson is enthusiastic about the first chance to see Mars so close, "down and personal in the dirt."

Imaging by Mars Express should last at least one Martian year (687 days), with further operations possibly extending into 2008. Using some of the same mission elements, future European explorers will head towards Venus in 2005 and Mercury in 2009.

Related Web Pages

Where is the Mars Express Now?
Mars Odyssey web site
Five Year Retrospective: Mars Pathfinder
Beagle 2
Open University: Beagle
Space Research Centre: Leicester
Mars Express PPARC
Water Signs
Microscopic Imager
Gusev Crater
Pancam – Surveying the Martian Scene
Mössbauer spectrometer
Alpha Proton X-ray Spectrometer
Mars Rover: The Owner’s Manual