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Planetary News: Mars Express and Beagle 2 (2005)

Mars Express: MARSIS Collects First Subsurface Radar Data

MARSIS, the sounding radar onboard the European Space Agency's Mars Express and the first instrument designed to look below the surface of the Red Planet, began science operations July 4 following the first phase of its commissioning. Since then, it has been collecting data about the surface, subsurface, and the ionosphere, and by all accounts, the instrument is working well, and the data looks good.

"During the commissioning we have worked to test all transmission modes and optimize the radar performance around Mars," Giovanni Picardi, MARSIS principal investigator, of the University of Rome 'La Sapienza,' reported late last week. "The result is that since we have started the scientific observations in early July, we are receiving very clean surface echoes back, and first indication about the ionosphere."

"The instrument is working very closely to our expectations," Enrico Flamini, Solar System Programs director for the Italian Space Agency (ASI), confirmed today during the pre-launch press conference for the Mars Reconnaissance Orbiter (MRO), which will carry another radar instrument, the Shallow Subsurface Radar (SHARAD) that will complement the subsurface findings to come from MARSIS, and will be operated by essentially the same Italian-American team.

MARSIS features three long antenna booms -- a dipole and monopole -- that broadcast very long wavelength radio waves toward the planet. Those waves or 'echoes' are then reflected back from the surfaces they encounter, and the instrument observes and records the time of the reflections, collecting 'signatures' of the elements and materials that are hidden underneath the surface, a technique known as ground-penetrating radar and basically the same type of radar used on Earth by geologists to prospect for water, oil, rock layers, or rock faults underground. In addition to characterizing the subsurface layers of sediments and searching for underground water or ice, MARSIS team members are using the instrument to conduct large-scale altimetry mapping and gather data on the planet's ionosphere.

From the beginning of the commissioning, the two 20-meter long antenna booms have been sending radio signals towards the Martian surface and receiving echoes back. "The commissioning phase confirmed that the radar is working very well, and that it can be operated at full power without interfering with any of the spacecraft systems," said Roberto Seu, MARSIS instrument manager for MARSIS, of the University of Rome 'La Sapienza', Italy.

Since MARSIS is the first instrument to be able to detect what lies beneath the surface of Mars -- as far down as 3 miles or 5 kilometers in the best of conditions, theoretically it has the capacity to locate underground habitats hospitable to microbial Martian life, if, that is any exist in the areas searched. Not surprisingly, planetary scientists and Mars enthusiasts around the world are anxiously awaiting the findings. But they'll have to wait a while longer, because the team is still analyzing the first data returns, and so far, 'mum' is pretty much the word on exactly what MARSIS has found.

"We're very excited about what we're seeing," Jeffrey Plaut, MARSIS co-principal investigator, of the Jet Propulsion Laboratory (JPL), told The Planetary Society in an interview this morning. Noting that it is "just too early" to discuss results, Plaut said: "We are analyzing the data now to try and understand what is coming from the surface and what's coming from the subsurface."

"The biggest part of our work just started, as we now have to be sure that we clearly identify and isolate those echoes that come from the subsurface," expounded Picardi. "To do this, we have to carefully screen all data and make sure that signals that could be interpreted as coming from different underground layers are not actually produced by surface irregularities. This will keep us occupied for a few more weeks at least."

MARSIS was initially supposed to be deployed in April 2004 toward the end of Mars Express' commissioning phase. Last minute computer simulations, however, indicated a potential risk of the booms lashing backward and harming the spacecraft and/or its instruments during deployment. So ESA delayed deployment until experts from JPL, which supplied the boom, and Astrium (France), the spacecraft prime contractor, could meet with ESA's engineers to conduct further analyses. Once the magnitude of the risk involved had been assessed and relevant mitigation scenarios defined, ESA began deployment of the MARSIS antennas in mid-May, and completed the procedure in late June.

Because of the late deployment, the MARSIS team decided to split the commissioning phase, originally planned to last four weeks, into two phases -- the first of which has just ended, with the second phase to be started by December this year. The first phase of the commissioning was dedicated to testing the MARSIS electronics and software, and the two 20-meter long antennas (dipole). The second phase of the commissioning is slated to last about 10 days and will be dedicated to the calibration of the 7-meter 'monopole' antenna, which will be used in conjunction with the dipole to correct any surface roughness effects caused by the radio waves emitted by the dipole and being reflected by an irregular surface. The monopole will excel during the investigations of areas where the surface roughness is higher.

Since MARSIS is charged with a number of different scientific tasks, ranging from the ionosphere to deep into the subsurface, it was designed to be capable of operating at different frequency bands, making it a very complex instrument. Lower frequencies are best suited to probe the subsurface and the highest frequencies are used to probe shallow subsurface depths, while all frequencies are suited to study the surface and the upper atmospheric layer of Mars.

Splitting up the commissioning phase allowed the team to begin scientific observations earlier and while still in the Martian night, the environmental condition favorable to subsurface sounding. During the daytime, the ionosphere is more 'energized' and that 'disturbs' the radio signals used for subsurface observations.

For subsurface probing, MARSIS must operate between 186 miles/300 kilometers and 497 miles/800 kilometers from the Martian surface; therefore, the instrument operates around the orbit 'pericenter,' when the spacecraft is closer to the planet's surface. For ionospheric sounding, MARSIS has already provided acceptable results from a distance of up to 1864 miles/3000 kilometers. The radar vertical resolution is about 492 feet/150 meters (in the free space), while the horizontal resolution, in the range of few kilometers, depends on the spacecraft altitude.

In each orbit to date since science operations began last month, the radar has been switched on for 36 minutes around the pericenter point, dedicating the central 26 minutes to subsurface observations and the first and last five minutes of the slot to active ionosphere sounding.

Although all three orbiters at Mars right now -- Mars Global Surveyor (MGS), Mars Odyssey, and Mars Express -- are in a polar orbit, Mars Express is in a highly elliptical orbit; therefore, where MGS and Odyssey complete 12 orbits a day, Mars Express completes about 3.5 orbits per day. "So the spacecraft spends most of its time away from the planet, and the only time that it can do close-up science on Mars is during the close approach of the orbiter to the planet," explained Plaut.

Over time, the position of that close approach of the orbit migrates around the planet, in latitude, and also with respect to the boundary of the day and night-side. "It kind of hovers around that boundary and spends a bit more time on the dayside of that boundary than on the night-side of that boundary," Plaut said. "During the periods when it's on the night-side, which are usually these seasons that last about three to four months, is when MARSIS has the best conditions for collecting data from the subsurface. We're just coming out of one of those seasons, and we will enter another one of those seasons in December."

Since the science operations began in July, MARSIS has been using the low frequencies mainly to investigate the northern flat areas between 30° and 70° latitudes, at all longitudes. "We are very satisfied about the way the radar is performing. In fact, the surface measurements taken so far match almost perfectly with the existing models of the Mars topography," said Picardi. Thus, these measurements provided an excellent test."

The scientific reason to concentrate the first data analysis on flat regions lies in the fact that the subsurface layers there are in principle easier to identify. Still, it's tricky. "As the radar is appearing to work so well for the surface, we have good reasons to think that the radio waves are correctly propagating also below the surface," added Picardi. That said, the analyses continue.

The first ionospheric measurements performed by MARSIS have revealed some interesting preliminary findings team members are willing to talk about now. The radar responds directly to the number of charged particles composing the ionosphere (plasma), and MARSIS' observations have found that number of particles to be higher than expected at times. "We are now analyzing the data to find out if such measurements may result from sudden increases of solar activity, like the one observed on July 14, or if we have to make new hypotheses," said Plaut. "Only further analysis of the data can tell us."

MARSIS will continue send signals to hit the surface and penetrate the subsurface for about another week, when the nighttime portion of the observations will end. After that, observation priority will be given to other Mars Express instruments that are best suited to work during daytime, such as the HRSC camera and the OMEGA mapping spectrometer. MARSIS will continue surface and ionospheric investigations, however, during daytime, with the ionospheric sounding being reserved for more than 20% percent of all Mars Express orbits, in all possible Sun illumination conditions.

Since finding underground water is a key -- and very high-profile -- objective, the MARSIS team has selected a number of potential targets based on data and findings from previous missions. "We have a number of areas we call targets, but it's not so easy to steer the spacecraft to a particular target," noted Plaut. "We get whatever's directly beneath us. Our nighttime data is in general at the higher latitude, so the equatorial areas [such as Meridiani Planum where the Mars Exploration Rover Opportunity found signs of past water, and in regions above which concentrations of methane have been detected] are difficult for us to catch on the night-side," he explained. "We have captured some data from Meridiani Planum, but I don't believe we went directly over the landing site, and we were in the Sun when we took those data, so if there is something in the subsurface it will probably be difficult for us to pull it out."

In December, the Mars Express orbit pericenter will enter the night-side again, and by then, the pericenter will have moved closer to the south pole. That will allow MARSIS to restart optimal probing of the subsurface, this time in the southern hemisphere. There, the team is interested in the high latitudes in the south polar region. "The icy deposits in the polar regions are very high priority for us, so that's one of the targets we're very interested in," Plaut said.

Following that, at some point next spring, MARSIS will scan the Hellas Basin. "The Hellas Basin is very low elevation terrain where, potentially, if there were liquid water, it could be closer to the surface than just about anywhere else on the planet," Plaut explained. "That's because the low elevation means it's warmer at the surface and therefore warmer below the surface and you don't have to go as deep to get to the warm pockets below the surface."