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Planetary News: Mars (2005)Mars Express: MARSIS Radar Conducts Ionospheric SoundingNovember 17, 2005
Since Mars Express’ radar – the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument known simply as MARSIS -- was deployed last summer, it has conducted its first ionospheric sounding, and the plan now is that it will continue operations looking for pockets of water in the Martian subsurface in December. Although Mars Express officially completed its primary mission this past summer, the European Space Agency (ESA) in September extended the mission by one additional Martian year (23 Earth months) in part to allow MARSIS to continue subsurface observations in December this year. By then, the pericenter – or the point of closest approach to the planet -- of Mars Express’ orbit will be in the Martian night-time when the time is best for the instrument to collect data from the subsurface. For the MARSIS operational period up to now, Mars Express has been making its closest approaches to Mars predominantly in the daytime portion of its orbit, ideal for the radar scientists to collect data about the upper layers of the Martian atmosphere, or ‘ionosphere,’ the highly, electrically conducting layer that is maintained by sunlight. Meanwhile, the MARSIS team members have also continued the laborious analysis of all data gathered during the first night-time observations last summer, especially in the search for and interpretation of possible signals from subsurface layers, including the search for a possible signature of underground water, in either a frozen or liquid state. Radar science is a complex business that is based on the detection of radio waves reflected by boundaries between different materials. By analysis of these “echoes,” it is possible to deduce information about the kind of material causing the reflection; thereby allowing informed projections about its composition and physical state. Different materials are characterized by their “dielectric constant” -- or the specific way they interact with electromagnetic radiation, such as radio waves. When a radio wave crosses the boundary of different layers of “material,” an echo is generated that produces a sort of “fingerprint” of the specific materials. From the time delay for an echo to be received by the radar instrument, the distance or the depth of the layers of material producing the echo can be determined. During the last several months when Mars Express’ closest approach has been in daylight, MARSIS has been operating only at higher frequencies within its capability, because the lower-frequency radio signals get disturbed. With these higher frequencies, MARSIS can study the ionosphere and the surface, but only some shallow subsurface sounding can still be attempted. When MARSIS moves to nighttime observations, like those performed briefly last summer immediately after deployment, it will be possible for the instrument to utilize all frequencies for scientific measurements, including the lowest ones that are most suitable for penetrating under the soil of Mars. There is a lot more to the process than just tuning MARSIS to different frequencies for different targets in different conditions. Since the instrument responds to signals reflected from any direction, it requires the scientists do a huge amount of analysis work to remove any interfering signals from the echoes or “clean up” the signals. So, the MARSIS teams members have spent a substantial amount of their time on this “clean-up” work, aided by “surface echo simulator” computer programs. A typical example of what the scientists look for is something called “clutter backscattering” -- reflections that appear to come from the subsurface but actually are produced by irregularities in the surface terrain that delay the return of the echo. During the first months of operations while MARSIS was performing its first ionospheric sounding, team members converted the data into typical plots, called “ionograms,” where the altitude at which the echo was generated, deduced by the echo time delay, is given for each transmitted frequency. The intensity of the various echo signals detected is indicated in different colors. In parallel to the analysis of surface and subsurface signals, the MARSIS scientists are studying all ionograms to draw the first conclusions on the nature and behavior of the ionosphere of Mars, and of its interaction with the planet and the surrounding environment. |
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