Searching for Water with MARSIS
Many Mars scientists believe that a lot of water must still be on Mars,
locked into frozen or liquid underground reservoirs. Recent results from
the Mars Global Surveyor orbiter may indicate that liquid
water may still burst out onto the surface from underground before
disappearing into the atmosphere. The 2001 Mars Odyssey
orbiter has also discovered large amounts of subsurface ice.
If aquifers are present in the upper 3 kilometers (about 2 miles) of the crust,
a radar signature would reveal it. Near-surface aquifers may be present
due to active thermal processes or low-thermal-conductivity sediments.
Detection of these sites could provide targets for future instruments
designed to detect life and to provide water resources for future human
An underground zone of liquid water will have very different electrical
properties from the surrounding rocks and it will reflect very strongly.
Scientists should be able to see the top of a liquid zone somewhere in the
upper 2-3 kilometers (1-2 miles) fairly easily, and may be able to go down to
5 km (about 3 miles) or more. The radio waves will be reflected at any interface,
not just that between rock and water, so MARSIS should reveal much about
the composition of the top 5 kilometers (about 3 miles) of crust in general. It
should, for example, pick out layers of rock interspersed with ice, which are
more likely to exist close to the Martian surface than liquid water.
Scientists should be able to measure the thickness of sand deposits in
sand dune areas, or determine whether there are layers of sediment sitting
on top of other material in areas hypothesised to be the sites of ancient
lakes or oceans. They may even see the boundaries between different lava
Studying the Upper Atmosphere with MARSIS
Scientists will also use this instrument to study the ionosphere to see
how the solar wind affects the Mars' upper atmosphere. Known as the
ionosphere, the upper atmosphere of Mars at altitudes greater than 80
kilometers (about 50 miles) contains free electrons that are produced by the
ionization of gases in the martian atmosphere. During daytime, the peak
concentration of these free electrons occurs at around 130-150 kilometers
(80-93 miles) above the surface of Mars. Low-frequency radio waves cannot
penetrate high concentrations of free electrons and if the frequency is
sufficiently low, the radio wave reflects back. This principle is used in the
radar to map the electron density profile of the ionosphere by transmitting
a number of pulses at different frequencies.
The best ground-penetrating studies are made during the night when
the martian ionosphere is least active and when the spacecraft is less than
800 kilometers from the martian surface, a condition that occurs for 26
minutes during each orbit.
During the day, sunlight ionizes the upper atmosphere
(charges it up electrically) and long wavelength radio waves bounce off it.
Those that are reflected from the ionosphere can reveal much about its
structure. MARSIS will measure the electron density in the ionosphere and
hence quantify the effect of charged particles streaming out from the Sun
(the solar wind) on the upper atmosphere. Such measurements will
complement those of other instruments whose prime task is to find out
whether the unremitting depredations of the solar wind over billions of years
have stripped Mars of much of its atmosphere.