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Geophysical Detection of Subsurface H2O and CO2 On Mars

Adding dimension to the debate about Mars and its watery history - or lack thereof. See Ancient Drainage Basin Became Aquifer On Mars
by Nick Hoffman
Melbourne - Oct 10, 2001
In early August some eighty international scientists from around the world met in August in Houston, Texas, at the NASA-sponsored GeoMars conference to discuss current and future missions to the Red Planet and instruments, data and observations relevant to the subsurface distribution of volatiles.

There was a large contingent of Italian scientists, representing ESA radar missions, and a variety of experts in permafrost, remote sensing, and geophysical technology. Experts came from around the globe to contribute their ideas and experience to the meeting, co-ordinated by Stephen Clifford of the Lunar and Planetary Institute.

The plan was to develop a roadmap for the next 5 or 10 years of Mars missions and define a comprehensive geophysical programme to map the planetwide distribution of subsurface water. But a few unplanned things happened on the way to the forum...

Problems with Mars
Two basic problems are emerging with our current models for the subsurface of the red planet. The first one is a geophysical problem. We know that the surface soils and rocks are relatively rich in iron minerals, and that a fraction of the windblown dust sticks to magnetic targets on the landers.

From this, one suspects that there is a significant metallic and magnetic mineralogy to Mars which will strongly effect radar and electical sensing methods, although we do not yet have detailed observational data to quantify this deduction.

The other is a more fundamental issue of exactly what liquids we should be searching for, anyway, and will be addressed in an opinion piece by this author next week.

Radar Missions to Mars
Working from the magnetic/conductive mineral aspect, several contributors (Gary Olhoeft, Assam Heggy and others) suggested that the figures currently being quoted for depth of penetration and resolution of radar systems on Mars were probably a very optimistic figure.

Therefore, actual results would be patchy and often an order of magnitude poorer than hoped, so plans to see down to 5km might result in penetration typically of only 500m - far too shallow to find liquid water in any but the most intense geothermal areas.

There was, of course, considerable discussion on this point and eventually the concensus was that although the planned radar missions (MARSIS and the planned higher frequency 2005 MHRAR) would not give the results expected on a global basis, but they would have local areas of better penetration.

This is especially true in the polar regions and the pole caps themselves, where there is less rock (and hence less of the troublesome magnetic and conductive minerals), and more ice which is largely transparent to radar at Martian temperatures - allowing deep penetration to the full 5km expected.

On a philosophical note, other contributors (notably Bob Grimm) observed that the goal to map the subsurface distribution of liquid water on Mars should be compared to the planet we know most about - the Earth. Here, despite decades of satellite and surface geophysics and remote sensing, we do not yet have an accurate view of the subsurface distribution of fluids such as oil and fresh vs salt water. Each area where we have conducted detailed surveys reveals a complex pattern of fluids.

Permafrost on Earth
As a Mars analogy permafrost studies (by Steve Arcone and others) showed how complex and varied the ice/water situation can be on earth, with films, veins, and lenses of liquid water within permafrost zones, and segregation of soil and ice into layers and lenses by slow processes over millenia.

Surface patterns of polygons and ridges, and topographic features such as pingos are often associated with the evolution of ground ice, with seasonal or longer-period freeze/thaw action or with the physical expansion and contraction of fully frozen ground. There are no easy and simple answers on Earth, and Mars will likely be much the same.

Understanding the subsurface of Mars will require radar missions at a variety of frequencies to probe to various depths and analyse the reflectivity of the surface at different wavelengths to give global reflectivity maps that will help map terrain units and geomorphic processes. In addition, surface callibration will be required from rover and lander based ground-penetrating radar (GPR) such as is planned for Britain's Beagle 2 lander in early 2004.

Other Geophysical Methods
Better answers may come from electrical sounding of the planet, which will require low altitude balloons or airplanes, or surface landers and rovers to deploy large wire coils and transmit signals into the ground.

On Earth, these methods have reasonable success in finding equivalent conductive targets, as that represented by subsurface brines on Mars, so plans for the next decade or so of Mars missions may begin to include electrical sounding arrays.

Seismic Tests for Water on Mars
The most significant near-term experiment that can be done for Mars is to land a sensitive seismometer. Gary Olhoeft described the Apollo seismometers which demonstrated unequivocally that the Moon was solid and dry, since Moonquakes trembled and quivered for hours after the initial shock, while even slightly wet rocks would have absorbed the energy and muted the echoes.

A similar test for Mars is planned in the Netlander mission which will deploy a series of groundstations with highly sensitive seismometers. We expect a reasonable number of Marsquakes, and these will immediately show if Mars has no water at all, by similar reverberation.

What is harder to analyse is how much water it does have, and where and in what form it is.

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Mars: A World Riven By H2O or CO2
Cameron Park - May 1, 2001
There are now so many puzzles and contradictions in the most popular interpretation of a watery Martian history that a radical new alternative -- proposed by Nick Hoffman of Latrobe University in Australia -- is starting to catch on among a growing number of planetary geologists.

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