Workshop Summary

The Mars Deep Water Sounding workshop was held at NASA Ames Research Center on January 28,1998. The focus of the workshop was to review the possible approaches for searching for deep subsurface water on Mars and recommend a strategy for performing the search. Invited participants included experts in the field of terrestrial geophysical exploration with an emphasis on field work in polar environments, planetary scientists with an interest in water on Mars, and scientists who have instrument concepts for sounding the Martian subsurface. A small number of invited presentations were given on the above topics, and considerable time was devoted to discussion.

Key Summary Points

Liquid water may exist below the Mars subsurface.
Theoretical analysis indicates that the Martian subsurface is probably a reservoir for ice, and, provided that the global inventory of water is sufficiently large, there should also be liquid water at depths of from 2-10 km. The minimum threshold value of the global inventory of water on Mars, above which there should be a globally distributed liquid water aquifer, has been estimated to be equivalent to a global layer 400 m deep.

The shallowest liquid water is likely to be found at sites that are at both low elevation and low latitude. Good candidate sites include: the northwest interior of Hellas Basin, Amazonis Planitia, Chryse Planitia, Utopia Planitia, Isidis Planitia, East of Elysium Planitia, and the confluence of Ganges, Capri, and Eos Chasmas.

Use terrestrial geophysics insights to find Martian water.
Experience in terrestrial geophysics yields the following lessons:

Active seismic methods are most likely to reach the depth of penetration required to reach liquid water, but are unlikely to identify water uniquely.

Electromagnetic sounding is plausible, but studies are needed to determine the radar penetration characteristics in Mars analog materials. Ambient electromagnetic and seismic noise levels are unknown on Mars, limiting the ability to predict the performance of these methods.

The use of multiple techniques that corroborate each other is the best approach.

The closest terrestrial analog study to the proposed Martian study is the survey of the Antarctic ice sheet by airborne radar-echo sounding performed in 1974-75 by the SPRI/NSF/TUD (Scott Polar Research Institute, UK, NSF, USA and Technical University of Denmark).

Radar sounding to find an aquifer under the Mars polar caps may be the best choice. Using the Lake Vostok study as a direct analog, the method considered "most likely to succeed" by terrestrial geophysicists is a radar sounding of an aquifer under the Mars polar caps. Assuming that the polar caps are pure ice, a radar signal could be uniquely interpreted because the dielectric radar properties of ice are well characterized. However, the effect of impurities in the ice, such as dust or clathrates, is not understood. Thus, subsurface radar profiling in permafrost regions may serve as a more useful analog for Mars application.

There are several different approaches to searching for Mars water. Instrument techniques that have been proposed (or are being proposed) to look for subsurface water on Mars include:

An orbital radar survey. If successful, this method would provide a global map of the vertical distribution of subsurface ice and water.

A radar instrument housed in the guide rope of a balloon. This method would perform a regional transect that would result in the vertical profile of ground ice and water along the transect.

A radar instrument carried on a Mars airplane. If successful, this method would perform a regional transect that would result in the vertical profile of ground ice and water along the transect. This method

A radar instrument carried on a rover. If successful, this method would result in a local-scale (0-10 km) transect of the vertical profile of ground ice and water along the path of the rover.

An active seismic network. If successful, this method would result in a local-scale (0-10 km) vertical profile of ground ice and water.

The ESA Mars Express strawman payload includes an orbital radar sounding instrument that, if successful, would map the global distribution of subsurface water on Mars.

Terrestrial geophysicists were skeptical whether any radar system would penetrate to the depths required to see liquid water on Mars Reflections from the surface will dominate the signal from orbital or airborne radar systems. The balloon guide rope system is extremely mass and power constrained. A rover system will necessarily only survey over a short range and is also very constrained in mass and power. Estimating the performance of radar penetration into the Martian subsurface involves considerable uncertainties. The dielectric properties of the Martian surface and subsurface are poorly known. The Martian dust appears to be a ferromagnetic material which might strongly absorb a radar signal.

Key Recommendations:

1) A terrestrial field research program in Mars analog environments should be used to determine whether radar or other electromagnetic sounding techniques can detect subsurface aquifers at depths and subsurface temperature ranges relevant to Mars. There are good analog field sites in the Arctic and Antarctic where aquifers of known depth underlay permafrost. Instrument concepts should be tested in such sites.

2) Characterizing the ambient seismic and electromagnetic noise environment is an important supporting measurement for geophysical exploration on Mars.

3) Active seismic methods may be required to sound to the depths expected for liquid water on Mars. A mission analysis for applying active seismic techniques to the search for water on Mars should be performed.