If you’re going to Mars, which do you bring: water or a shovel? This question was raised at a joint meeting of the two Mars radar sounder instrument science teams hosted by the National Air and Space Museum’s Center for Earth and Planetary Studies this past June.
The question may sound a little tongue-in-cheek, but it actually goes right to the heart of a critical need for future human exploration of Mars – accessible water. We know there’s abundant water at the poles of Mars, in the form of ice in the polar caps, but those areas are not the best places on the planet to visit. In fact, 50 degrees latitude north or south poleward could be an exclusion zone for human missions because of elevation considerations. If the elevation is too high, there is too little atmosphere; or if it’s too low, there’s too much atmospheric dust. So, the “Goldilocks” zone for human exploration is where there’s just the right amount of elevation and accessible, abundant water ice. While we know the topography of Mars very well, we still don’t know the location of deposits of ice at low latitudes.
One important method of detecting ice on Mars is the data obtained by orbiting radar sounder instruments, like the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). The MARSIS instrument on the European Space Agency’s Mars Express spacecraft has the first radar sounder sent to Mars, with a goal of detecting water. The MARSIS radar sounder transmits low-frequency, long-wavelength radio pulses that penetrate into certain geologic materials. These frequencies are reflected back at places where the radio pulses encounter a change in either the material’s bulk density or composition. The detection of subsurface reflectors can be used to determine a key electrical property of materials called the dielectric constant, by measuring the travel time delays between radar pulses reflected by the surface and subsurface. Pure water ice has a low dielectric constant, so deposits on Mars with low dielectric constants could be interpreted to be ice-rich. Indeed, some non-polar deposits on Mars with low dielectric constants are thought to be full of ice.
At this point you may be asking yourself, “What does any of this have to do with Meridiani Planum?” Meridiani Planum is a region on the equator of Mars currently being explored by the Opportunity rover. Opportunity has shown that minerals at the surface of Meridiani formed in the presence of, or were altered by, liquid water. However, many planetary scientists believe that subsequent, sustained evaporation likely left Meridiani Planum dry long ago.
The MARSIS radar sounder has detected subsurface reflectors deep below the surface of Meridiani Planum. Analysis of the delay time between the surface and subsurface returns indicate that the deposits of Meridiani Planum have a relatively low dielectric constant. Does that mean the Meridiani Planum is full of ice? Not necessarily. The dielectric constant is strongly influenced by the bulk density of the material. Without the presence of pore-filling ice, a thick sedimentary deposit will undergo significant self-compaction, increasing the bulk density and the dielectric constant. Newly derived compaction models for Mars that predict how different geologic materials compress with depth show that the relatively low dielectric constant of the deposits of Meridiani Planum can be explained by a thick sequence of porous, windblown sand, containing little or no ice at all. This is because unlike other geologic materials that have been suggested for Meridiani Planum such as volcanic ash or silicate dust, a thick deposit of sand-sized particles on Mars does not compact nearly as much.
Meridiani Planum, already important for the discoveries made there by the Opportunity rover, is teaching us an important lesson about the search for ice on Mars. We must be cautious about attributing non-polar deposits with low dielectric constants to the presence of water ice.
So, would I bring water or a shovel on a trip to Mars? I’d take both!