New Findings Published by Smithsonian Scientist Tom Watters
New findings reveal deposits on Mars that could be interpreted to be ice-rich may contain little or no ice at all, based on an analysis of radar sounder data for Meridiani Planum—an area on the planet’s equator being explored by the Opportunity rover. This new insight into Meridiani Planum may help identify areas with and without accessible water ice, a resource critical to future human exploration and possible colonization of Mars. The paper, “Radar Sounder Evidence of Thick, Porous Sediments in Meridiani Planum and Implications for Ice-Filled Deposits on Mars,” is published in the September issue of Geophysical Research Letters by lead author and Smithsonian senior scientist Thomas R. Watters.
Watters and colleagues present new compaction models for materials on Mars that suggest the electrical properties of the deposits of Meridiani Planum, derived from data collected by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument, can be explained without the need for pore-filling ice.
The MARSIS instrument transmits low-frequency radio pulses that penetrate into certain geologic materials and are reflected back where the radio pulses encounter a change in bulk density or composition of the materials. The detection of subsurface reflectors can be used to determine the dielectric constant, a key electrical properties of the materials, by measuring the travel time delays between radar pulses reflected by the surface and subsurface interfaces in Meridiani Planum. This analysis indicates that the deposits of Meridiani Planum have a relatively low dielectric constant. Pure water ice has a low dielectric constant and deposits on Mars with low dielectric constants are often interpreted to be ice-rich. However, newly derived compaction models for Mars indicate that the derived, relatively low dielectric constant of the Meridiani Planum deposits are consistent with a thick sequence of ice-free, porous, windblown sand. 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.
“It’s very revealing that the low dielectric constant of the Meridiani Planum deposits can be explained without invoking pore-filling ice,” said Watters. “Our results suggest that caution should be exercised in attributing non-polar deposits on Mars with low dielectric constants to the presence of water ice.”
Watters is a scientist in the Center for Earth and Planetary Studies at the National Air and Space Museum. The MARSIS radar sounder, on board the Mars Express spacecraft, is managed by the Agenzia Spaziale Italiana (ASI) and NASA. The Mars Express mission is managed and operated by the European Space Agency. MARSIS was developed by the University of Rome, Italy, in partnership with NASA’s Jet Propulsion Laboratory in Pasadena, California.
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A subsurface view of Miyamoto crater in Meridiani Planum from the MARSIS radar sounder. Located west of the Opportunity rover landing site, Miyamoto impact crater is approximately 160 kilometers in diameter (about 100 miles). Miyamoto is partially fill by the deposits that make up Meridiani Planum, leaving a portion of the crater floor exposed (about half the crater is shown in this 3D view). MARSIS sounder data, collected along a track cutting across Miyamoto, shows a curved subsurface feature (upward pointing arrows) that is offset in time delay from the surface reflection of the radar pulses (downward pointing arrows). This subsurface reflector is interpreted to be the crater floor extending beneath the Meridiani Planum deposits. With knowledge of the thickness of the deposits, the radar sounder data allows the electrical properties of the deposits to be determined. The curvature of the buried crater floor is an artifact of the time delay representation of the sounder data.
The MARSIS radar sounder instrument on the European Space Agency’s Mars Express spacecraft is comprised of a 40 meter (about 131 feet) dipole antenna. MARSIS transmits long wavelength, low frequency radio waves that can penetrate certain geologic materials and are reflected back where the radio waves encounter a change in bulk density or composition. The radargram, a time delay rendering of the reflected radar pulses, shows a subsurface feature or reflector (upper pointing arrows) separated from the surface reflection (downward pointing arrows) in Meridiani Planum. The subsurface reflector is interpreted to be the base the Meridiani Planum deposits, about 1 kilometer (about 0.62 miles) beneath the surface.
What lies beneath the Opportunity rover? Much of the plains of Meridiani Planum as seen by the Opportunity rover are flat, featureless, and covered by sand dunes. In May 2005, while the Mars Express spacecraft orbited high above, the Opportunity rover on route to Endurance crater and driving through “Purgatoy Dune” got stuck in the deep sand for more than a month. The MARSIS radar sounder has probed deep below the surface of Meridiani Planum revealing deposits of windblown sand are as much as 1,000 meters (about 3,280 feet) thick in some areas.