PROBING UNDERFOOT. MARSIS radargrams from two north-south orbital passes show a reflection (arrows) under the surface of Meridiani Planum. Scientists interpret this as marking the bottom of the topmost two sedimentary units. (Image taken from the abstract.)
Landed upon and explored by the Opportunity rover, the rocks of Meridiani Planum have been intensively studied, both from orbit and from ground level.
Now scientists in a team led by Thomas Watters (Smithsonian Institution) have used the MARSIS ground-penetrating radar on Mars Express to probe how thick these layered sedimentary units are. (The MARSIS radar tracks they used in the study passed about 230 kilometers (140 miles) to the east of where Opportunity is driving.)
Reporting (PDF) at the 45th Lunar and Planetary Science Conference in The Woodlands, Texas, the Watters team concluded that taken together, the uppermost two rock units have a thickness in the range of 600 to 900 meters (2,000-3,000 feet), with a best value of about 860 m (2,800 ft).
The range in possible thickness comes from uncertainty in knowing the dielectric constant (an electrical property of materials) for the sedimentary deposits. The scientists found that if they assumed a dielectric value of about 3 (consistent with a large amount of water ice in the sediments), this agreed with the MARSIS-measured time delay for the radar signals. (If there is less water ice in the deposits, the dielectric constant would be higher and the radar data would then be indicating a thinner sedimentary layer thickness.)
Looking at the geological setting of Meridiani, the team concludes that the MARSIS results are “consistent with a substantial component of water ice in the deposits. This is an intriguing possibility in light of the evidence that the deposits either formed in, or were altered by, liquid water.” They caution, however, that a very porous, low-density, ice-poor deposit cannot be ruled out.