Strong reflections not necessarily evidence that Mars' SPLD contains liquid water: Study

Recently, the European Space Agency's Mars Express orbiter detected bright reflections beneath the surface of Mars' South Pole Layered Deposit (SPLD) - an ice-rich geologic unit - with some scientists interpreting the observations as evidence of liquid water. 

Now, scientists have come up with an alternative explanation. Dan Lalich, a research associate with Cornell Center for Astrophysics and Planetary Science in the College of Arts and Sciences (A&S), said that while such a possibility is exciting, he and other Cornell researchers argue the strong reflections are not necessarily evidence that the SPLD contains liquid water.

Using computer simulations, the research team, which includes Alexander Hayes, associate professor of astronomy, director of CCAPS, director of the Spacecraft Planetary Image Facility and the Louis Salvatore ’92 Faculty Leadership Fellow, and Valerio Poggiali, CCAPS research associate, demonstrate that similar strong reflections can be generated by interference between geological layers, without liquid water or other rare materials.

"On Earth, reflections that bright are often an indication of liquid water, even buried lakes like Lake Vostok [under the surface of the East Antarctic Ice Sheet]. But on Mars, the prevailing opinion was that it should be too cold for similar lakes to form," Lalich said in a statement. He used a one-dimensional modeling procedure commonly used to interpret MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) observations.

For the unversed, MARSIS is a multi-frequency radar sounder and altimeter on the Mars Express orbiter, using synthetic aperture techniques and a secondary receiving antenna to enhance reflections from the Martian subsurface and ionosphere.

Lalich created simulations with layers composed of four materials - atmosphere, water ice, carbon dioxide (CO2) ice and basalt - and assigned each layer a corresponding permittivity, an intrinsic property of the material describing its interaction with electromagnetic radiation passing through it. Simulations using three layers - two CO2 layers, separated by a layer of dusty ice - produced reflections as bright as the actual observations.

"We have shown that it is possible to create bright reflections without liquid water, the researchers wrote in a paper published September 26 in the journal Nature Astronomy. The work was funded in part through NASA grants.