Mars Express, a spacecraft launched by the European Space Agency in 2003, is the second longest surviving continually active spacecraft in orbit around a planet other than Earth, behind only NASA's still active 2001 Mars Odyssey. As this spacecraft orbits Mars, it continues to provide important data on the Red Planet’s interior, subsurface, surface and atmosphere, and environment.
Onboard this spacecraft is an instrument called the Mars Advanced Radar for Subsurface and Ionosphere Sounding, or MARSIS for short. This instrument uses a radar sounder to assess the composition of the subsurface of Mars.
Scientists studying Mars, using the MARSIS instrument data, have previously reported a regionally strong radar reflection under Mars’ south polar ice sheet and have interpreted these bright radar reflections at the base of the south polar ice cap as being caused by liquid water.
But a team of scientists, including researchers with UC Santa Cruz and Arizona State University, using data from this same instrument, have determined that Mars’ south polar ice sheet may be made of clays, metal-bearing minerals, or saline ice. Their findings have been recently published in AGU’s Geophysical Research Letters.
“Our fresh look at the radar reflections from the bottom of the ice cap on Mars builds on my recent Antarctic research into electrical conductivity of sub-ice materials there. It is a great example of how understanding of polar regions on Earth informs our interpretations of cold environments on other planetary bodies,” said Professor Slawek Tulaczyk.
A radar reflection can be bright due to a large contrast in either dielectric permittivity (how a material responds to an electric field) or electric conductivity (the amount of electrical current a material can carry). While previous studies only considered contrasts in dielectric permittivity, Bierson and his team found that contrasts in electric conductivity between materials could also explain the brightness of the reflection.
Using Earth as an example, under the large ice sheets of Greenland and Antarctica, many materials have high electric conductivity, including very salty water (brines), salty ice deposits, and clays.
“We wanted to check if this same wide range of materials might be able to explain the bright radar reflection under Mars' south polar cap,” said planetary scientist Carver Bierson, with ASU’s School of Earth and Space Exploration.
Bierson led the study and co-authored the paper with Tulaczyk, graduate student Sam Courville of ASU, who conducted orbital radar modeling; and Mars radar measurements expert Nathaniel Putzig of the Planetary Science Institute.
Together, they were able to determine what level of electric conductivity the material below the ice would need to have to match the observed signal from MARSIS. Then, they identified materials that are both conductive and present on Mars including clays, metal-bearing minerals, and saline ice.
“Salty ice or conductive minerals at the base of the ice sheet are less flashy, but are more in line with the extremely cold temperatures at Mars poles,” said Bierson.
While not explicitly excluding a liquid brine, the results open new potential explanations for the observed strong radar reflections, some of which do not require liquid brine beneath the Martian south polar ice cap.
“Our results are a reminder that there is often more than one way to explain an observation,” said Bierson.
