This image's bright white area shows the icy cover that surrounds Mars south pole. It is made of frozen water, frozen carbon dioxide and frozen water. Credit: Bill Dunford/ESA/DLR/FU Berlin/Bill DunfordTwo teams of researchers have published recent results using data from the European Space Agency's Mars Express orbiter. They suggest that subsurface lakes thought to exist on Mars might not be lakes.Scientists using data from Mars Express orbiter made a shocking discovery in 2018. They discovered that signals from a radar instrument reflecting off the red planet's south pole revealed a liquid subsurface lake. Since then, several more reflections of this nature have been reported.Aditya Khuller, a graduate student at Arizona State University's School of Earth and Space Exploration, and Jeffrey Plaut of NASA’s Jet Propulsion Laboratory (JPL) are the lead authors of a paper that was published in the American Geophysical Union’s Geophysical Research Letters. They describe the discovery of dozens of radar reflections near the south pole. This was after studying a wider range of Mars Express data. Many are located in areas where water should not be liquid.A team of scientists, led by Carver Bierson, a postdoctoral scholar at ASU School of Earth and Space Exploration is looking into the question of whether these signals are liquid water. Their research was published in AGU Geophysical Research Letters. It revealed that bright reflections could be caused by subsurface clays or metal-bearing mineral minerals, as well as saline water.Mars Express is NASA's second-longest-surviving continuously active spacecraft orbiting a planet other that Earth. It trails NASA's 2001 Mars Odyssey, which is still in operation. Mars Express continues to orbit Mars and provide valuable data about the surface, atmosphere, and subsurface of Mars.An instrument known as the Mars Advanced Radar For Subsurface and Ionosphere Sounding (or MARSIS) is aboard this spacecraft. This instrument uses radar sounders to determine the composition of Mars' subsurface.Since 2004, MARSIS has been collecting data about Mars, including the south pole. This allows scientists to create a three-dimensional view. Khuller stated, "We wanted to see beneath the south polar ice" and characterize the terrain underneath with MARSIS data.Researchers have also found that reflections beneath the surface are brighter in other recent studies using MARSIS data. This is contrary to what scientists would expect.Khuller is currently completing an internship at JPL, under the direction of Plaut. Although there are many possible causes for bright subsurface reflections (which can be caused by a variety of factors), these studies found that the bright reflections were caused by a liquid water component. This is because radar sees liquid water as bright.The colored dots indicate sites where ESA's Mars Express orbiter spotted bright radar reflections at Mars southpolar cap. Credit: ESA/NASA/JPL - CaltechFrozen time capsuleThese radar signals were originally thought to be liquid water. They were discovered in the South Polar Layered Deposits region on Mars. This is named after the layers of dry ice (frozen CO2) and dust that have accumulated over many millions of years. These layers may contain a record of Mars' tilt over time. This is similar to how Earth's tilt has created ice ages or warmer periods in our planet's past. The thick, layered ice sheet that is found today was formed when Mars had a lower angle of tilt.These areas were originally thought to be filled with liquid water and are located in a small area of the Martian South Polar Layered deposits. Khuller and Plut expanded the search for strong radio signals to 44,000 measurements, spread over 15 years of MARSIS data covering the entire Martian south polar area.Unexpected 'lakes’: A muddy image?Khuller's new study with Plaut showed that there were many more bright radar reflections in a wider area and at a deeper depth than ever before. Some of them were located less than one mile from the surface. Temperatures are estimated to be below 81°F (minus 63° Celsius). Water would freeze if it contains salty minerals called perchlorates which can lower the freezing temperature."We don't know if these signals are liquid or solid water, but they seem to be more widespread than what our original paper found," stated co-author Plaut who is also co-principal investigator for the orbiter's MARSIS instrument. These signals could indicate that liquid water is either common below Mars' south pole or something else.Khuller also noted a 2019 paper where researchers calculated heat required to melt subsurface glaciers in the region. They found that there was no recent volcanism beneath the surface, which could explain the possibility of liquid water below the south pole.Khuller stated that they found it would require twice the Martian geothermal heat flow in order to keep this water liquid. Volcanism is one possible method to obtain this heat. We haven't seen any evidence of recent volcanism at South Pole, so it is unlikely that subsurface liquid water would be available in this region.Khuller, Plaut and their next steps are to examine the discovery of a second layer beneath Mars' south pole. Scientists believe this is an older buried terrain known as the Dorsa Argentea Formation. They believe it was modified by ancient glaciers that once existed in the area. They plan to try to determine its age and composition.Continue exploring Explore more Study Mars' underground water signalsMore information: Aditya Khuller and colleagues, Characteristics the Basal Interface the Martian South Polar Layered deposits, Geophysical Research Letters (2021). Aditya R.Khuller and colleagues, Characteristics the Basal Interface the Martian South Polar Layered Deposits. (2021). DOI: 10.1029/2021GL093631 C. J. Bierson et. Al, Strong MARSIS Radar Reflections From the Base of Martian South Polar Cap Could Be Due to Conductive Ice or Minerals. Geophysical Research Letters (2021). DOI: 10.1029/2021GL093880 Michael M. Sori and co, Water on Mars with a Grain Salt: Local Heat Anomalies are Required for Basal Melting Ice at the South Pole Today. Geophysical Research Letters (2019). Geophysical Research Letters Journal Information: DOI: 10.1029/2018GL080985
