A new article finds more radar signals suggesting the presence of underground “lakes”, but many are in areas too cold for the water to remain liquid.
In 2018, scientists working with data from ESA’s (European Space Agency) Mars Express orbiter announced a startling discovery: signals from a radar instrument reflected from the south pole of the red planet appeared to reveal a lake underground liquid. Several other such reflections have been announced since then.
In a new article published in the journal Geophysical research letters, two scientists at NASA’s Jet Propulsion Laboratory in Southern California describe finding dozens of similar radar reflections around the South Pole after analyzing a larger set of Mars Express data, but many are in areas that should be too cold so that the water remains liquid.
“We don’t know if these signals are liquid water or not, but they appear to be much more prevalent than what the original article found,” said Jeffrey Plaut of JPL, Orbiter’s co-principal investigator. MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding), which was jointly built by the Italian Space Agency and JPL. “Either liquid water is flowing under the south pole of Mars, or these signals point to something else. “
Frozen time capsule
Radar signals originally interpreted as liquid water were found in an area of Mars known as the South Pole layered deposits, named for the alternating layers of water ice, dry ice ( frozen carbon) and dust that has been deposited there for millions of years. These layers are believed to provide a record of how the tilt of the axis of Mars has changed over time, just as changes in the tilt of the Earth have created ice ages and warmer periods while throughout the history of our planet. When Mars had a lower axial tilt, snowfall and dust layers accumulated in the area and eventually formed the thick layer of ice that is there today.
By broadcasting radio waves to the surface, scientists can observe beneath these icy layers and map them in detail. Radio waves lose energy as they pass through materials in the subsoil; when reflected back to the spacecraft, they usually have a weaker signal. But in some cases the signals returning from the subsoil of this region were brighter than those from the surface. Some scientists have interpreted these signals to imply the presence of liquid water, which strongly reflects radio waves.
Plaut and Aditya Khuller, a doctoral student at Arizona State University who worked on the paper during his internship at JPL, are not sure what the signals point to. Areas believed to contain liquid water extend for about 10 to 20 kilometers (6 to 12 miles) in a relatively small region of the Martian South Pole. Khuller and Plaut extended the search for similar powerful radio signals to 44,000 measurements spread over 15 years of MARSIS data over the entire Martian south polar region.
Analysis revealed dozens of additional light radar reflections over a much wider surface and depth range than ever before. In some places, they were less than a mile from the surface, where temperatures are estimated to be minus 81 degrees Fahrenheit (minus 63 degrees Celsius) – so cold that the water would be frozen, even though it contained salty minerals called perchlorates, which can lower the freezing point of water.
Khüller rated an article from 2019 in which the researchers calculated the heat required to melt underground ice in this region, finding that only recent volcanism below the surface could explain the potential presence of liquid water below the South Pole.
“They found that it would take double the estimated Martian geothermal heat flow to keep this water liquid,” Khuller said. “One of the possible ways to get this amount of heat is through volcanism. However, we haven’t really seen strong evidence of recent volcanism at the South Pole, so it seems unlikely that volcanic activity will allow underground liquid water to be present throughout this region. “
What explains the brilliant reflections if it is not liquid water? The authors cannot say for sure. But their article offers scientists a detailed map of the region that contains clues to the climatic history of Mars, including the role of water in its various forms.
“Our mapping brings us a few steps closer to understanding both the extent and the cause of these confusing radar reflections,” Plaut said.
Reference: “Characteristics of the Basal Interface of the Martian South Polar Layered Deposits” by Aditya R. Khuller and Jeffrey J. Plaut, June 16, 2021, Geophysical research letters.
DOI: 10.1029 / 2021GL093631