In 1666, famed Italian astronomer and mathematician Giovanni Cassini (the man who discovered four of Saturn’s largest moons) observed the Martian polar ice caps for the first time. However, it was not until the late-18th century, when Sir William Herschel recorded his own observations, that the connection to Earth’s own ice caps was established. In his subsequent treatise, “On the remarkable appearances at the polar regions on the planet Mars” (1784), noted how the southern cap grew and shrunk due to seasonal changes.

With the development of modern telescopes and robotic explorers, scientists have learned a great deal more about these polar deposits. In 2011, they learned that unlike the northermost ice sheet, the southern cap is largely composed of frozen carbon dioxide (aka. “dry ice”). According to new research led by the Planetary Science Institute (PSI), glaciers of carbon dioxide ice have been moving and carving features in the southern polar region for more than 600,000 years – and are on the move right now!

The research team was led by Isaac Smith, a former PSI research scientist and an assistant professor of Earth and Space Science at York University in Toronto (where he also holds a Canada Research Chair in Planetary Science). He was joined by geologists, glaciologists, and engineers from the PSI, York University, the Institute of Low-Temperature Science and Arctic Research Center at Hokkaido University in Japan, and the NASA Jet Propulsion Laboratory (JPL).

Ice at Mars’ south pole. Credit: ESA/DLR/FU Berlin/Bill Dunford

The presence of carbon dioxide glaciers in Mars’ southern polar region was first confirmed in 2011 by the Mars Reconnaissance Orbiter (MRO). Along with data obtained by the ESA’s Mars Express, scientists noted that they were flowing like glaciers of water ice do here on Earth, based on the surface features they carved in their wake. As Smith described in a recent PSI press release:

“Approximately 600,000 years ago, CO2 ice started forming at the Martian south pole. Due to climate cycles, the ice has increased in volume and mass several times, interrupted by periods of mass loss through sublimation. If the ice had never flowed, then it would mostly be where it was originally deposited, and the thickest ice would only be about 45 meters thick. Instead, because it flowed downhill into basins and spiral troughs – curvilinear basins – where it ponded, it was able to form deposits reaching one kilometer thick.”

These observations also showed that the portion of the ice cap composed of water ice appears to be stationary, remaining at high altitudes. In previous research, which Smith helped contribute to while still with the PSI, scientists investigated the strength properties (aka. flow laws) of the carbon dioxide glaciers to determine why this was happening. Their results indicated that under the types of conditions that exist around the southern polar region, carbon dioxide ice flows were almost 100 times faster than water ice glaciers.

For this, they concluded that the CO2 ice behaves like glaciers here on Earth, which makes the slow-moving water ice cap appear stationary. “The glaciers have enough mass that if sublimated, they would double the atmospheric pressure of the planet,” Smith added, citing a 2018 paper by Than Putzig, a PSI Senior Scientist and co-author on this paper. “The longest glacier is about 200 kilometers long and about 40 kilometers across. These are big!”

The South Pole on Mars. Image: NASA.
Artist’s impression of the ice cap around the South Pole on Mars. Credit: NASA.

For this study, Smith and his colleagues