In the 17th century, astronomers Giovanni Domenica Cassini and Christian Huygens observed the presence of white ice caps while studying the Martian polar regions. These findings confirmed that Mars had ice caps in both polar regions, similar to Earth. In the 18th century, astronomers began to notice how the size of these poles varied depending on where Mars was in its orbital cycle. In addition to discovering that Mars’ axis was tilted like Earth’s, astronomers realized that Mars’ polar ice caps underwent seasonal changes, much like those on Earth.
While scientists know that Mars' polar caps change with the seasons, it was only in the past 50 years that they realized that they are largely composed of frozen carbon dioxide (also known as “dry ice”) that flows in and out of the atmosphere, and questions remain about how this happens. recent studya team of researchers led by the Institute of Planetary Sciences (PSI) synthesized decades of research with more recent observations of the poles. From this, they determined how the Martian poles differ in terms of their seasonal accumulation and release of atmospheric carbon dioxide.
The team was led by Dr. Candice Hansena senior scientist at the Institute of Planetary Sciences (PSI) and a member of the HiRISE imaging team. She was joined by researchers from the Lunar and Planetary Laboratory (LPL) at the University of Arizona, the University of Nevada, the U.S. Geological Survey Astrogeological Sciences Center (USG-ASC), the Laboratory of Atmospheric and Space Physics At UC Boulder, IUCLA, the Queen's University Belfast Astrophysics Research Centrehe German Aerospace Center (DLR) and NASA's Jet Propulsion Laboratory. The paper detailing their findings recently appeared in the journal Icarus.
For their study, Hansen and his colleagues relied on data acquired by Mars orbiters over the past few decades. They then compared them with more recent data from the High-resolution imaging experiment (HiRISE) instrument in the Mars Reconnaissance Orbiter (MRO). This allowed them to follow the growth and recession of Martian ice caps, which recycle about a quarter of the planet's atmosphere over the course of a Martian year. The ultimate goal was to learn more about the processes that shape the planet's surface and environment at large. As Hansen summarized in a PSI article Press release:
“Everyone knows there’s a difference in the way carbon dioxide interacts with the poles, but how many people understand why? That’s what I set out to describe. And fortunately, I have a bunch of really talented co-authors who were willing to fill in their own bits.”
Like Earth, Mars experiences seasonal changes due to its axial tilt — about 25 degrees relative to the orbital plane, compared to Earth's tilt of about 23.5 degrees. But because Mars has a much longer orbital period (about 687 days), the seasons last about twice as long as they do here on Earth. In addition, Mars has a higher orbital eccentricity (about 9% compared to 1.7%), meaning its orbit is more elliptical. Because of this, Mars is farther from the Sun when its northern hemisphere experiences spring and summer, while the southern hemisphere experiences fall and winter.
This means that summer in the Southern Hemisphere is shorter (while winter is longer in the North), coinciding with the dust storm season. As a result, the North Polar seasonal cap contains a higher concentration of dust than the South Polar cap. “So ultimately, southern fall and winter bring the most freezing and the lowest atmospheric pressure, as much of the atmosphere is frozen as dry ice,” Hansen said. “These are the main drivers of the differences in the seasonal behavior of carbon dioxide between the hemispheres. They are not symmetrical seasons.”
There are also significant differences in terms of elevation between the northern and southern hemispheres, namely the northern lowlands and the southern highlands. Differences between the northern and southern polar terrain also influence seasonal change. For example, black dust fans are distributed across the southern landscape, resulting from dry ice sublimation and the formation of dust plumes. As Hansen explained:
“In the fall in the Southern Hemisphere, a layer of carbon dioxide ice forms, which thickens and becomes translucent over the winter. Then, in the spring, the sun comes out and light penetrates this layer of ice all the way down to warm the ground beneath. Now, the gas is trapped under pressure. It will seek out any weak spot in the ice and break like a champagne cork.”
Once the gas finds a weak spot and breaks through the ice, it sends dark plumes of dust into the atmosphere. The dust drifts in different directions depending on the wind direction and falls into fan-shaped deposits. This process shapes the landscape by creating channels, colloquially called “spiders” (araneiformes) because of their arachnid-like appearance. While the Northern Hemisphere also experiences dust plumes in spring, the relatively flat terrain causes them to form dune-like features. Hansen said:
“When the sun comes out and starts to sublimate the underside of the ice sheet, there are three weak points: one at the crest of the dune, one at the bottom of the dune where it meets the surface, and then the ice itself can crack along the slope. No spidery terrain has been detected in the north because, although shallow grooves develop, the wind smooths the sand in the dunes.”
These findings demonstrate that Mars is an active place, not just over eons, but also on a seasonal and even daily basis.
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