The equatorial region of Mars is home to the solar system’s tallest volcanoes, which — in addition to standing as tall as three Mount Everests in some cases — likely hide an unexpected frosty phenomenon, a new study has found.
The biggest one — Olympus Mons — is 16 miles (26 kilometers) high and a whopping 374 miles (602 kilometers) in diameter, making it about 100 times larger than Earth’s biggest volcano, Mauna Loa, in Hawaii. In fact, the entire Hawaiian islands chain would fit inside the Martian volcano, according to NASA.
These giants are topped by large calderas — bowl-shaped depressions caused by the collapse of the top of the volcano after an intense eruption.
The sheer size of the calderas — up to 75 miles (121 kilometers) across — creates a special microclimate inside them. Using cameras fitted on probes orbiting Mars, researchers have observed morning frost forming inside the calderas for the first time.
“The deposits are forming on the caldera floor, but we also see a bit of frost on its rim. We also confirmed that it’s ice and likely water,” said Adomas Valantinas, a postdoctoral researcher at Brown University who made the discovery as a doctoral student at the University of Bern, Switzerland, and the lead author of the study.
“It’s significant because it shows us that Mars is a dynamic planet, but also that water can be found almost everywhere on the Martian surface.”
5,000 images
The team of more than two dozen researchers spotted frost in four volcanoes: Arsia Mons, Ascraeus Mons and Ceraunius Tholus, as well as Olympus Mons, according to the study published Monday in the journal Nature Geoscience.
The deposits are extremely thin — just one-hundredth of a millimeter thick, or one-sixth of a human hair, according to Valantinas — but they are spread over such a large surface area that they amount to a lot of water. “Based on rough estimates, it’s about 150,000 metric tons of water ice, the equivalent of 60 Olympic swimming pools,” he said.
To observe the deposits, the team first looked at about 5,000 images taken by CaSSIS — the University of Bern’s Colour and Stereo Surface Imaging System — a high-definition camera that has been photographing Mars since 2018. It’s among the instruments aboard the ExoMars Trace Gas Orbiter, a spacecraft launched in 2016 as a collaboration between the European Space Agency and the Russian space agency Roscosmos.
“This is also the first discovery coming from CaSSIS, which is quite exciting,” Valantinas said.
The team validated its observations with two other instruments: NOMAD, a spectrometer also on board the Trace Gas Orbiter, and HRSC, or high resolution stereo camera, an older camera onboard the ESA Mars Express orbiter, a spacecraft launched in 2003.
Serendipitous find
Valantinas says that the discovery came with a degree of serendipity, because he was originally looking for carbon dioxide frost but didn’t find any. The deposits were not spotted until now because they only form during the early morning and in the colder months, making the observation window narrow.
However, it’s unlikely that the frost could one day be harvested by human astronauts on Mars. “It would be quite difficult, because although it’s a large deposit it’s also very thin and ephemeral, meaning it’s only there during the night and early morning, then it sublimates back into the atmosphere,” Valantinas said.
The volcanoes are near the Mars equator, the warmest area of the planet, which makes a water discovery particularly intriguing, Valantinas said.
“Mars is a desert planet, but there’s water ice in the polar caps, and there’s water ice in the midlatitudes. Now we also have water frost in the equatorial regions, and equatorial regions are quite dry in general. So this was quite unexpected,” he said.
He added that in the past, when Mars had a thicker atmosphere and a different climate, there might have been glaciers on these volcanoes. The team now wants to expand the search for frost to all of the more than a dozen named volcanoes on Mars.
A notable achievement
If humans are ever to explore the red planet, we’ll need to know where the water is, so the Martian water cycle is an important field of study, said John Bridges, a professor of planetary sciences at the University of Leicester in the United Kingdom, who was not involved with the study.
“This paper is a fantastic use of the CaSSIS camera on the ExoMars Trace Gas Orbiter, which gives both visible color and infrared light reflected from the martian surface,” Bridges said, calling the results a “notable achievement.”
In addition, the water cycle on Mars is nowhere near as active as it was billions of years ago, so it’s challenging to measure how water moves around the surface, noted J. Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology. Perron also was not affiliated with the new research.
“If the frost on these volcanoes is confirmed to be water (and not carbon dioxide), it would be surprising,” he said.
Everywhere on the surface of Mars is cold and dry, Perron added, but the area around the equator is drier and less cold than the poles, so it’s one of the last places you’d expect to see water frost. It would also raise the question, he concluded, of where the water vapor that forms the frost comes from — out of the volcanoes, even though they’re dormant, or from much farther away, like the polar ice caps.