Mars was likely a cold and glaciated planet early in its history, a new Canadian-led study suggests, which has implications for the ongoing search for life on the Red Planet.
A new Nature Geoscience paper shows that many of the Martian valley networks were caused by liquid water formed under melting glacial ice – not by rivers flowing under the open air, which is a hypothesis many scientists have supported in recent decades.
The study was rigorous, analyzing 10,000 Martian valleys using a novel algorithm to determine their erosion processes. Notably, valley formation on Mars principally involved subglacial and fluvial erosion, but other mechanisms were also implicated such as groundwater sapping erosion, the study noted.
This work was supplemented by observations in regions such as Canada’s Devon Island, where valley networks were also formed due to glaciation, along with models of steady-state erosion. The team worked hard to go beyond qualitative observations, said co-author Gordon Osinski, a planetary geologist at Western University who studies cratering and astrobiology, among other interests.
Anna Grau Galofre, a former Ph.D. student at the University of British Columbia, led the quantitative research, and also participated in three Western-led analogue space missions in the Canadian Arctic to gather data from subglacially formed valley networks.
“I do a lot of analog work, and it’s really easy to fall into the trap of looking at an image from Mars and an image of Earth,” Osinski said, and then coming to the conclusion that just because you see similar features on both planets, they must arise from similar processes. Rather than relying just on observations, though, the research team used six metrics to quantify what they saw through a principal-component-based analysis approach, along with the observations and physical models.
The valley network shape produced by glaciation appears different than the valley network shape produced by river flow, noted Grau Galofre in a statement from Western. (For example, glaciated valley networks have finer features than river valley networks, as there is typically reduced water flow.)
“There are hundreds of valleys on Mars, and they look very different from each other,” Grau Galofre said. “If you look at Earth from a satellite you see a lot of valleys: some of them made by rivers, some made by glaciers, some made by other processes, and each type has a distinctive shape. Mars is similar, in that valleys look very different from each other, suggesting that many processes were at play to carve them.”
The icy explanation put forth in the peer-reviewed paper contradicts the favored explanation in recent decades for flowing water on Mars roughly 4 billion years ago. The scientific consensus favoured by NASA and others say the planet likely had a thick atmosphere that allowed for a warmer surface and for liquid water to flow freely, although not all scientists agree with that conclusion.
Today, Mars has a thin carbon dioxide atmosphere that forces all surface water into the ice state, although there may be exceptions for briny liquid that allow water to flow above the freezing point. Liquid water deposits are suspected underground, however, including a 2016 find of a subsurface lake believed to have the capacity of Lake Superior.
Yet habitability searches on Mars have been ongoing for decades, as scientists look for signs of water from orbit and also from numerous landing missions. The most famous water hunters are the NASA rovers operating continuously, across several missions, since 2004 – which doesn’t include a brief mission by the NASA Sojourner rover that operated for three Earth months in 1997. Taken as a whole, the missions have found rock chemistry altered by water, as well as extensive evidence of liquid water flow.
Water not only tells us about the geological history of a planet, but also can act as a pointer to habitability. Indeed, NASA’s Perseverance rover lifted off days ago, on July 30, on a quest to search for signs of microbial activity in rocks. The rover is expected to land on Feb. 18, 2021 and do a systematic investigation of its landing site at Jezero Crater.
With the assistance of a science team including the University of Alberta’s Chris Herd, Perseverance will cache some of the rocks it finds for a future “sample-return” mission that will bring the rocks to Earth, for analysis in laboratories. These facilities have far more instrumentation available than a Martian rover can carry, allowing for more rigorous investigation of samples than would be possible in situ.
Osinski added that Martian life is not necessarily impossible under glaciated conditions, as water still existed in liquid form in the valley networks. “It probably doesn’t mean there was no liquid water, but that it was beneath ice sheets,” he said.
Indeed, an ice sheet may actually be a boon for life, given that Mars has no global magnetic field to protect the surface against potentially deadly radiation, the study notes. Radiation would have more difficulty penetrating a glacier or ice sheet. Life could thus exist quite happily under the ice in liquid water, somewhat safe from the radiation baking the Martian surface.
For those in the “Mars was warm” camp, scientists are investigating a long-standing hypothesis about how the Martian climate changed; the theory suggests the atmosphere used to be a lot thicker, and then thinned over the eons. It is thought that activity from the sun pushed out lighter molecules from the atmosphere. This phenomenon is coming under scrutiny from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which arrived in planetary orbit in 2014 and is still operating.
In 2017, MAVEN determined that most of the Martian atmosphere was indeed lost to space by measuring the abundance of two different isotopes of argon gas, in the atmosphere. Argon was used as a proxy to measure how much atmosphere was lost overall over time, and through models and observation, the study showed that the lighter isotope of argon is underrepresented in the atmosphere now, as opposed to when the Martian atmosphere was formed. The team used measurements from MAVEN’s neutral gas and ion spectrometer and supplemented them with surface measurements taken from the Curiosity rover’s Sample Analysis at Mars instrument.
The result from MAVEN means that there still is a chance that atmospheric thickness played into the history of water on Mars. Osinski also acknowledged that glaciation may not be responsible for all running water in the Martian past, as clays – which are abundant on the Red Planet – are not typically known to form in glaciated water. He said that the team plans to conduct more study and analogue investigations to learn more about the planet’s water history.
There’s another positive for the new study, in that it could explain how water flowed on Mars even when the sun was believed to be only 70 percent to 75 percent as intense during the early part of the solar system, when water flowed on the Red Planet’s surface. The “faint young sun paradox” has puzzled astrophysicists for decades because it was difficult to imagine how the surface of Mars could be warm under such conditions 3.8 billion years ago, even with a thicker atmosphere, Osinski said. The new study may resolve the paradox by taking the sun’s activity out of the water question completely, although more work is required to confirm or deny this.