Highlands of Early Mars

Ice Sheets Covered Southern

The southern highlands of Mars are dissected by hundreds of ancient valley networks (3.9-3.5 billion years old), which are evidence that water once sculpted the Martian surface. According to new research, these valley networks were carved by water melting beneath glacial ice, not by free-flowing rivers as previously thought.

A view of Mars showing the planet’s northern polar ice cap. Image credit: ISRO / ISSDC / Emily Lakdawalla.

“For the last four decades, since Martian valleys were first discovered, the assumption was that rivers once flowed on Mars, eroding and originating all of these valleys,” said lead author Dr. Anna Grau Galofre, a researcher in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia and the School of Earth and Space Exploration at Arizona State University.

“But there are hundreds of valleys on Mars, and they look very different from each other.”

“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 similarity between many Martian valleys and the subglacial channels on Devon Island in the Canadian Arctic motivated Dr. Grau Galofre and colleagues to conduct this new study.

“Devon Island is one of the best analogs we have for Mars here on Earth — it is a cold, dry, polar desert, and the glaciation is largely cold-based,” said co-author Professor Gordon Osinski, a researcher in the Department of Earth Sciences and the Institute for Earth and Space Exploration at the University of Western Ontario.

A glacial valley network on Mars. Image credit: Grau Galofre et al, doi: 10.1038/s41561-020-0618-x.

In the study, the scientists analyzed 10,276 Martian valley segments, using a novel algorithm to infer their underlying erosion processes.

They used data from the Mars Orbiter Laser Altimeter (MOLA) instrument on NASA’s Mars Global Surveyor spacecraft and the High Resolution Stereo Camera (HRSC) on ESA’s Mars Express orbiter.

“These results are the first evidence for extensive subglacial erosion driven by channelized meltwater drainage beneath an ancient ice sheet on Mars,” said co-author Professor Mark Jellinek, a researcher in the Department of Earth Sciences at the University of Western Ontario.

“The findings demonstrate that only a fraction of valley networks match patterns typical of surface water erosion, which is in marked contrast to the conventional view.”

“Using the geomorphology of the Martian surface to rigorously reconstruct the character and evolution of the planet in a statistically meaningful way is, frankly, revolutionary.”

Distribution of Martian valley networks (purple) and those analyzed (white) by Grau Galofre et al. Image credit: Grau Galofre et al, doi: 10.1038/s41561-020-0618-x.

The team’s theory also helps explain how the Martian valleys would have formed 3.8 billion years ago on a planet that is further away from the Sun than Earth, during a time when the Sun was less intense.

“Climate modeling predicts that Mars’ ancient climate was much cooler during the time of valley network formation,” Dr. Grau Galofre said.

“We tried to put everything together and bring up a hypothesis that hadn’t really been considered: that channels and valleys networks can form under ice sheets, as part of the drainage system that forms naturally under an ice sheet when there’s water accumulated at the base.”

These environments would also support better survival conditions for possible ancient life on Mars.

A sheet of ice would lend more protection and stability of underlying water, as well as providing shelter from solar radiation in the absence of a magnetic field — something Mars once had, but which disappeared billions of years ago.

The research was published online today in the journal Nature Geoscience.

_____

A. Grau Galofre et al. Valley formation on early Mars by subglacial and fluvial erosion. Nat. Geosci, published online August 3, 2020; doi: 10.1038/s41561-020-0618-x