Massive Mars water discovery gives clues to the Red Planet’s past

Scientists believe that newly discovered layers of ice buried a mile beneath Mars’ north pole are the remnants of ancient polar ice sheets and could be one of the largest water reservoirs on the planet.

A view of Mars showing the planet’s northern polar ice cap. A new study led by The University of Texas at Austin has found remnants of ancient ice caps buried in the north polar region. Credit: ISRO/ISSDC/Emily Lakdawalla

The team from The University of Texas at Austin and the University of Arizona made the discovery using measurements gathered by the Shallow Radar (SHARAD) on NASA’s Mars Reconnaissance Orbiter (MRO). SHARAD emits radar waves that can penetrate up to a mile and a half beneath the surface of Mars.

The findings — published May 22 in Geophysical Research Letters — are important because the layers of ice are a record of past climate on Mars in much the same way that tree rings are a record of past climate on Earth.

Studying the geometry and composition of these layers could tell scientists whether climate conditions were previously favourable for life, researchers say.

The team found layers of sand and ice that were as much as 90% water in some places. If melted, the newly discovered polar ice would be equivalent to a global layer of water around Mars at least 1.5 meters (5 feet) deep.

The lead author, Stefano Nerozzi, is a graduate research assistant at the University of Texas Institute for Geophysics (UTIG) who is completing his PhD at the Jackson School of Geosciences. He explains: “We didn’t expect to find this much water ice here.

“That likely makes it the third largest water reservoir on Mars after the polar ice caps.”

The findings were corroborated in an independent study — also published in Geophysical Research Letters — conducted by researchers at Johns Hopkins University using gravity data instead of radar.

The layers formed when ice accumulated at the poles during past ice ages on Mars, each time the planet warmed, a remnant of the ice caps became covered by sand, which protected the ice from solar radiation and prevented it from dissipating into the atmosphere, according to the study’s authors.

Glacial events on Mars — driven by variations in the planet’s orbit and tilt have been known to scientists for some time. Over periods of about 50,000 years, Mars leans toward the sun before gradually returning to an upright position, like a wobbling spinning top. When the planet spins upright, the equator faces the sun, allowing the polar ice caps to grow. As the planet tilts, the ice caps retreat, perhaps vanishing entirely.

Until now, scientists had thought that the ancient ice caps were lost, but these new papers show that significant ice sheet remnants have survived under the planet's surface — trapped in alternating bands of ice and sand, like layers on a cake.

A composite image showing alternating layers of ice and sand in an area where they are exposed on the surface of Mars. The photograph, taken with the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter, was adjusted to show water ice as light-coloured layers and sand as darker layers of blue. The tiny bright white flecks are thin patches of frost. Credit: NASA/JPL/University of Arizona

Co-author Jack Holt, a professor at the Lunar & Planetary Laboratory of the University of Arizona, said that the study provides new, important insights into the exchange of water ice between the poles and the mid-latitudes, where his research group previously confirmed the presence of widespread glaciers, also using the SHARAD instrument.

Holt, who was a UTIG scientist and research professor for 19 years before joining the University of Arizona in 2018, has been a co-investigator with SHARAD since the spacecraft arrived at Mars in 2006, says: “Surprisingly, the total volume of water locked up in these buried polar deposits is roughly the same as all the water ice known to exist in glaciers and buried ice layers at lower latitudes on Mars, and they are approximately the same age.”

Nerozzi adds that studying this record of past polar glaciation could help determine whether Mars was ever habitable: “Understanding how much water was available globally versus what’s trapped in the poles is important if you’re going to have liquid water on Mars.

“You can have all the right conditions for life, but if most of the water is locked up at the poles, then it becomes difficult to have sufficient amounts of liquid water near the equator.”

Original research: Buried ice and sand caps at the north pole of Mars: revealing a record of climate change in the cavity unit with SHARAD, Nerozzi. S, et al, Geophysical Research Letters (2019). DOI: 10.1029/2019GL082114