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

The new findings, published in the journal 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, according to the researchers. Studying the geometry and composition of these layers could tell scientists whether climate conditions were previously favorable for life.

The team found layers of sand and ice that were as much as 90 percent water in some places. If melted, the newly discovered ice would be equivalent to a global layer of water around Mars at least 1.5 meters (5 feet) deep, which could be one of the largest water reservoirs on the planet, according to the researchers.

Observations revealed horizontal slabs rich in ice that were sandwiched by alternating layers of sand. The distribution of water ice ranged from 61 to 88 percent by volume, which means the cavi unit is primarily composed of frozen water. As noted in the press release, if melted and brought to the surface, "the newly discovered polar ice would be equivalent to a global layer of water around Mars at least 1.5 meters (5 feet) deep." That’s a lot of water.

This composite image shows alternating layers of ice and sand in an area where they’re exposed on the surface of Mars. The water ice appears as light-colored, while the sand is darker blue.

"We didn’t expect to find this much water ice here," said Stefano Nerozzi, a graduate research assistant at the University of Texas Institute for Geophysics (UTIG) and lead author of the new study. "That likely makes it the third largest water reservoir on Mars after the polar ice caps."

The findings were corroborated by an independent study using gravity data instead of radar, led by researchers at Johns Hopkins University and also published today in Geophysical Research Letters, of which Nerozzi is a co-author.

Layers of sand and ice

The scientists suspect 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.

Scientists have long known about glacial events on Mars, which are driven by variations in the planet’s orbit and tilt. 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 thought the ancient ice caps were lost. The new findings show that in fact significant ice sheet remnants have survived under the planet’s surface, trapped in alternating bands of ice and sand, like layers on a cake.

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

"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," he said.

Sudying this record of past polar glaciation could help determine whether Mars was ever habitable, according to Nerozzi.

"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," Nerozzi said. "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."