NASA’s Mars Odyssey spacecraft has discovered evidence of significant hydration near the Martian equator.

Dr. Jack Wilson of the Johns Hopkins University Applied Physics Laboratory and his colleagues analyzed archival data collected from 2002 to 2009 by Mars Odyssey’s Neutron Spectrometer (MONS).

In bringing the lower-resolution compositional data into sharper focus, the researchers spotted unexpectedly high amounts of hydrogen — which at high latitudes is a sign of buried water ice — around sections of the Martian equator.

By applying image-reconstruction techniques often used to reduce blurring and remove ‘noise’ from spacecraft imaging data, the team improved the spatial resolution of the MONS data from around 320 miles to 180 miles (520 km to 290 km).

“It was as if we’d cut the spacecraft’s orbital altitude in half, and it gave us a much better view of what’s happening on the surface,” Dr. Wilson said.

“MONS can’t directly detect water, but by measuring neutrons, it can help us calculate the abundance of hydrogen — and infer the presence of water or other hydrogen-bearing substances.”

Mars Odyssey’s first major discovery, in 2002, was abundant hydrogen just beneath the surface at high latitudes. In 2008, NASA’s Phoenix Mars Lander confirmed that the hydrogen was in the form of water ice.

But at lower latitudes on Mars, water ice is not thought to be thermodynamically stable at any depth. The traces of excess hydrogen that Odyssey’s original data showed at lower latitudes were initially explained as hydrated minerals, which other spacecraft and instruments have since observed.

Dr. Wilson and co-authors concentrated on those equatorial areas, particularly with a 600-mile (1,000-km) stretch of loose, easily erodible material between the northern lowlands and southern highlands along the Medusae Fossae Formation.

“Radar-sounding scans of the area have suggested the presence of low-density volcanic deposits or water ice below the surface, but if the detected hydrogen were buried ice within the top meter of the surface, there would be more than would fit into pore space in soil,” Dr. Wilson said.

How water ice could be preserved there is a mystery.

A leading theory suggests an ice and dust mixture from the polar areas could be cycled through the atmosphere when Mars’ axial tilt was larger than it is today. But those conditions last occurred hundreds of thousands to millions of years ago.

“Water ice isn’t expected to be stable at any depth in that area today, and any ice deposited there should be long gone,” Dr. Wilson said.

“Additional protection might come from a cover of dust and a hardened ‘duricrust’ that traps the humidity below the surface, but this is unlikely to prevent ice loss over timescales of the axial tilt cycles.”

“Perhaps the signature could be explained in terms of extensive deposits of hydrated salts, but how these hydrated salts came to be in the formation is also difficult to explain,” he added.

“So for now, the signature remains a mystery worthy of further study, and Mars continues to surprise us.”

The team’s findings are published in the journal Icarus.

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Jack T. Wilson et al. 2018. Equatorial locations of water on Mars: Improved resolution maps based on Mars Odyssey Neutron Spectrometer data. Icarus 299: 148-160; doi: 10.1016/j.icarus.2017.07.028