Scientists have discovered substantial deposits of water ice buried in shallow soils near Mars’s equator. The find could spur hopes for astrobiologists seeking life on Mars or future colonists seeking a supply of water, but it also raises a mystery for climate scientists.
The findings come from a reanalysis of data from NASA’s Odyssey spacecraft, which began orbiting Mars in 2001 and is the oldest functioning mission at the planet. One of Odyssey’s instruments measures the neutrons kicked up from the martian surface by cosmic rays striking the planet. From these neutron counts, scientists can infer the amount of hydrogen—and thus, presumably, the amount of water—present in the uppermost meter or so of soil. In small amounts, the water can take many forms—either in hydrated minerals or as small ice particles stuck between particles of sand or silt. But when the inferred abundances rise above 26%—as they do in some regions—scientists are pretty sure that bulk ice sits just below the surface, says Jack Wilson, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory in Laurel, Maryland.
That Mars holds stores of ice is not news. At the poles, there are permanent caps made up of frozen water and carbon dioxide. And as far back as 2002, researchers using Odyssey’s neutron instrument reported evidence of water in the subsurface at high latitudes. The evidence was strong enough that NASA sent the Phoenix lander to explore these regions. When the lander’s robotic arm scratched at the hardscrabble soil in 2008, it found ice just below.
Now, Wilson and his colleagues have come up with a way to map the previously collected Odyssey data at a resolution of 290 kilometers, almost twice as good as earlier maps. They looked for smaller, more concentrated patches of water ice that earlier analyses might have missed—and found many such patches at far lower latitudes than expected, the researchers report online this month in Icarus.
“This is a really wonderful example of how data, once collected, can be analyzed with new techniques,” says Jim Head, a planetary geologist at Brown University. “When we eventually send people to Mars, we’ll want to go where the water is.”
But the ice patches also present a puzzle. According to current models of Mars’s climate, equatorial ice can’t persist for more than 125,000 years or so, Wilson says. That’s because it would gradually sublimate into the atmosphere, even if buried beneath a shallow layer of insulating soil. If ice truly exists there, it could be evidence of a shift in Mars’s rotational axis within that time window, Wilson explains. Unlike Earth, Mars doesn’t have a large moon to help stifle the long-term wobble of its orbital axis. And if the planet’s axis were tilted more than its current 25°, some polar cap ice would have sublimated and moved to lower latitudes. Wilson acknowledges that the explanation is unlikely, however, given that Mars’s rotational axis shouldn’t wobble on such fast time scales. Another possibility, he says, is that the soil also provides a vapor barrier to help stifle sublimation as well as physical insulation.
Regardless of how the equatorial ice got there, if it finds a way to the surface and occasionally melts, it could provide a welcoming habitat for microbes. That’s why Mars scientists are so excited about so-called recurring slope lineae, dark streaks on steep slopes that are thought to be potential seeps of water, either from melting ice or from underground aquifers.
But the same habitats that could support martian microbes could also support earthly ones that survived spacecraft sterilization and hitchhiked to Mars. That’s why NASA has strict rules for sterilizing its spacecraft under its planetary protection office. “You don’t want to take anything to an area where it could live or be preserved,” says John Rummel, a biologist who once served as NASA’s planetary protection officer but is now at the SETI Institute in Mountain View, California.
Despite new evidence for near-surface ice in unexpected places, Rummel suggests that none of the NASA-designated “special regions” that require extra spacecraft sterilization protocols need to be expanded. That’s because, he contends, Mars’s current climate isn’t hospitable to Earth organisms. He says he’d be concerned only if equipment provided a stationary, long-term source of heat that could melt the subsurface ice and create a “warm pool” that could host life.