Geologist’s Martian Snow Theory May Hold Water
Water flowed across Mars in the last 100,000 years and could still be flowing today, a principal investigator for NASA said Wednesday as he announced a solution to the tantalizing mystery of how water could have carved fresh gullies without immediately freezing in temperatures 150 degrees below zero.
Water may exist just a few inches underground, beneath blankets of insulating snow that can last for tens of thousands of years, said Phil Christensen, a geologist at Arizona State University who analyzed a series of detailed photos taken by the Mars Odyssey spacecraft. The snow remnants that Christensen believes persist today could emit enough water to fill a swimming pool in a week, offering a “wonderful abode for life” and a clear target for NASA’s missions in the coming decade to determine whether extraterrestrial life could ever have existed on our neighboring planet, he said.
If you were to land near one of these gullies, “you would be shoveling snow,” Christensen said at a news conference at NASA headquarters in Washington that was nearly canceled by this week’s Earthly snowstorm. “If life ever existed on Mars, I can’t think of a more exciting place to look.”
After decades of disheartening findings suggesting water could exist only miles beneath the freezing surface of the planet, Wednesday’s announcement resurrects hope that Mars is not a barren, lifeless orb, but that it may contain refuges for life near the surface that could be easily accessible to robot explorers.
“The depressing thing was we were talking about drilling a kilometer or two, which is very expensive and difficult,” said Lynn Rothschild, a biologist and expert on extreme life forms at NASA’s Ames Research Center in Mountain View, Calif. “The real key is finding ... liquid water. Even a little bit would be enough.”
On Earth, primitive life forms such as “extremophile” bacteria have been discovered in acid baths, deep ocean trenches and under Antarctic ice. In the Sierra Nevada, fuchsia-colored snow algae can flourish in harsh mountain environments, creating “watermelon snow” in the winter, then going dormant when snow and water are unavailable in summer, Rothschild said.
Previous attempts to detect life on Mars, most notably the Viking missions in the 1970s, found no evidence. Astrobiologists point out, however, that the planet appears to offer the three ingredients required for life: a heat source (the sun and geothermal energy), organic chemicals and water. “You’ve expanded the envelope of places on Mars where life could conceivably be alive even today,” Rothschild said.
When photos of the mysterious gullies were first splashed onto front pages of newspapers in the summer of 2000, they overjoyed scientists who had long sought evidence that water necessary to support current or past life existed on the planet. The implications of the images, taken by the Mars Global Surveyor, were considered so important that NASA briefed the White House before releasing them.
A news leak set off a media frenzy. But ecstatic scientists soon became frustrated. They had no good way to explain how water could have flowed on the parched and frozen world.
Theories on what carved the deep, miles-long gullies abounded: leaking underground aquifers inexplicably perched high within crater rims; pressurized geysers of water that somehow didn’t freeze; high-pressure bursts of carbon-dioxide vapors strong enough to carve rock; invisible volcanic heat sources lurking below the surface.
Scientists knew the gullies were fresh because they were not pockmarked with craters like much of the battered planet. Estimates on their age range from 1 million years -- almost yesterday in geologic time -- to literally yesterday.
Creative as they were, none of the theories really fit. “They were getting pretty exotic and stretching a bit,” said David Senske, a Mars geologist at the Jet Propulsion Laboratory in Pasadena. Most perplexing of all, the gullies were on the coldest, shadiest sides of craters, not the warmer sides where melting water might be expected. Any water would freeze instantly, if it did not evaporate into the thin atmosphere.
Christensen’s new snow theory explains why the features are on the colder side: that’s where the most snow would accumulate. His findings are being published today in the journal Nature.
Like many Mars scientists, he believes snow has come and gone repeatedly on the planet, at times blanketing the mid-latitudes where the gullies have been found and at other times disappearing altogether, leaving only icecaps at the frozen poles. The mid-latitude regions of Mars are thought to alternate between very, very cold and just plain cold on a 100,000-year cycle that is related to how the planet tilts toward the sun as it orbits.
Models show that Martian ground is probably frozen solid for more than half a mile down because of the low temperatures. But when snow falls in the mid-latitudes, it acts as insulation or a “miniature greenhouse” and keeps the ground warm enough a few inches below the surface to allow water to exist, Christensen said. This meltwater then carves out the gullies as it runs downslope, eroding rock or pushing aside loose material, he added.
Christensen had gazed at the close-up images of the gullies but got nowhere in determining their origin. It wasn’t until he looked at larger-scale, lower-resolution images taken by the THEMIS camera aboard Odyssey that he could see what appeared to be snow patches “pasted on” to the steep slopes of craters and adjacent gullies.
“I saw it and said, ‘Ah-ha!’ It looks for all the world like these gullies are being exposed as this terrain is being removed through melting and evaporation,” he said.
The snow patches are difficult to make out because they are covered with the planet’s notorious red dust and dirt. “It’s dirty snow with this layer of dust,” Christensen said. The dust and dirt also serve as insulation and can slow or stop the snow from melting, he added. The snow could be 30 feet thick in some places, he speculated.
The snow, created from moisture released by the polar icecaps, likely fell sometime in the last 100,000 years, said Bruce Jakosky, a planetary scientist at the University of Colorado at Boulder and an expert on Mars water cycles. The snowfall cycle could repeat in the future.
Theories on how much water exists on Mars have become more conservative as data flow back from the planet. Ideas from a century ago that canals and civilizations crisscrossed the planets are now laughable. The theory that Mars was once wet and warm and may have supported huge oceans has also started to recede. Geologists are still trying to explain vast canyons and lake beds that appear to have been created by water. But those features existed in the distant geological past, perhaps billions of years ago.
