Today's Mars is a cold, arid desert planet. But a long time ago, scientists believe that its surface was covered with lakes and streams, cutting through the rock and dirt and possibly even providing favorable conditions for the development of life. Ancient evidence for this water persists on the surface in the form of dry lake beds and streams dotting the landscape of the Red Planet.
The unsung hero of Mars's more watery age: a thick atmosphere of carbon dioxide, a greenhouse gas that would have warmed the planet in order to allow the liquid to flow across the surface. At least, that was the prevailing theory.
But a new study based on data collected from NASA's Curiosity rover indicates that the truth about Mars's past may be a little more complicated than scientists initially thought. Analysis of rock samples collected by the probe – including some of the same rocks that indicated a past presence of water – turned up no definitive carbonate, a salt derived from carbonic acid that one would expect to find in abundance in underwater rocks in a CO2-rich atmosphere.
"Like many other greenhouse gases, CO2 acts to trap heat and insulate a planet's surface," M. Darby Dyar, professor of astronomy at Mount Holyoke College in South Hadley, Mass., who was not involved in the research, tells The Christian Science Monitor in an email. "Mars climate models have conventionally assumed that early Mars had both a hydrologic cycle and comparable levels of CO2 to what Earth has."
But according to the study, that might not have been the case. When carbon dioxide interacts with water on the surface of the Earth, it undergoes chemical reactions to produce carbonate, which can then interact with other elements and compounds to produce carbonate minerals at the bottom of bodies of water. Even millions of years later, scientists should be able to examine the levels of carbonate in a rock and work backward in order to determine how much CO2 would have been present in the ancient Martian atmosphere.
Instead, the international team of researchers found that the levels of carbonate were at least tens, possibly hundreds, of times lower than they should have been to indicate a warm, CO2-rich atmosphere that would have allowed liquid water to flow.
"It would be really hard to get liquid water even if there were a hundred times more carbon dioxide in the atmosphere than what the mineral evidence in the rock tells us," said Thomas Bristow, principal investigator for the Curiosity's Chemistry and Mineralogy (CheMin) instrument and lead author of the study, in a NASA statement.
Since Curiosity landed in Gale Crater in 2011, it has made no definitive detection of carbonates in any rock samples. Prior to that, orbital spectrometers had also been unable to detect significant amounts of carbonate on the Martian surface.
The researchers determined that for no carbonate to be detected by the rover today, the level of CO2 in the atmosphere could only have been in the tens of millibars, a unit of atmospheric pressure, when the lake still existed. A millibar is only one-one thousandth of the sea-level air pressure on Earth, not nearly enough to keep water liquid.
This presents a paradox for researchers. Evidence has been building for years pointing to a lack of carbonate on the Martian surface, yet there is clear evidence that strongly suggests that Mars has a wet, warm past. And reconciling those two things could be difficult, says Martha Gilmore, a professor of geology at Wesleyan University in Middletown, Conn., who has studied Martian geomorphology and was also not involved in the study.
"It's a wonderful problem to have," Dr. Gilmore tells the Monitor in a phone interview. "The issue is that the sun was cooler at the beginning of the solar system, and therefore to have a warmer Mars, or warmer any planet, you need a lot of greenhouse gas. And CO2 is the most available, most abundant, greenhouse gas."
Gilmore says that a number of possible alternative warming mediums have been proposed, such as other greenhouse gases or molecular water, but these theories have other problems. Gases like sulfur dioxide, methane, or nitrous oxide, for instance, would have only lingered in the atmosphere for a very short time, but short-lived warming periods these gases may have powered do not appear to be consistent with evidence indicating that many Martian bodies of water appear to have existed for extremely long periods of time.
The paper suggests that perhaps layers of ice over Martian lakes could have kept carbonates from forming, but Curiosity has found no geological evidence of an ice layer in Gale Crater. Another possibility is that volcanic areas around recent impact craters may have temporarily heated the atmosphere long enough for liquid water to form, says Gilmore, but that is not an entirely satisfactory explanation either.
"There are no really good [explanations]," says Dr. Dyar. "This study rests on a very firm scientific foundation. However, it may be important to remember that although Curiosity has traveled far (for a rover), there may well be a sampling bias in where it has sampled."
For now, the many unknowns associated with an alien planet means that this question will remain a paradox, at least for now. But while the new study will force researchers to reconsider what they thought they knew about Mars's past, Gilmore says that it also provides an opportunity to learn about a planet completely different from our own – an exciting prospect for a scientist.
"On Earth, you have the benefit of time and tools," she says. "I can stand in a river and understand how that river formed, because we see the same processes happening today that happened 3 billion years ago.... We can throw every machine in the book at those problems on the Earth. On Mars, we have a limited tool set."
"We're doing what we can, but it's a hard problem," she adds.