Global deposits of ancient, clay-rich rocks on Mars – widely seen as evidence of a wetter, warmer, climate in the planet's past – may not be all they're cracked up to be.
A new study posits that the clays, which often form on Earth either from the action of water on surface rocks or from the passage of hot water through porous underground formations, could just as easily have formed from cooling lava, bearing no witness whatsoever to a warm, wet climate in Mars' past.
The study doesn't rule out the sporadic appearance of water in some places on the Martian surface – perhaps from near-surface water ice melted by the heat of a meteoric impact or volcanic eruptions. Some clays could well have been formed through the weathering action of water released in these ways.
But if Earth look-alikes and evidence from some Martian meteorites are any indication, a purely volcanic origin for much of Mars' clay-enriched deposits is as plausible as invoking a warm, wet climate during the planet's first billion years, according to the research team, led by Alain Meunier, a researcher at the University of Poitiers in France.
The bottom line: "Mars is a diverse place, and all of these processes happened at different points on the planet," says Bethany Ehlmann, a planetary scientist at the California Institute of Technology in Pasadena and a member of the team reporting the results Sunday in the journal Nature Geoscience.
The planet shows clear evidence for weathering and hydrothermal processes altering the surface in various areas, she adds. Indeed, Gale Crater and its looming Mt. Sharp, home to NASA's latest Mars rover, Curiosity, displays features that are hard to explain unless flowing or standing water were present on the surface.
The questions, she says are these: Which process dominated on a planetary scale at the time, "and what does that have to say about the overall climate?"
The team's eruptive proposition comes at a time when the notion of a persistent warm-wet-climate during the planet's first billion years has been taking hits in other ways.
For instance, a team of researchers from the United States and France has modeled Mars' early climate, when the sun would have been weaker. They conclude that the climate would have been too cold to sustain liquid water on the surface for very long. Instead, over time water ice would migrate to the poles, forming vast icecaps. Limited warming could occur from large impacts or volcanic eruptions, releasing enough water to carve features seen on the southern highlands.
Others have noted that water-carved features in widely spaced locations formed at vastly different times, weighing against a single span of wet climate as the source of flow that formed them.
Dr. Ehlmann and Dr. Meunier had teamed up on research last year that indicated clays could form through the interaction of water leaching through hot rocks. The clays resulting from hydrothermal activity would tend to show composition similar to clays formed at the surface.
But, Ehlmann recalls, Meunier mentioned the volcanic process, in which water trapped in lava interacted with the superhot material as it cooled and released its gases. Where water was involved, the interaction formed clays that filled nooks and crannies in the cooling rock. And its chemical signature was similar to the clays formed on the surface and via hydrothermal action. The evidence came from volcanic clays found in French Polynesia and in Brazil, which had experienced ancient eruptions where magma would flood the landscape from fissures in the rock.
Ehlmann offered that such clays wouldn't be abundant enough to yield a spectral signature orbiters could pick up. Meunier and colleagues went back to the lab, examined the samples with spectrometers similar to those on the orbiters, and found that such clays were in fact readily detectable from orbit.
In addition, the team found evidence for volcanically generated clays in some of the Martian meteorites that researchers have found on Earth, although the meteorites are younger than the period in question for forming the ancient, widespread clays that have intrigued researchers over the past five years or so.
For now, the notion that a volcanic origin could be the dominant source for clays widely seen as rain gauges for an ancient climate is a hypothesis, one that makes firm predictions about subtle differences in composition the clays should exhibit compared with the rock layers around them.
But such measurements will require additional Mars landers or rovers with the right capabilities near clay deposits in various locations on Mars. As a result, the work could help guide decisions on future landing sites, notes Brown University planetary scientist James Head III – budgets willing.