A perspective rendering of the Martian chloride deposit and surrounding terrain. 

LASP / Brian Hynek 

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Back in 2010, scientists discovered an 18-square-mile chloride salt deposit on the surface of Mars. Found in the planet’s Meridiani region — near the Mars Opportunity rover’s landing site — this salt deposit sits in a low point in the Martian landscape. A new analysis now confirms that the deposit most likely formed at the bottom of an ancient lakebed, right around the time the Red Planet dried up.

Bryan Hynek (University of Colorado, Boulder) and colleagues used spacecraft observations and terrain models to determine that the deposit sits at the bottom of a depression, fed by inflow channels from higher terrain and drained by what looks like a big outflow channel from the depression’s lowest point. As the team argues August 5th in Geology, the feature is potentially an ancient impact crater that was degraded and filled by water. The water then evaporated and left the chloride behind.

Currently, we know of more than 600 salt deposits on the surface of Mars. But determining their ages has been challenging. Scientists determine the ages of features on Mars by counting the impact craters on that particular surface and then calibrating to the carefully dated rocks collected by the Apollo astronauts and the lunar crater record. The problem with the chloride deposits on Mars is that they are generally too small to have enough craters for an accurate age estimate.

So the team used one of the oldest and most basic principles of geology: the principle of cross-cutting. This principle states that a geologic feature, such as a valley or a channel, cutting through a given terrain has to be younger than the terrain it cross-cuts — or at least the same age. In this instance, the outflow channel draining the depression cuts through a cratered landscape, enabling the team to estimate the channel’s age—and, therefore, the age of the lake it drained.

The results of the analysis indicate that the lakebed is no older than 3.6 billion years old. However, this upper limit slightly contradicts the suggested time period when Mars is thought to have been warm enough to sustain large amounts of water on its surface, an era than ended roughly 100 million years prior.

The presence of a lake during or after this time implies that Mars could still preserve some bodies of water shortly after the planet’s wet climate era supposedly ended, explains Mohamed El Maarry (Bern University, Switzerland). “We know from many studies that Mars is able to sustain liquid water activity even for short geological periods, at least in localized regions” he says. “This could happen right after a period of intense volcanic activity, for example, or shortly following a large impact event.”

Given the salt’s extent and thickness, as well as the lake’s volume (based on the terrain models), the researchers estimate that the lake was only about eight percent as salty as Earth’s oceans. That puts the lake on par with lakes currently on Earth.

Many studies have shown that the majority of the salt deposits on Mars consist mainly of chloride salts such as sodium chloride - normal table salt - and potassium chloride, which are the main salts found in oceans and seas on Earth. Furthermore, many of the dried-up lakes or playas on Earth - such as the numerous ones in California - show a similar predominance of chloride salts. This is telling us that ancient bodies of water on Mars had a similar chemistry and consequently could have been hospitable to microbial life.

But salinity isn’t the only factor that determines habitability. The water’s acidity, for instance, matters as well. The team left acidity levels out of the report, however El Maarry suggests this may have been due to the fact that that the researchers didn’t observe any minerals in the study region that form in highly acidic conditions. The lake basin contains signatures of a certain type of clay minerals that forms commonly in neutral or slightly alkaline conditions, he says. “Therefore, it is safe to assume that the lake had a neutral or slightly alkaline level, which as we know from our experience on Earth, is extremely habitable,” he adds.

These types of studies are exciting because they prove Mars had a rich diversity of hydrological regimes spanning a long period of time. Constraining the time of activity as well as the environmental conditions is a critical step in locating sites on the Red Planet that might have preserved ancient biosignatures. Previous research has uncovered promising clays and even glass that could serve as such time capsules. This site also looks like it could be a good candidate.

Quelle: Sky&Telescope