Mars’ ancient climate had a “wet-pass” filter

Volcanic eruptions — and the rocks they produce — dominate the surface of the Red Planet. Mars also shows evidence for activity by liquid water — sediments, channels, and valley networks — through much of its history. While volcanos can erupt under any climate regime, liquid water requires a narrower range of temperatures.

DEEP FREEZE AND THAW. Cyclical changes in Mars' orbit can warm the surface periodically to produce snowmelt. The scale at left shows predicted temperature swings over 700,000 years. The outtake centered on 300,000 years shows hypothetical changes in the sediments in Mount Sharp, the Gale Crater mound that's a target for NASA's Curiosity rover. Orange layers are dry episodes, blue ones are affected strongly by snowmelt. The left column shows a steady accumulation, on the right is the pattern made when accumulation occurs during wet intervals. (Image taken from Figure 17 in the paper.)

A team of scientists led by Edwin Kite (Caltech) developed a computer model to look at a particular subset of the geologic evidence — sedimentary rocks — and they concluded that these could be explained by infrequent, orbitally-controlled seasonal melting. Their work is published in Icarus.

“The pattern of sedimentary rocks on Mars,” the team writes, “is most consistent with a model Mars paleoclimate that only rarely produced enough meltwater to precipitate aqueous cements and indurate sediment.” Moreover, sedimentary rocks on Mars are not widely distributed across the planet, but rather narrowly concentrated into equatorial latitudes and low elevations.

The scientists explain that this implies long spells of globally dry climate with brief wet intervals. The result, they note, is “unfavorable for past life on Mars.”

In the team’s model, the factor that controls the rate of sediment formation is the supply of liquid water coming from seasonal melting of snow or ice.  The model assumes conditions that the researchers believe are reasonable estimates for early Mars. These include an atmosphere of pure carbon dioxide about 10 times thicker than today’s, dusty snow, and a solar luminosity reduced by 23 percent.

“Under these conditions,” the team explains, “snow melts only near the equator, when obliquity and eccentricity are high, and when perihelion occurs near the equinox.” Such situations occur relatively rarely, only about 20 percent of the time at most.

The team says, “This fraction of time is small, yet consistent with the geologic record of meta-stable surface liquid water acting as a ‘wet-pass filter’ for Martian climate history.”

In essence, they say, the sediments record only those orbital conditions that allow liquid water to exist at the surface.

When the researchers ran the model they found that viewed globally, snowmelt reached a maximum in Valles Marineris and Meridiani Planum. A third “wet spot” location is Gale Crater, where the rover Curiosity is currently exploring.

Rover scientists reported January 15, 2013, that outcrops on the Gale Crater floor which they are preparing to drill into appear to have been saturated with water. Water-related minerals have precipitated in cracks, while nearby cross-bedded dunes of coarse sand suggest shallow but flowing water.

In regard to Mt. Sharp, the giant mound in Gale Crater, the Kite team predicts that Curiosity will find, as it climbs the mound, a succession of layers, each with generally homogenous chemical changes. Curiosity should also find evidence of wet/dry orbital cycles, with wet events occurring only during optimal conditions.

Early Mars, the team suggests, was not very warm and not very wet. “The climates considered in this paper are extremely cold,” they write. “Meltwater production is comparable to the coast of Antarctica.”

They add, however, that the computer model is still incomplete: “Our model runs can’t explain big valley networks, so an angle to explore in future work would be to try slightly warmer climates, comparable to the coast of Greenland, and see if this produces enough melting and runoff.”

Looking at the big picture, the scientists conclude, “Seasonal melting on Mars is the product of tides of light and tides of ice, which move around the planet on time scales driven by orbital changes. The peaks of these tides infrequently intersect, and melting can occur when they do.”