Martian stairs suggest predictable ancient climate

“I’ll build a stairway to Paradise.
With a new step ev’ry day!
I’m gonna get there at any price;
Stand aside, I’m on my way!”
— “I’ll Build a Stairway to Paradise,” lyrics by Ira Gershwin

They may not be the steps to paradise, but several outcrops of Martian rocks do resemble stairs, showing a regular pattern that suggests the ancient climate on the Red Planet wasn’t always a hellish amalgam of cataclysmic floods, volcanic eruptions and crater-gouging impacts, planetary scientists report.

Instead, the evenly spaced, ancient formations suggest that during a relatively small window of time several billion years ago, the Martian climate varied in a gentler, more periodic and predictable fashion. Moreover, those changes may be tied to periodic variations in the planet’s tilt.

Evidence has accumulated that Mars’ spin axis has wobbled significantly during the recent past, in the last several million years. This wobble periodically cooled the equatorial region and warmed the poles as they received more sunlight.

But the new findings provide some of the first hints that the axis may have had a similar wobble in the much more distant past, notes Kevin Lewis of the California Institute of Technology in Pasadena. He and his colleagues describe the findings, based on high-resolution stereo images of outcrops within four craters in the Arabia Terra region, in the Dec. 5 Science. 

Combining these images of layered rock formations taken from different perspectives by the HiRISE camera aboard the Mars Reconnaissance Orbiter, the team constructed finely detailed, three-dimensional views of the region. The outcrops have been eroded into remnant mounds on the floors of the craters, with many of the layered deposits showing a stair-stepped shape. Although the layering differed from one outcrop to the next, within any particular outcrop, “each layer had exactly the same thickness as the one above and below it,” says Lewis.

The team proposes that such uniform layering within one crater, called Becquerel, indicates that each layer, or strata, formed over a period of about 100,000 years and was produced by the cyclical change in climate that the wobble in the planet’s spin axis would cause. Every 10 of the staircaselike layers appears to be bundled into a larger unit, which the team calculates was laid down over a million-year period. (Becquerel contains 10 such bundles.) One million years is the same duration as the periodic variations in Mars’ tilt, suggesting that climate variations induced by the tilt produced the layering. Each bundle, then, represents climate processes as the planet tilted and its equatorial region received more or less sunlight.

The rocks’ placement suggests they are a northern extension of the planet’s southern highlands, known to be from Mars’ distant past. But planetary scientists are uncertain exactly how ancient they are, notes Lewis. However, the study provides a new window on how the Martian climate may have operated during a roughly 10-million-year interval several billion years ago. It appears likely that liquid water was present during that time period, helping to cement the different layers, Lewis adds.

“These are fascinating deposits that clearly record repeated depositional events of about the same size on early Mars,” says planetary scientist Taylor Perron of the Massachusetts Institute of Technology.

But it’s unclear, he adds, “whether the sedimentary layers formed in response to periodic changes in Mars’ climate, or just repeated events that may have happened at irregular intervals” unrelated to climate changes. Without an absolute age for each layer formed or an understanding of how the layers formed, “it is difficult to make that distinction,” he says.