Mars’s polar ice layers signal climate changes

Understanding the relationship between Mars’s climate and orbital dynamics is among the most important goals in its science. For decades, researchers have hypothesized that polar deposits contain layers of ice whose thickness is controlled by variations in Mars’s axial tilt, precession angle, and eccentricity—much as variations in Earth’s orbit push our planet into and out of ice ages (see the article by Mark Maslin, Physics Today, May 2020, page 48 ). Those orbital parameters affect the temperature and sunlight on both planets. But distinguishing the climate signal from stochastic variability has proved challenging. In previous studies, scientists have found correlations between Mars’s orbital dynamics and the layering of its polar ice caps. But the extent of that connection has remained unclear, possibly because the ice caps are billions of years old and the extent of the ice has grown and shrunk over time.

A group led by Purdue University’s Michael Sori has now examined high-resolution images taken by NASA’s Mars Reconnaissance Orbiter spacecraft of newly discovered ice deposits. Those deposits were spotted in impact craters that are smaller, younger, and located at lower latitudes than those at the polar caps. Sori and his colleagues realized that orbital signals might be more straightforward to identify at the lower latitudes, where they are more sensitive to obliquity variations. In particular, the researchers analyzed the topography visible on the perimeter of Burroughs Crater, pictured here. The protrusion of that crater’s icy layers turned out to be a reliable paleoclimate proxy. The researchers used Fourier transforms to search for periodicities in the profiles across the ice layers. They found that two wavelengths in the power spectrum—one at 15.6 m and one at 6.5 m—had signal power higher than 95% of the randomly generated profiles at those wavelengths. The 2.4 ratio of those wavelengths in the stratigraphy is the same as the ratio between the period of Mars’s obliquity (120 000 years) and precession (51 000 years).

The obliquity cycle corresponds to the 15.6 m pattern, and the precession cycle corresponds to the 6.5 m pattern. Both give the same rates for ice accumulation: 15.6/120 000 ≃ 6.5/51 000 ≃ 0.13 mm/year. The detection of that match will help Mars researchers understand what controlled the planet’s paleoclimate. That understanding, in turn, is essential for predicting when the planet might have been inhabitable. Mars is a natural laboratory for investigating the connection between climate and orbital parameters. Unlike Earth, Mars does not have such complicating factors as biological processes and plate tectonics, which can erase past surface features. (M. M. Sori et al., Geophys. Res. Lett. 49, e2021GL097450, 2022 .)