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Univ. of Arizona/JPL-Caltech/NASA

Ancient craters on Mars reveal how the planet’s tilt has changed over time

Earth is a tilted planet—it lists by 23.5°. This “obliquity” causes the North Pole to sometimes tilt toward the sun and sometimes away, giving us the seasons. Now, researchers have revealed how Mars’s obliquity has changed over the last 3.5 billion years, results that could reveal how frequently water ice and snow would have melted to liquid on the Red Planet.

To conduct the study, scientists assembled computer models of Mars with different obliquities and bombarded each version of the planet with asteroids. Elliptical craters—created by asteroids hitting the planet at shallow angles—tended to be more evenly distributed over the models with larger obliquities. On more upright planets, the elliptical craters tended to cluster around the equator.

By comparing their models with the distribution of more than 1500 elliptical craters on Mars’s surface, the researchers concluded that, in its past, Mars listed between 10° and 30°, similar to its present-day obliquity of 25°. Gravitational interactions with other planets changed Mars’s obliquity over time, but the planet probably spent less than 20% of the last few billion years tipped over by more than 40°, the team reports in Earth and Planetary Science Letters.

Mars’s relatively shallow tilt over cosmic time might have led to more underground aquifers drying out, the team suggested. But not all of them—this week, a submerged body of water was discovered on Mars.