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Let's go to Mars

Mars's tilt turns out to be chaotic

Two 1993 calculations showed the Martian axis wanders chaotically — one integration ran it from 11 to 49 degrees over 80 million years. The present 25.19 degrees is a low value, not a constant.

An oblique orbital view of a wide rust-coloured crater whose inner walls are skirted by smooth, rounded flows spilling toward the floor, with a broad grooved valley cut through the rim on the right.
Debris-covered glaciers banked against the walls of Deuteronilus Cavus — ice that gathered far from the poles, when the axis leaned much further over than it does now. · Image: ESA/DLR/FU Berlin

On 18 February 1993 Nature published Jacques Laskar and Philippe Robutel on the chaotic obliquity of the planets; on 26 February Science published Jihad Touma and Jack Wisdom's direct numerical integration of the rotation of Mars, submitted independently in December 1992.

William Ward had shown in Science in 1973 that the Martian tilt oscillates, and a 1991 Ward and Rudy paper had it reaching 45 degrees; the character of the motion was new. Mars has no large moon, and its spin axis precesses at about 7.58 arcseconds a year, close to the solar system secular inclination frequency governing Venus's orbit plane tilt, so the resonance is crossed repeatedly rather than cleanly entered. Touma and Wisdom's integration ranged from roughly 11 to 49 degrees over 80 million years, nearby starting conditions diverging in three to four million years. A companion paper in the same Nature issue noted that Earth's tilt holds inside about 22.1 to 24.5 degrees only because of the Moon.

Laskar's group published long-run statistics in 2004. Precise solutions reach 10 to 20 million years, no further. Over five billion years the mean obliquity is 37.62 degrees, standard deviation 13.82, maximum 82; the standard model's most probable value is 41.80 degrees. The present 25.19 degrees is a low outlier.

On 18 December 2003 James Head and colleagues reported, from Mars Global Surveyor data, a layered, dusty, water-ice-rich mantle metres thick, draped symmetrically in both hemispheres from about 30 degrees latitude to the poles. They dated it to 2.1 to 0.4 million years ago, when obliquity reached 30 to 35 degrees, and read its pitted, retreating state as decay. Mars is in an interglacial, and its ice ages run the opposite way from Earth's: warmer poles, ice carried toward the equator.

From elliptical crater orientations, Holo, Kite and Robbins argued in 2018 that mean obliquity since the late Hesperian was probably low: 10 to 30 degrees, under a fifth of the time above 40.