Mars's spin speeds up and slows down on a mysterious 26-year rhythm

01The question

Mars turns once every 24 hours and 37 minutes or so. Scientists care about that number down to tiny fractions of a second. Knowing exactly how Mars is tilted and how fast it spins is what lets spacecraft land in precise spots, and the spin also carries clues about what's happening inside the planet and in its thin air.

In 2023, the team behind NASA's InSight lander noticed something odd: Mars seemed to be spinning very slightly faster during InSight's mission (2018–2022) than during the Viking landers of the 1970s and '80s. They described it as a slow, steady speed-up. This new study asked whether that picture holds up once far more data is added.

02What they did

Researchers at NASA's Jet Propulsion Laboratory and in Belgium combined radio tracking from six spacecraft spanning nearly 50 years: three landers (Viking, Pathfinder and InSight) and three orbiters (Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter).

The trick is the Doppler effect, the same thing that makes a passing siren change pitch. When a spacecraft moves, its radio signal arrives on Earth at a slightly shifted frequency. A lander sitting on the ground moves only because Mars is turning, so its signal tracks the spin directly. Orbiters are tugged by Mars's lumpy gravity field as it rotates beneath them, which also reveals how the planet is turning.

In all, more than 47 million radio measurements went into one giant calculation that solves for Mars's gravity, tilt and spin together.

03What they found

All three orbiters, each analyzed on its own, told the same story: Mars's spin isn't changing at a steady rate. Instead it drifts ahead and then falls back in a cycle about 25.8 years long. The orbiters detected the change at 20 to 30 times their measurement error, so it isn't just noise.

The effect is tiny. At its biggest, a spot on Mars's equator ends up only about 4 meters ahead of or behind where a perfectly steady spin would put it. The most recent tracking data suggests the spin has started slowing down again after the InSight years, which a simple steady speed-up can't explain.

Along the way, the team also produced a sharper map of Mars's gravity and a recommended official model of how Mars is oriented in space. It pins down the direction of Mars's spin axis to within about 1 meter from 1950 to 2050.

04Why it matters

A planet's spin changes when mass moves around on it, just as a figure skater spins faster by pulling in their arms. Mars already shows a yearly version of this: carbon dioxide frost piles onto the poles in winter and turns back into gas in summer. Finding a new, unexplained 26-year rhythm means something on or inside Mars may be moving mass around on a much longer schedule. The authors suggest the atmosphere or the planet's interior as possible causes.

The cycle is also intriguingly close to the Sun's 22-year magnetic cycle, although no link has been shown. Solving the mystery could teach us something new about how Mars works, and the improved maps will help future missions navigate.