The moons of Mars are puny little things – small, oddly shaped chunks of rock that orbit close to the planet. But Phobos and Deimos could be indirect descendants of a much larger moon – one that was up to a third as massive as Earth’s moon. And a recent study says that early moon could explain something about Mars itself.

Mars is oddly shaped. It has two big “bulges” that line up roughly opposite each other on the globe. One of them is the highest landscape on the planet, and it’s crowned by giant volcanoes.

The study says the shape could be the result of the gravitational pull of a big moon. Researchers named the moon Nerio for a goddess who was an early ally of Mars.

Nerio would have stayed over the same spot on Mars. If it was there early on, when the Martian surface was molten, then its gravity could have created the bulges. Mars then cooled quickly, locking the bulges in place.

The moon wouldn’t have lasted long. It could have been kicked away by a close encounter between Mars and another large body, or shattered in a giant impact. A chunk of the moon could have survived. Then it, too, was shattered, leaving Mars with only Phobos and Deimos.

The scenario raises a lot of questions. So it’ll need much more study before scientists can confirm that Mars once had a big moon.

Mars is in view all night. It looks like a bright orange star. It’s in the east-northeast at nightfall, and climbs high across the south during the night.

Script by Damond Benningfield