Mars may once have had a much larger moon

A crater on Mars may once have contained water that moved with the tides. If this is true, it follows that Mars must have had a moon massive enough to exert sufficient gravitational pull on the planet’s seas to create tides. Neither of the two moons it currently has is large enough for this task.

Suniti Karunatillake of Louisiana State University and colleagues found that traces of tidal activity appear to be preserved in thin layers within the sedimentary rocks of Gale Crater.

They analyzed the sediment layers to obtain the period of the tides and the properties of the moon that helped cause them. If it actually existed, it would be 15 to 18 times more massive than Phobos, the larger of the Red Planet’s two current moons. This would nevertheless make it hundreds of thousands of times less massive than Earth’s Moon. Today’s two Martian moons could actually be remnants of the larger moon.

Karunatillake will present the team’s results at the American Geophysical Union annual meeting next week in New Orleans, Louisiana.

The rocks on which the researchers based their conclusions were photographed by NASA’s Curiosity rover. They contain alternating layers of different thicknesses and colors. Such layers are called rhythmites, because they indicate that material has been brought in by a wind or current of regularly varying strength. In the case of tides, the rising tide brings sand, which is then covered with fine mud when the tide turns and the water is still.

Gale rhythmites contain thin dark lines suggesting such “mud curtains,” which “show a very close similarity to Earth’s tidal patterns,” says team member Priyabrata Das, also at Louisiana State University.

To strengthen the team’s hypothesis, Ranjan Sarkar of the Max Planck Institute for Solar System Research in Germany used a standard mathematical technique called a Fourier transform to analyze the layering pattern of Martian rocks. This helped identify additional periodicities in the thickness of the layers, suggesting that the sun and moon once directed the tide, just as on Earth.

With this analysis, the researchers may have confirmed an idea first raised by Rajat Mazumder of the German University of Technology in Oman. An expert on rhythmites, he suggested in 2023 that the layered formations observed by NASA’s Perseverance rover in another Martian crater, Jezero, could be tides. But these images did not have sufficient resolution to perform a Fourier transform. Excited by the analysis of Gale rhythmites, Mazumder points out that, on Earth, the discovery of such rhythmites “is very strong evidence for tidal activity. In other words: marine conditions.”

But not everyone is convinced. The lakes inside Jezero and Gale craters, with their respective diameters of 45 and 154 kilometers, were too small to have tides, says Nicolas Mangold of the Laboratory of Planetology and Geosciences in Nantes, France, a member of NASA’s Perseverance Mars team. “So even with a larger moon in the past, I don’t think those two places are the right ones to record tidal deposits.”

Christopher Fedo of the University of Tennessee, who works with NASA on Curiosity’s explorations, also sees problems with the idea of ​​a larger moon and notes that tidal-like rhythms can be formed by regular variations in the flow of a river into a lake.

But Sarkar thinks there might be a way out of interpreting the tides. “Maybe an ocean was hydrologically connected to Gale. Even subsurface porosity can connect bodies and cause tides. On Mars, you have a very fractured and cratered surface, so porosity is not a problem there.”