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Curiosity rover finds surprising evidence of ancient continental crust on Mars

Graham Templeton

A few months ago, NASA scientists noticed some surprisingly light- coloured rocks in a photograph. They shouldn’t have been so light, by their understanding of the area’s geology — and so naturally, these scientists sent Curiosity to go and blast those rocks with a laser. The onboard “ChemCam” experiment then looked into the chemical composition of the vaporized rock this produced — and what it found looked surprisingly familiar. Though it has long been believed that Mars never had distinct continental plates, this new evidence suggests that Mars may once have had continents much like those on primordial Earth.

Some commentators are jumping from this observation to the idea that with continents must come liquid oceans between those continents, but this study doesn’t actually present any evidence in favour of that idea. What it does say is that spectral analysis of 22 light-colored rocks found near the Gale Crater show a high feldspar content, and likely quartz content too, and that the samples bear a striking overall resemblance to a type of Earth rock called Tonalite-Trondhjemite-Granodiorite (TTG). TTG is characteristic of Earth continental crust formations from this planet’s Archaen period, which ended some 2.5 billion years ago.

A figure from the study, showing some of the rocks they sampled for this study.

Mars had been widely theorized to be mostly “basaltic,” made of dark, relatively dense igneous rock of the sort found on Earth’s ocean floors. There was little support for the idea of distinct continental plates on Mars, even in the planet’s ancient history, but that seems to be the simplest explanation for this week’s reported findings. The light, granite-like rocks seem to suggest that around 4 billion years ago, Mars could have been much more Earth-like than previously believed.

These light-colored rocks were only visible to Curiosity because the rover landed near the enormous Gale Crater. This ancient pock-mark provides a several kilometers-deep cutaway of the layers of the Martian crust, but it’s also surrounded by the rocky debris thrown off by the original asteroid impact that formed it. Rather than scale the sides of the impact crater, Curiosity rounded up readings from almost two-dozen of these light rocks, right where they fell some 3.5 billion years ago.

The Gale Crater: 3.5 billion years old, and with the 18,000-foot mountain Aolis Mons in the middle.

Orbital satellites could not have gathered this information. They can only look at the composition of rocks right near the surface of the planet’s crust, and even then only when the rock is reasonably homogenous and covering a sizable area of the planet. Analyzing bits of rock this small and uncommon required boots on the ground. By rolling to within 20 feet of the specific target rocks, Curiosity was able to laser-blast its way to a better understanding of Martian history — hopefully.

The more astronomers learn about the history of Mars, the more it becomes clear that the planet has gone through many markedly different developmental stages. Evidence suggests that it once had an atmosphere — now that’s gone. It once had a functioning magnetic dynamo at its heart — that’s now silent, and the planet’s protective magnetic field along with it. Now, it turns out that around 4 billion years ago, Mars may also have once have had a more dynamic surface than previously believed.

The coming months will reveal if this insight will spur updates to existing theories of Martian planetary development, or any changes in strategy for the continuing search for life on the Red Planet.