“Purple” spots on Martian rocks intrigue scientists

When we think of Mars, the first color that comes to mind is the reddish-orange of its surface, caused by the high concentration of iron oxide (literally rust) in the soil. But the Perseverance rover has found frequent signs of another color, which is puzzling scientists.

These are mysterious purplish patches, abundant in the rocks in the region of Jezero Crater, which the rover is exploring, and which have never been found in past missions. The color is visible in rocks of all shapes and sizes. In some it forms a thin, uniform coating, in others it looks like splatters of paint.

Dark, purplish spots on the surface of the rocks are visible on the right side of this image, taken by Perseverance on May 10, 2021. Image: NASA/JPL-Caltech

Their cause? “I don't really have a good answer for you,” says Ann Ollila, a geochemist at Los Alamos National Laboratory in the US, who presented an early analysis of the patches at an American Geophysical Union (AGU) conference.

Ollila and his colleagues are using the camera on the Perseverance's mast, called Mastcam-Z, to photograph and analyze the phenomenon. Another instrument that is helping the study is a laser, capable of vaporizing the surface layer of a rock and determining its composition. With it, scientists have already been able to determine that the coating is less hard and has a different chemical composition from the rock below it.

Mastcam-Z images suggest that the stains may contain many types of iron oxide (again, rust). And according to Ollila, analyzes done with the SuperCam suggest they are rich in hydrogen and, occasionally, magnesium.

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The presence of hydrogen and iron oxide points to our old friend: water. We know that the Jezero Crater region was once a lake where the delta of an ancient river flowed. Studying the spots can give us a lot of information about Mars' past, including how long there was water in the crater and, perhaps, the lake's own chemical makeup.

“The existence of coatings could be a key part of this story,” says Bradley Garczynski of Purdue University, who also presented an analysis of the phenomenon at the AGU conference.

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