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The United Kingdom-based Daily Mail interviewed Hap McSween, a professor emeritus of earth and planetary sciences, on a recent development on the surface of Mars.

A black and white picture taken on December 20, 2016 shows the same ball in the same place, although Martian dust appears to have covered surrounding rocks since then. The pictures were taken by Nasa’s Curiosity rover–which has been examining the surface of Mars since it landed in 2011–using its right mast camera. The images are most likely to show an example of a Martian spherule.

There are many theories surrounding the discovery of strange objects on Mars from groundwater-caused reactions to optical illusions.

“A number of straightforward geological processes can yield round shapes,” says McSween.