Now you don't see it, now you do.

Ten years into a mission that was originally going to only last a few months, NASA's Opportunity rover continues to turn up surprises on Mars. In this case quite literally. Take a look at the following pair of images from the rover's location at 'Solander Point', up on the rim of Endeavor Crater.

Before...and after (Credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.)

See anything different between them?

Yep, 12 sols (martian days) later there's a bright looking rock sitting on the surface that wasn't there the first time Opportunity looked.

What is it? Did some sneaky martian creep up and put it there to flummox us?

Probably not. But the appearance of this pebble, now named 'Pinnacle Island' (it's only about the size of a small clenched fist, or jelly donut as the rover scientists described it), is certainly surprising given the usually desolate and static nature of the landscape. Since the rover did not drive directly over this location, there are two plausible theories. The first is that there was a nearby meteor strike and this rock was lofted into the air to land here. The second is that the rover itself, now somewhat clumsy as motors on its wheels have degraded and failed, churned up and 'tiddlywinked' the rock a meter or so from its original spot.

Close up of the rock (Credit: NASA/JPL-Caltech/Cornell/Arizona Univ.)








This second explanation seems most likely. Since the rover is the only mobile entity on this bit of Mars (Curiosity being a long ways away) the simplest explanation is almost certainly the correct one.

It may also turn out to be a wonderfully serendipitous bit of environmental damage - the rock has a high sulfur, magnesium, and manganese content, placing into a new category compared to previous mineral sightings. It looks like we're seeing the underside of material that was, until 12 days earlier, snugly buried in the martian regolith and therefore unexposed to the atmosphere and weather.

But could the rover have really kicked a rock out like this? It's important to remember that the surface gravity on Mars is about 38% that on Earth. That means that if something is thrown it will spend about 2.6 times longer aloft than an equivalent throw on Earth (equivalent in the sense of the same velocity vector, and ignoring drag). It will therefore travel horizontally 2.6 times further than it would on Earth.

We tend to forget about this kind of simple physics, and can probably thank Hollywood for that. I've yet to see a fictional movie about Mars that properly imagines the basic mechanics of being in 0.38 g's. The fact is that stuff on Mars is going to be a lot more readily dispersed and thrown around than we're used to, and Opportunity itself is indeed the likeliest culprit for this sudden rock appearance.

Nonetheless, such attention to what is, let's face it, a rock, does also bring to mind a marvelous TED talk parody by The Onion.