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A recent picture of Curiosity rover depicts a surprisingly accurate sphere on the surface of Mars. What is it, and how is it possible that it is so perfecly spherical?

NASA photo
Photo by NASA Mars Science Laboratory, Mastcam: Right, taken 2014-09-11 14:46:58 UTC. Source

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    $\begingroup$ It is possible for objects to form naturally that almost resemble a perfect sphere: en.wikipedia.org/wiki/Concretion $\endgroup$ – called2voyage Sep 30 '14 at 13:41
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    $\begingroup$ An astronomer, a physicist and a mathematician are on a train in Scotland. They look out the window and see a lone black sheep in a field. "Amazing", says the astronomer, "all sheep in Scotland are black!" The physicist rolls his eyes and says "you astronomers, always jumping to conclusions without sufficient data. There exists a field in Scotland which contains a black sheep." The mathematician looks at his colleagues and says "allow me to add some true rigor: there exists in Scotland a field which contains a sheep which is black on one side!" $\endgroup$ – Schwern Oct 1 '14 at 3:43
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    $\begingroup$ Ahhhh, so I never did hit that windmill . . . $\endgroup$ – imallett Oct 1 '14 at 4:20
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    $\begingroup$ at least one side, @Schwern $\endgroup$ – GreenAsJade Oct 1 '14 at 8:58
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    $\begingroup$ @Schwern Reminds me of the mathematician who stood outside an empty building, and a guy came out of it. He concluded that if he entered the building it would become empty again. Btw, the ball is a little baby Mars! $\endgroup$ – LocalFluff Oct 2 '14 at 13:21
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As called2voyage♦ pointed out in a comment, this looks like a concretion. It is not the first concretion observed on Mars, and it was most likely formed back when Mars had liquid water. More information from an article on Discovery News, dated 24 September:

According to MSL scientists based at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., the ball isn’t as big as it looks — it’s approximately one centimeter wide. Their explanation is that it is most likely something known as a “concretion.” Other examples of concretions have been found on the Martian surface before — take, for example, the tiny haematite concretions, or “blueberries”, observed by Mars rover Opportunity in 2004 — and they were created during sedimentary rock formation when Mars was abundant in liquid water many millions of years ago.

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Curiosity photographs thousands of rocks every week. It does that for years. You will find rocks with almost any shape you can think of. This one happens to be somewhat round in one direction. From the shadows and the way it stays on what is apparently a slope, I would infer that it is not a sphere, but is actually ragged on the other side.

I'm no geologist (do we have one?), but I don't think this requires further explanation.

If I did have to venture a guess I would say that maybe its porous and thus light. Together with Mars' low gravity it might have tumbled in the wind to dull the edges a little.

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  • $\begingroup$ What both you and @gerrit explain this with seems the most plausible, but isn't there a chance that lightning in the past caused this kind of formation? Don't we get spherical granules like that from arc welding or other discharges? $\endgroup$ – mechalynx Sep 30 '14 at 19:46
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    $\begingroup$ @ivy_lynx, lightning strikes produce fulgurites, which are decidedly non-spherical. $\endgroup$ – Mark Sep 30 '14 at 21:37
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    $\begingroup$ @ivy_lynx, parsimony: the simplest solution that explains all observations is preferable over more complicated ones. $\endgroup$ – Rikki-Tikki-Tavi Sep 30 '14 at 21:51
  • $\begingroup$ @Rikki-Tikki-Tavi No disagreement here, but why is lightning a "complicated" explanation? I'm not even arguing it's the explanation - I even said the existing answers are more plausible, just adding a note. $\endgroup$ – mechalynx Sep 30 '14 at 21:55
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    $\begingroup$ @ivy_lynx, the arc-welding spheres are droplets of molten metal that are small enough for surface tension to be the dominant shaping force (ie. microscopic). The sphere in the picture is decidedly macroscopic; if it had been melted by lightning, gravity would be the dominant shaping force and it would look like a solidified puddle. $\endgroup$ – Mark Oct 1 '14 at 5:32
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Recently I have found a similar ball-shaped stone (limestone or dolomite?), 4 cm in diameter, in a "gully" on a hillside in Budaörs, Hungary. I think it is Mother Nature furnishing curious things like this. enter image description here

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