That seems extremely unlikely. If a few dozen planes crashed on Earth (on land), how likely do you reckon you'd be to ever run across significant debris from any of them just by randomly driving around?
Devil's advocate: those are of the successful landings, it doesn't include the other Mars "landings" that were unsuccessful or unintentional. Such as Beagle 2, MPL, MCO, Deep Space 2, Mars 2, and Mars 6. Here's a full list: http://en.wikipedia.org/wiki/List_of_artificial_objects_on_M...
Most of the impact sites of those objects are known except for a few. MCO's impact point is unknown and whether or not Beagle 2 hit the surface is unknown (it may have skipped out of the atmosphere into a heliocentric orbit, or into an elliptical orbit and reentered elsewhere, or burned up entirely).
Even so, the chances are astronomically low that Curiosity has managed to stumble upon the wreck of another spacecraft.
The problem is the shape, not just the material. As a layman, I'm not used to rocks having that shape, except in very rare circumstances but in those cases "abnormal" shapes are usually found in clusters (Grand Canyon etc). This is a seemingly-solitaire rock of abnormal material AND abnormal shape, which is possible, but almost as unlikely as random probe debris.
Just to put a bit of context here, you should visit Death Valley. It is the strangest and most wonderful place you can walk around and find amazing mineral deposits, geologic formations, and generally odd sorts of things.
Mars has had during its life a very active volcanic system, these systems imply magma, and magma implies melting and cooling of basalt. 'Shiny' objects in volcanic fields are typically obsidian or some other form of basaltic glass. Erosion by wind of a piece of obsidian can carve it into a pretty bizarre shape.
So all said and done, the geology is fascinating. And one of the differences between having a rover up there and having humans up there was that this would have been a 15 minute news story with humans, "Oh look, shiny!", "What is it?", photo, photo, chip, chip, "Some sort of <analysis result>, what is over where you are?"
Now its 'next day' at best, and 'in a month' if there is a long list of things to do before you get a break at worst. Sigh.
If Earth were devoid of vegetation and instead uniformly rocky, running into a sliver of a downed plane might've been far more probable. Especially with volunteers scouring every taken image for anything that doesn't look like a rock.
> Especially with volunteers scouring every taken image for anything that doesn't look like a rock.
I don't think anybody is confusing the likelihood of finding a piece of a plane due to not looking at the picture well enough. It's a question of if there would be anything in that picture to begin with.
