Some of them are excavated in the impacts by all the craters.
Others, especially in the Northern plains, were brought there on icebergs floating on the ancient lakes and seas of Mars.
New Evidence For Ancient Ocean on Mars - Astrobiology Magazine
Others erode out of cliff faces. Here is a rather dramatic image of a bouncing track left by fall of just such a boulder (Diagonally top left to bottom right, there's another one in the bottom left corner).
Bouncing Boulders on Mars
Some are meteorites. With its thin atmosphere, meteorites don't burn up in the atmosphere and the smaller ones just end up lying on the surface, if they're too small to make a big crater.
This is the first meteorite ever found on another planet
Roughly the size of a basketball, iron -nickel meteorite,. discovered by NASA's Mars Exploration Rover Opportunity in 2005
and here is another.
Huge Meteorite on Mars Discovered by NASA's Curiosity Rover
There may be other reasons for forming boulders, those are just some of the ways I know about.
Mars also has volcanic eruptions. No volcanoes active now, but it's recently active and will be in the future, chances are. So - when volcanoes erupt that's another source of boulders. That's likely to be the source for these boulders found by Spirit for instance:
APOD: 2006 May 15
Others were formed from sedimentary rock, which then is eroded by water or wind and boulders or stones moved by the water. (wind can't move pebbles larger than 1 cm or so on Mars).
Basalt, Shale, Sandstone, Conglomerate
There are probably other ways boulders can form on Mars. These are just a few that I know of.
For instance dry ice processes or splitting by ordinary ice, or gouged out by glaciers.
It is a planet with a rich and varied geology.
Others, especially in the Northern plains, were brought there on icebergs floating on the ancient lakes and seas of Mars.
New Evidence For Ancient Ocean on Mars - Astrobiology Magazine
Others erode out of cliff faces. Here is a rather dramatic image of a bouncing track left by fall of just such a boulder (Diagonally top left to bottom right, there's another one in the bottom left corner).
Some are meteorites. With its thin atmosphere, meteorites don't burn up in the atmosphere and the smaller ones just end up lying on the surface, if they're too small to make a big crater.
This is the first meteorite ever found on another planet
and here is another.
Huge Meteorite on Mars Discovered by NASA's Curiosity Rover
There may be other reasons for forming boulders, those are just some of the ways I know about.
Mars also has volcanic eruptions. No volcanoes active now, but it's recently active and will be in the future, chances are. So - when volcanoes erupt that's another source of boulders. That's likely to be the source for these boulders found by Spirit for instance:
Others were formed from sedimentary rock, which then is eroded by water or wind and boulders or stones moved by the water. (wind can't move pebbles larger than 1 cm or so on Mars).
Basalt, Shale, Sandstone, Conglomerate
There are probably other ways boulders can form on Mars. These are just a few that I know of.
For instance dry ice processes or splitting by ordinary ice, or gouged out by glaciers.
It is a planet with a rich and varied geology.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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