Looks like someone’s been having fun on Mars without telling anyone. Trails in slopes that seem suspiciously like sled tracks have been documented by research photographs. They weren’t made by little green men though. Blocks of dry ice are the suspects this time.
These tracks are called linear gullies and you can see a much larger photograph on the Apod.gov site. The speculation is that the blocks of dry ice evaporate by the time they reach the bottom of the slopes, so there is no longer any physical evidence remaining, other than the gullies themselves. They are about two kilometers long and end suddenly in a pit.
Researchers on Earth tested blocks made of various materials on sand dune slopes. The ice and wood ones didn’t move much, but a block of dry ice slid down the dune all the way to the bottom with ease. The researchers explained that when it heats up, dry ice emits a gas that pushes against the sand. The CO2 it emits functions like a lubricant, which allows the block to move smoothly downwards. Even on a test dune that was much less steep, a block of dry the researchers used moved easily downwards.
Would it be possible someday for astronauts to ride a slab of dry ice down the slopes of Mars? In theory, yes, but it might take someone like Richard Branson to try it first. Though it might be difficult to quaff his favorite beverage at the same time.
Mars is also very cold – the warmest it gets at noon is about 70 degrees Fahrenheit, but at the poles temperatures can be minus 225 F. (Not exactly the best snow boarding weather.)
If these tracks had been photographed in the 1950s, there might have been wild speculation about human-like aliens having ski resorts on Mars. Thankfully, we seem to have progressed on the awareness front somewhat since the 1950s.