Dr Malin and Dr Edgett suggest that the gullies are being created by water that is either seeping slowly to the surface or building up behind barriers of ice and occasionally bursting through.

ECONOMIST: Martian chronicles

These are usually one day affairs and are typically based on a running race of 3-12 miles, but those miles are fraught with hazards not found on any normal marathon or half-marathon course, from mud pits to vertical walls, fiery flame barriers to electric shocks, ice plunges to crawls under barbwire.

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In order to get people to farm, thus providing local food (you know, local is good, right?), in a place which is snow and ice for half the year it is necessary to put up huge trade barriers to food coming in from places with more grass and more cows.

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