For all their dramatic visual appeal, the gullies of Mars are proving mighty enigmatic. They look as if they were cut the other day by rivulets of water seeping from crater walls and cliff faces. But in geology, looks aren't everything. Seven years after discovering gullies, planetary geologists still disagree about where the water comes from and even whether water was involved at all. Add in the even more contentious dark streaks that mark other martian slopes, and you've got no end of debate over the recent history of water on the Red Planet.
展开▼
