these variety of features related to water activity, one of the
most prominent are 'slope streaks', stains that run down the slopes
of valleys and canyon walls. They are the fastest changing feature
on the Martian surface. For every hundred exising slope streaks
there are seven more every Martian year.
2005 streaks were found which had not shown up in photographs of
the same areas taken in 1999, suggesting that there has been liquid
water running on Mars' surface, not even thousands of years ago,
but within the previous 6 years.