Rain and snow were regular occurrences on Mars in the past; especially in the Noachian and early Hesperian epochs.[2][3][4][5][6][7] Water was theorized to seep into the ground until it reached a formation that would not allow it to penetrate further (such a layer is called an aquitard and is believed to be impermeable). Water then accumulated forming a saturated layer. Deep aquifers may still exist.[8]
^Grotzinger, J.P.; Arvidson, R.E.; Bell, III; Calvin, W.; Clark, B.C.; Fike, D.A.; Golombek, M.; Greeley, R.; Haldemann, A.; Herkenhoff, K.E.; Jolliff, B.L.; Knoll, A.H.; Malin, M.; McLennan, S.M.; Parker, T.; Soderblom, L.; Sohl-Dickstein, J.N.; Squyres, S.W.; Tosca, N.J.; Watters, W.A. (2005). "Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars". Earth and Planetary Science Letters. 240 (1): 11–72. Bibcode:2005E&PSL.240...11G. doi:10.1016/j.epsl.2005.09.039.