Layered geologic outcrops on Mars, that will be described in this week’s issue of the journal Science — may be composed of sedimentary rock that dates from the earliest span of martian history, between 4.3. and 3.5 billion years ago. Images of these sedimentary rock exposures, captured by the Mars Orbiter Camera (MOC), suggest that parts of ancient Mars may have resembled a land of lakes, and that the geology of early Mars was much more dynamic than previously suspected.