Download higher quality image. If life existed on Mars during this time period, researchers believe that the fossil remnants of that past life may be sandwiched within the sedimentary rock layers, just as they are on Earth. The martian outcrops, in some cases a few kilometers thick, appear to be made of fine-grained materials deposited in horizontal layers, the hallmark of sedimentary rock. These outcrops are found inside craters, between craters, and within chasms, said Michael C. Malin and Kenneth S. Edgett of Malin Space Science Systems in San Diego, California. Download higher quality image.
Massive units appear as on bulky rock layer with no clearly defined horizontal bedding.
In a few cases, these types appear together, with the massive unit always perched on top of the bedded unit like a thick, indistinct coat of frosting on a layer cake. Thin mesa units, with surfaces ranging from smooth to pitted to ridged and grooved, are almost always found on top of eroded massive or layered sedimentary rock. Under this scenario, sediments may have been transported into the lakes in regular, swift pulses, building up thin layer units. Massive units may have been deposited when the lake became stagnant or deep enough to cause sediments to sift down through the water over longer intervals. Some of the MOC images of these outcrops show hundreds and hundreds of identically thick layers, which is almost impossible to have without water, said Malin.