However, the ground is cut by vast dendritic networks and huge channels that appear to have been carved by flowing water, leading scientists to conclude that at least in the past, liquid water was abundant on Mars. There is even some evidence that an ocean once existed in the Northern Hemisphere of Mars. This clement period in Martian history is thought to have occurred over three billion years ago. The alteration products had to have something to alter, so they must have formed during the past 1.3 billion years, long after the suspected wet and warm climate on Mars had changed to the dry, cold conditions found today. These are the questions Baker, Swindle, and their colleagues are trying to answer.
Martian meteorites possess an interesting, though complicated, array of alteration products.
One type of Martian meteorite, the shergottites, are much like terrestrial basalts , and formed in lava flows. They consist mostly of the minerals plagioclase feldspar and pyroxene , but contain patches and veins of calcium carbonate and calcium sulfate, the alteration products. The nakhlites are quite different in composition. They are made mostly of pyroxene and olivine , with only a small amount of feldspar. Lafayette has prominent occurrences of rusty-brown veins that are a mixture of clay minerals, iron oxide, and iron hydride.