Answer to Question 1
Liquid water cannot exist now on the Martian surface because it would boil away in the extremely low atmospheric pressure, so evidence that water once flowed on Mars means that surface conditions there must have been quite different long ago. It must have had a thicker atmosphere.
Answer to Question 2
The four-stage history of Mars is a case of arrested development. The planet began by differentiating into a crust, mantle, and core. Studies of its rotation reveal that it has a dense core. The Mars Global Surveyor spacecraft detected no planet-wide magnetic field, but it did find regions of the crust with fields a bit over 1 percent as strong as Earth's. Apparently, the young Mars had a molten iron core and generated a magnetic field, which became frozen into parts of the crust. The core must have cooled quickly and shut off the dynamo effect that was producing the planet-wide field. The magnetic regions of the crust remain behind like fossils. There are no folded mountain ranges on Mars and no signs of plate boundaries. The large size of volcanoes on the planet is evidence that the crust does not move. Planetary scientists therefore conclude that as Mars's interior cooled its crust grew thick and immobile. For Mars, the fourth stage of planetary development has been one of moderate activity and slow decline. Volcanoes may still occasionally erupt, but this medium-sized planet has lost much of its internal heat, and most volcanism occurred long ago. The atmosphere is thin, and the surface is a forbiddingly dry, cold desert.
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