ESA Science & Technology30-Jun-2005 05:48:25

Planetary Evolution

Last Update:  30 Aug 2004

Origins of Mars

Planetary Nebula in Beta Pictoris.
(Credit: J.-L. Beuzit et al. Grenoble Obs.), ESO

When the Solar System was born, 4.6 billion years ago, solid bodies condensed out of the hot gas that swirled around the infant Sun. The gravitational pull between these bodies attracted them to each other and they eventually coalesced into the planets. The young planets continued to undergo intense bombardment from the numerous small bodies that still roamed the Solar System. But as the Solar System aged, the rate of bombardment decreased as the number of small bodies reduced.

This hypothesis about the formation of the Solar System forms the basis for the best method we have so far for estimating the age of rocky planetary surfaces. Roughly speaking, the more craters a surface has, the older it is.


Geologists divide the ages of Mars into three epochs, based on the density of cratering. The diluvian floods are thought to date from the oldest time when the planet was still undergoing heavy bombardment. So this era is called the Noachian: it runs from about 4.3-3.8 billion years ago. Next is the Hesperian which dates from about 3.8-1.8 billion years. And finally is the Amazonian, dating from about 1.8 billion years ago to the present. As nobody can be precisely sure how the rate of cratering declined with time, however, the timing of the ages is open to dispute.

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