Introduction

The origin and early evolution of our atmosphere is a subject that has been repeatedly explored during the last four decades, without a definitive conclusion having yet been reached. Recent reviews have been published by Hunten (1993), who included the other terrestrial planets, and Kasting (1993), who focused on the Earth. The volume edited by Atreya, Pollack, and Matthews (1989) contains a number of relevant chapters by specialists in several fields, with a comprehensive bibliography.

The fundamental problem can be divided into a search for sources of the volatiles that ultimately became the oceans and the atmosphere, and an evaluation of the processes that could have changed the initial composition to produce the mixture we observe today. This chapter will concentrate on the sources. We will adopt a comparative approach, using new information about Mars and comets to put the terrestrial atmosphere in a larger context. The publication of possible evidence for ancient life on Mars (McKay et al. 1996) makes the inclusion of this planet particularly relevant. The following chapter by Kasting will examine some of the processes involved in subsequent atmospheric evolution.

Ever since Brown (1952) emphasized the depletion and fractionation of atmospheric noble gases, it has been clear that our atmosphere is not composed simply of captured solar nebula gas. Ideas about the origin of the oceans and the atmosphere have therefore focused on two principal sources: the rocks that formed the bulk of the planet's mass (the internal reservoir), plus a late-accreting veneer of material that originated well outside the Earth's orbit (the external reservoir).

The formation of the internal reservoir is a natural consequence of the accretion of the planet itself.