ATMOSPHERIC SAMPLE RETURN

T. Owen

It will be impossible to reconstruct the origin and early history of the Martian atmosphere without a detailed knowledge of the present composition, including isotope ratios of the various elements. This knowledge must be extended to gases trapped in rocks, adsorbed on soils and condensed as ices. Only in this way can we determine the importance of various escape processes and identify current volatile reservoirs.

The study of the Martian atmosphere has greater significance than simply learning more about the atmosphere itself. We need to know the early history of the atmosphere in order to understand the early history of water on the planet and the associated probabilities for the origin and early evolution of life. The discovery of a trace constituent such as methane, which is far from chemical equilibrium in the present Martian atmosphere, would be a clue that life might actually exist on Mars today. Because Mars is relatively inactive geologically, it preserves a record of the first billion years that is almost completely missing on Earth. We can therefore hope to use the study of the Martian atmosphere to help us understand the early history of all inner planet atmospheres.

As an example of how that can be done, I can offer an outline of a model Akiva Bar-Nun and I have developed for bringing in volatiles with icy planetesimals.* This is illustrated in the accompanying copies of transparencies. One aspect of our argument that is worth noting here is the importance of many Martian samples. We use noble gases from the SNC meteorites to support the idea that inner planet atmospheres may be a mixture of volatiles brought in by rocks and those contributed by icy planetesimals. If we only had the meteorite called Chassigny, we would never have seen this connection.

We can certainly make further progress in understanding the Martian atmosphere by means of remote investigations from Earth and by additional measurements of SNC meteorites. However, these investigations cannot substitute for direct analyses of a sample of Martian atmosphere that has been brought to Earth in an undisturbed state, accompanied by suitable samples of soils, rocks and ices. We can only achieve the necessary accuracies for stable isotope abundance determinations in suitably equipped terrestrial laboratories. These atmospheric samples should therefore be an essential component of any sample return program.

*Owen, T. and Bar-Nun, A. Icarus 116, 215-226 (1995). TCO:dst4/8/96

Vugraph 1. Atmospheric Sample Return

Vugraph 2. Mars: Present Atmosphere

Vugraph 3. Hypothesis

Vugraph 4. Did They Really Do It?

Vugraph 5. The Missing Xenon Problem

Vugraph 6. Planetary Component of Noble Gases in Meteorites

Vugraph 7. Sources of Atmospheric Volatiles

Vugraph 8. Mixing Two Reservoirs

Vugraph 9. Logarithmic Plot

Vugraph 10. Comets as the External Source

Vugraph 11. Results: Bar-Nun Experiments