What Can We Learn from Other Planets?

Abstract

Venus (especially) and Mars have atmospheric compositions strikingly similar to Earth’s near surface volatile inventory. The relative amounts of carbon dioxide and nitrogen, the major gases, not only resemble Earth’s but also resemble the volatile fractions of a putative meteorite mix. The dominance of oxidized gases (even disregarding the oxygen in Earth’s atmosphere) suggest parallels and presumed support for the standard model for terrestrial planet degassing. Detailed consideration of processes that probably affected Venus indicate loss of enormous amounts of hydrogen from its hot atmosphere, to the extent of largely removing it from the planet. The oxygen (from water) left behind is responsible for oxidizing both the surface materials and the reduced carbon compounds that would have been present on early Venus if the current model (highly reduced primordial surface) were applied to Venus.

Mars is a more complicated situation. Applying the reduced early surface model to Mars can explain several features that are otherwise difficult to understand. The lack of a proportionately comparable volatile inventory on Mars may be due to a lower level of degassing because of less thermal energy available for degassing. The warm, wet early Mars (evidenced by imagery showing the effects of flowing surface water) may have been due to methane and ammonia production in hydrothermal systems affecting a proto-ocean containing organics (as for early Earth), and the decrease in hydrothermal activity led to a sufficient drop in these important gases to result in a transition to cold, dry conditions in spite of increasing solar luminosity. Such a scenario suggests the preservation of a frozen ocean and organic-rich sediments underlying the Northern Plains of Mars. What organics remained exposed to Mars atmosphere following the transition would have been readily oxidized (as would the rocky surface) by oxidation from continued loss of hydrogen to space from decomposition of atmospheric water. Because these early conditions resembled early Earth, the question of life on Mars is particularly interesting. If life did not arise on Mars, why not? Comparison with conditions on Earth suggest that differences in length of ocean coastline and probable tidal ranges resulted in limited availability of enough prebiotic reaction pools, explaining the lack of life on Mars (assuming life is not there!).