CLATHRATES AND CARBON DIOXIDE ON A DRY COLD MARS

Conventional models of Mars' evolution emphasise the role of water despite the present day cold dry climate and the dominance of CO₂ in the atmosphere and seasonal polecaps. Evidence of surface flood features is used to support "Blue Mars" models in which a former warm and wet climate is assumed. The implications of such models are often paradoxical in terms of surface chemistry and mineralogy, and the relationship of Mars to its sibling terrestrial planets. For instance, the embarrassing lack of carbonate minerals on Mars is a fundamental challenge to liquid water in the past, and a greenhouse-driven warm wet Mars is at variance with the relatively cool history of Earth, given their respective insolation.

A new volatile model for Mars explicitly includes CO₂ clathrate as the main H₂O-bearing phase, and also subsurface CO₂ in a variety of phases - solid dry ice in polar permafrost, vapour phase CO₂ in shallow underpressured pores and most importantly liquid CO₂ in deep and pressurised "liquifers".

The escape of volatile liquid CO₂ from deep overpressured reservoirs is an ideal mechanism for generating voluminous terrain collapse and catastrophic-scale gas-supported density flows. A CO₂-driven gas-supported density flow would be a cryogenic analogue of a pyroclastic flow on Earth. Strong analogues in scale and mechanism can be drawn from submarine turbidite flows on the floor of Earth's oceans, but the flows on Mars do not involve liquid water. Instead, the outburst "flood" channels of Mars were carved by cold and dry flows.

In the new "White Mars" paradigm, the volatile history and surface chemistry of Mars can be matched in a single self-consistent model that involves no arbitrary warm and wet episodes, and is consistent across the family of Venus, Earth, and Mars.

When we cease looking at Mars through blue-tinted spectacles, the planet becomes uniquely and simply understandable.