Passage VI

In the Chryse Planitia area of Mars's surface, numerous winding channels are visible. The channels, some of which are 100 km long, were formed more than 3 billion years ago (bya). Two scientists discuss whether these channels were formed by liquid H₂O or by liquid CO₂.

Scientist 1

Unlike today, Mars was warm enough prior to 3 bya for liquid H₂O to exist on the surface. Prior to the formation of the channels, a large volume of liquid H₂O slowly accumulated in a basin within the Chryse Planitia. Once the volume became great enough, the liquid H₂O was released suddenly from the basin. The rapidly flowing liquid H₂O carried away rocks and soil, forming the channels. On Earth, similar features called outflow channels are produced by the sudden release of a large volume of liquid H₂O from a basin.

Grey hematite (a mineral) is abundant on Mars's surface within the Chryse Planitia. On Earth, this mineral is associated exclusively with liquid H₂O . Even today, large quantities of frozen H₂O are still present in Mars's polar regions.

Scientist 2

Before 3 bya, large volumes of CO₂ gas were present a short distance below the surface in the Chryse Planitia. Eventually, the subsurface pressure due to the weight of the overlying rocks and soil became great enough to change the CO₂ to a liquid. When a later major event such as an asteroid impact removed most of the overlying material, the liquid CO₂ quickly moved up to and then along the surface. The rapidly flowing liquid CO₂ carried away rocks and soil, forming the channels. On Earth, gases or liquids that have been under pressure belowground are released to the surface during some volcanic eruptions. These pyroclastic flows move rapidly along the ground, forming channels by carrying away rocks and soil.

Since Mars's formation, its average surface temperature has never been greater than -50°C. This has made it impossible for a significant amount of liquid H₂O to be present on the surface. If any liquid H₂O had been present on Mars's surface, it would have reacted with CO₂ to produce carbonate rock (a rock composed of carbonate minerals). No carbonate rock has been found on Mars.