CARBONATE GLOBULES

Recall that the grandaddy of ALH84001 is thought to have been fractured by a large impact around 4 b years ago. Since Mars's atmosphere was thought to be considerably wetter then than today, water was able to seep into ALH84001. Because the water was saturated with atomspheric carbon dioxide, carbonate materials were deposited into the fissures forming the carbonate globules (the formation of the carbonate globules could be assisted by life). The age of the carbonate globules are thought to be 3.6 b years. Then there was a second large impact on Mars roughly 16 M years ago which ejected ALH84001 into space. Some carbonates show shock faulting due to this second impact so that we know that the carbonate are older than 16 M years.

• Age
• Structure

The interesting things are:

• the carbonates may be old, 3.6 by old ===> formed on Mars a long time ago (not in Antarctica).
• A sticky point is that independent teams have suggested that the carbonate formed by high T metamorphic or hydrothermal processes (~700 C) (based on equilibrium phase relationships and "microprobe" chemistry) or the carbonate formed at lower T (0 - 80 C) (based on oxygen isotopic ratios). The high T is not good as biological remains would have a tough surviving the high T.
• McKay et al. suggest that the complex structure of the carbonate suggests that the enironment was altered as the carbonate was laid down (perhaps by biology).

  Comment--Finding evidence for life at 3.6 by on the Earth is not an easy job.