Commentary by NASA ARC’s Chris McKay on Magnetotactic Bacteria on Mars

The recent paper by Imre Friedmann and co-authors in PNAS (PNAS vol 98 page 2176, 2001) reports finding chains of magnetite that are consistent with a biological origin.

This agrees with the results on the size and shape of the individual magnetite grains as published in the same issue of PNAS by Kathie Thomas-Keptra and co-authors (PNAS vol 98, 2164, 2001). The size and shape is also consistent with a biological origin.

In the original publication (1996) of the ALH84001 paper by David McKay and co-authors they listed 4 lines of evidence suggesting biological origin 1. microfossils, 2. PAH organics, 3. carbonate isotopes, 4. magnetite crystals. At the time it seemed to me that of these four only the magnetite crystals provided a credible link to biogenic origin. Since then it has been widely recognized that the magnetite had the only possibility of providing good evidence for biogenic origins. The present papers therefore follow up on this important lead.

1. At the present time then there are two lines of evidence that indicate biological origin of the magnetite in the ALH84001:
2. the size and shape of each individual magnetite crystal
   the presence of chains of magnetite.

To me this evidence is now strong enough that we should begin to consider the implications of a biological origin for this magnetite on Mars 3.9 Gyr ago. These implications include:

1. what was the O2 level on Mars at the time these magnetotactic bacterial were present. On Earth magnetotactic bacteria use their magnetitic navigation to position themselves with respect to a O2 gradient
2. what were the sources of magnetic fields
3. what would be the expected density of magnetotactic bacteria such that their remnants are widespread enough that they are found in volcanic rocks
4. is there any biological origin of the magnetic fraction of the current
   martian soil.

Astrobiology