Scientific paper review

Fossil evidence for past life on Mars?

MacKay, D.S., et al., 1996 Search for past life on Mars: Possible biogenic activity in martian meteorite ALH84001. Science 273:924-930

Allan Treiman (Lunar & Planetary Insititue) has assembled a great web site describing various aspects of this paper and related subsequent work:

• Main site
• About ALH84001
• Evidence for life in this meteorite
• Some "technical" explanations about this
• Discussions of early papers (1996-1998) related to this work
• Discussions of later papers (1998-2000) related to this work

Observation : Evidence for fossil remains of life on a martian asteroid?

This is a very controversial paper, in which the authors claim to have found evidence for life on Mars in the form of the transformed fossil remain of microbes in a meteorite from Mars. This paper was in large part the cause of a resurgence in interest in the possibility of life on other planets, and no wonder; if proven to be correct, it'd be the discovery of the century. However,...

The history of the meteorite can be reconstructed from the geology and chemistry of the rock itself. The rock unquestionably originated on Mars; gas inclusions contain gases identical to those on modern Mars in chemical and isotopic composition, found nowhere else that we know of. It is volcanic (igneous), and solidified 4.5 Bya (about the same time the Earth formed, but Mars had already been around for a while). The rock went through at least two shocks on Mars, probably nearby impacts, about 4 Mya. The carbonates that the authors interpret to be fossils originated later, about 3.6 Bya, in fissures in the rock, (although this is disputed - see below). The rock was recently (16 Mya) ejected from Mars by an asteroid impact. The rock drifted in space for most of the time after that, then descended to Earth 13,000 years ago, landing in what is now called the Allen Hills ice field in Antarctica. These fields are routinely searched by scientists looking for asteroids - they're easy to see on the ice and essentially all of the rocks found here are asteroids. The rock is unweathered, and is coated in the 'crust' that formed during entry (not re-entry!) into the Earth's atmosphere.

The evidence for life in this rock, described by the authors, is primarily three-fold: the PAH's (polycyclic aromatic hydrocarbons, a.k.a. soot), the fissure carbonates, and magnetite beads.

PAH's
The authors apparently thought this was their best evidence for life, but this has completely fallen apart and even the authors (most of them, at least) have since agreed that the PAH data is not relevant. It has since been shown that the concentration and chemical diversity of the PAHs is entirely consistent with a non-biological origin.

Carbonate deposits
The striking evidence for life in this asteroid is what appear to be fossil microbes in the carbonates that formed there 3.6 Bya. They look just like rod-shaped Bacteria, and this is probably what caused the intense interest in this paper. There are 2 major concerns about these apparent fossils: their size, and the origin of the carbonates. The fossils are small - much smaller than typical Bacteria - at 20-100nm. The size distribution of the fossils is problematic - Earthling microbial populations typically are fairly narrow in size range, but the 'fossil' martians vary more than 100-fold in volume. Although the authors work very hard to convince the reader that the carbonates are very unlikely to have formed chemically alongside the sulfides, there are alternative explanations for their formations.

Magnetite
Probably the best evidence that these really are fossils is the presence of magnetite beads. These look exactly like the magnetite beads in magnetotactic Bacteria in size and composition (say the authors), and if found on Earth would be considered to be uncontroversial "magnetofossils". Magnetite can be produced synthetically, but is produced naturally, as far as we know, only biologically. The larger granules (20-100nm) are single-domain magnets, like the internal magnetosome granules, and the smaller ones are supermagnetic, like the periplasmic crystals produced by some magnetotactic species.

Although the PAH and carbonate evidence has become less and less convincing, the magnetite data has fared better. More recently, strings of magnetite beads that look just like those found in the cytoplasm of magnetotactic species have been found in other samples from ALH84001 and in other related martian asteroids. In fact, the shapes of these magnetite beds are remarkably like those of living magnetotactic Bacteria, to the point that the authors have more recently claimed to be able to determine the strain of Bacteria that created the fossils!

However, all of the claims about the magnetite beads in ALH84001 are disputed, and similar beads have been synthesized by recreating alternative paausible histories of the meteorite in the lab. The apparent chains seem to be not real - 3D scans of the asteroid show that the beads don't form chains any more that would be expected at random. Mineralogically, many say the magnetite beads were clearly formed where they are, in the rock, in solid state.

One aspect of the ALH84001 asteroid (and others) that is clear is the fact that it traveled from Mars to Earth. Other asteroids have made the journey in much less time that ALH84001, and so it seems likely that if there is (or was) life on Mars, it could have been transported here, perhaps alive. It now seems possible (but not likely, by any reach of the imagination, in most peoples minds) that life on Earth may have originated on Mars and 'seeded' to Earth about 3.6 Bya. Movement of asteroids outward from the Sun, against the gravitational field, is extremely rare, and so it is considered unlikely that life could move from inner to outer planets.