Astrobiology's favourite martian meteorite, Alan Hills 84001 is shown alongside a Magnetosome, a bacteria that fixes iron within its body to form magnetite and gives itself a free ride (or orientation) courtesy of the Earth's magnetic field. Could the tiny magnetite crystals serve as biomarkers long after the host is gone? And what is the origin of these kinds of features in ALH84001?
Recently, we kicked off the CATP seminar series for the 2011-2012 season! The first talk of the year comes from McGill University's Hojatollah Vali. Dr. Vali's work is mainly concerned with the ability of iron-fixing bacteria to serve as biomarkers long after the bacteria have long since passed away. It's just one way that we can answer the question of how to detect past biological activity in the absence of well-developed morphologically distinct fossils.
For most planets, there are several ways that we can get information on whether they supported a past biology. First, many creatures build hard internal structures or armor plate their exteriors for protection. This kind of biomineralization will be familiar to anyone who has seen a dinosaur skeleton. We can also gain some information from what has been left behind in the rock. Many leaching features in surrounding rock have specific morphologies that suggest a biological origin and certain minerals make better foodstuffs for life than others do. We can also use life's tendency to extract energy from its surroundings and cheat entropy by looking for the coexistence of incompatible mineral phases that would not typically form together. Sometimes we can even get lucky and find organic residues themselves inside the rock. Though these may no longer show much trace of having once been life, they can be suggestive in context. This is especially true when they are combined with isotopic composition. The chemical reactions that organisms use tend to be very sensitive and the slight differences in the zero-point energies of the specific isotopes involved can upset the balance. Thus life selects for one and not others and excesses or depletions of that isotope suggest that life has been active. Though not a direct biomarker, evidence of prebiotic chemistry can also be suggestive.
But on planets with significant magnetic fields, like the Earth, there may be one more trait that can act like a biomarker: magnetic signatures. For many bacterial creatures, it makes sense to precipitate a specific iron mineral called magnetite within or along their bodies. This transforms these creatures into little magnets and allows them to travel effortlessly along magnetic field lines, lending them a mobility they would not otherwise possess or providing them with a way to align themselves to a magnetic field for swimming. What distinguishes the iron crystals created by life from other forms? Their domains are typically aligned, they form in chains of grains and the crystals themselves are elongated compared to their inorganic kin. And the size, between 30 and 100 microns is very particular: this is the easiest size over which you can form strongly magnetic materials easily, yet have the entire crystal be made up of just one magnetic domain.
A schematic of the Earth's magnetic field is shown in this USGS graphic. Note that as long as you are away from the equator, the magnetic field lines have a vertical component as well as a horizontal one. Travel along it far enough and you can either head up in the world or use your orientation to get down.
The existence of such bacteria also tell us a great deal about their environment. While they need some oxygen to form the magnetite in the first place, most magnetosomes are either anerobic or microaerobic (tolerating small amounts of O2) meaning that for many of these creatures, free oxygen can be lethal. Since magnetic field lines have a vertical component, if you are away from the equator, this gives the bacteria another advantage. By orienting themselves along these field lines, they are able to travel back down to lower levels in a pond or marsh, where the oxygen levels are reduced and conditions are more clement.
So what is Dr. Vali's connection with all this? He has done a little work with Astrobiology's favourite martian meteorite, ALH84001. This particular meteorite is the site of many a hotly contested battle about the meaning of the small yet enigmatic shapes found within. Like the Martian dunes and permafrost polygons, they look very much like their terrestrial counterparts despite the problems in scale.
Perhaps tellingly, the meteorite has a significant fraction of nice elongated single domain magnetite crystals of about the right size to have been from magnetosomes. This has been used in the past to argue for a biogenic origin for the other deposits seen in the rock. For Vali, what makes this even more compelling is two major factors. First, the magnetite crystals are located similarly to where you would find them on Earth. They are on the rims of the carbonate deposits, rather than shot entirely through the rock. As well, the magnetite crystals are found distributed randomly within these rims which would not have been the case, had the material cooled from high temperature.
This idea of finding magnetofossils is not far fetched - on some parts of the Earth, such crystals made from uncountable numbers of magnetosomes are the major magnetic mineral present and can persist in the right environments for tens of millions of years at a minimum.
But if these "magnetofossils" are indeed indicative of past magnetosomes on Mars it has far reaching implications. First and foremost, Mars would have needed to have evolved life to the bacterial level. But even more tantalizing are the details of such an environment. For magnetite to have any value to a bacterium, Mars would have needed to have had a significant magnetic field and a fluid in which a creature could travel. Secondly, there would have had to have been a survival advantage to that bacterium of being able to rise or fall within its environment. The most obvious example would be an atmosphere that locally had significant amounts of free oxygen.
Paleomagnetism on Mars today as measured by Mars Express. Note that the highest levels (blue and red) correspond to the ancient highland terrains in the South. If Mars has not had a magnetic field for a long time, the magnetite in ALH84001 must be extremely old to have been a biomarker from a magnetosome.
It's a great picture, but there are problems. Mars has no magnetic field today and it is possible that has been the case for a long time. We know that Mars did originally possess such a field since we can detect small amounts of "paleomagnetism" frozen into the ancient rocks of the martian southern highlands. These are some of the oldest rocks on Mars (Noachian in age) and this suggests that Mars did not maintain a strong magnetic field for long. Furthermore, we know from the history of the Earth that it takes a long time for even wide-spread life to create an oxygen rich atmosphere. This suggests that both factors that together give a survival advantage to magnetosomes on the Earth may not have co-existed on Mars.
As well, over the years, the results found in ALH84001 have been challenged and refined. There are abiogenic origins that have been proposed and as they say: extraordinary claims require extraordinary evidence. The best that can be said, as of today, is that the issue has not been settled definitively. Perhaps the Mars Science Laboratory will be able to help. If Mars ever did sport free oxygen in its atmosphere, there will likely be some quite obvious signatures just waiting to be found in Gale Crater!
Some Background on Magnetosomes:
You can also check out the original paper to which Dr. Vali contributed here:
You can listen to Dr. Vali's lecture and others in the CATP Series here: