Is There Life on Mars? Ask a Magnet.
Between three and four billion years ago, Mars was a lot like Earth. Both planets are believed to have had surface water. Those similarities make it a prime candidate for extraterrestrial life. “The assumption is that if bacterial life emerged on Earth at that time, then why not on Mars?” says Soon Sam Kim, principal member of technical staff at NASA’s Jet Propulsion Laboratory.
Mars may also have had a magnetic field much like the one on Earth. Kim believes that the presence of an ancient martian magnetic field could be the key to tracing signs of ancient bacterial life. He has developed methods to detect two forms of the mineral magnetite (Fe3O4) that could act as mineral signatures of bacterial life. One is produced by Earth bacteria to assist with spatial orientation. The other is a respiration byproduct generated by bacteria that use iron in their metabolism instead of oxygen.
Terrestrial bacteria have evolved to make magnetite crystals of a very precise size range: 35 to 60 nanometers in size (a human hair is about 50,000 nm thick). These crystals act as magnets that can help bacteria align themselves with the Earth’s magnetic field – for example, to orient themselves in the direction of higher oxygen concentrations. Bacteria make this precise size range of magnetite because outside of this size range, a particle’s magnetic field points in more than one direction and is therefore useless as a navigational tool. On Earth, that distinct size range makes biogenic magnetite easily recognizable because non-biogenic minerals tend to occur in a haphazard range of sizes.
There is no shortage of iron oxides on Mars — oxidized iron is what gives the planet its distinctive red color. If bacteria evolved on Mars, Kim reasons that they may have used the martian magnetic field in a similar manner, and left behind telltale biogenic magnetite. Such a geologic record can last billions of years because magnetite crystals are quite stable.
Kim wants to use magnetite as the basis for a miniaturized detector that could be carried aboard future missions searching for signs of ancient life on Mars. His detector is based on ferromagnetic resonance (FMR) and electron paramagnetic resonance (EPR) spectroscopy. FMR detects the unique internal magnetic fields of biogenic magnetite. It works because a particle’s internal field is directly related to its size and shape – and because biogenic magnetite crystals fall into a very precise size range, they have a distinctive FMR signature.
The spins of the particle’s electrons orient themselves along the particle’s internal magnetic field. In the presence of just the right frequency of microwave radiation, a resonance occurs that results in absorption of the radiation at that frequency, which can be easily detected. The instrument scans a broad range of microwave frequencies and its detector records absorptions. The result is an FMR spectral signature that is unique to biogenic magnetite.
The magnetite respiration product can be detected through a similar process, but it requires the application of an external magnetic field to create resonance and microwave absorption (EPR). Respiration-product magnetite particles are smaller than those used for orientation: less than 10 nm. At this size, the particles show narrow spectral lines. Some non-biogenic particles show a similar spectral signature, but such samples usually also contain particles with broader lines. A narrow signature alone is a strong hint of bacterial life, Kim says.
Kim and his team are working on a prototype miniaturized detector that is 50 x 12 x 10 centimeters in size, 2 kilograms in mass and consumes 5 Watts of power.
Kim already has applied his method to the famous meteorite ALH84001, which made headlines in 1996 when scientists suggested that it showed evidence of ancient Martian fossils. Much of that evidence is now disputed, but the meteorite contains magnetite that could be evidence of bacterial life. Kim and his team applied EPR/FMR to the magnetite particles in the meteorite in 1999.
The results were tantalizing but inconclusive, as the EPR/FMR signature looked about midway between what would be expected for biogenic and non-biogenic magnetite. “It was very funny,” Kim recalls. He adds that most researchers now believe that the magnetite in the meteorite is not biogenic, but the question remains unresolved. Kim has no plans to conduct further tests. “The amount of sample (that is available) is so limited – with the amount of sample we were given, that is the (best) we can do.”
Still, the result continued to fuel an ongoing debate, according to Henry Sun, a professor of Earth and ecosystem sciences at the Desert Research Institute. “There are other lines of evidence that were supportive of life, but most of them didn’t hold up under close scrutiny. Magnetite is the one that is still standing, though it’s controversial.”
Regardless of the origin of magnetite in ALH84001, its very presence is good news for future Mars missions. “If magnetite is present in one meteorite, (that implies that) it might be widespread on the planet. It shouldn’t be an isolated event,” says Sun, adding that it should be comparatively easy for a martian lander to find samples for Kim’s instrument to analyze: “We know it’s there, but we don’t know what it is telling us.”
Kim’s research is supported by a grant from NASA’s ASTID (Astrobiology Science and Technology Instrument Development) program.