Life on Mars

M. Dudouyt’s creepy Martians from the 1917 edition of H.G. Wells’ classic novel, The War of the Worlds

The idea that extraterrestrial life may be out there is one that has captured our species’ collective imagination for centuries. It’s possible that being able to share this incomprehensibly vast universe with someone else makes us feel a little less alone. It’s equally possible that we just want advanced alien technology without the effort and expense associated with inventing it ourselves. Perhaps that’s why the theories suggesting that there is, or ever was, bacterial life on Mars is such an attractive one to Terrestrial scientists.

The presence of even unintelligent life on a planet as nearby as Mars would mean that, however improbable it may be, life somehow found a way to exist twice in the same tiny corner of the galaxy. If that’s the case, what’s to say it’s as rare as we think it is? If life could be found just next-door, we reason, the void stretching between us and another intelligent species might not be as expansive as we previously thought.

Speculation that life could be found on the Red Planet is not a new one – from Edgar Rice Burrough’s John Carter of Barsoom to the Looney Tunes’ Marvin the Martian, little green men (or huge green men, as the case may be) have played a prevalent role in science fiction since its inception. However, it wasn’t until the mid-1990s that fantasies about Martian life began to bleed into the realm of possibility.

HALLO SPACEBOY

On December 27th, 1984, the scientific community received the best Christmas present it had been given in a long time when a National Science Foundation team found a meteorite in the Allan Hills of Antarctica. The meteorite, now named ALH84001, was described by the team as being grayish-green, highly-shocked, and the rarest find of that season. Additional remarks in the field notes include simply, “Yowza-yowza.”

Nine years of analysis revealed that the meteorite was both of Martian origin and incredibly ancient; radiometric dating determined that ALH84001 is approximately 4.1 billion years old, meaning it formed very shortly after Mars itself came into being. After an impact event launched it from Mars’ surface, ALH84001 had spent nearly 16 million years dancing out in space before crash-landing in Antarctica somewhere around 13,000 years ago.

In 1996, ALH84001 found its way into the spotlight when NASA scientist David McKay reported a shocking discovery: this meteorite may hold evidence of ancient microbial life on Mars.

The carbonate disks found in ALH84001, rimmed with black magnetite rings. Image: Thomas-Keprta et al. (2009)

The carbonate disks found in ALH84001, rimmed with black magnetite rings.
Image: Thomas-Keprta et al. (2009)

LITTLE WONDERS

While examining the meteorite, McKay’s group found that ALH84001 contains flat carbonate disks rimmed with rings of tiny magnetite crystals. Magnetite is a commonly occurring iron oxide that is a natural magnet. Terrestrial magnetite is known to form in igneous and metamorphic rock when under pressure in high temperatures. However, some magnetite is of biogenic origin – it is created by living organisms.

In 1975, Richard Blakemore discovered a group of magnetite-producing bacteria that he named “magnetotactic bacteria.” Magnetite derived from magnetotactic bacteria is very distinct; they produce very fine (as small as 0.000000005 meters!) magnetite crystals bound by intracellular membranes. These tiny organelles are called “magnetosomes.”

The magnetosomes of magnetotactic bacteria are aligned in chains that act like a bacterial compass needle – they orient the bacteria into perfect alignment with the Earth’s geomagnetic field. The bacteria use this process, called “magnetotaxis,” to narrow their search for an ideal growing environment.

Interestingly, magnetotactic bacteria exist in numerous forms in as many varied locations around the planet. Magnetotactic cocci, rods, vibrios, spiriella, and even multicellular forms have been found in diverse aquatic environments, from oceans to lakes to rice paddies. The only things these tiny, living magnets seem to have in common is that they are all Gram-negative, contain magnetosome chains, and live in watery habitats.

A transmission electron micrograph of a magnetotactic bacterium. The dark spheres are the magnetosomes, lined up in a chain along the bacterium's axis. Image: Nature Education

A transmission electron micrograph of a magnetotactic bacterium. The dark spheres are the magnetosomes, lined up in a chain along the bacterium’s axis.
Image: Nature Education

The magnetite from magnetotactic bacteria is so finely, purely, and consistently produced, that to date, no lab has been able to synthesize magnetite of the quality made by magnetotactic bacteria. However, scientists have been able to use both magnetotactic bacteria and isolated magnetosomes in a variety of medical and scientific applications.

SPACE ODDITY

That magnetite was found in ALH84001 is in itself not very exciting; magnetite is made all the time near terrestrial volcanoes and hydrothermal vents. In fact, about 75% of the magnetite crystals ringing ALH84001’s carbonate disks may have been produced by those same processes. What excited the scientific community were the unique chemical and physical properties of the remaining 25% of the crystals.

These magnetite crystals are chemically pure and very fine, measuring a tiny tens of nanometers in size. In terms of size, shape, purity, and magnetic properties, these crystals match the characteristics of magnetite produced by terrestrial magnetotactic bacteria – the same properties that neither humans nor geological processes could imitate.

Many experts therefore believe that these magnetite crystals are a Martian biosignature: a physical or chemical marker of the presence of life. If this is true, these Martian bacteria are the earliest forms of life known to man.

A side-by-side comparison showing the similarities between magnetite crystals found in ALH84001 and those produced by the Earth magnetotactic bacterium strain MV-1. Image: Kathie Thomas-Keprta, NASA Johnson Space Center

A side-by-side comparison showing the similarities between magnetite crystals found in ALH84001 and those produced by the Earth magnetotactic bacterium strain MV-1.
Image: Kathie Thomas-Keprta, NASA Johnson Space Center

LIFE ON MARS?

Whether or not these magnetite crystals were actually made by ancient Martian magnetotactic bacteria is a subject of hot debate. Other potential Martian biosignatures exist, though none so definitive as a mineral of biogenic origin. Methane found in Martian rock samples may imply active biological processes are taking place on Mars, perhaps just below the planet’s surface.

Further, analysis of the carbonate disks in ALH84001 showed that they were formed during what is known as the Noachian epoch on Mars, when high numbers of asteroid and meteorite impacts formed the oldest Martian surfaces that exist today, and water was possibly an abundant resource on Mars. These disks precipitated 3.9 billion years ago in a shallow, sub-surface watery environment near a temperature of 18°C. This means that this magnetite was produced in an environment similar to ones where some terrestrial magnetotactic bacteria are found.

Some experts believe that, rather than being made by bacteria, the magnetite may have been produced by the geological process of thermal decomposition, as often seen on Earth. However, experiments have shown that none of the currently proposed scenarios for geological production of these crystals could have resulted in magnetite crystals with these properties. This doesn’t prove that the crystals definitely resulted from a biological process; it just means that we still can’t rule out the possibility that they did.

MOONAGE DAYDREAM

It may not be a giant face on the surface of Mars, but ALH84001 has brought us closer to finding extraterrestrial life than ever before. Will our search for neighbors somewhere in the cosmos prove fruitless, or miraculously reveal that we are not alone in this vast universe? We may never know for sure, but at least the clues hidden in ALH84001 have given mankind a real reason to hold on to hope, and Congress a real reason to give NASA funding. Perhaps that’s a big enough miracle in itself.

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