Let's face it: discovering where something came from can be confoundingly difficult. Tracing human lineage is hard, even with gene sequencing at our fingertips. Microscopic life unexpectedly pops up every day in places it shouldn't, like the outside of the space station.
As for picking ancient nanobacteria out of billion-year-old space rubble: even if we found nanobacteria, how could we be certain about its origins? After all, for almost a decade, we didn't even know that the now-famous Alan Hills Martian meteorite came from Mars. To answer the long-standing question, "Is there life on Mars?" humankind needs to level up its sleuthing skills or make fundamental changes in how we look for exobiology. Or, perhaps, we can do both.
Leveling Up by Looking Inward
Knowing what you're looking at is a good first step toward finding what you're looking for. After being spotted during an Antarctic snowmobile ride in 1984, the oldest Mars meteorite discovered to date was erroneously identified as a fragment of an asteroid named 4 Vesta. The space rock, thereafter known as "ALH 84001," remained mislabeled until 1996, when NASA geochemist David Mittlefehldt found orthopyroxenite in the 4.5 billion-year-old sample.
Orthopyroxenite is known to have formed when magma overflowed onto the surface of Mars when that planet was very young. So young, in fact, that this particular magma was on the surface of Mars at the same time that water might have been there in abundance. Indeed, ALH 84001's chemistry is such that, as this particular piece of magma cooled, it interacted with water and captured gasses from the Martian atmosphere.
When it was discovered that the ingredients for Martian life — water, warmth and a (comparatively) thick atmosphere — were all in the same place at the same time as ALH 84001, that mislabeled meteorite went on to become one of the most studied rocks in history. Shortly thereafter, extremely tiny structures that resembled Earth's own nanobacteria were found within the rock. All at once, "Is there life on Mars, and did we just find it?" became the question of the hour, the week and the entire career of some astrobiologists. Though some still believe that these worm-like forms, which are only 0.00002 millimeters wide, may be evidence of ancient life on Mars, many astrobiologists agree that it's less likely that these structures represent once-living nanobacteria than nanobes: 500 nanometer-wide structures that form inorganically.
Looking for life in ALH 84001 turned out to be a lesson in how looks can be, if not deceiving, at least confusing. The more carefully we looked at smaller and smaller slices of the meteorite, the less certain it became that what we were looking at had ever been alive, or perhaps had been a part of something that was once alive. In other words, structures alone couldn't tell us. The chemistry alone couldn't tell us, either. The same chemicals often appear both where life both is and where it is not. When a rover finds methane on the surface of Mars, we wonder, "Did a life form make this or not?"
In simple terms, trying to answer the question, "Is there life on Mars?" based on what life looks like to someone from Earth has thus far been difficult and unsatisfying. Looking for chemical traces of what life might have left behind billions of years ago brings its own challenges. Methane from a peat bog on Earth may, chemically, look indistinguishable from methane retrieved from the surface of an asteroid, comet or Mars rover sample. Fortunately, there may be a way to look at the chemistry not in terms of what's there, but in terms of what shouldn't be there. A potentially necessary shift in the search for life may involve asking the question, "Is there any way to get these molecules without life being involved?"
A Shift Toward Energetic Evidence
The next big leap in finding nanobacteria and other life — fossilized or otherwise — may be Agnostic Biosignatures. Agnostic biosignatures are signs of chemical complexity. Any body in the universe may have carbon, hydrogen, nitrogen and oxygen on, over or beneath its surface. The energy necessary to transform raw materials like carbon and oxygen into complex byproducts like sugar often comes from what we call life.
This definition of life as we know it doesn't have a particular form. It doesn't come from a certain type of place. In this case, the definition of life is boiled down to its absolute essential function: a self-replicating, self-contained series of processes that uses energy to turn simple ingredients into something less simple. In other words, by this method, we look for lifeforms without form — or, at least, without forms we can easily recognize — by focusing on their functions instead.
Practical application of the agnostic biosignatures approach requires a thorough understanding of the planetary environment and a bit of energetic accounting. Astrobiologists like Heather Graham at NASA's Goddard Space Flight Center take a thorough look at the basic chemistry of an environment, examining how much carbon, oxygen, etc. is present. Then they look at what minerals and complex molecules are present.
Comparing the basic ingredients with the complex components they find, Dr. Graham, Dr. Johnson and others who work in the field of life detection then ask: Is this assemblage possible given the amount of energy in the environment? Or would it require more energy? If complex molecules are present, but the energy in the system isn't sufficient to have formed them, the next best answer for how those molecules may have gotten there is… life! But life from where?
Interplanetary Family Trees
What complicates the search for life more than anything else? If it could be summed up in a word, that word might be panspermia. Panspermia is the idea that all living things have a common origin — that the tree of life arose from a single seed capable of taking root in the wider universe. Panspermia may sound like it grew out of science fiction. In fact, the idea of life everywhere in the Universe having a single origin has been considered by many cultures, philosophers, scientists and even a Nobel Prize winner, Svante Arrhenius. Arrhenius wasn't an astrobiologist, an astronomer or a geologist. He was an electrochemist who laid the groundwork for studies of what we now know as the greenhouse effect. Svante Arrhenius was a master of the interplay between chemistry and energy, and it seemed reasonable to him, and to many scientists since, that if all matter and energy in the Universe had a common origin, all life might too.
Whether or not all life everywhere has a common ancestor, extant life on Earth and past (or present) life on Mars might have one. Lithopanspermia — the idea that rocks could have carried the blueprints for life back and forth between the third and fourth planets from the Sun — means that, on some level, life on Mars might be difficult to distinguish from life on Earth.
Carbon-dating might tell us how long ago life on Mars began, how long it hung on and when it ceased. However, carbon dating cannot tell us if life on Mars started on Earth, or vice versa. If the samples currently being collected by Curiosity return to Earth with evidence of life, energetic or otherwise, nanobacteria, or something bigger, the next task may be to determine if that long-gone life arose on Mars, or if it started on Earth and made its way to Mars. If fossilized life from Mars ends up looking like something that descended from the oldest branch of Earth's family tree, we may have the opportunity to ask ourselves who the aliens among us are.
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