What can astrobiologists study here on Earth?

The emerging science of astrobiology seems to be lacking its primary object of study.  It seems a little premature to talk about the scientific study of aliens, when we haven't found any yet, and can hardly be considered a space-faring species ourselves.  Despite a name hinting of "space life," the scope of astrobiology includes more than just extraterrestrials.  It is the study of life within a universal context, and Earth is indeed part of the universe.  Earth-based life is the only form of life we currently know of, and in order to make predictions about the potential for life elsewhere, astrobiologists study this data point of one.  Especially...

Extremophiles -- organisms that live in extreme environments, like hot springs or dry Antarctic valleys.  (although.. the name 'extremophile' highlights our bias, as a hot spring environment can hardly be considered extreme to the organisms dwelling there.)  By studying life that thrives in environments normally considered hostile, scientists have expanded our awareness of where we might find life.  Extremes of temperature (both hot and cold), pressure, salinity, acidity, radiation... life on Earth can cope with a wide variety of conditions.  The most extreme survivors tend to be bacteria, but tardigrades are remarkable little animals that have even survived exposure to outer space.  Some extremophiles live in environments similar to what can be found in locations on Mars.  This raises the possibility that if life evolved on an ancient, wetter and warmer Mars, it might still exist there, thriving in a vast ocean of subsurface ice.

Biosignatures -- What constitutes evidence of life?  Before we can find proof of life on other planets, we need to know what we're looking for.  Most of the life on Earth, for most of its history, has been microbial.  From this, we can assume there will be more single-celled life in the universe, making detection somewhat more difficult.  Even the evidence for the earliest life on Earth is still debated, because microbes don't leave obvious bones in the ground.  Microfossil structures have that resemble contemporary bacteria have been found in sedimentary deposits dating back 3.5 billion years; but morphology is not a sufficient indicator of biogenicity.  Processes leading to the preservation of structures on the microscopic scale are not well understood, and what looks like an ancient cyanobacteria could be an abiotic artifact.  Even if Mars is dead now, it might have once harbored life.  To plan future missions and prevent overstating any evidence, astrobiologists attempt to define what makes for clear indicators of past life.

The Origins of Life -- one of the greatest ponderables of all time!  Science has yet to advance a complete theory regarding the origins of life on Earth.  The theory of evolution covers the speciation of life once it has developed, but does not address the question of life's origins.  Darwin wrote to a colleague about the possibility of life forming in a "warm little pond" initiating speculation about the primordial soup, and prebiotic chemists have since tried to recreate the recipe, without success.  The most famous experiment occurred in the 1950's, when Stanley Miller was able to synthesize some of the building blocks of life, including amino acids, from simple molecular precursors.  Currently, the RNA World hypothesis seems the main contender for the origins of life, positing simple life forms of ribonucleic acid, before the development of proteins and DNA (deoxyribonucleic acid).  An understanding of the conditions necessary for the origins of life on Earth can help guide the search for extraterrestrial life, by predicting where the transition from chemistry to biology might also have occurred.

NASA defines astrobiology as "the study of the origins, evolution, distribution, and future of life in the universe."  It is the study of the extent of life in the universe, which includes life on Earth.  Indeed, the study of Earth-based life forms the foundation of how to look for alien life, even if we have yet to find any.

Maybe we're all aliens...

Did life necessarily start on Earth?  What if we were seeded here from another world?


The origin of life on Earth remains an unresolved question.  Fossil evidence suggests the presence of life almost as soon as the planet cooled enough, following a period known as the Late Heavy Bombardment when inner solar system was a veritable shooting gallery of asteroid impacts.  The short amount of time between planet-sterilizing collisions large enough to boil off the oceans and the emergence of life has suggested to some that the early Earth was seeded with life.  This notion that life got its start elsewhere before coming to Earth is known as 'panspermia'.  Despite seeming somewhat fanciful, several prominent scientists have written seriously about the idea -- including Lord Kelvin, Svante Arrhenius, and Francis Crick, one of the co-discoverers of the structure of DNA.  


Panspermia comes in a few different varieties, depending on the mode of transportation.  The most commonly considered being ballistic panspermia, in which microorganisms catch a ride inside a meteorite to another world.  A large enough impact can blast small chunks off a planetary surface into space, where it might drift for several hundred thousand to millions of years before colliding with another planet.  This process of exchange is well documented, as several meteorites have been identified as originating on Mars.  The most famous of these being ALH84001, a Martian meteorite found in the Allan Hills region of Antarctica in 1984.  This small rock rocketed into infamy in 1996 when President Clinton announced the discovery of "microfossils" inside, as seen with an electron microscope.  The scientific community has gone back and forth over the ultimate nature of the shapes, whether biogenic or not, such that there is no consensus.  One upshot of the debate, additional studies have made the case that ballistic panspermia is at least theoretically possible... exactly how likely is still unknown.

Supposed nano-fossils seen in the Martian meteorite ALH84001

Another flavor can be described as diffusive panspermia, in which small spores drifting in the wind reach the upper atmosphere, then continue into space, which drift along solar winds until settling on a new habitat and taking hold.  Spores are reproductive structures well adapted to extended survival in unfavorable conditions, and are released in the billions upon billions of spores by plants, bacteria, fungi, algae, etc..  Given astronomically long odds as a means of interplanetary distribution, this seems almost ridiculously unlikely; but the possibility can't be completely ignored either.


Complicating both of these methods of interstellar distribution are the harsh conditions and vast distances of space.  Ultraviolet light and other cosmic radiation is highly destructive to DNA and the complex molecules of life.  Then there's surviving the ultracold temperature of 3 K (temperature in Kelvin, named for Lord Kelvin, mentioned above as a proponent of panspermia; also -270 ºC or -454 ºF), in the vacuum of space.  Plus, the difficulty of remaining viable after the unimaginably long durations required to travel such vast distances.  Directed panspermia gets around these complications by proposing an intelligent transfer of life between planets by extraterrestrials in spaceships.  As with other theories that fall under the auspices of pseudo-science, this hypothesis provides an explanation without offering any realistic means of testing -- indeed, the lack of evidence is often hailed as definitive proof.  That said... if humanity ever colonizes Mars, directed panspermia will become a reality; and thus it probably exists among any interplanetary species, if any exist.


(I have an irrational fear of scorpions.  They're evil, and I want nothing to do with them.  In my moments of levity, I like to rant about scorpions being on Earth as a result of unintentional panspermia.  Scorpions are like the Norway rat of spacefaring civilizations.  I mean look at them: scorpions are aliens, and they shouldn't exist here.  They're an invasive species.  The only possible explanation is that they were stowaways on a flying saucer, and a visiting intelligence accidentally left them here millions of years ago.)


As fascinating it might be to consider panspermia, it ultimately feels unsatisfying as a hypothesis regarding the origins of life, as it merely deflects the question.  If life got here from elsewhere... how did it get started elsewhere?  Prebiotic chemists study the conditions of early Earth, and try to cook up an analogue prebiotic soup to explain the origin of life.  Out of this study has come a softer version, what might be described as contributive panspermia.  Many of the building blocks of life are fairly common in the universe.  Carbonaceous chondrites, meteorites containing organic compounds, probably contributed ingredients to the prebiotic soup, including amino acids and polycyclic aromatic hyrdocarbons.  An experiment simulating cometary ice formation produced lipids, which can spontaneously form vesicle structures resembling a cell membrane when dissolved in water.  The delivery to Earth of extra-terrestrially derived molecules could have been an essential step in the origin of life.