What about life as we don't know it?

The search for extraterrestrial life seems focused on life that is similar to Earth-based life: with strategies like 'Follow the Water,' and looking for carbon containing compounds.  What about life as we don't know it?

The main reason behind expecting aliens to be similar to Earth life, is that we know carbon-based life works.  Given how little we know about the universe at large, it's reasonable to imagine "strange" forms of life could evolve, completely different than life as we know it.  But without examples, any assumptions are mostly speculative.  How do you look for something when you're not exactly sure what you're looking for?

At first glance, it seems easy to distinguish what is alive from what is not.  Most definitions of life consist of a list of qualities -- mobility, growth, reproduction, etc. -- and anything possessing these qualities is alive.  Loose interpretation of qualitative lists can potentially identify non-living processes as alive; after all, fire consumes resources and reproduces itself.  Borderline cases confuse matters further.  Viruses do not possess a metabolism of their own and must hijack the cellular machinery of a host in order to replicate.  Depending on who you ask, viruses can be considered alive or not; there is no consensus on this basic question.  Lacking a realistic definition of life makes it difficult to predict what other forms of life may or may not be possible.

Silicon-based life is sometimes mentioned as an alternative to carbon.  Chemically, this substitution is entertained as possible because silicon falls directly beneath carbon on the periodic table, so the two elements have similar reactivities.  Carbon and silicon both have four electrons in their outermost shell, and tend to form bonds with four neighboring atoms.  It seems doubtful silicon life could exist as a direct analogue of carbon-based life however.  A primary waste product of our metabolism is carbon dioxide, which normally exists as a gas.  The equivalent, silicon dioxide, or sand, is a little more difficult to excrete at standard Earth temperature and pressure.  If it exists, any silicon life would probably exist in conditions alien to anything we could reasonably survive; perhaps a permanently molten surface too close to it's host star.


Earth life depends on water, such that a cell can be roughly described as a bag of water.  At a basic level, more important for life than water is the existence of a liquid solvent.  Alternate solvents for the chemistry of life have been considered, which would be necessary on worlds where water does not exist as a liquid.  Ammonia is normally a gas on Earth, but is a liquid at temperatures where water is a solid.


In my mind, the most exciting astrobiological target within our solar system is Titan, as it posses the most potential for strange biochemistry.  Titan is the largest moon of Saturn, and is very cold far from the Sun.  All the water is locked up as ice, hard as granite.  But there is a liquid cycle, with lakes of methane and ethane -- which we burn as natural gas on Earth.  The Cassini probe recently spotted seasonal rain on Titan.  Theoretical models have described the potential for life utilizing acetylene as a primary food source, and some calculations suggest we should see more acetylene at the surface of Titan.  That doesn't automatically imply strange alien life forms exist on Titan... but it's one possible answer out of the four or five easiest explanations.


Lacking clear examples of alternative forms of life, we primarily strive to find worlds that could harbor the one kind of life we know for sure exists in the universe.  Carbon based life in a water solvent, like us.

Is there life on Mars?

Mars, the Red Planet, has inspired endless fascination and over forty exploratory missions (even if most have failed to reach their destination).  From Giovanni Schiaparelli's early maps of Martian canali -- interpreted by Percival Lowell as a global network of channels built by extraterrestrials -- to H.G. Wells' War of the Worlds, in which the denizens of a dying Mars invade Earth, mankind has imagined life on one of our closest celestial neighbors.


The Viking missions in the mid-1970s carried four life detection experiments to Mars, to look for signs of bacteria in Martian soil.  One of the tests reported evidence of metabolic activity, but none of the other tests could confirm; most discouraging, a GC-MS analysis found no evidence of organic compounds.  Scientific consensus deemed the seeming metabolic activity a false positive, a result of highly reactive chemicals in the soil, and Mars a lifeless planet.

Or is it?

Interest in the possibility of extraterrestrial life on Mars has been growing, especially since the unambiguous detection of water -- potentially in large amounts -- existing just beneath the surface.  Water is required for all life on Earth, and one primary strategy in the search for extraterrestrial life is 'Follow the Water.'

Satellite images revealed what appear to be water carved gullies, but debate circled around the age of these features.  Four billion years ago, Mars was wet, covered with oceans.  At roughly half the size of Earth, Mars cooled much more quickly and was not able to sustain a thick atmosphere.  The atmosphere of Mars today is too thin for liquid water to persist, any water would sublimate directly from a solid to a gas and likely escape into space.

In 2008, a robotic arm on the Phoenix lander scraped away a small area of soil and found ice.  There could well be oceans of permafrost hiding beneath a dusty red veneer, and deep subsurface aquifers with liquid water heated by geothermal processes, that occasionally bubble to the surface as geysers.  The question now is not whether water exists on Mars, but how much?

Some scientists speculate there is greater biomass existing within the Earth than on its surface.  Even in mine shafts carved a mile deep, there is life.  Bacteria buried in the rock, living in slow motion, with high heat and punishing pressures, extracting energy from chemicals normally considered poisonous.

Given what we know, it seems reasonable to suspect that if life ever managed to become established on Mars, it probably still exists there.  Conditions on Mars are not so different from what certain extremophiles here on Earth tolerate.  I imagine the first mission to drill into the surface will find a thriving Martian community of microorganisms, protected from the harsh UV conditions by a meter or two of rock.  Perhaps not the aliens promised by science fiction, but still infinitely fascinating.

The Phoenix lander also detected a high concentration of perchlorates in the Martian soil.  Recent studies with Earth soil suggest the levels of perchlorate present in Martian soil would break down any organic material under the conditions of the GC-MS experiment performed by the Viking lander.  Maybe... we did detect actually life back in the 70's, and just didn't recognize it.

Which came first, the chicken or the egg?

The chicken or egg riddle is an apt analogy when considering the origins of life.  Just as it's difficult to imagine eggs without chickens or chickens without eggs; what gave birth to the first living thing?  Evidence suggests that the earliest lifeforms on Earth were single-celled organisms that reproduce asexually by dividing in two -- but where did the first cell capable of replicating itself come from?

"Understanding the origin of life may be profitably explored by decoupling the origins of different features of life." - Leo Buss

A living system can be described as having three essential components: a metabolism, an information program, and a boundary dividing it from the environment.  All living things need to eat, to take in nutrients from the environment and generate the chemicals needed to maintain cellular function.  Life requires some way to copy itself, to transmit information about its specific chemical makeup and processes to the next generation.  The collection of metabolic function and genetic instructions are set apart from the outside environment, contained within a semi-permeable membrane.


Origins of life scientists can generally be described as "information first" or "metabolism first." (full disclosure: I work with self-replicating RNA, which puts my primary research in the information camp.)  And here we come back to the chicken and egg paradox...  How were information containing molecules capable of copying themselves without a system for extracting energy and transforming chemicals?  How was a complex set of chemical reactions able to develop without a set of instructions to orchestrate the operation?


I imagine the answer will be found in the synthesis, as that would mark the true origin of life as we know it.  In the meantime, much insight can be gained by attempting to study the systems in isolation.


There is also the notion of "membranes first," in that a separation from the outside is required to maintain the chemicals of life in high enough concentration.  Cells are mostly bags of water and the first life probably formed in water; a semi-permeable membrane allows the inside to take in nutrients and prevent useful chemicals from diffusing away.  It's difficult to imagine "naked chemicals" drifting about in a dilute primordial soup as life.




My apologies if you were expecting a final answer to the origin of life.  Nobody knows.  The best scientific answer is that it probably occurred about 3.5-4 billion years ago.  There's debate over a deep hot origin, with life beginning at geothermal vents on the ocean floor, and surface origins in a warm lagoon with the tides helping mix the first chemical cycles.  The earliest fossil evidence is debatable, since single-celled organisms don't necessarily fossilize well nor preserve the same shape after 3.5 billion years buried in rock.  From phylogenetic assays, LUCA (the Last Universal Common Ancestor) is presumed to be a hyperthermophilic (high temperature dwelling) bacteria.  But nobody knows if that represents the first life on Earth, or merely what was able to survive a horrible cataclysm during the Late Heavy Bombardment of planet sterilizing meteor collisions.


I should also mention panspermia, the idea that Earth was seeded from life elsewhere.  Totally plausible in my opinion, but that just begs the question -- how did the first life on another planet come about?