Life. Life. What is it?

Biology is often described as the scientific study of life. Which is a pretty good definition, so long as you understand the biological definition of life. What is the biological definition of life? Well, I'm not sure there is one. We are pretty sure humans are alive, as are our individual cells. Plants, fungi, bacteria, these we know are alive. They have cells bounded by membranes. They have genetic material. They grow and reproduce. They exchange gasses with their environment and maintain their internal order by taking in low entropy material and putting out high entropy material.

In high school biology, I was told that viruses are not alive. They are like pieces of paper that say "Photocopy me!" They cause themselves to be replicated, but they don't actually self replicate. They don't grow. They don't exchange gasses. They don't take in low entropy material and put out high entropy material. They don't have a membrane.

Prions, misfolded proteins which cause other similar proteins to misfold in the same way, are even further from the traditional definition than are viruses. Viruses at least have their own genetic material. Prions don't. Therefore, not alive.

But we must be careful about phenomenological definitions. If we define Zebra as "an equid with stripes" then we can make any horse into a zebra by painting stripes on it, and an albino foal born to a zebra mother and zebra father would not a zebra. But an albino zebra is a zebra, and so while stripes are a prominent characteristic of zebrasity they are not a defining one.

So we must be wary of defining things in ways that they have to have some arbitrarily chosen us-like characteristic to qualify. Anthropocentrism is so uncool, and so limiting. Why does life need to have cells bounded by membranes? When earth is invaded by aliens who's tissues don't consist of cells bounded by membranes, are we going to insist that they aren't alive? I hope not. If on Mars we find some manner of critter that self replicates, exchanges gases and takes in low entropy matter/energy in order to maintain a low internal entropy state, but is without genetic material, are we going to say, "damn, we thought we might have life here, too bad?" No. We would say how amazing yet expectable it was that Martian life does it differently than we do.

So I don't quite know what life is, but looking again at that list from my high school biology class, there are certain criteria that make more sense than others. The "cells bounded by membranes" criterion is foolish, based only on the fact that we have only ever observed one related group of living things. It is like defining humans as light skinned after a visit to one family in Norway. The "has genetic material" criterion makes a bit more sense. I would rephrase it as "makes offspring that are more similar to themselves than would be expected randomly." The "grow and reproduce" thing is really two criteria. Growth, I think, is like the zebra's stripes. If we found beings that were built at full size by their parents, and then went on to make offspring that at the beginning of their lives were also full sized, but had high alivitude in every other way, I think we would recognize them as alive. Reproduction, on the other hand, seems a necessary component of a living system. Individuals who don't reproduce can be alive, but it is difficult to imagine a system in which life persists without reproduction. The "exchange gasses with their environment" malarkey is pure chauvinism. Most living things we know do, but is this a defining characteristic? No. "Maintains internal order by taking in low entropy mass/energy and putting out high entropy mass/entropy" is I think, inescapable for any lifeform.

So biology is (maybe) the study of entities or phenomena which make offspring that are more similar to themselves than would be expected randomly (in other words reproduce) and maintains internal order by taking in low entropy mass/energy and putting out high entropy mass/entropy. I'll have to think about this definition and see if I can think of counter-examples.

Musings on the Oceans of Europa

In the sequels to Arthur C. Clarke's "2001, A Space Odyssey" humans discover, and are told to not interfere with, life forms on Europa, the watery moon of Jupiter. Europa is thought to have some tens of kilometer of water ice over (potentially) 100 kilometer of liquid water, with a rocky core that is kept at least partially molten by the stretching and straining caused by the tidal pull of nearby Jupiter, around which it zips every 3.55 days.

It is thought quite possible that the ocean bottom of Europa, as the ocean bottom of earth, could have hydrothermal vents, and as on earth, these vents could potentially support life. On earth, ecosystems around the vents rely on chemosythesis (the production of energy bearing organic compounds by the oxidation of high energy chemicals, in this case hydrogen sulfide) rather than photosynthesis.

A wide range of methods have been proposed for testing the hypothesis that their are lifeforms in the waters of Europa, from biological testing of the waters to robotic submarines to a search for freeze dried lifeforms orbiting near Europa, thrown off into space by collisions with other bodies.

All of these seem like long shots to me, even assuming there is life there to find. Hydrothermal vent lifeforms on earth do not venture far from the vents. The concentration of hydrogen sulfide drops off rapidly as one moves away from the vents. It is likely that a Europan ecosystem based around hydrothermal vents would be very localized, soaking up every scrap of whatever the chemical was that fueled their physiology. Just as there is very little sunlight reaching the dirt in a mature forest, very little of the chemical energy source would likely escape beyond the immediate surroundings of the vents. The autotrophs would be in tight knots around each vent, and the heterotrophs would tend to follow. Unlike our oceans, in which the entire surface is covered with a sheen of plankton, the top of Europa's waters would likely be dead.

This line of thinking points to two other proposed methods of looking for life. One is to map out beforehand where the hydrothermal vents are, and send a probe directly there. This may be doable, if we can use magnetic and gravitational data to map the plates and faults of Europa's rocky interior's crust, and predict the mostly likely spots.

Perhaps simpler as a first step would be to look for the breakdown products of chemosynthesis in the waters. Just as the earliest sign distant aliens would have had of life on earth was the buildup of excessive oxygen in our atmosphere as a byproduct of photosynthesis, perhaps the easiest way to tell if there are Europans is to look for their waste gases.
This of course, is made more difficult that we have no idea what chemicals they would use as an energy source, or how they would process it. We would basically be looking for any sort of surprising chemical imbalance, with the assumption that we can thermodynamically predict what a balance should look like.

Of course bringing back a Europan in a jar would be much more exciting than saying, " we have a significant deviation from our predicted chemical balance." But checking the chemical balance is probably a lot more doable as a first step.