Life: Beyond Carbon and Water

We generally make the assumption that carbon and water are fundamental ingredients required for the advent of living systems in the universe. Given that all the data we have to work with is based on the behavior of biologic systems operating on the surface of a single planet, this conclusion, from a purely scientific perspective, is based on insufficient understanding. If we define life as a pattern of physical behavior that is independent of its specific physical and chemical ingredients, then our current universal view of life’s required chemistry and supporting environment is truly suspect. One consequence of this change in perception will be seen in the Sagan-Drake “equation” for the estimation of intelligent life in a galaxy.

The estimation of the number of worlds harboring intelligent life would increase due to an increase in the number of planets capable of supporting biological systems (n_e; see below) if we remove the carbon and liquid water requirements (which implies specific temperature ranges) for the advent and evolution of life. There is no good reason why Carbon and water should be deemed universally necessary ingredients for the advent of living systems up to and including life forms that are capable of thinking in unusally abstract ways. After all, thinking is an electrochemical process (so we think, anyway). There is, however, one obvious reason to think Carbon and water are required components for life. It's what we have found to be the case here on Earth. So, better to rephrase what we are looking for out there: Life like ours. That focuses the question. It's always easier to look for things when you know what you're looking for. Still, if you want to look for something as broad as the notion of life in the universe you certainly can't base your search criteria on what works for a single planet. On to the Sagan-Drake equation (which is necessarily composed of some rather subjective variables).

The Sagan-Drake equation:

N = R^*f_pn_ef_lf_if_cL

Where

N is the number of intelligent communicating civilizations in the galaxy at present

R* is the average rate of star formation in our galaxy (stars/year)

f_p is the fraction of stars that have planetary companions

n_e is the number of planets per planet-bearing star that have suitable ecospheres (that is, environmental conditions necessary to support the chemical evolution of life)

f_l is the fraction of planets with suitable ecospheres on which life actually starts

f_i is the fraction of planetary life starts that eventually evolve to intelligent life-forms

f_c is the fraction of intelligent civilizations that attempt interstellar communication

L is the average lifetime (in years) of technically advanced civilizations

The notion that microbial life is abundant in the universe is certainly a compelling possibility ( requires a high value for  n_e ) if life is in fact an endemic planetary surface property with an evolutionary pattern that is tightly coupled to that of its planet. Perhaps the degree of bio-environmental coupling is a significant factor in determining if the development of intelligent complex life is possible. Certainly, if a planet harbors substantial life (present globally like here on Earth) then the evolution of the planet’s surface will be strongly coupled to that of its biology and the planet will maintain a surface environment capable of supporting life for periods of geologic time. Time, and lots of it, is a critical ingredient in advanced biological evolution. Or is it? I just ranted about the inherent problems with requiring Carbon and water for the advent and evolution of biological systems. Why this geologic time business? That's a good question. I don't have the answer. For now, let's just say that, regardless of specific chemistry and physics, it takes a long time for life starts to blossom into thinking creatures. I agree that this restriction may be too harsh.

Perhaps Mars is an example of a rocky planet that had sparse microbial life (relative to Earth) and therefore Martian biology had little net effect on the evolution of the Martian surface and atmosphere leading to a relatively short geobiologic lifespan and therefore no chance for the advent of complex life. Clearly, this is wild conjecture, but in the next 5 or so years we will probably know the answers to these Martian questions.

For detecting extraterrestrial life we should not only focus on whether or not carbon/water-based life forms can be supported on a rocky planet (geologically active and rocky surface (not a gas giant), like Earth, Mars, Venus and Saturn's wildly interesting moon, Titan), but whether or not a planet possesses surface properties that demonstrate a predictable pattern of behavior over time ( for example, a substantial and dynamic atmosphere (like the consistent addition and removal of Methane from Earth's or Titan's atmosphere) ) which is independent of the specific chemistry and physics operating on the geobiologic level. Now, if places like Jupiter's Europa support biological systems, then throw this paticular solution out of the nearest window since there is no way using this technique to remotely detect the presence of life that lives beneath the surface of a moon with no atmosphere in an ocean of salty water. It's certain that just looking at geologic patterns on the surface of a place like Europa will not provide enough evidence for the existence of life.

Comments

[Anonymous Coward]

> The estimation of the number of worlds
> harboring intelligent life would increase
> due to an increase in the number of planets
> capable of supporting biological systems

It's tough to beat our current odds, though. 100% of the planets we know that has life also has intelligent life. Hopefully, those odds would decrease some in the future.

[Carmine]

AC,

Titan may very well decrease these odds to 50%. It is an artificial restriction to couple the existence of biologic systems with the presesnce of intelligent life. Consider the early Earth, for example. Of course, most of this thinking will continue to be conjecture until we have more data to work with, but it's just so fun to speculate. In fact, speculation is one of the coolest side effects of being intelligent and creative systems.

[Anonymous Coward]

> If we define life as a pattern of physical
> behavior that is independent of its specific
> physical and chemical ingredients

I have some problem with this definition of life. Take one of our virus for example. Its behavior is dictate entirely based on its chemical make-up. Yet, it's managed to even reproduce. If we found a virus on Titan, surely, we'd declare that as the greatest discovery ever. With the lack of a satisfactory definition, perhaps life should be treated the same as porn -- "I'll know it when I see it."

[Carmine]

The problem with the porn analogy is that the life signatures we will be looking for around our galaxy, for example, will be contained in points of light on which spectral analyses will be performed. This means that we have to have some preconceived ideas of what we're looking for. This is why the notion of a life pattern (extended to atmospheric chemical state, for example) is so compelling. Of course, this still falls in the life-like-ours category since, theoretically, life doesn't require the existence of an atmosphere (consider Europa and the interesting possibilities there).

Re virus: a virus by definition can not "live" without the aid of a host which makes copies of the virus as a result of dna hijacking. If we were to detect viruses on Titan, you are correct that this would be the greatest discovery of all time as it would mean that there are cellular genetic systems present on a world besides our own. The things that these Titan viruses infect would be what we classified as life, however.

[Anonymous Coward]

> are points of light on which spectral analyses are performed.
> This means we have to have some preconceived idea of what
> we're looking for. This is why the notion of a life pattern
> is so compelling.

Well, yeah. Spectral analyses can only give us so much. Now, if a planet shows up w/ 80% nitrogen & 20% oxygen, that would be exciting news. Better yet, an "I Love Lucy" episode from a far-off star would be nice. Is it me, but space has became very exciting all of a sudden? Mars Rover & Cassini came out of nowhere.

> Re virus: a virus by definition can not "live" without the aid
> of a host which makes copies of the virus as a result of dna
> hijacking

I used that because it's not clear even here whether that's life. That's definitely more life than not. Plus, I'm not even sure that all us homosapiens are "independent of our specific
physical and chemical ingredients."

[ikari]

As far as I know, there is no *definition* of life yet. We can't be even sure if we want to find a planet with simmilar conditions to Earth's - even on Earth we've found bacterias able to live in extremely high temperatures and feed or breathe with sulfur. Why then do we want to find conditions for *humankind*? There are so many possibilities... They don't have to have cells, DNA, anagolic life-cycles... Everything may be different. The truth is that we know ONLY ONE planet and ONE possibility of life developement. Can we describe the climate knowing only one day weather? No, that's not enough.
All we want from the life we seek for is to exist - we can't define what else would it do, because it doesn't have to correspond with life on Earth.
Well, then.... we don't know what we are looking for?

[Tom Servo]

The search for life in the universe is a nice thing, but we should get FTL travel working first, or at least FTL communications.

[Carmine]

ikari, you are right: there is no completely satisfactory definition of life at this point in time. However, if you look at one form of life as a planetary surface property (like what we have here on Earth, cumulatively) then you have something to work with and you have something to look for that you understand. This is why the atmospheric observation of distant worlds via spectral analysis (to see if the atmosphere is in a state of chemical disequilibrium) is a good idea. In fact, this is exactly what ESA and NASA will be doing over the next 10 years. James Lovelock, though he won't be credited by ESA and NASA, really deserves a lot of kudos for both his definition of life as a planetary property that is tightly bound to the planet upon which it goes about living and devising a means to remotely detect the existence of this type of geobiologic system.

The search for life can be thought of as the search for geobiologic life signatures (atmospheric entropy gradients). Yes, life like ours in the sense that, if it exists on a broad enough scale on the surface of a rocky planet with an atmosphere and interacts with surface geology and atmospheric processes in a cybernetic way (adaptive feedback), then we can find it.

[Carmine]

Tom, faster than light technology will probably be achieved sometime after we detect other worlds bearing life. We already have most of the technology to look for life around our galaxy. In 10 years, we will definitely be able to find life similar to what we have here (if only similar in how the cumulative effect of the extra-solar life perturbs planetary surface properties like atmosphere and geology). And, as I've mentioned in an earlier post, there is still the possibility that living systems are present on Titan. And, we can get there without the need for FTL technology.

[Paul Golovin]

"Big Numbers aren't so Big"
I am Amazon-reviewer "pg--az", who synchronicity-wise just endorsed David Swift's "Evolution Under the Microscope". A pointed quote from page 243:
"the frequency of individual (DNA) mutations( about one in a billion ) is comparable numerically with the years of geological time( a few billion )...if the large size of bacterial populations and their short generation time means they can find three simultaneous mutations in one year( due to about 10**27 search attempts ), then comparable populations would have a good chance of finding four simultaneous mutations in the whole of geological time, but probably not five."

Pgolovin believes everyone who chats about astronomical numbers should read this book - Swift tries to point out the implications, if you really believe that mutations are random-independent, how awesomely hopeless the statistics of dead-dumb-trial-and-error actually is. This book is perhaps four times as detailed and throrough as "Darwin's Black Box", which itself is a great read, sez me.