hallucinating sapience

While it is certainly not conceptually impossible for large, fast growing, fast moving and otherwise energetic lifeforms to arise exploiting chemistry without O2, using oxygen certainly seems to be the easiest way for life to accomplish these things, and so we would expect, that, if multicellular life is at all common, oxygen dependent multicellular life would be the most common among whatever variations are allowable.

#1,2, etc: I suppose it might be possible to look for the signatures of pollutant gases produced by industry as a signature for technological intelligent life. One would have to distinguish these from the products of non-intelligent biochemistry, which would require knowing something about what is and is not possible for non-intelligent biochemistry – a tall order. Perhaps things like CFCs, maybe. One could also try detecting the absorption spectra of non-natural building materials, like concrete and asphalt, which would probably require a planet to be significantly more urbanized that earth is at present to work, or maybe to tease out uniformity in the vegetation signal that might represent agriculture monocultures, though again, distinguishing the signal from something produced in nature without intelligence would be difficult (as would defining ‘intelligence’ – if one detected a water/gas/vegetation signal produced by a planet with 90% of its land surface riddled with beaver ponds, would one call that intelligent?)

Technically, also, life alone could not have produced the O2 signature of today’s earth, because photosynthesis and respiration are in chemical equilibrium – you can’t go from 0 O2 to 20% O2 just by having photosynthetic organisms evolve because when these organisms die and decompose, exactly 100% of all the O2 they produce is recombined with their organic remains to produce CO2 and water. You also need sequestration of organic carbon somewhere inaccessible to oxidation, which probably means some equivalent of plate tectonics and possibly a few chance events like global glaciations to reset the O2/CO2 equilibrium.

Hmm, I really don’t know, they generate energy by photosynthsys and are multicellular, so I would say they are, even though they lack alot of the other features that make a plant a plant.

They can be counted as plants in two respects: for methods such as those discussed (where it is practical to glom all oxygen producers together), or phylogenetically as above (where it is practical to distinguish clades).

Googling a phylogenetic tree, red algae is among the oldest separation early on after uptake of a cyanobacterium (primary plastids), together with or after glaucophytes (unicellular algae, I think, which have putative early remains from plastid bacterial heritage in the form of membrane components).

Then green algae separates out, which some places among plants [TOL], others not [Plant Physiology Online].

For how many of the Earth’s 4.5 billion years would this test have shown life on this planet?

Depends on the sensitivity. Photosynthesizers leaves the earliest traces (both as stromatolites, now known to be organic all the way down, and as isotope traces). Oxygenating photosynthesizers seems to have been early too, see early BIF remains et cetera, but the reducing atmosphere quickly subdued the release oxygen.

I’ve seen models published where oxygen slowly increased to a few tens of percent as ‘excess’ reducing hydrogen has been lost to space. And other models which then have a bistability, switching from these scarce amounts to a substantial part of the atmosphere. (The mentioned oxygen catastrophe.)

But I’m a layman and don’t have insight into the full research here. Though IIRC at 0.2 % there is a “Pasteur limit” or something such, after which yeast finds it energetically favorable to switch to oxygen dependent metabolism. That is, I take it, the biological separation between a “non-oxygen” and “oxygen” atmosphere, and is consistent with the bistability models.

we just don’t know how likely or unlikely the formation of life is.

But we do know the order of likelihood, as life is observed in the oldest available rocks.

It is very unlikely that such a short period of time was a coincidence, but instead shows that life is an easy process. That is also consistent with observations of organics processes in the universe, as well as with the many pathways to life now proposed.

[So many pathways in fact that I've seen biologists claim that they expect the analysis will turn to which of them are the likeliest. The question of if any realistic pathways at all are possible seems passed.]

If seen as a process instead of a coincidence a model would tell us how few habitable planets would be without life.

At the very least a Poisson model would do it. Hmm, say 1 Gy as an upper limit for life on Earth. (Though, as I understand Miller, 10 ky is likely.) That would give P(no life on Earth analog) = P(no life at 5 Gy) ~ 0.7 %, unless I’m mistaken.

Using Miller’s estimate, I get something like 10^(-250 000) as the no-life likelihood. Low enough unlikelihood for ya’?

Unfortunately, while we seem to know the likelihood, a test of these models correctness must await observations. So while we arguably know what to expect, we don’t know what we will actually find. But that is but another thing what makes science exciting!

I have to disagree with you that ozone is more stable than molecular oxygen. … Lastly, ozone can only be made through a radical reaction of oxygen so you have to have some levels of molecular oxygen in the environment in order to make oxygen.

That’s what I thought, or at least that the ozone from oxygen was sufficiently different in scale to enable life detection. I’m quite sure that it has been proposed as an observation of life, because as we can suspect from the spectrogram it gives a huge signal.

Maybe we should ask Phil in the skeptics spririts to produce another “old planet” where ozone has been observed but not an oxygen atmosphere?

What would be the easiest best test for intelligent life then assuming you can’t see artifacts from orbit or there are no radio emissions? Lights on the dark side of a planet?

I think that is a good guess. But also common pollutants such as anomalous large amounts of nitrous oxides and so forth.

There was this astrobiology webcast where they looked at future detectors with potentially sizes comparable to the solar system (from separation of smaller units), and how they could resolve continental size areas on (some) exoplanets. Light pollution on continents was IIRC mentioned.

@ mike burkhart:

Of course, but we also have to develop remote observation methods of life. And we have only this one reference object to look at…

I was trying to point out to DA that the presence of O2 in a planet’s atmosphere definitely indicates life on that planet, since the only processes we know of that can produce O2 in large quantities (which is extremely reactive and will go away very quickly if not replenished) are biological. However, the lack of O2 does *not* imply the lack of life. For the first 2 billion years of Earth’s history, there was no O2 in the atmosphere, even though there was life for most of this time, and O2-producing life for a good part of it. When cyanobacteria first started producing molecular Oxygen (in the oceans), it immediately combined with disolved Iron and precipitated out, resulting in the banded iron formations that are the most common iron ores on Earth. Only after all the disolved iron had been removed (about 2.5 billion years ago) did O2 start accumulating in the atmosphere.

there will be intelligent life detected on earth, which ones?

Mice and dolphins?

42

J/P=?

BTW there are only 81 stable elements, hydrogen to lead, less technicium. All the heavier elements are unstable, though some have long enough lifetimes they can still be found on Earth.

“Kepler is not capable of spectroscopy but should find hundreds of earth sized planets.”

Kepler will find nothing. He died dozens of years ago, killed by the stench of Tycho’s festering chopped-off nose!

But a future space telescope, TPF, should by 2016 be able to do spectroscopy like LCROSS did, but for these exosolar planets, and it might be able to detect the signature molecules of life.

Still, please remember that we do not know for sure two related things: (a) exactly how did life originate on earth, and how complex was the process (b) are there enough planets “out there” so that life would be statistically able to form on them, and that’s the rub, we just don’t know how likely or unlikely the formation of life is.

A case could be made that there are enough planets so it’s inevitable that life is out there.

A case could be made that the probability of the formation process of life is so infinitesimally small that all the planets in the universe times this probability would give a very low probability of life being out there.

Even Earth didn’t need O2 for life for the first billion years or so of life’s existence – all the O2 was originally a poisonous by-product of other biological processes.

The point is not that you need O2 for life, it’s that you don’t get O2 without life. Which is not to say that all life must produce O2, merely that if you see O2, you can be pretty sure that there’s life there. There may be other forms of life which would not be detected by this approach.

Probably in other Galactic systems it could be Hydrogen or Nitrogen or some completely new element.

Probably not – the chemistry is rather different. And it’s not going to be “some completely new element”, because we already know what all the naturally-occurring elements are and there is nowhere in the periodic table for a new one. Chemistry is the same wherever you go, because physics is the same wherever you go.

Oxygen is a particularly useful chemical for life because it enables all sorts of energetic chemical reactions.

Gregg:
Just out of curiosity: For how many of the Earth’s 4.5 billion years would this test have shown life on this planet?

For at least the last 2.4 billion years, since the The Great Oxidation.

El Satélite de Observación y Detección de Cráteres Lunares (LCROSS), parte de la misión del Orbitador de Reconocimiento Lunar (LRO), realizó una calibración de rutina de sus instrumentos hace unos días, apuntándolos hacia la Tierra y viendo qu…

We have to look for what we already understand and what we know is that for energetic life to exist, O2 is essential.

There are only 92 stable elements in this universe and only a very few seem capable of entering into biological processes. Silicon is chemically similar to carbon in it’s chemical reactivity and “as common as dirt” but life here “chose” to use carbon as it’s basis(DNA made from Carbon, Hydrogen, Oxygen and Nitrogen). One would think, if silicon were a possible basis for life, we would find examples of such buried somewhere, in a geothermal vent or deep mine or,,,well, you get the picture. The chemical processes of life are not exactly random. They depend on how stable molecular structures need to be in order to form a more perfect union as well as how easily they can be remade into other structures. Most of that reactivity is dependent on the particular atomic/molecular valence levels of life elements. Before life stumbled on O2 as a reducing molecule, it was dependent on slow chemical processes and evolution was equally slow. A billion years of one celled, anaerobic life went into evolutionary hyper-drive when O2 came along, which produced,,,us and that’s what we’re really interested in,,,critters similar enough to us that we would recognize them. H2, Silicon based or some other exotic life chemistry is likely beyond our ability to detect until we can actually hang out on such a planet long enough to figure out how these rocks manage to make more rocks. Ah HAH! it’s because they’re ALIVE,,,in other words, what is the sound of two rocks humping??? and do they produce more rocks???

GAry 7

http://www.dailymail.co.uk/sciencetech/article-1204254/Has-mystery-Mars-Monolith-solved.html

Buzz Aldrin stokes the mystery of the monolith on Mars

*sigh*