A good on-line resource for information on Astrobiology is the NASA Astrobiology Institute home-page.
Thursday December 4
I will begin this brief summary by outlining some of the things that had to happen for us to be here, having this class:
But now let me ask a question: "What is `life'?"
I ask because we have only one example of a biosphere. Is terrestrial life a typical example of life? Is it unique? When we look for other life, what should we be looking for? How do we search for life? For intelligence?
Here's a possible definition: Life is something that extracts energy from its environment, and uses that energy to modify its environment in order to help preserve and reproduce itself.
This is intentionally broad. But when pressed to decide how and what to look for, we tend to narrow our scope a bit. Specifically, we tend to consider life in the context of Terrestrial Life. Terrestrial life is based on carbon-chain chemistry in aqueous solution.
Carbon is a very good element to use as the basis for biology. It is relatively abundant, and it forms strong double-bonded chain molecules. Such molecules are very good a serving many of the needs of organisms. They are good for building structures (proteins). They are good for storing and transfering energy (fatty acids and carbohydrates). And they are good for storing and transfering information (DNA and RNA).
No other element does these things as well as carbon. Silicon is the next best thing. But the abundance of silicon in the Earth's crust is substantially higher than that of carbon. And yet Terrestrial biology uses carbon, not silicon.
So we typically assume the other life would follow the same chemical template as does Terrestrial life. But you should keep in mind that this may not be the case.
Another reason why we favor the notion of carbon-based life is that basic organic molecules are fairly easy to make. This was first demonstrated in the 1950s in what is now known as the Miller-Urey Experiment. In this experiment, a sealed, evacuated apparatus was then filled with H2O, CH4, NH3, and H2 (to imitate what was then thought to be the composition of the early terrestrial atmosphere). They heated the apparatus, and discharged sparks into the mixture (to imitate the effect of lightning).
After allowing the system to run for a week or two, they opened it up to see what they had. And they had LOTS of organic compounds. Much of it was just long carbon-chain compounds. But they had also synthesized fatty acids, urea, and amino acids. Thus they had shown that the basic constituents of organic chemistry could be produced abiotically (that is without living organisms) in conditions similar to those that obtained early in Earth's development.
Further evidence that carbon-based chemistry is the way to go in general comes from analysis of carbonaceous chondrites, revealing them to have a number of complex organic compounds in them, including amino acids. We can tell that these are not due to terrestrial contamination because one feature of amino acids is that they demonstrate chirality, or handedness. That is, one can make two versions of any amino acid that are as similar to one another (and as different from one another) as your right and left hands. They are mirror-images of one another.
All terrestrial life uses ONLY the left-handed amino-acids.
This is another indication that all terrestrial life had a common origin. It also points back to a comment I made above about finding complex carbon compounds in meteorites. One set of such compounds are amino acids. But the amino acids we find in meteoritic samples are racemic. That is, they are equal parts right- and left-handed. This is clear evidence that the amino acids are NOT due to terrestrial contamination.
We also detect a number of moderately complex organic compounds via molecular line emission from dense molecular clouds in interstellar space.
So the basic chemistry for life is ubiquitous. It occurs in environments much harsher than anything we see on the Earth. This gives us some hope that life may be common in the Universe.
We know from the fossil record that life on Earth began very soon after the Earth formed. The Earth formed about 4.6 Gyr ago. The oldest known fossils are stromatolites that are about 3.5 Gyr old. Stromatolites are basically fossilized algae mats. So there was organized life on Earth within 1 Gyr of Earth's formation. This includes the time when the planet was still so hot it was basically molten. Thus life must have begun on Earth nearly immediately once conditions allowed for it.
When we say "Search for extraterrestrial life", we are usually thinking about little green men with ray guns. But if we use our own example, we see the situation is quite a bit different. For about 85% of the history of terrestrial life, there was nothing around more complex than single-celled organisms:
There are three main places that people hold out any real hope for as potential spots for life (past or present) elsewhere in the Solar system.
One of the really historic events of the last decade is our discovery that other planetary systems exist. The question "Are there planets around other stars?" is thousands of years old, and we finally know the answer is "YES!"
I discussed this a month or two ago. And while this is a huge advance, we still don't have any idea how common planetary systems are. Do all stars have planets? Only a few? We just don't know yet.
This is relevent for our present discussion, as life as we understand it requires a planet.
The problem of timescales comes up again here. If most of the life in the Universe is single-celled organisms, we don't have any good way of finding it outside our Solar System. The only life we are likely to find is life we can communicate with. This brings us to the Search for Extra-Terrestrial Intelligence, or SETI.
One of the only relevant questions we can ask in SETI is "How likely is it that there are other civilizations in the Galaxy for us to communicate with?"
Now that the question is on the table, how do we go about answering it?
We can't answer it directly. We don't know enough. But we can at least figure out a way of asking the question that tells us what we need to try to figure out. This is typically couched in terms of something called The Drake Equation:
Nc is what we're after: The number of other technologically advanced civilizations in our Galaxy, with which we could potentially communicate. What are the inputs that go into that estimate?
So the Drake Equation doesn't give us predictive power, so much as it gives us a structure for studying the problem. It tells us what we don't know, and what we have to try to understand, in order to make any progress.
The most promising part of the spectrum to use for this is the radio regime. This is because radio waves are not absorbed by interstellar dust, so signals will make it clear across the Galaxy (given enough time).
Now, think about the implications of this for a minute. This means that there could be guys sitting in an extra-terrestrial frat house on a planet around some star 35 light years away, tuning into Gilligan's Island. Or opera fans 90 light years away listening to Enrico Caruso. We've been leaking low-power radio signals for the better part of a century.
There have been small pilot programs designed to search for such signals from other stars. But we haven't found any yet. This isn't much of a constraint though. There are LOTS of stars, and there is a lot of bandwidth (you have to tune your radio to the right station, after all).
In closing, a sales pitch: If you think this is an interesting topic, we offer an entire course on Life in the Universe -- Astronomy 115.