McKay presented this lecture, entitled "Drilling in Permafrost on Mars to Search for a Second Genesis of Life," at a NASA Astrobiology Institute Director's Seminar on November 29, 2004.

In this part of his lecture, McKay defines one of the most interesting topics to astrobiology: how would one know an organic relic when it appears?

How would we decide if any relic organics were ever alive? We know the solar system is full of organic material - lots of gooey brown stuff - but it was never alive. If it is like us, we can put it in a PCR and amplify the DNA. But if it is alien, we may not be able to detect DNA.

Here we need some standard methods to detect alien life. The first standard method is to use a tricorder, of course. The second method is just, "We'll know it when we see it."

Neither of these is going to work very well. We don't have any tricorders, obviously, and we don't have any guesses as to how one might work.

I have a suggestion, which I call the LEGO principle. Biology is built from a small number of components, just like LEGOS.

Biology is going to pick a few common building blocks - some amino acids, sugars, etc. - but not the whole spectrum of available biomolecules.

Different life forms could have a different LEGO kits. As a kid, I used Lincoln Logs. You could build anything that you could with LEGOS, but you couldn't mix the two kits.

They won't work together, but they can each be used to build equivalent construction projects.

Will alien life be that different? At the very least, when we go to Mars, we're not going to be surprised if life depends on carbon and water.

At a higher level, we are also okay. Darwinian evolution is going to apply. Big fish are going to eat little fish. There will be photosynthetic organisms capable of using sunlight.

So at the ecological level, life will be similar. At the chemical level, it will be similar. These will converge. It is the middle level, the level of the biomolecules, where life might be different. At DNA, at ATP, etc., it might diverge.

To test the LEGO principle, we plot distributions of the types of molecules. If non-organic, for instance, we will see equal mixtures of right- and left-handed biomolecules.

But if the distribution is biological, it will be unusual. It will be a series of delta-functions, spikes of right- and left-handed molecular types.

So in principle, if we could get a complete molecular picture from the subsurface, we should be able to see this biological signature. Even if martian life is different from Earth's, it still may have a biological distribution.

We may not know now how to do this analysis, but if you had a sample in the lab, you would probably do a gas chromatography/mass spectrometer study. It is not entirely satisfactory, because GCMS is destructive.

Think of it as characterizing a building by blowing it up, rather than going into it.

What you would rather do is not just see structures, but also functions.

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