Definitive confirmation of life without taking a physical sample?

jarroe

I don't think it’s possible, but just wanted to confirm since there are some lofty IQ's floating around the forum that would know for certain. More of a fundamental Astrobiology question.

Can we determine life 'from a distance' with a spectrometer or by any other means?

I believe we have the ability to detect organic compounds that would indicate life process' going on, but it would not be definitive for life itself is this correct?

Mass spec, or a culture are the only ways I know of, and both involve a physical sample. Some animals/bacteria do give off a fluorescent light after UV excitation however, and that would be detectable from a distance, so perhaps I've answered my own question in part, but limited to only species that 'glow in the dark', and I'm not sure how far we could detect that?

So, can we detect life from a distance on a planet/moon/planetoid/asteroid with current technology?

Ygggdrasil

Well, if aliens were looking at Earth, they could infer the presence of life on Earth from the high concentration of oxygen in the atmosphere. Oxygen is a relatively unstable compound that would disappear from the atmosphere through reactions with other molecules if it were not being constantly synthesized by life.

Of course, we have no idea if other forms of life would produce oxygen, so while seeing oxygen in an exoplanet's atmosphere would signal the likely presence of life, its absence would not be proof of the absence of life.

jarroe

Thinking more from an orbiting probe prospective. I guess infrared analysis might give some clues as well if larger life forms were present and gave off a heat signature. I doubt that would be detectable from orbit however with today’s technology?

As far as microbial growth/life I would imagine that it would be impossible to definitively determine without a physical sample being obtained?

Like you mentioned, we could look for clues from chemical composition analysis from the atmosphere/surface that would indicate life could exist.

phinds

I'm not sure just how good the resolution is, but I'm pretty sure orbiting satellites around earth can see things as small as a house-cat w/ IR, and if you HAVE a satellite in orbit why would you not also use regular vision in daylight?

Animal life smaller than a mouse might congregate in a way as to be identifiable from IR (e.g. plankton)

Darwin123

You are obviously interested in the remote detection of microorganisms. Macroscopic creatures could in principle be photographed by telescopic methods. So you apparently are most interested in more general methods of detecting life at a distance.
There is probably no one measurement by itself that would establish the existence of microorganisms from a distance. However, there is a method that in principle could detect microorganisms. The method is based on the hypothesis that regardless of what form the life takes, it creates optically active mixtures of compounds.
Life on our planet tends to prefer molecules of a certain chirality. Although most (not all) chemical processes create racemic mixtures, self catalyzed chemical processes can amplify the optical activities of compounds. Life is one of those self catalyzed chemical processes. The only known processes that exclusively select one chirality over the other on a laboratory time scale are biological. In a compound, the selection of one chiralaty over the other causes a set of optical properties in that are collectively known as optical activity. Optical activity has been studied as a possible means of detecting microorganisms with low probability of false alarm.
The optical activity of biological molecules can hypothetically detect organisms at a distance. Optical properties that depend on chirality include circular birefringence and circular dichroism. Polarized scattering measurements and polarized transmittance have been suggested as possible methods of detecting chirality.
The practical problems in detecting optical activity at a distance are very daunting. The remote detection of chiral molecules is a bit beyond current technology. However, it is theoretically possible that in the future remote biodetectors will use manifestations of optical activity to detect microorganisms.
No practical devices are available, but there are studies. Here are references to articles that discuss the possibilities of using optical activity to remotely detect microorganisms.


Speculation and a table top experiment simulating a remote experiment is presented in the following article. Not much, but I know the author.
“Remote Biodetection Method Using Circular Dichroism”, by David L. Rosen, Applied Spectroscopy 47(11), 1887-1891 (November 2003).


The following two are presentations of speculation on the remote detection of life through optical activity.

http://www.stsci.edu/institute/itsd/...eaming/archive
“Life Signature Remote Sensing”
“Remote Detection of Biological Activity via Circular Polarization of Light” by William Sparks 7th May 2009.”


http://origins.mcmaster.ca/conf/astr...ogy_manset.pdf
“Remote sensing of life using homochirality of amino acids and polarization” by Manset et. al.

jarroe

Very interesting. I never considered that angle. I knew of chiral molecules and isomers mainly with respect to Pharmaceutical products, but never imagined they could be used as biologic markers. I knew there had to be a way, but figured technology just hadn't caught up yet.

Thanks for the good info and links, that's exactly the info I was looking for!

Darwin123

I made a mistake in my citation. The strange thing is that it was my article.
I wrote that article in 1993, not 2003.
Anyway, here is the correct citation.
Speculation and a table top experiment simulating a remote experiment is presented in the following article.
“Remote Biodetection Method Using Circular Dichroism”, by David L. Rosen, Applied Spectroscopy 47(11), 1887-1891 (1993).

Ygggdrasil

I'm not sure that optical activity would be a good indicator of the presence of life. Many in the origin of life field have hypothesized that an enantiomeric excess of certain sugars and amino acids preceded the life and lead to the homochirality of life, not the other way around. Although no one has found any definitive answers yet, a variety of mechanisms have been proposed for the non-biological origin of such an enantiomeric excess through purely chemical or physical means. For a review, see for example Blackmond 2011. Phil Trans R Soc B. 366:2878. doi:10.1098/rstb.2011.0130.

Darwin123

I am on record as saying that there probably is no one spectroscopic marker that is totally unique to biological processes. That being said, there are very few nonbiological processes that would cause an enantiomeric excess.
Scientists have considered the idea that enantiomeric excess could have proceeded organisms. Therefore, an enatiomeric excess could possibly be an indication of prebiotic conditions as well as current life.
If a probe were to determine an enantiomeric excess on Mars, then it would be extremely interesting. I would say that a large enantiomeric excess would cause more impact in chemistry if it wasn't associated with life. It would be a clue to how life started.
On earth, the detection of an enantiomeric excess over a large area would be somewhat reliable biodetector. If one could really determine the spectra associated with these enantiomeric excesses, then one could find out a lot about the living things that caused it.
The real problem would be the optical detection of optical activity. There are optical artifacts that make currently difficult to detect optical activity at a distance. These optical artifacts are a bear. As one example out of many, a small amount of linear birefringence anywhere could create a huge background of circularly polarized light. Unpolarized light also becomes circularly polarized at oblique scattering angles. The optical artifacts, not nonbiological sources of enantiomerism, are the main barriers against to using light to remotely probe distant life forms.