The title of the article might sound weird but astronomers have used the Very Large Telescope (VLT) in Chile to conduct an experiment that searches for evidence for the presence of life on Earth. They observed the light that illuminates the moon, which is predominantly made of the light from Sun (sunshine) and a small amount of sunlight that Earth reflects onto the moon (earthshine). Analysis of the components of earthshine enabled the astronomers to confirm the presence of life on Earth. This experiment is important as the same technique could be used to search for evidence of life on other planets.
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| Credit: ESO/L. Calçada |
When light from the sun hits the earth, the gas molecules in Earth's atmosphere absorb light at wavelengths before it is reflected onto the moon as earthshine. Each element has its own characteristic wavelength at which it absorbs light. Due to this absorption, when the earthshine is observed using a spectrometer, certain wavelengths are missing. The dark lines that can be seen in the above picture in the earthshine which are not there in the sunshine are due to this absorption. These dark lines are called Fraunhofer lines. Observation of Fraunhofer lines in the spectrum of light reflected from planets like Jupiter and Saturn has helped scientists identify the components of their atmospheres.
The light reflected by the moon (moonshine) is made up of both sunshine and earthshine. But the amount of sunshine reflected by Moon overwhelms the amount of earthshine reflected. Astronomers have now managed to observe this small amount of earthshine reflected off the moon in the background of intense sunshine by a sensitive method called spectropolarimetry (measurement of Polarization at different wavelengths). This method is important because it is difficult to observe the light reflected by exoplanets because the reflected light is extremely dull in the intense background of the light from the stars that they orbit. The sensitivity of this experiment will be useful in identifying the components of these exoplanets (To understand the techniques used in detecting exoplanets, do read this article).
How does identifying the components of the exoplanets provide any evidence for the presence of life in those planets? On Earth, before life evolved, there was no free oxygen in the atmosphere. All the oxygen was present only in the form of oxides. Therefore the presence of free oxygen can be attributed directly to the presence of life on Earth. Similarly most of methane in the atmosphere is due to the presence of living organisms on Earth. These gases are called biosignatures. By using the spectropolarimetry method if such biosignatures can be identified in the exoplanets, it can give us an indication that these exoplanets may support life.
Along with the Kepler mission, which has been identifying more and more exoplanets in the habitable zone, the spectropolarimetry method provides us with a powerful tool to search for evidence of life on alien worlds.
Very cool article!
ReplyDeleteBut how does presence of oxygen indicate life on a planet? I don't get that part :-/
Thank you for the comment.
ReplyDeleteFree oxygen is not stable in Earth's atmosphere. It reacts with various elements to form oxides. It is only because of life that there is a carbon dioxide-oxygen equilibrium which has given rise to an oxygen rich atmosphere.
If we assume that life on other planets is similar in nature to that on Earth (we have to make this assumption because this is the only kind of life we know), then searching for oxygen rich atmospheres on other planets is a good way to start.
So using this method, have they found life on any other planet/moon?
ReplyDeleteand also, don't volcanoes release huge amounts of carbon dioxide, water vapour, methane and other gases? I mean how can the presence of certain gases confirm life - how accurate are the biosignatures?
ReplyDeleteIt is true that methane is produced in volcanoes. Till recently scientists thought that methane could be used as a biosignature because biotic methane was produced 300 times more in quantity than abiotic methane. But recent studies have shown that this is not true. So methane is no longer used as a biosignature.
ReplyDeleteWhat scientists assume is that if there is high concentration of both oxygen and a reduced gas in a planet's atmosphere, there is a good chance of life on that planet. This is because of the fact that free oxygen is unstable and would immediately react with the reduced gas. On Earth, due the Cardon dioxide-Oxygen cycle, we have high levels of oxygen and other reduced gases.
Thanks for the comment. No, this method hasn't been used to find life on other planets. Only is the last couple of years we have started discovering exoplanets. This method is pretty difficult to use and astronomers are trying to perfect it. They even used the Venus transit to test this method (http://the-bohred-biologist.blogspot.com/2012/05/looking-at-moon-during-transit-of-venus.html). I would say that it will take a couple of years before we start getting results.
ReplyDeleteNice, thanks. As interesting as this sounds, I feel there are way too many assumptions. It's a cool method, but even to assume that all life forms will use oxygen/carbon dioxide is a little far fetched don't you think? Life on earth started using carbon dioxide because it was available, and not the other way around I feel..
ReplyDeleteYes I read your venus transit article related to this.. well, good stuff!
ReplyDeleteThat is true. There are way too many assumptions. Currently the only type of life that we know is what we find on Earth. Since we don't know what else to expect, like I said previously, a good start for the search of extraterrestrial life would be to assume that there could be life on other planets that behave just like the ones on Earth.
ReplyDeleteThanks! :)
ReplyDelete