So how do scientists find out what is in an atmosphere? Yes, this can be really interesting because when an extrasolar planet that is earth like is found in would be nice to find out if it could support life by looking at its atmosphere and the way it is made up. Obviously a nice oxygen nitrogen ratio would be nice like Earth but you can’t have everything so it is worth checking before we take a trip there.
The hardest way to look at what is in an atmosphere is to actually go there and sample the atmosphere. The only way we can do that at the moment is to send a probe there. Obviously an extrasolar planet discovered will be much further away. Alpha Centauri for instance is 4.6 light years which would take 4.6 years at light speed. Even if we give the best probe the biggest boot it’s not going to exceed a fraction of light speed so we are confined to the solar system at the moment for this method.
Gas chromatography is a way of separating a gas into its components and measuring the amounts of each component. This is an excellent way of working out what an atmosphere holds but it is one of those methods that needs to sample the gas directly.
Gas chromatography works by pumping a gas (a planet’s atmosphere) along a tube that is filled with packing and coated with a liquid. The soluble liquids dissolve and are released later by applying a gas. The soluble gases that are slowed down take longer to come out from the end of the tube than the insoluble ones. This means that each gas will come out of the tube at a different time. To identify which gas is which the gas chromatography instrument must be calibrated with authentic samples so that we can tell what each test is when it comes out of the tube.
NASA had a program called the Viking program to probe Mars, Viking 1 and Viking 2. There were two components to the spacecraft one part would orbit Mars and the other landed on Mars. The one that landed on Mars carried out gas chromatography just on the atmosphere but on the surface as well. They didn’t find much but the information they did find was what they had until the 1990s. As mentioned before the output of the gas chromatograph had to be identified and this was identified by a mass spectrometer. This is called gas chromatography-mass spectrometry and can only be used if a sample is introduced into the instrument (yes, it is just one instrument).
It is probably a good idea to say here that spectrometry is the same as spectroscopy which is the same as spectrography. There are a few differences but they are very, very small differences.
Mass spectrometry which I mentioned has been used on the Viking missions and is the way that the gases are recognised from the output of the gas chromatography instrument. There is another way though!
Infrared spectroscopy measures the light or other electromagnetic radiation given off or absorbed by the gas. Because the light and the electromagnetic spectrum are used telescopes and spacecraft can find out the makeup of an atmosphere from a distance. This is looking more useful isn’t it?
There are a lot of different types of spectroscopy measuring different parts of the spectrum. This includes the visible, infrared, ultraviolet and x-ray. If you want to see all the different types then try this article spectroscopy.
Infrared spectroscopy is particularly useful. It is used for a variety of things such as forensic science but we are more interested in using it to find out what is in the atmospheres of planets at great distances.
When a beam of light is passed through a prism it is possible to see the visible spectrum and if the beam came from a thermal source the beam would contain all visible wavelengths and the prism would show a multicoloured band. There are two ways of identifying the output of the spectrometer. If you look at the absorption spectrum then you will see dark lines where the molecules of gas have absorbed the light and not interfered with the other light that has not hit any molecules. The other way of identifying gas molecules is to look at the emission spectrum which is the light given off by the gas itself.
For an example of a spectrometer on a spacecraft then the ion and neutral mass spectrometer (INMS) on the Cassini is a good example. It is collecting data to identify the components of Titan’s atmosphere and the magnetosphere of Saturn. It is also having a look at Saturn’s icy rings and moons.
Infrared spectroscopy is used by the wise (widefield Infrared survey Explorer) telescope. That’s a pretty good name for something that is giving fantastic images. Wise is looking at asteroids, cool stars and the most luminous galaxies. Wise is giving some great images such as this one of Comet sliding spring.
When it comes to much further off planets it looks like spectroscopy will be a very good indicator of whether life exists. The more advanced spectrometers get the more information they will give and the better the results so it will be only a matter of time until there is a very similar planet to Earth mentioned in the news.