In previous posts I’ve looked at some likely, and some less likely, candidates for planets or moons in our solar system that could harbour life, including Jupiter, three of it’s moons, two of Saturn’s moons, and Mars and Venus.
Now its time for Earth. Yep, you read right.
In 1990 NASA used the Galileo spacecraft to look for life on Earth. Why bother you scream, whilst hurling your cup of tea violently against the wall? Well, NASA did it to test how well spacecraft like Galileo can find life on planets and moons from space. Call it a proof of concept, if NASA can find life on Earth, then at least they know the tech works, and hopefully won’t miss signs of life on other planets.
Here’s what they found on Earth:
1. Atmospheric ozone. Galileo used a technique called spectroscopy to scan the Earth’s surface and atmosphere. Spectroscopy looks at the light that is either emitted or absorbed by something, and uses this light to work out what that something is made out of. Galileo found that the Earth’s atmosphere emits light at a very specific wavelength that indicates a molecule called ozone is present (you’ve probably heard of the ozone layer). Ozone is simply a molecule made up of three atoms of oxygen; its chemical symbol is O3. Why is this important though?
Well, ozone is present high in our atmosphere as a gas, and molecules of ozone absorb ultraviolet (UV) light emitted by our Sun (this is why the ozone layer is important, it limits the amount of harmful UV light that reaches the Earth’s surface). As ozone molecules absorb UV light they are destroyed, therefore ozone needs to be continually replenished. Now this is the important bit, ozone is created from oxygen, so something must be continually creating oxygen on Earth. Yep, green plants. Finding ozone in a planet’s atmosphere is proof of photosynthetic life, as photosynthesis is the process by which plants trap the Sun’s energy, and it creates oxygen as a by-product. A planet with ozone, and therefore abundant oxygen, is a planet that probably has lots of life.
2. Atmospheric methane. Methane is another gas that Galileo detected in our atmosphere using spectroscopy, and it’s another good sign of life. Methane is a pretty reactive gas and in an atmosphere with lots of oxygen it quickly turns into carbon dioxide and water. So like ozone, it must be continually replenished. Methane can be produced by a variety of processes, but the sources that produce the majority of methane are biological. Certain types of bacteria produce the most methane on Earth as a by-product of metabolism, and these bacteria are common in the guts of animals that eat grass, like cows. Quite frankly, these bacteria produce lots of methane inside cows, and cows “expel” the methane, by farting and burping.
Other animals produce methane too; termites are responsible for quite a lot, as are humans, although more from burning fossil fuels than from farting and burping. But basically, as with ozone, methane is a gas that needs to be replenished, and it’s mostly produced by life, so if you can find both ozone and methane in an atmosphere, then you’ve likely found life.
3. The red edge. Galileo also used spectroscopy to look at the light reflected from the Earth’s surface, and found a particular wavelength of light that is reflected strongly by parts of the Earth, mostly the land parts of the Earth, particularly the parts with lots of plants, life rain forests. Galileo had found chlorophyll.
Chlorophyll is a molecule used by plants to absorb sunlight in photosynthesis. It’s a green molecule, and it’s what gives plants their green colour. In the visible spectrum (wavelengths of light that the human eye can detect, basically different colours) chlorophyll absorbs blue and red wavelengths, and reflects green light, this is why plants look green to humans. Chlorophyll also reflects a specific type of infrared light, whilst humans can’t see infrared light (if we could it would probably be a deep dark red), Galileo could, and it detected a large amount of this light being reflected by Earth’s plants.
If we can find a similar red edge being reflected by exoplanets then we’ll have discovered chlorophyll, or a chlorophyll-like molecule, and likely alien photosynthetic life. This may be a bit less likely than the previous two lines of evidence though. Chemistry and biology suggests that methane and oxygen are two gasses that lots of different types of life could produce, so it’s quite likely that alien life would produce these gasses. However, life may be able to exist without something like chlorophyll, or may have a very different type of chlorophyll, for instance, alien plants may be black rather than green, and would reflect light at a different wavelength; the type of star a planet orbits will likely have a big influence on this. Still, finding evidence of chlorophyll on a distant planet would be a huge find.
4. Distinct radio waves. My favourite sign of life Galileo found on Earth were radio waves that were constrained to very narrow wavelengths and occurred in patterns of modulation that looked complex and unnatural. Galileo had discovered radio and TV on Earth. This is a really big one. The previous lines of evidence indicate life, but radio waves of this type indicate a specific kind of life, intelligent life with electronic technology. If astronomers’ can detect similar radio waves emanating from a distant planet then its pretty good evidence that we’ve found intelligent alien life. They’ll probably be airing repeats of Friends though, Friends always seems to be on TV, no matter where you go.
This might all sound a bit dry and technical, but the really exciting part is that all of these lines of evidence can be found, in theory, using telescopes on Earth or in Earth’s orbit. As telescopes like Kepler and COROT discover more and more exoplanets, and as the tech becomes more sophisticated, then soon astronomers should be able to train their telescopes on these distant planets and begin looking for similar signs of life. I cannot wait until this happens. If evidence of alien life is discovered, then the implications for humanity are huge, and if its not discovered after searching countless exoplanets, then the implications may be even huger.
After its search for life on Earth, Galileo then looked for life on our Moon in 1992 (spoiler alert, it didn’t find any), then, it went flying off towards Jupiter to have a look at the beautiful gas giant and some of its moons. It totally found signs of alien life, but that’s all a big government/NASA conspiracy, so we don’t like to talk about it.
Oh, and if you were hoping for a post on the history of life on Earth, then try here.