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Buckets of Methane

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How do you look for methane in Mars atmosphere from an orbiter?

It seems a bit weird to have a satellite measure the composition of an atmosphere. You'd think that in order to measure the composition of something you'll need to get very close to it, even into it. For a gas however, this isn't really true. In fact, for a gas there are ways to analyse it from far away.

The way this works is spectroscopy.

Light as we see it is an electromagnetic wave, just like the radio waves our smartphone uses to communicate, X-rays to see broken bones, and a lot of other useful stuff. The differences is the wavelength, or frequency of this waves. Light is the part of the electromagnetic spectrum we can see with our eyes. Different colors also are different wavelengths. That is what gives objects color: having a specific color means that the surface of the object reflects a certain wavelength. White objects reflect all wavelengths, black reflect none, blue reflect blue and so on.

If you are looking at a gas, you are looking at very small objects. In a gas, the light interacts with gas-molecules on a molecular level. A gas doesn't make up a big, coherent object like a solid, but a cloud of small objects with the same properties. And that means that a gas mostly is transparent. There is enough space between the molecules for light to shine through.

The interesting part is what happens with the rays of light that don't shine through, but hit a molecule.

Electromagnetic waves interact with things that are sized like their wavelength.

That means long wavelengths interact with big things, short wavelengths with short things. Your phones antenna interacts with radio wavelengths that are short, so the antennas are short. Your ham radio uses much longer wavelengths, so the antennas are much longer. Our eyes work with wavelengths that are much shorter than radio waves and that is reflected in the size of the cells on the retina that detect them.

With our light rays going through a gas we have the same effect. Our light ray carries all colors of the spectrum. If it hits a molecule, only the colors with a wavelength fitting the size of the molecule interact with it. Those colors are blocked by the molecule, while the others aren't. That means our white light ray, composed of all colors, going into the gas on the one side, comes out missing some colors. It is no longer white, but has a different color.

Using a spectrometer, we can now split up this light ray into it's colors (the wavelengths) and for each color we can measure how much intensity we get. Comparing that to the original intensities of the white light ray, we now know which colors got filtered out. Knowing the color, we know the wavelength, and with that, we know the size of the molecule that filtered it out. The more of a certain wavelength is missing, the more molecules of it's size are in the gas.

This is how such a spectrum looks like:

spectrum3.0.jpg

You see all the colors of the rainbow are there, but there are some dark lines. Those lines are the colors that got absorbed by the molecules in the gas. They are called absorption lines.

As mentioned above, light is just a certain interval of the electromagnetic spectrum. For the detection of different molecules a bigger part of the spectrum is used. On the right side of the visible range you go into the infrared spectrum, which we know as heat radiation, on the left is the ultraviolet spectrum, which gives us cancer. UV light is usually filtered by Ozon in the Earths atmosphere - which is a very lucky thing for us, because otherwise live on earth may not be possible.

Infrared light is reflected by what we call greenhouse gases. In fact, this is what makes them greenhouse gases. They reflect infrared - which carries heat energy back on earth, making it warmer. Glass, a transparent solid, does the same - it let's visible light through, but reflects infrared. Methane also is a greenhouse gas, that means it absorption lines will be found in the infrared.

ESAs Schiaparelli spacecraft carries a spectrometer to look for Methane on Mars.

It will simply look down at Mars and analyze the radiation it sends out. If it finds dips in the intensity of the infrared wavelengths that correspond to Methane, we know that Mars atmosphere has Methane. We can see Mars from earth, which means we receive radiation from Mars, but we still need to go there to get better measurements. Because there might only be very little amounts of Methane on Mars, the dips in intensity will be very small. The radiation we receive on Earth is already to weak for us to find a dip that small, so we try to get closer to get a better signal.

But why do electromagnetic waves interact only with molecules their own size?

For the same reason your guitar strings emit a certain tone. If you ever listened to good music, you heard guitar feedback. Usually, a guitar string emits a certain tone when it's plucked, because it vibrates at a certain frequency. Feedback happens when the tone makes the string vibrate. The tone is a vibration in the air and the same tone that the string emits when plucked also can make the string vibrate.

The same thing happens with the light and the molecules. If the light has the same color as the molecule, it makes the molecule vibrate with it. And in that moment, the energy of the light ray is transferred to the molecule. The light ray get's weaker because some of it's energy was used to stimulate the molecules vibration.

I quickly wrote this after listening to the Vergecast. I already forgot why. I may or may not come back to fix typos tomorrow (it's midnight now where I am). If there is anything badly explained so you can't understand it, I'm sorry. If there is anything wrong, please give me shit in the comments. I may or may not correct it, but please point it out so others can see it.