Comets and X-rays? What's Up with THAT?

Comet ISON seems to float against a backdrop of stars and distant galaxies. Astronomers studied it using the Chandra X-ray Observatory and found x-ray emissions it created as it moved through the solar wind. STScI/NASA/ESA

We've all heard about comets, those icy dirtballs from the outer solar system that occasionally appear in our sky. For the longest time — well, before the advent of modern telescopes and astronomy — people didn't know what to make of them. Comets were often called "evil omens" because of their ghostly appearance over many weeks or months. They were more in the realm of soothsayers and magicians, and not really thought of as a scientific problem.

Today, all that has changed. Comets are routinely observed by both professional and amateur astronomers. We can track them easily and study their atmospheres and ices using special instruments on both ground-based telescopes and orbiting observatories. Astronomers have actually landed a small probe called Philae on the Comet Churyumov-Gerasimenkov and the Rosetta spacecraft was put into orbit around it to study the comet as it rounded the Sun and headed back to deep interplanetary space. Based on that work, plus the many observations made by other spacecraft, we now know quite a lot about these once-mysterious icy bodies.

In 1996, the ROSAT (short for Roentgen Satellite) detected x-rays in the vicinity of Comet Hyakutake for the first time ever. Before that time, no one expected to see x-rays at comets. This form of radiation only seems to occur in very energetic and hot circumstances, and an icy body was the last place anybody thought to look for x-rays or to do x-ray astronomy.

But, there it was, despite the fact that nobody really expected to find x-rays. Still, Comet Hyakutake,which was bright and close to Earth at that time, was a perfect target for the ROSAT mission. Much to everyone's surprise, the x-ray sensitive satellite's instruments picked up a signal a hundred times stronger than everyone expected.

The next step was to figure out WHY the x-rays were appearing in conjunction with a comet.

Why X-Rays at a Comet?

Since that time, astronomers have pointed a number of x-ray telescopes at comets and found emissions nearly every time. And, as a result, people have figured out the reasons why x-rays form. Basically, the comet's gases interact with the solar wind — the magnetized stream of charged particles that constantly flow from the Sun and interacts with magnetic fields to form such phenomena as aurorae. Essentially, the charged particles from the Sun slam into gases in the comet's atmosphere. Those gases come from the comet itself as it is warmed by the Sun.

The comet's ices sublimate(similar to what dry ice does in sunlight), and the atoms and molecules of gas (plus dust particles) flow away from the comet's nucleus. So, what's really happening is very similar to what happens when you heat gases in a test tube in a chemistry lab: they glow. 

Scientists know that comets produce x-rays during these high-speed, high-energy encounters. Although most of the particles in the solar wind are hydrogen and helium atoms, the observed x-ray emission is from "heavy" atoms (that is, elements heavier than hydrogen and helium, such as carbon and oxygen).

They have lost most of their electrons. When they collide with neutral atoms in the comet's atmosphere they "exchange their charge", and in the process, energy in the form of x-rays is given off. 

Now, here's a fascinating tidbit: the amount of x-ray emission tells astronomers about the particles in the solar wind. A high amount of x-rays indicates that there are more of the heavy particles in the solar wind. 

Comets as Solar Wind Socks

If you watch a comet for any length of time, you'll notice that its tail seems to change. Actually, comets have two tails: a dust tail (which is what you see when you look at a comet with the naked eye), and a plasma tail (the fluorescing gases that interact with the solar wind). The plasma tail always points down the direction of the solar wind as it flows away from the Sun.

It also is very sensitive to conditions in the solar wind and can be affected by changes in wind speed, electrical polarity and by solar outbursts. For this reason, comets are an excellent way to get an "in situ" (local) sample of what's in the solar wind and what it's doing. The study of x-rays at comets are one more method to understand just what our star is doing and how it affects objects in interplanetary space. 

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Petersen, Carolyn Collins. "Comets and X-rays? What's Up with THAT?" ThoughtCo, Mar. 2, 2017, Petersen, Carolyn Collins. (2017, March 2). Comets and X-rays? What's Up with THAT? Retrieved from Petersen, Carolyn Collins. "Comets and X-rays? What's Up with THAT?" ThoughtCo. (accessed January 20, 2018).