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Uncovering the chemistry of interstellar space

Many people imagine the space between the stars as an empty, cold infinity. In reality, it is teeming with extraordinary molecules: More than 300 different types have already been discovered. For her Ph.D. thesis, chemist Kim Steenbakkers studied a number of molecules here on Earth and contributed to proving the existence of at least one of them in space. She will defend her research at Radboud University on 3 June.

"The conditions in space are completely different from those here on Earth," says Steenbakkers. "It is very cold, around -240°C, and the pressure is very low. There are far fewer collisions between molecules than on Earth: here, there are a billion collisions per second, while in space there is one every 10 days."

This means that certain molecules that occur in space cannot survive here on Earth. There are too many other molecules here that they would collide with immediately, causing them to ignite in the air or form new molecules. But how do you study those molecules?

Steenbakkers says, "At the HFML-FELIX laser and magnet lab, they have an enormous refrigerator that can cool down to -270°C and in which you can reduce the pressure. This creates conditions similar to those in space." The chemist then sent a powerful infrared laser through the molecule to see how it would react.

She did this with the charged molecules C₂H⁺ and HC₂H⁺, which are thought to occur in space. "We don't have these molecules on Earth because they react immediately with other molecules here. But we do have laser gas, which is used for welding. That's C₂H₂ and is very similar to HC₂H⁺ and C₂H⁺."

Welding gas is highly flammable and reacts immediately with air during welding. But if you take that welding gas, put it in a machine and fire a lot of electrons at it, it breaks down and you can extract HC₂H⁺ and C₂H⁺.

By then firing an infrared laser at these charged molecules, Steenbakkers was able to obtain a kind of "fingerprint" of these molecules. To do this, she had to set up entirely new experimental methods and develop advanced theoretical models to understand the data. "Once you have such a fingerprint and understand it, you can see if we can find it in the data collected by telescopes."

The method used by Steenbakkers has already helped find another of these exotic ions: CH₃⁺, which is methane (CH₄) with one less H. This molecule was observed in the Orion Nebula with the James Webb Space Telescope, in an area where stars are born. "But we expect this molecule, and the others I investigated in my thesis, to occur in many more places in space."

Steenbakkers states, "If we know exactly what the chemistry of space looks like, we can deduce how stars and planets are formed and how far a nebula is in its life cycle. Ultimately, it could also tell us something about how life on Earth originated and whether life can arise on other planets."