But it turns out we can pick out aspects of a comet’s chemical fingerprint without leaving the planet – and those “dirty snowballs” are quite the diverse bunch.
Planetary scientists from the US and Japan, led by Johns Hopkins University’s Neil Dello Russo, examined the chemicals in 30 comets, focusing on the coma – the hazy halo of material around the solid nucleus – and tail of each.
Writing in Icarus, they found each comet had its unique chemistry – but those that whiz around the sun more often tend to have lower levels of the most volatile substances.
“When NASA or [the European Space Agency] sends a mission to a comet, we can learn a tremendous amount of detail on that specific comet,” Dello Russo says.
His work, he adds, puts those findings into a larger chemical context: “We can help answer where an individual comet fits into the population of comets.”
Comets are thought to be largely unchanged clumps of rock and ice leftover from the formation of the solar system 4.6 billion years ago. Knowing where and how comets formed, evolved and were stored can give insights into the solar system’s youth.
The solar system has two main comet storage reservoirs.
Just beyond Neptune is the Kuiper belt – a region thought to contain mainly icy bodies such as comets. It’s a disc roughly 30 to 50 astronomical units away (where one astronomical unit is the distance between the sun and Earth).
But far beyond that is the Oort cloud, a vast spherical shell icy bodies – such as comets – that surrounds the sun between 2,000 and 50,000 astronomical units away.
Sometimes, a comet makes the journey towards the sun, loops around and whizzes back out again.
Those with a round trip of more than 200 years are called long-period comets; if the journey is made in less than 200 years, it’s in the short-period group. (Comet 67P/Churyumov-Gerasimenko, which skirts around the sun every 6.5 years, is a short-period comet, while the long-period Comet Hale-Bopp takes more than 2,500 years.)
Uncovering the chemical composition of these primordial bodies has only recently been made possible with advances in infrared detection methods.
Seeing comets in the visible part of the spectrum is pretty, but it’s the infrared that unveils their chemistry.
Different infrared wavelengths correspond to different molecules – particularly symmetric hydrocarbons such as methane.
As volatiles sublime (transition from solid to gas) they form the coma and tail, giving planetary scientists a proxy for the chemical composition of the frozen nucleus.
So from 1997 to 2013, Dello Russo and colleagues gathered data from four Earth-based telescopes on molecules such as methane, formaldehyde, ammonia and carbon monoxide from 21 Oort cloud comets and nine Kuiper belt comets.
While each comet had its own chemical fingerprint, they noticed short-period comets were relatively depleted in the most volatile molecules measured: acetylene, ethane, methane and carbon monoxide.
It may seem logical that short-period comets, with more passes close to the sun, would have fewer volatiles. But previous work shows no correlation between those molecules and number of orbits.
So Della Russo and colleagues conclude that the comets were “built” with less of those chemicals in the first place – or were kept in conditions that enabled those molecules to react into different ones more easily.