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[♪ INTRO]
Few things in the night sky are as beautiful as comets.
They’ve been recorded by ancient people all over the world for thousands of years.
And understandably so!
No other astronomical object visible to the naked eye shines so colorfully.
And scientists have been interested in their color for quite a while.
One question, in particular, has puzzled astronomers for nearly a century:
if comets’ heads are often green, why is that never true for their tails?
Now, thanks to cutting-edge research, scientists may finally have the answer.
A comet’s green color isn’t just beautiful;
it’s also unstable and easily destroyed by the Sun’s powerful rays.
Comets are icy objects that were left behind
after the solar system formed four and a half billion years ago.
Today, these space snowballs largely orbit the Sun in the vast expanse
beyond Neptune, in areas called the Kuiper belt and the Oort cloud.
And when they’re pulled onto paths that carry them into the inner solar system,
they become visible from Earth.
As a comet gets closer to the Sun, it begins to heat up, which transforms it
from basically a ball of ice into the more extended object we’re familiar with.
In general, comets, when we see them, can be broken down into four distinct parts.
The most substantial is the nucleus, which is the hard, icy object at its center.
Around the nucleus swirls the coma, an envelope of gas and dust
that has evaporated from the nucleus as the comet heats up.
Then, there are the tails.
Most comets have two distinct tails.
The dust tail consists of bits of the coma that have drifted away
as the comet hurtles through space, kind of like
the dust behind a car as it drives down a dirt road.
But when the Sun’s ultraviolet light hits particles in those bits of coma,
it can knock some of their electrons off, forming electrically-charged ions.
This creates the ion tail.
And because it interacts with the Sun’s powerful solar wind, the ion tail always
points directly away from the Sun, regardless of which way the comet is moving.
What makes comets so intriguing is that
each of these components can have a different color.
A dust tail reflects sunlight, giving it a yellow-white hue, while the
carbon monoxide molecules in the ion tail emit
their own light with a ghostly blue color.
The coma also emits its own light, but rather than blue, it can shine bright green.
Astronomers have puzzled for more than a century
over why all these colors can exist at the same time.
After all, if both the ion and dust tails form from material in the coma,
why aren’t they green?
The answer seems to come down to chemistry.
You see, as the comet gets closer to the Sun, it warms up, and the coma forms.
The rising gas expels complex carbon-based molecules into space,
where they are torn apart by the Sun’s ultraviolet light.
As the molecules are broken down, they rearrange,
creating an unusual molecule called dicarbon,
which is two carbon atoms bonded to each other and nothing else.
Scientists were able to figure out as early as 1868 that dicarbon was the
likely source of a comets’ green color,
but they couldn’t figure out why it didn’t spread to its tail.
Dicarbon shines green because of the arrangement of its electron energy levels.
Molecules can emit light as their electrons fall
from a higher energy level to a lower one.
The exact change in energy corresponds to
a particular color of light, in this case, green.
But a potential answer on how this color doesn’t reach the tail
wasn’t suggested until the 1930s:
Perhaps UV light didn’t just create dicarbon, it also destroyed it.
And if that destruction happened fast enough,
the molecule wouldn’t survive long enough to enter the tail.
But the technology to actually test that hypothesis didn’t exist at the time.
That’s because dicarbon is unstable
and can only be produced in energetic, low-oxygen environments.
Space has plenty of those; Earth, not so much.
But, in a 2021 paper, researchers set up an experiment
to simulate the space environment around a comet.
The team shined ultraviolet lasers on the molecule perchloroethylene,
which knocked off the chlorine atoms and left behind a dicarbon molecule.
And the experiment was run in a vacuum chamber
to simulate the conditions in a comet’s coma.
At that point, another UV laser was turned on,
mimicking the intense light of the Sun on the coma.
The researchers observed that the light split apart, or photodissociated,
the dicarbon, exactly as had been predicted in the 1930s.
They were able to calculate the average lifetime of a dicarbon atom in the coma,
which turned out to be around two days,
which was also in line with previous estimates.
And, voilà, the resolution of a 150-year-old cosmic mystery!
As a comet hurtles through space, it forms dicarbon,
but it can’t drift too far into the tail because it gets destroyed way too fast.
A result like this can seem kind of unimportant, after all,
scientists had the basic idea figured out almost a hundred years ago!
But getting confirmation of our predictions is a really big deal
because it gives future researchers the confidence to make their own.
Astronomers have made all sorts of predictions that may not be
testable in our lifetimes, so every success is an encouragement
that the scientific method is leading humanity in the right direction.
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