Titan has been tormenting me. It's an otherwise perfectly
likeable world, with its cold methane lakes and an atmosphere filled with
little haze particles that look like they come from someone playing with the
Tesla coils a bit too long. Admittedly, it might not be ideal for a holiday if
you want to survive, but it's a pretty lively place with lots of interesting
chemistry. But, like many objects of study, it has its secrets that it wants to
hold onto very tightly. I became slightly obsessed with one of these secrets.
As is a common theme in my work, this one involves clouds.
Titan: livelier than
it looks [Cassini Imaging Team/ISS/JPL/ESA/NASA]
Clouds and a trick with the light
Titan is like a planet-sized chemistry kit, where lots of
funky molecules are continuously produced, mainly from breaking down of methane
(CH4) and nitrogen (N2) molecules by ultraviolet
radiation from the sun, and by energetic particles near Saturn. Titan is so
cold that many of these newly-created molecules eventually freeze over and form
clouds of ice crystals. So, unlike the Earth with its water clouds, there are
clouds made out of many different molecules in Titan's atmosphere. We already
know of different Titan clouds that are made out of methane (CH4),
ethane (C2H4), acetylene (C2H2),
hydrogen cyanice (HCN), benzene (C6H6), cyanoacetylene (HC3N),
and dicyanoacetylene (C4N2)*.
Some of the molecules grow bigger and bigger in all this
chemical mayhem, until they become one of the small haze particles that completely
fill up Titan's atmosphere. Because this thick haze layer is obscuring everything
from view, you can't just take normal pictures to learn about clouds on Titan.
You need something else.
If you cannot reach the place yourself, that "something
else" is generally spectroscopy. This means that you disentangle light
into its contributions at different wavelengths or frequencies. Water droplets
do this if they make a rainbow from the part of the sun's light that you can
see with your eyes. Beyond what you can see with your eyes there is more light,
such as the ultraviolet and the infrared. At some of these wavelengths, you can
see right through Titan's haze and look at other things that might be
interesting.
Spectroscopy is sorting
the light by wavelength [BYJU'S]
The great thing about spectroscopy is that different things,
such as different molecules, interact with light in different ways. Each different molecule or cloud particle
leaves a different, and almost unique, fingerprint in the light if you unravel
it into a spectrum.
The misfit
My obsession started around 2005, when the Cassini
spacecraft just started doing its laps around Saturn, with regular passes of Titan.
Cassini could unravel the light all the way from the ultraviolet (very short
wavelengths) to the far-infrared (very long wavelengths). And there in the
far-infrared, in the far reaches of the electromagnetic spectrum, a beautiful
bump could be seen. A previous generation of scientists had already met this
bump when the Voyager 1 spacecraft whizzed past Titan in 1980. Voyager's
instruments weren't as good as Cassini's, and we knew less about what different
molecules and cloud particles looked like in this part of the spectrum, and so scientists
were unsure what to make of this bump. Cassini, on the other hand, showed that
it was not composed of a set of thin spikes that would indicate a gas, but that
its broad appearance means that we're looking at a cloud. But what kind of
cloud?
The mysterious bump in
Titan's far-infrared spectrum. Wavenumber is a measure for the frequency of the
light, radiance is a measure for the amount of light.
This is where things become frustrating. You can normally
work out what a cloud is made out of by comparing measurements of the spectrum
to spectra taken from a laboratory on Earth. In the lab, you can make a certain
amount of the molecule you want, make it freeze by cooling things down, and then
measure its spectrum. Although it is harder than it sounds, this has been done
now for many molecules. Often, the bumps in the measurements can be matched very
well with those that are measured in the labs. In this way, you can identify
what molecule a cloud is made out of and how much of the stuff is there. This often
works, but not always. All the molecules that you would expect to form clouds
on Titan have their spectra taken, but none of these match the bump seen in the
Cassini data. That leaves a hard question: if it's not any of the things that
are measured in the lab, then what could it possibly be?
Collecting clues
We might not know directly what kind of cloud gave that
mysterious bump, but Cassini has given us some clues. For one, we do know where
in the atmosphere this bump is seen. We also know the temperatures and gas
concentrations of the atmosphere at these places, which shows us in what
conditions this cloud formed. It turns out this cloud is located at the poles,
where many clouds form due to the cold temperatures and high gas concentrations.
No surprise there. More useful is that the cloud is located pretty high up in
the atmosphere. This actually rules out a lot of possible suspects, since many
gases cannot form clouds in Titan's atmosphere this high up.
At these heights, the most important gases you expect to
form clouds are HCN, HC3N, and C6H6. And
indeed, Cassini found the fingerprints from these three clouds in places where
you roughly expect them. The far-infrared lab measurements of these frozen
molecules do not resemble the mysterious bump, so these gases cannot possibly
be the origin of the mysterious bump. Or can they? Here, something weird is
going on again. The most abundant of these three gases, HCN, shows cloud
signatures at wavelengths in between visible light and far-infrared light. If
you trust the lab measurements, there should be a huge HCN bump in the far-infrared,
right next to the mysterious bump. But Cassini doesn't see the HCN bump in the
far-infrared, despite seeing other HCN cloud features at the places where you
expect HCN clouds.
So, where is the far-infrared HCN bump? And is it a coincidence that, at the places
where you expect a huge HCN bump, you find the huge mysterious bump? I think
not.
Possible solutions
There are a few possible ways out of this conundrum. In any
case, I think HCN must play an important role. One possible solution is that
the HCN mixes up with the haze particles and other frozen molecules, such as HC3N
or C6H6, which then changes how the HCN bump looks like. Because
this is a likely scenario, lab measurements have actually been taken of such
mixtures of cloud material. Some weaker far-infrared bumps (Titan has several
weird cloud features) do seem to match with Cassini measurements, but there has
been no luck yet for the big mysterious bump.
Another thing that might be happening is chemistry. For
instance, we think that C4N2 clouds might form from
chemical reactions within the mixture of HCN and HC3N cloud
particles. Such an elaborate scenario was proposed, because we do not see any C4N2
gas that could form the C4N2 clouds directly (I told you,
Titan has several weird cloud features). Maybe another weird molecule is formed
that is responsible for the mysterious bump. If that would be the case, there
must be a lot of the stuff, and somehow the stuff only affects the fingerprint
of the HCN cloud in the far-infrared and not at the other wavelengths.
My own personal bet is that the bump is just plain and
simple frozen HCN. I think the frozen HCN just looks a little different on
Titan than it does in the lab. How can this be? This might have to do with the
way the individual HCN molecules are organised into the frozen HCN. A recent
paper** indicates that a different orientation of the molecules might indeed give
a bump that looks a lot more like the bump on Titan than the bump from the lab,
but so far, this has not yet been shown in the lab.
Frozen HCN might be a boring and disappointing solution,
since we already know it's there. We actually have nice pictures of clouds on
Titan that have a lot of frozen HCN in them. It might be boring in some sense, but
because it's different from the normal lab measurements, we might learn
something about the orientation of the molecules and how these clouds are
formed. Maybe the interaction with the haze particles is really important, or
maybe something else is going on.
Are we looking at the
culprit right here? [Cassini Imaging Team/ISS/JPL/ESA/NASA]
Cassini ended its mission in 2017 by intentionally burning
up in Saturn's atmosphere, but we can still make new lab measurements to solve
this mystery. It might be HCN, or it might be something much more exotic. In
any case, we will learn something new.
*There is a nice (free!) scientific review paper here,
although their interpretation of the data is slightly different from mine.
Reacties
Een reactie posten