they've caused earth to undergo several ice ages in the past.
And global warming is of course a man mad
climate change that could dramatically alter our way of life.
Yet I believe Titan's atmosphere may have undergone something far more drastic than
anything Earth has ever experienced, here's why.
So nothing is destroyed at a staggering rate on Titan without resupply.
The present abundance of methane in the atmosphere of Titan should be irreversibly
destroyed by photolysis by UV light from the sun.
And converted into higher order hydrocarbons
within a few tens of millions of years.
Now, that may seem like eternity to human lifespans but
geologically speaking compared to the 4.5 billion years of solar system evolution,
that's merely the blink of an eye.
It Titan's atmosphere somehow sustained its present amount of methane
throughout it's 4 billion year history.
One would expect to find a huge global ocean of hydrocarbons on Titan,
a global ocean.
Maybe a kilometer deep, covering the entire surface of Titan.
This global ocean would serve to resupply the atmospheric methane
throughout the billions of years and also be a catchment for
all of the photoproducts of the intense photochemistry
going on on Titan right now that would've been sustained for billions of years.
But when Cassini arrived on Titan, we found something different on the surface.
We didn't find the entire surface completely covered in a global ocean.
We found, yes, lakes and seas, but
nothing compared to the global ocean we anticipated.
This suggests that most of the time,
Titan might've existed without a lot of methane in its atmosphere.
Furthermore, Titan's atmosphere might return
to such a methane depleted state in the future.
Where did all of Titan's current atmospheric methane come from?
Well, the jury's still out, it's an unsolved problem,
but one of the leading hypotheses is that it is periodically injected
into Titan's atmosphere from the interior.
The abundance or
presence of atmospheric methane has a huge implication on Titan's climate.
Recall that methane is one of the root
molecules in all of Titan's photochemistry.
This wild web of photochemistry that is actually simplified in this figure here
from Atreya et al.
All starts with the photolysis of methane on one side and
molecular nitrogen on the other side.
But without methane all of this stuff, really, disappears,
the haze that shields Titan from our eyes would be gone.
And the ability for Titan's atmosphere to be warm and
puffy would be profoundly diminished.
Also it would make Titan not such a good hiding place, okay,
so as part of my research, I ran a photochemical model to simulate an ancient
Titan atmosphere with less methane, and see what happens.
So what changes between a normal Titan photochemical model and
a photochemical model of this ancient atmosphere, well two things change only.
First of all is your temperature, it's a colder atmosphere so
you have to reduce the temperature.
And also the initial chemical abundance of methane is going to be less,
much more depleted.
Everything else really just stays the same, your reaction rates,
all the physics and chemistry that goes on in here, completely the same.
It has to be the same or else, we have a pretty big problem on our hands.
What do we find when we run our photochemical model of ancient Titan?
Well, we can look at the downward carbon mass flux or basically, the stuff that is
raining down from the atmosphere, raining or snowing down from Titan's atmosphere.
And we have a pie chart here for
the present day Titan with a break down of its constituents, and another pie
chart representing the stuff raining down out of ancient Titan's atmosphere.
I want you to notice to notice two things, one is that present-day
Titan's pie chart is about ten times bigger than ancient Titan.
That's because more stuff is being produced and
more stuff is coming down simply because there's more methane in the atmosphere.
But also look at the ratio between orange which denotes hydrocarbons,
molecules made up of Cs and Hs only.