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This is a model of how river networks evolve
and change a landscape over time.
It was developed by my advisor, Taylor Perron,
here at MIT, based on theory and measurements
of the effect of Earth's rivers on the rocky surface
of the continents—but I've been using the model
to study Saturn's largest moon, Titan.
Until recently we didn't know much about
Titan's surface at all. We knew that the atmosphere
was mostly nitrogen, like Earth's, and that there
was also a lot of methane in the atmosphere; we knew
that it was very cold, and so there was the possibility
that methane might exist not just as a gas, but also
as a liquid, and so it was speculated that there might
even be oceans of liquid hydrocarbons on Titan's surface.
But we didn't have much more information than that
because Titan is very far away, and even more
challenging is the fact that all of these organic molecules
in the atmosphere make it very difficult to see the surface.
So, effectively, Titan's stratosphere is just full of smog.
And, the observations from Earth can't see through
that smog at visible light wavelengths. So there's just
a couple of windows where you can take pictures through
the smog, and looking at it from very far away there wasn't
much detail about the surface at all.
In addition to the difficulty of actually seeing Titan's surface
and taking the images that we now have, another challenge
that we have to deal with is the fact that it's a very exotic
environment. And so, in addition to the difference in
gravity, there's a difference in atmospheric composition,
and so the liquid that we're dealing with, that rains
out of the atmosphere, runs off of the surface, and makes
rivers, and cuts into the surface, is methane and not water,
and the material that's being incised into is not rock
like it is in most cases on Earth, but mostly water ice.
And so it's not immediately clear that the landscape
on Titan should behave like it does on Earth, and yet
we see striking similarities between the landscapes
that have been imaged on Titan and river networks we see
here on Earth. And in fact, as long as you know what the
properties of those materials are, and you use the right
kind of theory that takes that into account, you can
study the mechanics of hydrocarbon rivers cutting into ice
just like you can rivers of water cutting into rock.
The problems Taylor described in actually seeing
what the surface of Titan looks like were partially solved
by the Cassini spacecraft, which entered into orbit
around Saturn on June 30, 2004.
It doesn't orbit Titan, but it flies by Titan every once in a while,
and every time it does a drive-by, you get a single image
of a piece of the surface, created by the synthetic
aperture radar instrument on board.
It's not a normal picture like the satellite images
in Google Earth, but rather a radar image —
the radar can penetrate the haze.
The radar pictures are the highest resolution view we have
of large areas of Titan’s surface — they’re still kind of
coarse, about 300 meters per pixel at best, but that’s what
lets us see the drainage networks, and they're what
I've been using to study how rivers on Titan have modified its surface.
Here are radar images taken by Cassini.
They reveal bright and dark features that stretch for
hundreds of kilometers in some cases across Titan's surface.
This is one of the images I worked with:
The dark Rorschach blobs are liquid hydrocarbon lakes
near Titan's north pole; Cassini has captured images of
sunlight glinting off the flat surface of a lake, which is
one of the ways we know that the lakes really are full of liquids.
The largest of the lakes in this image is called Ligeia Mare,
named after one of the sirens in Homer's Odyssey.
The white lines are valleys cut by rivers of methane that
I identified and measured. The longest river network in
this image is about 200 km long, though
you can also see dozens of smaller networks.
In this radar swath, which is 100km across, you can
see some of the most distinct groups of river valleys
at the opposite pole, at around 75 degrees S.
One of the valleys appears to meander, similar to how
some rivers on Earth meander.
This image hints at a diverse landscape.
Other studies have shown that Titan is home to 'mountains'
which can reach 2km in height, and vast deserts filled with dunes.
In our study, we compared the shapes of river networks
on Titan and Earth to the river networks in our model.
On the left, you can see a detail from one of the radar
images of Titan’s surface, and on the right, a snapshot from the model.
We found that many of Titan’s river networks are relatively
elongated and spindly, which suggests that in some regions of Titan,
the hydrocarbon rivers have produced surprisingly little erosion.
Based on these results, we conclude that either erosion on
Titan is much slower than on Earth, or Titan’s surface
has recently been renewed, perhaps by a process such as
the eruption of icy lavas, or by tectonic upheavals.
We have a lot more to learn about Titan,
and what we're learning could help us answer some
fundamentally cool questions about Titan's history.
Titan is one of the very few places besides Earth
where we've found active modification of the surface by flowing liquids,
and we're excited to learn more about this familiar process
on an entirely different world.