The Joy Of Science : 13.7: Cosmos And Culture A paper exploring the consistency and makeup of Saturn's moon Titan brings joy with discovery as we learn about our universe of wonders, says astrophysicist Adam Frank.
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The Joy Of Science

This composite image made from pictures taken on Nov. 13, 2015, shows an infrared view of Saturn's moon Titan from NASA's Cassini spacecraft By NASA/JPL/University of Arizona/University of Idaho hide caption

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By NASA/JPL/University of Arizona/University of Idaho

This composite image made from pictures taken on Nov. 13, 2015, shows an infrared view of Saturn's moon Titan from NASA's Cassini spacecraft

By NASA/JPL/University of Arizona/University of Idaho

In today's political climate, it's hard sometimes to remember that science, at its root, is about the fun of discovery. It's a kind of kids-play.

What animates the joy and the excitement and the inspiration so many folks feel in their encounters with science is a very pure sense of: "Wow, that is sooooooo cool."

I found this myself the other day when I stumbled across a scientific study of "dirt" on another world.

Back in 2004, the Cassini space probe eased its way into orbit around Saturn. Its mission was to study the beautiful ringed planet and its 53 moons. Of particular interest was Titan, the largest of Saturn's satellites. Titan is actually the 2nd largest moon in the whole solar system — and is larger than the planet Mercury. It's also the only moon in the solar system with an atmosphere (mostly nitrogen with some methane). About six months after Cassini arrived, it released a small lander called Huygens that dropped down to Titan's surface leaving a legacy of amazing data in the form of images, spectra and other measurements.

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As Cassini prepares for its final orbits of Saturn, it's remarkable to see how that data legacy continues to be mined. Even a decade after Huygens made its descent into Titan's frigid warrens (surface temperatures on the hazy moon are -179 C), we are still learning new stuff.

This reality hit me hard reading a paper published in February — by a group led by Stefan Schroder of the Max Planck Institute in Germany — focused on Huygens's probe's landing. The title of the paper lays out the basic idea: Bouncing on Titan: Motion of the Huygens Probe in the Seconds After Landing.

Using a bunch of instruments on Huygens — like the "accelerometer" to tell how the probe's motion was changing, and its descent imager/spectral radiometer (DISR) to tell its orientation — the researchers mapped out exactly what happened as the parachuting probe hit the ground. Here's the scientists' story:

"The most likely scenario is the following. Upon impact, Huygens created a 12 cm deep hole in the surface of Titan. It bounced back, out of the hole onto the flat surface, after which it commenced a 30-40 cm long slide in the southward direction. The slide ended with the probe out of balance, tilted in the direction of DISR by around 10 degrees. The probe then wobbled back and forth five times in the north-south direction, during which it probably encountered a 1-2 cm sized pebble."

So that's it. Using data that had never been analyzed in this way before, the team could "see" the descending Huygens drive a fist's distance into the ground, bounce back up and slide almost half a yard before coming to a tilted, wobbling stop.

Now I know what you may be thinking: "OK. Fine. That is quite an amazing little piece of scientific detective work Schroder and his collaborators pulled off. But really, so what? Why spend all that time working out the explicit, few-second story of a space probe hitting the ground and coming to rest?"

Well, the answer to that question comes when you ask yourself this seemly simple and innocuous question: What do we mean by ground?

See, Titan is an amazing place. It's the only other world besides ours in the solar system with liquid sloshing around on its surface. There are lakes on Titan and we've even seen it rain there. That's cool enough on its own — but here's the real kicker: Given the cold on Titan, the lakes and the rain aren't made of water but of liquid methane (and/or ethane!)

So in a world of super-cold hydrocarbon lakes and rain, what exactly is "the ground" like? It's not going to be like we have on Earth. Titan itself is a kind of icy-rock-mudball with ice volcanoes and who knows what else. So answering the question "What is the ground like?" was the point of the Schroder study. They weren't unpacking all those details of the Huygens landing just for fun, but to use those details to extract the properties of the surface Huygens landed in and skidded across. Telling that bounce plus skid story was just the appetizer. Using the story to learn something new about Titan's "dirt" was the main course.

So what did they find? Here is the all-important summary: "... the dynamics appear to be consistent with a surface consistency of damp sand."

So there you have it. Take a minute to let it sink in and give your imagination a moment to work its own path on this.

  • Damp sand.
  • Damp with liquid methane.
  • On a frigid but not frozen moon orbiting a giant ringed planet.
  • 746 million miles from Earth.

See, we do live in a universe of wonders. And science is the window that lets us thrill before the view.


Adam Frank is a co-founder of the 13.7 blog, an astrophysics professor at the University of Rochester, a book author and a self-described "evangelist of science." You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4

Correction May 3, 2017

A previous version of this post incorrectly said that Titan is 50 billion miles from the Earth. It's actually 746 million miles away.