Move Over, Mars: Saturn’s Moon Titan Is Surprisingly Earth-Like

Titan may be an outer solar system world, but it has inner solar system cred.

Jun 18 2015, 6:50pm

Titan. Credit: NASA

Over the last few years, scientists have discovered several Earthlike planets orbiting other stars, suggesting that our comfy planetary setup is not all that unique in the universe. In fact, it may not even be that unique in our solar system, as Saturn's fascinating moon Titan has repeatedly proved.

Titan which is larger than the planet Mercury, and it's the only terrestrial world outside of Earth and Venus to have evolved a dense atmosphere. It's also the only place other than Earth known to boast liquid bodies on its surface (though Titan's lakes and rivers flow with liquid methane and ethane, not water).

But the similarities between Earth and Titan do not end there, as evidenced by two new papers about this unusual Saturnian moon published this week. Indeed, the first of the pair actually used Earth as an analog for figuring out a long-standing question about lake formation on Titan's surface, according to the European Space Agency (ESA).

Size comparison of Titan, the Moon, and Earth. Credit: C White/NASA

The ESA team wanted to find out what geological forces had originally formed the numerous depressions around Titan's poles, most of which contain methane lakes.

To do so, the researchers studied karst topography formations on Earth—landscapes shaped by the dissolution of soluble rocks like dolomite, gypsum, and limestone. When these softer rocks sink into deeper turf, they create geological features like sinkholes, caves, and saltpans.

The team, led by ESA planetary scientist Thomas Cornet, calculated how long it would take for a similar process to produce sinkholes on Titan, swapping soft stones like limestone out for the rich hydrocarbons covering the moon's surface.

"We compared the erosion rates of organics in liquid hydrocarbons on Titan with those of carbonate and evaporite minerals in liquid water on Earth," said Cornet in a statement.

Their results indicated that it would take about 50 million years to produce a 100-meter-deep depression. This rate is much slower than its counterpart on Earth, but it still indicates that "dissolution is a major cause of landscape evolution on Titan, and could be the origin of its lakes," according to Cornet.

The depressions at Titan's poles. Credit: NASA/JPL-Caltech/ASI/USGS; left and right: NASA/ESA. Acknowledgement: T. Cornet, ESA

Right on the heels of this news, a different team published research today in Geophysical Research Letters identifying yet another Earthlike process on Titan. The authors, led by plasma physicist Andrew Coates, were also focused on natural phenomena at Titan's poles, but in this case, they investigated the mysterious atmospheric gas runoff generated by the moon's polar winds.

By trawling through several years worth of data from the NASA Cassini orbiter, Coates's team was able to track the complex interaction between solar radiation, Saturn's magnetic field, and the molecules in Titan's upper atmosphere.

"Although Titan is ten times further from the Sun than Earth is, its upper atmosphere is still bathed in light," Coates said in a statement. "When the light hits molecules in Titan's ionosphere, it ejects negatively charged electrons out of the hydrocarbon and nitrile molecules, leaving a positively charged particle behind."

"These electrons, known as photoelectrons, have a very specific energy of 24.1 electronvolts, which means they can be traced by the [Cassini Plasma Spectrometer] instrument," he added.

The orbiter's plasma spectrometer revealed that photoelectrons were being catapulted way out into space, reaching distances of about seven times as far as Titan's radius. This massive halo of negatively charged particles in turn powers an electric field that is strong enough to pull gaseous hydrocarbon molecules right out of the Titan skies with it.

In their study, Coates and his colleagues noted that more or less the same thing occurs when Earth's atmosphere is twisted around by the polar wind, solar radiation, and our planet's magnetic field. Their research marks the only time this mechanism of gas ejection has ever been observed beyond Earth, though theoretically it might occur on Venus and Mars too.

All in all, it seems that despite Titan's freezing temperatures of minus 179 degrees Celsius and its alien hydrocarbon rivers, this moon is shaping up to be one of the most unlikely Earth analogs ever studied.

Moreover, the fact that Titan is located well beyond our solar system's "Goldilocks zone"—the region in which liquid water is possible—suggests that the far-flung worlds of other solar systems may be just as interesting as those with closer, Earthlike orbits. It makes sense to prioritize exoplanets that resemble Earth as closely as possible. But Titan continues to be a great reminder of the true diversity of worlds both within our solar neighborhood, and beyond it.