Friday July 6th 2012

Titan’s Lakes and the Drive to Explore

What is it that makes us want the stars? Surely there are philosophical reasons that push us into the universe, and in his book Quest: The Essence of Humanity (2004), Charles Pasternak delves into ‘questing’ as a drive embedded in the species. But alongside a need to explore I can see two other drivers. One is the urge to know whether life exists elsewhere, and ultimately, whether there are other technological civilizations somewhere in the galaxy. The other is simple survival: We need to move into the universe as a backup plan in case of disaster here on Earth, whether that disaster is caused by an asteroid or a human activity gone awry.

This morning I’m musing on all this in the context of recent news from the outer Solar System, where the data we’re analyzing from the Cassini mission are matched only by our desire to have still further, more targeted explorations. We learn, for example, that Titan has lakes around its equator. Lakes on Titan aren’t a surprise: We’ve already known about lakes of methane and ethane like Ligeia Mare near the moon’s poles. But now we have evidence of a 2400 square kilometer body of liquid with a depth of a meter or more in Titan’s lower latitudes, down in the moon’s ‘tropics,’ where it was thought nothing but sand dunes were likely to be found.

Image: Saturn’s rings lie in the distance as the Cassini spacecraft looks toward Titan and its dark region called Shangri-La, east of the landing site of the Huygens Probe. Credit: NASA/JPL-Caltech/Space Science Institute.

This is a world that gets more interesting all the time. Titan draws our attention not only because of its thick atmosphere — which, coupled with low gravity, makes landing on it a straightforward affair — but also because its methane replaces water in Earth’s hydrological cycle to make it the only other place in the Solar System known to support liquid lakes. The newly found lakes, detected by Cassini’s visual and infrared mapping spectrometer, are found in the area called Shangri-La, not far from where the Huygens probe touched down in 2005. I hadn’t realized a point this NASA news release makes, that when Huygens landed, the heat of its lamp vaporized methane from the ground, an indication that the probe landed in a relatively damp area.

This week’s Nature has the paper, which reports not only on the larger lake but on small, evidently shallow ponds in the same region that can be likened to marshes on Earth, perhaps no more than ankle-deep. Current thinking on Titan’s global circulation had led us to believe that liquid methane in the equatorial regions would quickly evaporate and be carried by the winds to the poles, where it would condense to form the polar lakes, leaving the tropical regions arid. That model would suggest that the tropical lakes are being produced by an underground methane source that floods the surface, forming a kind of oasis.

Caitlin Griffith (University of Arizona) is lead author on the paper:

“An aquifer could explain one of the puzzling questions about the existence of methane, which is continually depleted. Methane is a progenitor of Titan’s organic chemistry, which likely produces interesting molecules like amino acids, the building blocks of life.”

The theory of an underground aquifer is also supported by the fact that rain has been detected falling in the equatorial regions only once, leading the researchers to believe the lakes are not being replenished by rain. Thus we are building the case for an active subsurface hydrology on the frigid moon as liquid hydrocarbons emerge on the surface to supply methane. The former image of equatorial Titan as a place of nothing more than sand dunes has to be reconsidered. Titan thus builds its reputation as a place of intriguing chemistry on which we’d like to spend some time, perhaps through an airborne probe like AVIATr or the floating Titan Mare Explorer.

Meanwhile, Cassini continues to return interesting information about another world of astrobiological interest, Enceladus. The latest is the observation of so-called ‘dusty plasma’ near the plumes spreading out from the moon’s south polar region. Saturn’s plasma environment is a lively one as Enceladus continues to spew ionized material into the planet’s magnetic field. About 100 kilograms of water vapor per second are known to be blowing out of the so-called ‘tiger stripes’ that mark the cracked surface of the south pole here. The plume is quickly converted into charged particles interacting with the plasma in Saturn’s magnetosphere.

‘Dusty plasma’ is filled with charged dust. Here, the dust behaves as part of the plasma cloud, differentiating it from dust that simply happens to be in the plasma at the time. Lead author Michiko Morooka (Swedish Institute of Space Physics) thinks the observation is a first:

“Such strong coupling indicates the possible presence of so-called ‘dusty plasma’, rather than the ‘dust in a plasma’ conditions which are common in interplanetary space. Except for measurements in Earth’s upper atmosphere, there have previously been no in-situ observations of dusty plasma in space.”

The work follows up on studies by Sven Simon (University of Cologne) and Hendrik Kriegel (University of Braunschweig) — reported in this JPL news release — that showed negatively charged dust grains had to be in the plume to account for the observed perturbation of Saturn’s magnetic field. The plume itself, known to contain water vapor, ice particles and organic molecules, presumably flags the presence of a subsurface ocean in which the basic ingredients for life to get a start may well be present. Mission concepts like the Enceladus Explorer, from the German Aerospace Center, would use a drilling probe to reach the liquid water reservoir.

The question of life in the universe is deeply stirring and compels us to search nearby worlds to learn whether life could have evolved under dramatically different conditions than here on Earth. Ultimately, our SETI efforts are directed toward slaking the same thirst for knowledge as we try to find out whether intelligence — or at least technological savvy — is common or a rare feature of the universe. Missions are always at the mercy of available resources and budgetary constraints, but the long-term forces of curiosity and survival compel us out into the system. Timetables are useless, but it’s hard to see these drivers being ignored by future generations.

The Titan paper is Griffith et al., “Possible tropical lakes on Titan from observations of dark terrain,” Nature 486 (14 June 2012), pp. 237–239 (abstract). On the Enceladus plasma work, see Morooka et al., “Dusty plasma in the vicinity of Enceladus,” Journal of Geophysical Research (Space Physics), Vol. 116 (2011), A12221 (abstract).


This article was originally posted on Centauri Dreams