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Planetary News: Cassini-Huygens (2005)

No Oceans on Titan, for the Present

Click to enlarge > Scientists have been searching for glinting reflections off of putative liquid bodies on Titan. The glints were seen by radio telescope. But despite a landscape that looks like one of continents and oceans, no such glints have been observed using optical telescopes, either from Earth or Cassini. Credit: NASA / JPL / Space Science Institute

Is Titan wet or dry? It's looking drier and drier. Since Voyager Titan has been theorized to have large bodies of liquid hydrocarbons, methane or ethane, on its surface. Those theories appeared to have been proven in 2003 when researchers using the giant Arecibo radio telescope in Puerto Rico reported that the surface of Titan echoed radio waves in a way that was with "consistent with hydrocarbon lakes" being present across widespread areas of the surface.

However, when Cassini-Huygens went into orbit at Saturn, the story changed. The orbiter looked very carefully for shiny "specular" reflections from liquid surfaces. It found plenty of details on the surface of Titan, large dark and bright areas that scientists were unable to stop themselves from calling "oceans" and "continents" or "islands." But Cassini saw no specular reflections in either of the two instruments that could have detected them, the cameras (ISS) or the Visible and Infrared Mapping Spectrometer (VIMS). Furthermore, when Huygens landed, it saw indisputable evidence for the past erosive action of a liquid substance on the surface, but its landing site in one of those dark regions looks as dry as Mars.

Another line of inquiry has now added to the increasingly firm conclusion that there are no widespread bodies of liquid on the surface of Titan. Scientists have reported in the journal Nature the results of a careful Earth-based survey of Titan using a near-infrared camera attached to the Keck II Telescope, which has recently been equipped with an adaptive optics system. (This is the same system that enabled the discovery of an active and interesting meteorology at Uranus last year.)

Of the 31 observations that were performed using the NIRC2 near-infrared camera on the Keck II telescope, not one observation contained an obvious sign of specular reflection. In fact, "I’ve not seen any hints of a specular reflection in any of hundreds of ground-based observations that I’ve looked at," said Robert West, who is the lead author on the Nature article and also a member of the Cassini ISS team.

The 31 observations were along one line of latitude at approximately 25 degrees south, but were spread out across Titan in longitude. Careful mathematical modeling of what the team could have detected in those images yielded the conclusion that "we can firmly rule out smooth surfaces covering more than 15% of any of the 31 regions surveyed," West and his coauthors state in the Nature paper. "A similar assessment for high-resolution Cassini VIMS images yields a limit of 2%."

So now scientists have two mysteries on their hands: where Titan's methane comes from, and why Arecibo saw specular reflections but Keck didn't. Titan's atmosphere contains lots of methane, a gas that should be destroyed rapidly by sunlight. Either there must be a reservoir on or near the surface that replenishes the atmospheric methane continuously, or we just happen to be observing Titan at the moment in its history after some major geologic event filled its sky with methane. With the evidence against surface reservoirs of liquid methane, scientists have to go back to the drawing board for a new theory.

Huygens produced interesting piece of evidence about where the methane might be coming from, West points out. "Something like 20 minutes after landing there was an increase in methane that was seen by Huygens' Gas Chromatograph Mass Spectrometer. That suggested that there was liquid only inches below the surface. So people were saying it seemed like only yesterday [that there was liquid on Titan's surface] -- within the last week or month or year. But I don't see how you can get liquid within inches of the surface over the long term, but nowhere at the surface. It’s a puzzle."

How did the Arecibo observations lead scientists astray? The key difference between Arecibo and the NIRC2 instrument on Keck II is the wavelength at which they operate. Arecibo is a radio telescope, employing a 13-centimeter wavelength. West and his collaborators on the Keck project -- as well as Cassini's VIMS -- look at Titan at a wavelength of 2 microns (2 millionths of a meter). Electromagnetic radiation only interacts with reflectors that are roughly the size of the wavelength, or larger.

Thus surfaces that look "smooth" to Arecibo could be rough on centimeter scales (think of a flat, dry lake bed with small rocks on the bottom). However, for Keck II or VIMS to see a specular reflection, the surface would have to be smooth on a scale only achievable by a liquid. The surface of the liquid could be roughened into choppy waves by Titan's breeze, but those waves would still have ultra-smooth, reflective facets that Keck and VIMS could have seen.

West and his collaborators propose four explanations for surfaces that would be smooth at Arecibo wavelengths but not at 2 microns:

The reflective surfaces seen by Arecibo could be flat, dry evaporite deposits left behind by the evaporation of a lake or ocean. Such deposits could be very flat over very long distances, but rough at small scales. They could even be porous, so there could in fact be liquid below the surface of such a deposit.

The reflective surfaces could be frozen ammonia-water "lava flows," which were extruded onto the surface as a liquid that froze into smooth, glassy ice. However, the radar properties of the reflective surfaces Arecibo saw match organic materials better than they do water-ice materials. That's not necessarily a problem, West says: "Pure ice would be bright in the beginning, but it would be covered rapidly by sedimenting haze of organic material."

In fact, organic gunk falling out of the atmosphere could have filled in topographically low areas, leaving a flat, smooth surface. Again, such a deposit could be porous and could be hiding the methane reservoir needed to keep the atmosphere in equilibrium.

The surface could actually be liquid, with some rougher solid particles floating on top. However, most solids proposed for Titan would sink in liquid methane, so this possibility is "pretty unlikely," according to West.

Any solid surface, no matter how smooth, would tend to be roughened over time as a result of asteroid impacts. Therefore, while the Arecibo observations may no longer be considered proof of Titan oceans, they do strongly support the idea that Titan has been geologically active relatively recently in the past. There is certainly evidence of dynamic conditions existing in one place on Titan in the present: Titan's south pole, now lit continuously by summer sunlight. Cassini has seen large fields of clouds form and disappear repeatedly near the south pole, raising the possibility that there is a methane cycle of evaporation, condensation into clouds, methane rain, and runoff into lakes.

In fact, Cassini's cameras recently observed a very small, smooth-bordered, very dark spot near Titan's south pole. It could be a liquid body. Proving that suggestion will be difficult, however, because "the places we can examine for specular features are dictated by the geometry of the tour," West said. "Cassini has not yet gotten in to position to see [a specular reflection from] that feature from the south. We don’t have the ability to dictate what longitude we go over. I think we’ve determined we’re not going to," at least not in Cassini's primary mission.

All of this may mean that Titan is not a place with continuous hydrologic activity, like Earth, but rather may be more like Mars once was: a mostly dormant world that is occasionally convulsed with a rainstorm or a flood bursting out from underground, which drains away into the ground or evaporates into the atmosphere. "It seems like there’s been a lot of geologic activity in the past," West says. "The big question is, how far in the past? There are people who think, based on Huygens observations, that it has been in the very recent past. But I would expect to see a lot of liquid on the surface if there has been recent activity."

That activity could be recent, but seasonal. And seasons at Saturn last a long time, given Saturn's 30-Earth-year orbital period. "Cloud activity on Titan is rare outside the southern high latitudes but is predicted to move north as Titan's southern summer turns to southern autumn, suggesting the possibility that these regions are seasonally refreshed with precipitation, which could work to smooth the surface," the researchers state in the Nature paper. They will continue to watch from Earth as the season advances at Titan, while Cassini will continue to watch from orbit. Maybe they'll get to see a new lake form!