Thursday, August 04, 2005

Present-day oceans on Titan put to rest?

A paper in today's issue in the journal Nature, puts to rest the idea that large parts of Titan's surface are currently covered in methane/ethane oceans. Several years ago, a paper by Campbell et al. in Science, presented evidence for wide-spread liquids on the surface of Titan from specular reflections seen in radar pulses sent from Arecibo. The paper, West et al., in today's issue of Nature uses near-infrared observations near 2 microns. Like Campbell et al. they used ground-based observations to look for specular reflections along a wide-range of longitudes near 25 South Latitude. They found no evidence of specular reflections in their data. Specular reflections at near-infrared wavelengths would be much more conclusive for liquids on the surface than at RADAR wavelengths. For specular reflections to be seen, a surface must be smooth at the scale of the wavelength used to observe the surface. So for RADAR specular reflections, a surface only needs to be smooth on the order of 13 cm (or 3 cm for Cassini RADAR), which can be achieved by such terrain as a mud flat that contains only small, smooth rocks (think Huygens landing site). For a near-infrared wavelength specular reflection, terrain has to be smooth on the order of 2 microns, which is VERY difficult to achieve with anything but a liquid surface. So the absence of a specular reflection at near-infrared wavelengths would seem to rule out liquids at 25 South latitude. The presence of a RADAR specular reflection may indicate that some of these areas are covered in mud flats (or regions without large boulders or rough terrain), perhaps indicating that liquids were present in the past.

There are some pathological cases where you could have a liquid covered in floating crude, but very few materials can float on liquid methane, except perhaps acetylene, but it still has to overcome the low surface tension of methane. Additionally, this study only looked at liquids at 25 South. This leaves open the possibility that liquids may exist seasonally at the poles as they receive enhanced rainfall.

UPDATE: 08/04/2005 12:45pm: New Scientist has a more in-depth article online for those without subscriptions to Nature.

2 Comments:

Anonymous JRehling said...

Another possibility is that the dark low-latitude areas on Titan also hold liquid in the present epoch, but don't hold liquid *today*. I'll submit the speculation that tides in a sub-mud liquier could move a wet spot around -- unlikely, but not excluded by what we know just yet. A separate possibility, also quite speculative, is that Titan, like Mars, might have a winter cap (as of right now, the northern one); if a lot of methane melts at the equinox, then we might see those global oceans form in the next couple of years before they dry as a southern cap forms. This hypothesis is not entirely wacky -- we do know that the stratosphere over Titan's north pole is cold enough for methane ice -- either there *is* a winter cap, or there's a very interesting system of thermal transport going on that slides warm atmosphere under a very wide stratosphere of colder atmosphere. Finally, and least wild of the ideas here, is the possibility that the seasonal clouds that are in the deep south now and were in the far north when Voyager flew by might spend some time in the low latitudes, raining there around equinox, and making the mudflats wet (to some degree) in that season. All told, we have some potential that a dry area in 2004/2005 might not be dry in 2010/2011.

8/04/2005 09:36:00 AM  
Anonymous Jerry said...

The liquid model was attractive, because it explained both the clouds and the radar-smooth surface.

Jason, are do you feel like we are close to an educated guess about when, if not how, Titan was last resurfaced?

8/05/2005 10:26:00 AM  

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