Ethane Lake Found on Titan
Liquid ethane has been detected in a lake near the south pole of Saturn’s moon Titan, reported JPL yesterday. This confirms long-held suspicions that ethane, a byproduct of methane disruption by the solar wind, accumulates on the surface of the large atmosphere-shrouded moon. A problem remains why there is so little of it. Pre-Cassini predictions envisioned a global ocean of liquid ethane.
Nature1 reported observations by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) of a lake near the south pole that had been discovered previously. The specular signal indicated an extremely smooth surface – smoother than any geological process could make. The spectrum of ethane, a hydrocarbon that is liquid at Titan’s surface temperature, confirmed that ethane is present.
Ethane is formed when atmospheric methane, CH4, in its gaseous form, is struck by the solar wind in Titan’s upper atmosphere. The bombardment strips off a hydrogen atom. The ionized methane molecules, CH3+, quickly combine into ethane, C2H6, which falls to the surface as rain. A stable molecule, ethane should collect continuously on the surface in pools and lakes. The chemical process is irreversible (03/11/2005). Ethane cannot evaporate back up into the atmosphere, nor can it dissociate back into methane. This should lead to the accumulation of liquid ethane on the surface of Titan over time.
Calculations prior to Cassini’s arrival showed that a global ocean of ethane and liquid methane should have accumulated over 4.5 billion years – up to a half-mile deep or more. Pre-arrival radar images from earthbound instruments cast doubt on the presence of a global ocean, but before the Huygens Probe parachuted to the surface (01/15/2005, 01/21/2005), scientists were still hoping for a splash. The probe landed, instead, on a dry lake bed of what appears to be methane-saturated icy sand, with only traces of ethane present.
“Before Cassini, scientists thought Titan would have global oceans of methane, ethane and other light hydrocarbons,” the JPL press release stated. “More than 40 close flybys of Titan by Cassini show no such global oceans exist, but hundreds of dark, lake-like features are present.” The southern lake, comparable to the Great Lakes in extent, has been named Ontario Lacus – Lake Ontario. Science Daily posted a picture of the 150-mile-long lake from orbit. It covers 7,800 square miles, slightly larger than its earthly counterpart. How deep is it? VIMS can only constrain the minimum depth to 3/4 of an inch. It could be much deeper. The presence of a beach around the perimeter, though, suggests that the lake is evaporating. Scientists wonder if the lakes migrate from pole to pole as the seasons change during Saturn’s 29.4-year orbit. Cassini has detected more and larger lakes in Titan’s northern latitudes (radar map) than in the south (radar map).
The original paper began with the mystery of the missing ethane ocean: “Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist.” The statement included references to five papers from 1982 to 1995 predicting an ocean (see 1983 and 1993 abstracts by Lunine), including two by Carl Sagan (1982, 2002). Francois Roulin, commenting on the paper in the same issue of Nature,2 noted that “Titan’s lakes are probably a liquid ethane�methane mixture together with dissolved nitrogen, as previously proposed for the speculative oceans” that turned out to exist only on paper. Jonathan Lunine, who had predicted a global ocean in 1983, told National Geographic News that the lakes “do not hold enough ethane to account for what can be produced over the age of the solar system. So we still have a mystery here.” See also the 02/15/2008 entry.
The lake may contain other organic molecules besides ethane. Hydrocarbons and nitriles such as propane, butane, acetylene and benzene have been detected in Titan’s atmosphere, as well as high-mass cluster ions. Cosmic rays bombarding the lakes may produce additional exotic molecules. Artwork tries to convey what an observer might see on this dim world with orange sky; here is another. The surface near the Huygens Probe was actually photographed in realistic color in this historic image from the landing site. Photographs of the descent have been put together into a stunning movie available from the Descent Imager website.
Titan turned out to be drier than expected. The equatorial regions are awash in dunes reminiscent of the Namib Desert – except that the particles are made of ice, probably coated with hydrocarbons manufactured in Titan’s bizarre chemical laboratory.
1. Brown, Soderblom et al, “The identification of liquid ethane in Titan’s Ontario Lacus,” Nature 454, 607-610 (31 July 2008) | doi:10.1038/nature07100.
2. Francois Roulin, “Planetary science: Organic lakes on Titan,” Nature 454, 587-589 (31 July 2008) | doi:10.1038/454587a.
Here is a science project crying out for some mathematically-minded person to work on. It can be done right at the desk without special equipment – just a literature search and a calculator. Figure out how long it would take to get the observed lakes on Titan. Inputs are conservative rates of ethane production in the atmosphere and the most optimistic estimates of lake volume. The result should place a severe upper limit on the age of Titan.
Scientists have downplayed the severity of this problem. They usually mention it in passing only to change the subject. Usually, they jump to the possibility that Titan’s organic chemistry might have something to say about the origin of life (07/26/2005) – a foolish distraction. Where is the ethane? There should be a huge ocean of it by now, according to their belief in billions of years.
Their only escapes now are less credible. They have to claim the chemical process started recently, or that the ethane remains in the atmosphere (how long could that last?), or that the ethane went underground. They never, ever question the billions-of-years age! Their problem would be solved by reducing the age of Titan (and Saturn, and the solar system) by a few orders of magnitude. That would fit this observation and many others (e.g., 04/08/2005, 06/29/2008, 05/05/2008, 03/10/2008, 12/03/2007, 08/04/2007, 07/18/2007, 05/04/2007, 03/31/2007).
So here is a challenge. The long-age believers made a prediction that appears to have been falsified. We want to know what they are going to do about it. Don’t let them sweep this problem under the crust.