Now that spring has arrived, there are more opportunities to observe. Among other observing targets is Saturn. I lost track how times I’ve seen Saturn through a telescope, but I never get tired of looking at it. After I’ve soaked in the image of Saturn’s disk and the rings, I start looking at the moons. The number of visible moons varies, but you can almost always see the largest of Saturn’s moons, Titan.
I bring this up because of a talk I attended this March: “The Methane Hydrological Cycle on Titan,” by Jonathan Lunine (Planetary Science and Physics at the University of Arizona). Also attending were Fred Adams (Physics, University of Michigan) and Sushil Atreya (Atmospheric, Oceanic, and Space Sciences, University of Michigan). I first met Dr. Atreya while I was an AOSS grad student. Some of you may remember that one of Dr. Atreya’s students give a talk at the February 2005 club meeting “Titan, as seen through the eyes of Cassini-Huygens.” Both talks discussed the Cassini-Huygens mission and what we have learned about Titan. Titan is not just a point of light, it is a strange world; like Earth in some respects but very different in other respects.
Before I talk about Titan, I should go over some basic atmospheric chemistry. The Earth’s atmosphere is mainly nitrogen and oxygen with smaller amounts of other materials including water vapor. The atmosphere is divided into “spheres,” for our purposes only two are important. The troposphere is the part of the atmosphere closest to the Earth. The stratosphere is farther out. In the Earth’s atmosphere very little water vapor escapes the troposphere into the stratosphere. This is important because materials in the stratosphere are often broken down by solar radiation. Water vapor is not affected in this way, but oxygen is (the later is converted to ozone). The Earth’s atmosphere has only very small amounts of hydrocarbons (these are compounds composed of carbon and hydrogen, for our purposes only two are important, namely methane and ethane).
Titan is one of only four solid bodies in our solar system with a significant atmosphere (the others are Venus, Earth and Mars). It is permanently covered in an orange haze, this haze made observations of the surface impossible until relatively recently. Our understanding of Titan has increased as a result of data collected by the Cassini-Huygens space mission. Cassini-Huygens has established that Titan’s atmosphere is mainly nitrogen and methane, and there is evidence for small amounts of ethane. Methane and ethane are normally colorless gases, though both gases will condense out as clouds in Titan’s atmosphere. Titan is cold, though somewhat warmer than you might expect due to the methane.
Cassini-Huygens has produced visible light photographs and radar images of Titan’s surface. The resulting images have given us a lot of information. The northern polar region looks similar to the Thousand Lakes region of northern Minnesota. There are many black areas, one about the size of Lake Michigan. These black areas resemble lakes. For these structures to appear black in the radar images, they must be flat. The most likely possibility: these are lakes of methane or a mixture of methane and ethane. Water ice has been ruled out as possibility. There are no lakes in the equatorial region, and only one lake in the south. Near the equator, there are “dunes” suggestive of sand dunes; they are probably made of water ice. Unlike Earth, there are no “oceans” (at least no visible oceans....) There are structures that resemble rivers, they suggest that periodic rainstorms result in rivers of methane. Even though the rivers dry up, you can still see the river channel.
On Earth, water goes from vapor to liquid water (“rain”) and back to vapor (“evaporation”). Something similar seems to happen on Titan, only with methane/ethane instead of water. The analogy isn’t perfect; for one thing methane mixes into the stratosphere where solar energy almost certainly breaks apart. The end result: methane is converted into ethane and hydrogen; the later escaping Titan’s gravity. This process is fast by astronomical standards and is one way; ethane cannot be converted back to methane. We expect all the methane to be converted to ethane in ten million years or so.
This prompts an obvious question, since Titan is over 4 billion years old, why is there is still methane in Titan’s atmosphere? While we do not have the data to be sure, it is possible that the atmospheric methane is replenished from underground reservoirs (similar to the aquifers of water under the Earth’s surface). Another possibility: methane could be produced underground. While it is too cold for liquid water on the surface, it is possible for liquid water mixed with ammonia to exist underground (the ammonia would act as an antifreeze). Some researchers have speculated that there could be large underground oceans of ammonia water. This could undergo chemical reactions to form methane and it might occasionally erupt; Titan’s version of a volcano. There is evidence that such volcanism may have occurred in the past.
Since Titan receives very little energy from the sun, there isn’t much energy to power a hydrological cycle. If Titan were like Earth, we should have seen many methane rainstorms. We haven’t; it is believed that they occur but infrequently and we haven’t been observing long enough to see any so far. It is unknown if a Titan equivalent of a thunderstorm is possible; none have been observed so far. There are winds on Titan, but they are weak (if the winds were stronger, the lakes should have waves and this has not been observed).
While it might seem far-fetched, some researchers have suggested life might be possible on Titan. If there is life, it could be responsible for the methane in Titan’s atmosphere. The evidence to date suggests there is no life on Titan, however it hasn’t been completely ruled out.
While I don’t want to go into much detail, I attended a number of other talks.
Jonathan Lunine gave a second talk later in the day “Exploring the Outer Solar System: Present and Future.” He discussed objectives in possible future space missions: Pluto, three moons of Jupiter (Io, Europa and Callisto) and two moons of Saturn (Titan and Enceladus). Titan is our best model of the early Earth (Titan’s atmosphere best approximates what we think the atmosphere of Earth was four billion years ago). Enceladus is one thousandth the size of Titan, it has geysers and it sheds matter forms the E-ring of Saturn. All would be interesting targets for space missions, however it seems unlikely there will be enough funding to allow space missions to each of these targets.
Margaret Geller (Smithsonian Astrophysical Observatory) gave a talk on dark matter. Among her many accomplishments, Dr. Geller produced a documentary “So many galaxies, so little time.” We may need to consider that as the club motto. She began her talk by saying that we don’t know what dark matter is, but are now know where it is. Dr. Geller’s work has involved mapping the location of dark matter. She presented her findings to the audience, which included the most detailed maps of the kind I have seen.
Saul Teukolsky (Physics, Cornell) gave a talk on gravitational waves and black holes. In brief he is exploring the following problem: Gravitational waves are predicted from the theory of General Relativity. However detecting such waves is very difficult. If we have some idea of what the signal of a real gravitational waves would look like, it might be possible to optimize our equipment to make the detection easier.
Following this logic, Dr. Teukolsky built a computer simulation of a binary black hole (BBH for short). It seems likely that BBHs would produce the best examples of gravitational waves. (Gravitational waves require a moving gravitational field. BBHs contain two moving fields, one for each black hole, and black holes produce the strongest gravitational fields known). The simulation was written in C++ (while it has taken a while, FORTRAN is no longer the only language used for such projects). The simulation takes thousands of CPU hours to run.
(Some of you will recognize the name; Saul Teukolsky is one of the co-authors of the “Numerical Recipes” books).
Sushil K Atreya. May 2007. “The Mystery of Methane on Mars & Titan.” Scientific American. Volume 296, No. 5, pp. 43-51.
Margaret Geller. April 2, 2008. The Eighth Annual Ford Motor Company Distinguished Lecture in Physics (the University of Michigan): “Newton Meets Einstein: Mapping Dark Matter in the Universe.”
Jonathan I. Lunine. March 27, 2008a. Michigan Center for Theoretical Physics Lecture (the University of Michigan): “The Methane Hydrological Cycle on Titan.”
Jonathan I. Lunine. March 27, 2008b. Michigan Center for Theoretical Physics Lecture (the University of Michigan): “Exploring the Outer Solar System: Present and Future.”
Jonathan I. Lunine and Sushil K. Atreya. March 2008. “The Methane Cycle on Titan.” Nature Geoscience 1, 159-164 (2008). [Published online: February 17, 2008].
Saul Teukolsky. April 3, 2008. Michigan Center for Theoretical Physics Colloquia (University of Michigan): “Black Holes and Gravitational Waves.”