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Listen to part of a lecture in an astronomy class. The professor has been discussing the planet Saturn.
Professor: As you know, we’ve been exploring the flat disk of rocky material that forms the rings which orbit the planet, but there are also over 16 moons orbiting Saturn, each one unique in its own right. Let’s start today by discussing one that’s particularly unusual: Titan. Does anyone remember what make Titan so unusual? Lauren?
Student 1: Well, it’s Saturn’s largest moon and it’s even larger than the planet Mercury.
Professor: Correct. Anyone else? Robert?
Student 2: It’s the only other world we know of, aside from Earth, that has something like a hydrologic cycle.
Professor: Ok, good and what does that mean exactly, hydrologic cycle?
Student 2: It’s a cycle where, where water is moved between liquid and gaseous states, so there’s evaporation, condensation, and precipitation.
Professor: Correct. Of course on Titan, it’s not water that’s involved in the cycle. It’s actually methane and ethane. I know we’re used to thinking of methane as a gas, but on Titan methane can take the form of a liquid and can be found in lakes and puddles. That’s because methane becomes a liquid at about minus 108 degrees Celsius. On Titan, that’s nothing. Titan is so cold, it can reach temperatures as low as minus 183 degrees Celsius. Now, for the past few years we’ve been observing Titan and its atmosphere from a spacecraft called Cassini that’s in orbit around Saturn. And one thing we’ve noticed about Titan’s surface is that it only has lakes at the poles. The equator on the other hand is unusually dry and covered with sand dunes, but there’s also what appear to be dried-up channels and rivers pointing to the possibility of rainfall at some point in the past, which may well be the case because recently we think we’ve seen evidence of rainfall near Titan’s equator. Of course, we can’t actually see it raining from Cassini because it would be obscured by the clouds, but we did see images of a large dark spot near Titan’s equator just after the clouds that passed over that area.
Student 1: Oh, I get it. It’s like when it rains on Earth and the wet sidewalk turns dark.
Professor: Exactly. Also, the dark spot started fading very fast, like when evaporating. Now, this evidence of rain is very significant. Why? Well, it’s the first example of extraterrestrial rain we’ve ever observed, and what may be even more significant is the timing of this rain. See, there’s a model of Titan’s precipitation that predicts that equatorial rain would only occur during spring or autumn, and it was spring on Titan when Cassini took these images. This model is based on something we call the Inter-Tropical Convergence Zone or ITCZ. Can anyone tell me what that is?
Student 1: Yeah, isn’t the ITCZ the area of a planet where like the surface winds from the northern and southern hemispheres meet or converge? It’s an area of precipitation.
Professor: That’s exactly right. Now, on Earth this zone stays within the tropics because the weather systems on both the north and the south sides of the equator block the ITCZ in and don’t allow it to move much. That’s why there’s rainfall on our equator year round, but on Titan, we think the ITCZ migrates almost from pole to pole every half year, so it basically never remains over the equator for a long time. That may be why we hadn’t seen scenes of equatorial precipitation until now when Titan’s ITCZ was passing over its equator at the same time Cassini was in the neighborhood.
Student 2: So what we have to address is what explains Titan’s ITCZ migration?
Professor: Well, most likely because of Titan’s slow rotation period. See, we think that slow rotations cause simpler atmospheric circulation patterns, and simpler circulation patterns mean that there aren’t large weather systems surrounding the ITCZ that would keep it from shifting, from moving from one pole to the other. Not like on Earth and Titan takes nearly 16 Earth days to rotate just once. At any rate, Titan, with its quasi-hydrologic cycle, is definitely a world that we should be doing research on. You see, even though it’s too cold to sustain life, Titan, with its methane clouds, its methane and ethane lakes, Titan may give us some clues as to what our planet was like before the development of life, so this make it imperative that we study Titan to better understand how Earth may have started out.