I know I just posted a global color map of Saturn’s moon Titan, but sometimes it’s cool just to take a step back and look at a picture that gives a little context… and it doesn’t hurt that it’s a moody grayscale shot, too:
[Click to encronosenate.]
This shot of Titan was taken by the Cassini spacecraft back in August, and shows the moon superposed on Saturn’s rings, seen here almost — but not quite — edge-on.
The fact that you can see surface detail on Titan is a dead giveaway this shot was taken in the infrared: optical light, the kind we see, can’t penetrate the thick, hazy, nitrogen/methane atmosphere blanketing this moon. Infrared light gets through, though, so surface features can be seen. In fact, this image was taken using a filter that lets through light at 938 nanometers (the reddest light the human eye can see is about 750 nm). Methane is pretty good at absorbing light at a bunch of different wavelengths, but at 938 nm it’s transparent, so this is a particularly good place in the spectrum to look at Titan — astronomers call it the "methane window". Not only that, but this image also employed a polarizing filter, which blocks a lot of light from the atmospheric haze, making the surface easier to see (it also makes rainbows appear and disappear, too).
Not that the atmosphere is completely invisible in this picture: look around the moon’s edge and you can just see some of the upper atmospheric layers, and at the top you can easily spot the north polar hood, which may have water ice crystals in it.
And that dark region on Titan’s surface? It may have once been the bed of a methane sea, but now it’s a dry, vast area of wind-blown dunes, hydrocarbon grains collected by the Titanian winds. It’s called Shangri-La, and that makes me smile. I’m not sure anything at -180°C could be called a human paradise, but for astronomers, it’s certainly a scientific one.
There has been a bit of an uproar the past day or so that scientists have found evidence of life on Saturn’s giant moon Titan. As soon as I saw the press release I knew this was going to be a problem. So let’s be clear:
First, have we found life on Titan? No.
Have we found evidence that there might be life on Titan? Sorta. The results are preliminary and not yet confirmed; in fact, some of the evidence is from computer modeling and has not been directly observed.
Bear in mind as well that evidence is not proof. Evidence just means an observation was made that is consistent with life on the moon, but doesn’t say much else. There are non-biological explanations for the observations as well.
Of course, speculation is running rampant, so much so that Chris McKay, an exobiologist who studies Titan, has released an article clearing things up.
I hate to make the obvious jokes, so I’ll simply say I was on this week’s Skeptic’s Guide to the Universe talking about methane on Mars and the tilt of Uranus. I’m glad they invited me on; I hadn’t heard of either of these stories until Steve Novella alerted me to them before we did the interview.
Basically, a new hypothesis has come out that the large tilt of Uranus (98°) is not from a collision, but instead had its natural tilt reinforced by a large moon that has since been ejected. Also, scientists tested the idea that the methane seen to change on Mars with the seasons might be from meteorites, and find that they don’t supply nearly enough to explain the observations. We also talk JREF, solar power, the Norway lights, and the usual nonsense. I just finished listening to the whole episode, and thought it was pretty good despite me being on it, so go give it a listen!
A peculiar flash of light glinting from Saturn’s largest moon confirms what’s been suspected for years: liquid lakes exist on the surface of Titan!
[Click to entitanate.]
The image above was taken on July 8, 2009 by the Cassini spacecraft. Light can reflect off the surface of liquids, producing a little sparkle or glint, called a specular reflection. Knowing that earlier images had shown what look to be lakes of liquid methane on Titan, they kept their eyes open for Cassini’s images of the moon to show such a glint. There are lots of lakes in the northern hemisphere of Titan, making the odds better it would be seen there, but it was only last year that spring sprung in Titan’s northern latitudes. That’s when it was finally possible to see sunlight plinking off of any purported standing liquid.
And that’s what we’re seeing here. They checked to make sure this wasn’t some other source of light like lightning or geologic activity, and were able to trace the position of this glint to the shores of a monstrous lake called, appropriately, Kraken Mare. It’s a sprawling 400,000 square kilometers, bigger than the Caspian sea!
Titan’s atmosphere is thick and hazy, so visible sunlight isn’t strong enough to produce a glint. The image was taken at 5 microns, well into the infrared, where Titan’s atmosphere is essentially transparent. Cassini was about 200,000 km (120,000 miles, about half the distance of the Earth to the Moon) away from Titan when the image was taken.
It’s a cool picture! It looks a lot like images of Earth taken from space. I don’t mean the color or fuzziness — both due to Titan’s smoggy air — but just the way our brain recognizes how a flash of light like that is from liquid. We have to double check our brains, of course, since we’re easily fooled, but the confirmation of it satisfies some part of my own brain that likes to categorize things.
And it also brings home, so to speak, just how Earthlike this alien world is. It has a thick atmosphere, weather, and a hydrological cycle… except where we have water, Titan has methane. And of course it’s incredibly cold there; water would be frozen into ice literally as hard as rock. But liquid on the surface harkens to another part of our brain, the piece that asks if life could arise in such a place.
The answer is, of course, we don’t know. Not for sure. But we can’t rule it out, either.
The more we learn about Titan — and everywhere in our solar system — the more intriguing and beguiling it gets. I know that even now scientists are planning the next generation of exploratory spacecraft. I hope one of them will take a much closer, and much wetter, look at this giant satellite world.
Last year, the Cassini spacecraft found solid (haha) evidence for the existence of lakes of liquid methane and ethane on the giant moon Titan. Of course, Titan is barely a moon at all — bigger than Mercury, it would be a planet in its own right if it weren’t orbiting Saturn. It has an atmosphere with almost twice the surface pressure as Earth’s, which is mostly nitrogen and a trace of hydrocarbons.
But that trace is important: because Titan is so cold, methane and ethane can rain from the Titanian sky, forming river systems and lakes. But there’s a problem: the north pole of the moon has far more lakes than the south pole. Seven times as many!
First, methane on Titan goes through cycles something like water does on Earth. During Titan’s summer, the northern lakes lose methane to evaporation, and the gas gets transported to the colder south pole, where it recondenses. The opposite happens in the southern summer. But that should balance out, so that each pole over the course of time has the same amount of liquid. They don’t. So, clearly, something else is going on. And it isn’t topography; the north and south polar regions of the moon have roughly the same overall shape, so you don’t expect liquids to flow into or out of one of those regions more than the other.
However, some scientists think they may now know what it is. Their idea, not yet proven but still very interesting, is that the reason is due to the seasons on Titan, coupled with Saturn’s elliptical orbit.
Saturn orbits the Sun once every 29 or so years. Its orbit is decidedly elliptical; it varies from about 1.35 to 1.5 billion kilometers from the Sun, a variation of 150 million or so kilometers! When Titan’s north pole is in summer, Saturn is farther from the Sun, and the southern summer is when Saturn is closest. That means that summers are cooler in the north, lowering the methane transport to the southern pole… and summers are warmer for the south pole, increasing methane transport.
In other words, Titan’s south pole is selling low and buying high. That’s not what you want to do if you want to keep yourself rich in hydrocarbon lakes.
Interestingly, again like the Earth, the geometry of Saturn’s orbit changes slightly over time. Over the course of tens of thousands of years, it changes such that the situation is reversed: Titan’s north pole will experience summer when Saturn is closest to the Sun, and the south pole when Saturn is farthest. If this idea of asymmetrical seasonal flow is correct, this will reverse that flow, putting more lakes at the Titanian south pole. On the Earth, these orbital variations are called Milankovitch cycles, and are most likely tied to very long term (like thousands of years long) global climate change. It’s possible, too, that the Milankovitch cycle may be tied to regions on the Earth periodically becoming deserts and then turning more humid once again.
Titan is sometimes seen as an analog of the young Earth, with methane taking the place of water in its geology and chemistry. It’s interesting to see that perhaps we have even more in common than we thought!