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Posted by: Cugel Sep 13 2005, 04:02 PM

For a long time in history Titan has been considered to be the largest of all moons in our solar system. And rightfully so! Unfortunately, somebody got the crazy idea that the atmosphere of Titan does not count, and indeed, the diameter of the rocky/icy body of Titan is roughly 100 km. less in diameter than that of a pitiful piece of debris called Ganymede orbitting 'That Other Planet'.

So, what would be the biggest planet in the solar system if we don't count atmospheres anymore, huh? Probably not that 'That Other Planet'! It's ridiculous.

I think Titan should be back on the #1 spot of Greatest Moons. ASAP.

What do you think?

Posted by: Rob Pinnegar Sep 13 2005, 04:18 PM

This kind of ambiguity also occurs in trying to define the radii of gaseous bodies (the Sun and the giant planets). Atmospheres don't have a clear-cut outer boundary.

One way of addressing it is to consider mass as being more important than diameter, since this _can_ be defined without significant ambiguity. In this respect, Ganymede wins by about 5% over Titan. Callisto comes third.

This doesn't mean Ganymede is more deserving of study than Titan. Larger doesn't mean more interesting. Compare the 1986 images of Miranda with those of Uranus.

Posted by: David Sep 13 2005, 04:24 PM

I believe that the height of Titan's atmosphere changes quite a bit over time (as does Earth's), according to time of day and season (and perhaps also solar output?), and also varies at different spots on Titan's globe. So you wouldn't have a consistent set of numbers to describe Titan's physical shape.

Posted by: Decepticon Sep 14 2005, 12:46 PM

Don't forget Ganymede has a atmosphere also.

If you want to count the atmosphere as part of diameter then count Ganymede's atmosphere also, No matter how sparse it is!

Posted by: Cugel Sep 14 2005, 04:02 PM

Well, I admit there might be some practical problems with this. However, since we do take the atmospheres of Jupiter and the likes into account...
Maybe we should simply put a density limit of where an atmosphere ends and space begins. This will indeed fluctuate over time but we can simply average over a few 1000 years or so....

Or maybe forget about this silly topic all together! I just mentioned it because I'm a bit amazed about the current "What is a planet?" discussion. Why make such a fuzz of it when basically every astronomical designation is 'ad hoc' ?

Posted by: mike Sep 14 2005, 05:14 PM

If people didn't disagree on definitions there would be nothing.

Posted by: Jeff7 Sep 15 2005, 09:27 PM

Seriously? Titan was considered the largest?

Never heard that. I had always heard that Ganymede is the largest moon.

Posted by: SigurRosFan Sep 15 2005, 09:57 PM

Before the Voyager encounters, astronomers thought Titan was the largest. Voyager measurements of Ganymede showed it is larger than Titan.

Posted by: Rob Pinnegar Sep 16 2005, 12:06 AM

That's right. Titan was always thought to be a fair bit bigger than Ganymede. I guess that the thickness of the atmosphere was what threw astronomers off -- they thought the orange cloud layer was the surface and didn't realize that the true surface was much deeper down. An understandable mistake, really. Remember there was no Hubble telescope back then.

Even more poorly constrained was Triton's diameter. I distinctly remember that one of the astronomy books in my hometown library ("Solar System" by Ludek Pesek, maybe?) claimed that Triton's diameter could have been as large as 6000 km, making it the largest satellite in the Solar System. It is of course slightly less than half that size in reality. I guess the moon's albedo was drastically underestimated.

Now that I think of it though, it might also have been due to the same limit-of-resolution problem that caused Pluto's diameter to be listed as "3,600 miles" for decades. 3,600 miles is almost exactly 6000 kilometres.

Posted by: alan Sep 16 2005, 01:17 AM

I remember before Voyager got close to Neptune there was speculation about liquid nitrogen on Triton. Once it was close enough to resolve Triton and determine its diameter and albedo it was apparent that Triton was too cold for liquid nitrogen to exist. (higher albedo = lower surface temperature)

Posted by: Decepticon Sep 16 2005, 01:33 AM

I clearly remember Triton is astronomy text books having artist renditions of methane lakes!

Posted by: tedstryk Sep 16 2005, 02:44 AM

Some old texts gave Pluto's diameter at 6,000-12,000 km. This was because it was hypothesized that Pluto and Triton would have very low albedos, and given the limits of telescopes of the day, the diameters we were using were simply formulated from their brightness and an albedo guess. The fact that they are extremely reflective and very small was a surprise. In the case of Titan, yes the atmosphere got in the way. In the case of Triton and Pluto, it was very poor data. Not to mention the fact that pre-Charon studies of Pluto lump Pluto and Charon together, confusing things even more.

Posted by: tedstryk Sep 16 2005, 02:47 AM

Of course Voyager did find frozen evidence of such things that were results of cryovolcanism. And no one expected geyers. As for the lakes, it is actually tied to the diameter issue. Had Triton been 3,000 km across and of a low albedo, it would absorb more solar energy and hence be a bit warmer, possibly able to sustain nitrogen lakes. The fact that it reflects almost everthing away meant that in addition to being smaller, it was colder than expected, causing nitrogen to freeze.

Posted by: BruceMoomaw Sep 16 2005, 05:11 AM

It's been pointed out in (I believe) "Icarus" that, if Uranus had a moon comparable in size to Triton, it probably WOULD have liquid nitrogen seas. Missed by that much...

Posted by: edstrick Sep 16 2005, 07:19 AM

*OPTICALLY*, Titan is the largest moon in the solar system.

The single most important science Voyager 1 did at Titan was the radio-occultation experiment. It precisely measured the diameter and (with one limitation) precisely measured the atmosphere pressure/density/temperature structure.

The limitation was they couldn't tell the % of Argon in the atmosphere, as much as 30% in some early post Voyager models. That changes the mean molecular weight of the atmosphere and slides the whole temperature scale up or down <don't remember which> by a few degrees.