Titanic Occultation

The Earth isn’t the only world in the solar system with an atmosphere. Heck, all the planets have atmospheres, with the possible exception of Mercury^*, but that asset is rare among moons. Only one moon orbiting a planet in our solar system has an appreciable atmosphere, and that’s Titan, the largest moon of Saturn.

Titan doesn’t just have an atmosphere, it has a thick one: the pressure at the surface is actually higher than Earth’s by about 50%! So, its atmosphere is denser even than ours.

There’s a funny thing about air: It bends light. This phenomenon is called refraction, and you’ve seen it countless times. It’s the same thing that happens when a spoon in a glass of water looks bent; the water bends the light from the spoon more than air does, distorting the image that gets to your eye.

This can be useful. For example, if a planet or moon with an atmosphere appears to pass directly in front of a star as seen from Earth, the starlight will get bent as it passes through that atmosphere. The way the light gets bent depends on the atmospheric thickness, temperature, and pressure… and those all change with altitude and position. So, the light gets bent differently as it passes through different parts the atmosphere. By studying just how the light bends, you can infer quite a bit about the world’s atmosphere.

Perhaps you see where I’m going with this…

Cassini image of Titan; you can see the haze layer over the north pole. The moon is shrouded in thick haze, giving it that yellowish appearance and hiding surface features. Credit: NASA/JPL-Caltech/SSI

Photo: NASA/JPL-Caltech/SSI

On December 20, 2001, the giant moon Titan was predicted to pass in front of the star NV0435215+200905 (in astronomy lingo this kind of event is called an occultation). Observations were planned so that astronomers could learn more about the atmosphere of Titan (mind you, this was several years before the Cassini spacecraft arrived at Saturn and started intense observations of Titan from up close).

One such observation was done using the 5-meter Hale Telescope at Mt. Palomar, which, for decades, was the largest optical telescope on Earth. The camera was designed to look in the near-infrared part of the spectrum, and had an adaptive optics system on it that compensated for smearing of the incoming light due to Earth’s own atmosphere. Our air roils around us, blurring out small details in observations, but an adaptive optics system correcting for that means very high resolution can be achieved.

As the observations began, the astronomers discovered something critically important: NV0435215 was not one star, but two! It was a binary: two stars of nearly equal brightness, separated by a mere 1.5 arcseconds — a very small distance (the full Moon on the sky is over 1000 times bigger). But that separation is wider than Titan’s apparent size, which means it was possible the stars could both miss Titan! This led to one of the most wonderful paragraphs I’ve ever seen in a paper:

The first result of these observations was the immediate realization, as the star NV0435215+200905 crept into the field of view of the PHARO camera, that it was a clearly separated binary. Predictions of the occultation had assumed it to be a single star, and placed Palomar Observatory 80±250 km from the occultation centerline. Brief panic thus ensued as we feared that the two stars would pass to the north and south of Titan, while their center of light passed behind the satellite’s disk. Happily this did not occur, and both stars were occulted in rapid succession…

That “brief panic” must’ve been quite something. As they say, though, circumstances worked out, and the two stars did pass right behind Titan. An animation was made of the event, and it’s truly wonderful. It’s also truly weird. Watch:

The two stars appear on the right, and Titan, well-resolved as a disk, is on the left. Titan fluctuates a bit due to our own air, which was not 100% compensated for by the optics. The stars also appear to have broken rings around them; this is an artifact due to the telescope optics (due in this case to diffraction, a different way light gets messed up as it passes around things).

During the second star's transit, two refraction spots appear on opposite sides of Titan. Credit: Bouchez et al.

Photo: Bouchez et al.

As the stars disappear behind Titan’s edge, you can see them fade out, then reappear on the other side. But if you look closer, you’ll also see that as the first star is occulted, a bright spot of light whips around the top of Titan, and the second star has one that whips around on the bottom (as well as a fainter one around the top moving in the opposite direction)! What’s that?

This is a well-known effect of refraction. The atmosphere of Titan is thick enough to bend the light entering it, a lot. The way the geometry works out is that you get light bent all around the moon, at every angle, but it’s brightest at the point on the edge connecting the center of the moon with the star. In other words, start at Titan’s center, then draw a line from it to the position of the star at any one time as it passes behind. Continue that line until it intersects the nearest edge of the moon, and that’s where refraction is brightest:

Diagram showing the geometry of the event. The refraction spots appear on opposite sides of Titan; the line drawn between them instersects both the position of the star behind the moon and the center of the moon. Credit: Phil Plait

Photo: Phil Plait

The second brightest spot is on the opposite limb. In reality, it gets bent all around the limb, but the amplification is too small to notice. If the stars had passed exactly behind Titan’s center, the light would’ve been bent in all directions around the atmosphere equally, and there would have been a bright ring all the way around Titan, what’s called a “central flash”. That would’ve been weird, too.

As odd as this is, it’s tremendously useful. A lot was learned about Titan’s atmosphere from this event by the way the stars’ light was affected (including the existence of jet streams distorting the air at lower latitudes). It also showed that adaptive optics could be used for phenomenally accurate high-resolution observations like this.

Sequence showing Titan occulting the two stars, with each frame taken roughly one minute apart. Titan has been digitally subtracted for clarity, but the solid surface is shown by the dashed circle. You can see the two refractions spots as both stars pass behind the moon. The box is about 3 arcseconds wide, roughly the width of a red blood cell held at arm’s length. Credit: Bouchez et al.

Photo: Bouchez et al.

We now know way more about Titan than we did back in 2001; nearly 13 years of Cassini data will do that. But this occulting method is useful in studying more than just Titan; it can be used on planets, too, and adaptive optics have in many ways revolutionized ground-based astronomy — you can get resolution as good as Hubble can, and in some cases over a pretty decent sized chunk of the sky.

We’ve come a long way in 16 years, but that doesn’t mean old tools are no longer useful. We won’t always have probes going to other planets, or huge space-based telescopes to point wherever we want (James Webb Space Telescope, which is due to launch next year, will be hugely oversubscribed for many years; heck, Hubble launched in 1990 and it’s still hard to get time on it). But cameras like the one used here are and will always be really important...and I’ll note that the 5-meter Hale telescope saw its first light in 1949.

New is good, but don’t let it occult the old.

^* Mercury does have gas lingering around its surface, but it’s at a pressure of one one-hundred trillionth that of Earth’s, so if you think that qualifies as an atmosphere then may I suggest no.

Tip o’ the bent spoon to Evil Mad Scientist for the link to this article.