The Cassini spacecraft orbiting Saturn recently had everyone worried when it unexpectedly went into safe mode, but fortunately it was brought back to life in time for a targeted flyby of Enceladus on Nov 30th. Images from that flyby are being released as I type (here’s a particularly nice one of a crescent Enceladus, with the plumes at the top of the image), but today I’m going to show you another interesting moon called Hyperion, that was also imaged as part of the flyby.
Hyperion’s was discovered by earth-based astronomers in 1848, but like all of saturn’s moons we didn’t get a good look at it until Voyager 2 flew past Saturn in 1981. Unfortunately Hyperion was pretty far from Voyager 2 at the time and it couldn’t get very high-resolution images, but the images did reveal that Hyperion was irregularly shaped. Fortunately, Cassini got some better images – here’s a sequence of images taken at the end of November 2010, showing Hyperion in all its glory (click to expand, and then magnify it – it’s a big image!):
Hyperion is a grand-sounding name, but the satellite itself is only a few hundred kilometres across (since it’s non-spherical, it’s actually 328 km × 260 km × 214 km) – but that’s pretty big for an asteroidal body. In fact, if it was in the asteroid belt, Hyperion would be the 11th largest asteroid! As you can see, it looks a bit like a giant sponge, suspended in space! So could it be a captured asteroid perhaps? Or is a fragment of a formerly spherical icy moon that was broken up by some cataclysmic impact?
All of these images are false colour, constructed using images taken in infrared, green, and ultraviolet filters (see Postcards from a Distant Moon for more details about this process). Apart from some cleaning of the UV images (which had lots of bright speckles in them), they are otherwise unaltered. Before I continue though, I’m going to digress a little and discuss another important aspect of image interpretation.
| Digression: Incidence, Emergence, and Phase angles
There are three important angles to be aware of in imaging – the incidence angle, emergence angle, and phase angle (I’m not sure that there’s a collective name for all three!):
Putting this all together, high phase angles are the best way to see topography on the surface, since the sun is low to the ground and casting lots of shadows (this is why you can see lots of mountains and craters when you look at the boundary between the light and dark sides of the moon). A low phase angle combination is the ideal way to see differences in albedo on the surface because then all of the variation in brightness that you see are due to differences in the colour, material or reflectivity of the surface. When the phase angle is exactly zero, you get an opposition surge where the object becomes significantly brighter than at other angles because of the complete lack of shadows being cast by grains on its surface (see this page for more info), but I won’t dwell on that here since that’s getting into complicated stuff about photometry ;). High emergence angles give you nice oblique views if you want an impressive perspective view of something, and have been used to good effect in Apollo images of the moon (like this classic Apollo 17 image of Copernicus crater). You might also notice that the emergence angle is independent of phase angle – the sun can be behind the viewer (low phase angle) but the viewer (and sun) can be looking obliquely at the ground (high emergence angle) or directly overhead relative to the surface (low emergence angle). In both of these cases, the albedo variations of the surface will be dominant because of the low phase angle, but the view will be affected by perspective differently. |
OK, now that’s out of the way, you can look again at the imaging sequence at the top of the post and see that we’re going from high phase angle (crescent Hyperion – I’d say the phase angle is about 150° there) to low phase angle (full Hyperion – phase angle is close to 0°) as the image sequence progresses in time from left to right. You can see in the first couple of images of the sequence (on the left) that the topography dominates what you see – you can’t tell much about the actual brightness of the material that Hyperion is made of, or the colour of its surface – and that’s typical of high phase angle images. In the last couple of images (on the right), the incidence angle is lower, which means there are less shadows and we can see a lot more of the surface. Here’s one of those images, in which we can see that many of Hyperion’s craters are filled with dark material.
Interestingly, this dark material is reddish, and suspiciously similar to the dark material coating an entire hemisphere of Iapetus, the next moon out from Saturn – so it seems that some of that stuff is getting onto Hyperion too. This leads me to the other interesting feature of Hyperion – the dark material is found all over satellite and is not concentrated on one hemisphere, because unlike pretty much every other major satellite in the solar system it is not tidally locked to its primary (Saturn’s outer moon Phoebe and Neptune’s satellite Nereid being the other exceptions). Tides generally act to rapidly slow a satellite’s rotation so that it matches its orbital period, so that one face permanently points toward its primary planet – this is what happened in the case of our own moon for example, and the satellites of Jupiter. If an object is small enough and far enough away from its primary (as is the case with Nereid and Phoebe) then the timescale in which this happens is longer than the age of the solar system, which means that today they have rotation periods independent of their orbital periods – but this isn’t what has happened in Hyperion’s case.
The reason for this is Titan, Saturn’s largest moon and the next satellite in from Hyperion. Hyperion is in a 3:4 orbital resonance with Titan, which means that Hyperion completes three orbits around Saturn in the same time that Titan takes to complete four orbits – so they reguarly get back to the same configuration relative to eachother. This means that Hyperion receives repeated, regular gravitational “tugs” from the much more massive Titan, which may explain why Hyperion’s orbit is eccentric too. All of these factors combine to prevent Hyperion from tidally locking to Saturn – but it gets even crazier than that. Hyperion actually has chaotic rotation as a result of this – not only is it not tidally locked, but its rotation period also varied, and what’s more the orientation of its axis of rotation varies too! Hyperion is literally tumbling through space as it orbits Saturn, with no two rotations being the same length or even around the same axis! This explains why the dark material is found in craters all over the satellite, since it lands on a random surface when it falls onto Hyperion from the outer Saturnian system.
All in all, Hyperion is a rather fascinating little satellite. There are still at least five more flybys of Hyperion scheduled for the rest of Cassini’s (very) extended mission, so hopefully we’ll be seeing more interesting images in the coming years! I’ll leave you with a gallery of (false-colour) images from the flyby sequence at the top of this post, and a rather nice animation that I found showing the whole sequence!
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