Small, Bright Moon…of Saturn

Today I’m going to talk about one of Saturn’s many moons. This one is named Enceladus and is smaller than the state of Arizona, though quite bright because it reflects 80 to 90 percent of the light that hits it. Despite Enceladus’ small size, it is a very interesting moon. The Cassini spacecraft that orbits Saturn completed flybys of Enceladus, and made some surprising discoveries, starting in 2005. For one, Enceladus has an atmosphere. That’s not unheard of: one of Saturn’s other moons, Titan, has an atmosphere. But because it is so much smaller, Enceladus has a lot less gravitational pull than Titan; in other words, it would not normally be able to hold onto an atmosphere. So there must be something actively replenishing the atmosphere. Sure enough, plumes of gas and ice were later found to emanate from Enceladus’ south pole. The plumes come from the region near long, dark stripes nicknamed Tiger Stripes. Additionally, the south pole is much warmer than expected, as you can see in the image below. This implies that there is some sort of internal heating.
The thermal anomaly present with Enceladus. The left shows what was predicted: the right shows what was actually seen.  Image credit NASA/JPL/GSFC.
The thermal anomaly present on Enceladus. The left shows what was predicted: the right shows what was actually seen. Image credit NASA/JPL/GSFC.
These discoveries started in 2005. Naturally, scientists started observing Enceladus as much as possible. I often think of science as trying to logically explain the weird, and Enceladus is weird. Did I mention that the ejecta from the plumes seems to be creating the one of Saturn’s rings, the E ring?
Enceladus is the small dark circle near the bright spot. The white ring is Saturn's E ring, replenished by Enceladus' plumes. Image from Astronomy Picture of the Day. Image credit CICLOPS/ESA/JPL/NASA.
Enceladus is the small dark circle near the bright spot. The white ring is Saturn’s E ring, replenished by Enceladus’ plumes. Image from Astronomy Picture of the Day. Image credit CICLOPS/ESA/JPL/NASA.
Enceladus’ surface appears to be very geologically young. In this case, ‘geologically young’ means it might even have reached its 100 millionth birthday! Small bodies like Enceladus tend to have internal heat cool down quickly, which stops geologic activities (for example, Mars, a smaller planet than Earth, is geologically dead while humans on Earth report volcanoes and earthquakes). So why is small Enceladus still doing things? The theory I have seen repeatedly is that it may have to do with the push-and-pull of gravitational tidal forces resulting from Enceladus’ oval-shaped orbit.
So, now you know the basics of why scientists are paying close attention to the results of Cassini flybys past Enceladus. Among these scientists are Valeriy Tenishev, Doğa Can Su Öztürk, Michael R. Combi, Martin Rubin, Jack Hunter Waite, and Mark Perry, who worked together on today’s paper, “Effect of the Tiger Stripes on the water vapor distribution in Enceladus’ exosphere“. This work was published in 2014 in Journal of Geophysicsal Research: Planets. Exosphere can be your vocabulary word of the day: it is the outermost part of an atmosphere.
Tenishev et al used what they call a multiplume model to simulate the activity on Enceladus. This model determines the density of the ejected gas by looking at a uniform source within the moon, eight previously identified vents, and fissures along the Tiger Stripes. All of these would combine to explain the gas plumes. How much each of these contributed was determined by fitting the models results to actual results from the Cassini spacecraft.
When they went to look at the emissions from the Tiger Stripes, they found that there were too many jets to look at one by one. Instead, this was modeled as the length of the fissure all giving off the same amount of gas. The observations from Cassini suggest that’s not actually the case, so they compensated by putting extra fissures in the appropriate regions. The best match for the observerd data was when the modeled temperature was 180 Kelvin (-93 C, -136 F). The speed of the particles was somewhere in the range of 350 to 950 meters per second (783 to 2125 mph). That’s much faster than the speed needed to escape Enceladus’ small gravitational pull, about 235 meters per second (526 mph). As extreme as these numbers might seem to us Earth-dwellers, they match pretty well with the results from Cassini’s instruments.
The four bluish stripes in a row are the Tiger Stripes I keep refering to. Image from Astronomy Picture of the Day. Image Credit: Cassini Imaging Team/SSI/JPL/ESA/NASA.
The four bluish stripes in a row are the Tiger Stripes I keep referring to. Image from Astronomy Picture of the Day. Image Credit: Cassini Imaging Team/SSI/JPL/ESA/NASA.
The effect of the Tiger Stripes fissures are needed in order to match the observed results. Even so, the authors of this study could not get the model to match without adding in an additional source. They believe it is located within a fracture on the surface, suggesting that it had to do with geological activity.  In other words, both the Tiger Stripes and a yet-to-be-determined source is needed to complete the picture. The tiny moon is still partially a mystery because it does things like this:
The plumes from Enceladus' south pole. Image credit NASA/JPL/SSI.
The plumes from Enceladus’ south pole. Image credit NASA/JPL/SSI.
It goes without saying that scientists are going to keep studying Enceladus until we have it figured out. And then we will keep studying it, because there is always another question that comes up, or something to be nailed down exactly, or something.
Science never stops.
Bibliography
Sources used to directly write this post
Astronomy Picture of the Day. “APOD: 2006 March 10 – Enceladus and the Search for Water.” N.p., 10 March 2006. Web.
Image of Enceladus’ south pole, showing the Tiger Stripes.
Astronomy Picture of the Day. “APOD: 2007 March 27 – Enceladus Creates Saturns E Ring.” N.p., 27 March 2007. Web. 1 Apr. 2015.
Image of Enceladus and the E ring.
National Aeronautics and Space Administration. “Enceladus.” N.p., n.d. Web.
A page about Enceladus in general.
National Aeronautics and Space Administration “Enceladus Discovery Timeline.” N.p., n.d. Web.
A timeline of when things were discovered about Enceladus. While it shows the highlights, it unfortunately only discusses 2005.
National Aeronautics and Space Administration. “Enceladus ‘Feeds’ Ring.” N.p., n.d. Web.
Linked to within the main post, this discusses the structure of Saturn’s rings and how Enceladus is related to the E ring.
National Aeronautics and Space Administration. “Jet Blue.” N.p., n.d. Web.
The image of Enceladus’ plumes.
National Aeronautics and Space Administration. “Enceladus’ Stressed out Tiger Stripes.” N.p., 7 June 2010. Web.
A page about the Tiger Stripes. It discusses tidal forces and how they may effect the plumes in good terms. It was also linked to within the text of the main post.
National Aeronautics and Space Administration “Enceladus Temperature Map.” N.p., n.d. Web.
A page describing the temperature anomaly, from which I took the image used in today’s post.
Tenishev, Valeriy, et al. “Effect of the Tiger Stripes on the water vapor distribution in Enceladus’ exosphere.” Journal of Geophysical Research: Planets 119.12 (2014): 2658-2667.
The main paper for today.
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