# Saturn's biggest kids play peek-a-boo

By Phil Plait | July 1, 2011 11:30 am

I spent all day yesterday writing a 2000-word article for a print venue to be named later, and the weather outside is sunny and delightful and begging to be biked in, so I am disinclined to write something deep and philosophical today. So instead here is just a simply way-cool picture from Cassini taken in 2009, showing the Saturnian moon Rhea peeking out from behind the much larger Titan:

[Click to eneldergodenate.]

[UPDATE: I messed up here. In the original post I misread Titan’s radius when I looked it up, and was comparing it to Rhea’s diameter. This changes my numbers enough that I have simply corrected everything below; otherwise it would be too confusing to read. Thanks to the commenters for pointing this out!]

Rhea is a little over 1500 km (900 miles) across, and Titan 5150 km (3100 miles). However, in this shot, Rhea was almost two and a half times farther away than her big sister, so it looks smaller than it really is. Titan has a thick atmosphere, which is pretty obvious in the picture, while Rhea is basically a ginormous iceball.

Still, hmmm. Titan and Rhea are the two largest moons of Saturn, but to be honest Titan really is a lot bigger than Rhea, more than 3 times wider. Why such a big gap in sizes? Jupiter’s two largest moons, Ganymede and Callisto, are much closer together in size (5260 and 4820 km, respectively), and Ganymede is only 1.7 times bigger than Jupiter’s fourth largest moon, Europa. After that, though the rest are far smaller.

It seems to me that Saturn and Jupiter are telling us something about the physics of the way their moons formed. But what could it be? Titan orbits well over twice as far from Saturn as Rhea, while Ganymede is actually closer to Jupiter than Callisto. Is that important? Did those moons form at other distances and get their orbits jostled through gravitational interactions over billions of years, maybe even switching positions?

These are pretty basic questions, but it’s questions like these that lead to basic insights on how our solar system formed and changed as time went on.

And dangit! I guess I did get a little deep and philosophical here. Ah well, what can I say? Images like this are so pretty and so interesting to look at, they spark all kinds of thought processes in my head. And the more I do that, the more I want to do that. Science is like that: addictive, but in a good way.

Damn! Did it again.

Image Credit: NASA/JPL/Space Science Institute

Related posts:

1. Jason

That is a very cool picture, if the positions were swapped would we still see any fuzziness because of Titan’s rather thick atmosphere?

And I just happened to think of this, would Titan be able to hold onto its atmosphere if it were the same distance from the Sun the Earth is or would solar warming and other effects blast it away? I was under the impression (possibly mistaken) that we couldn’t get much smaller and still gravitationally hang onto our air.

2. I love how years later there are still tons of images I have never seen from Cassini.

3. When I worked on IBEX I was at conference where they brought in some folks that worked on Cassini because their researchers found essentially the same mapping of the heliosphere using MIMI that we did both by using detection of electrically neutral atoms.

This is just another one of those missions that will keep giving us awesome things for a long time, even after its gone. I really love Casini.

Casini and IBEX. . .

4. Alan C

Titan is not 2600Km across, it is about 5150Km.

5. Brody

It looks like a boob.

6. Pete Jackson

@1 Jason: I think it’s safe to say that Titan would have lost its atmosphere were it orbiting the Earth rather than Saturn. What’s more, it would be a lot smaller, reduced mainly to its rocky core as all the methane, ammonia, and even water (ice) sublimed away over the ages.

Even Mars, much more massive than Titan and farther from the Sun than the Earth has lost most everything.

7. Kochira

@1 Jason and @5 Pete Jackson As I understand it Pete is correct that Titan would not have retained its atmosphere, but the reason would have to do with the lack of internal dynamics creating a magnetic field for shielding. Saturn has a really big magnetosphere so this is what protects Titan from solar wind erosion of its atmosphere.

So really it has more to do with its association with a magnetosphere rather than the distance from the Sun. Though I’m sure there is a relationship between distance and strength of magnetosphere required at some level.

Append: Found an interesting article on this here: http://www-ssc.igpp.ucla.edu/personnel/russell/papers/sat_mag.html

8. Gaby

Click to what?

eneldergodenate?

9. Gonçalo Aguiar

Incredible how thick Titan’s atmosphere is.

“Rhea was almost two and a half times farther away than her big sister”
Dang… I’ve always pictured Titan has a male moon… Btw how do you figure moons’ gender? You play peek-a-… erm

10. Jason

@Pete Jackson

That is what I was thinking given the state Mars is in, but I wanted to be sure. Though is that due to size, or lack of a magnetic field, or both?

11. My favorite part was the moment of realization as I went from “oh, it’s a fuzzy picture of a moon” to “no, wait. Rhea’s sharp. It’s not the photo that fuzzy, it’s Titan!”

12. The “across” figures for both Rhea and Titan are the radius, not the diameter, but then the values for Jupiter’s moons are the diameter. Titan is actually only slightly smaller than Ganymede.

13. Pete Jackson

@7 Kochira: Mercury’s magnetic field hasn’t protected its atmosphere. And Jupiter’s whopping magnetosphere hasn’t protected its children.

14. kevbo

That’s a Titanic outie!

15. Andrew W

“Rhea is a little over 1500 km (900 miles) across, and Titan 2600 km (1550 miles).”

You’ve got Rhea’s diameter and Titan’s radius there, so the ratio of diameters is actually about 3.5:1

16. Kochira

@Pete Mercury does not have any internal dynamics and as a result had no magnetic field to protect it’s atmosphere.

Current research into Mars indicates that when it was young it had a liquid core and as a result a magnetic field that would have protected its atmosphere but it cooled and solidified, which resulted in the erosion of atmosphere.

While the lack of a protective magnetic field would indicate that erosion of atmosphere is likely, I don’t think we can make the inverse statement that the presence of a magnetic field ensures the presence of an atmosphere, at least in the case of moons. As a result it does still seem likely that the Saturn’s magnetic field is, at least partially, responsible for the presence and maintenance of atmosphere on Titan.

No doubt terrestrial processes on the moon also play a part as Titan appears to have some processes that produce gasses occurring.

The lack of atmosphere in Jupiter’s moon would be more indicative that those moons never had or developed atmospheres rather than show that Saturn’s magnetic field has no relation to Titan’s atmosphere.

17. Yeah, about 3, not 1.7.

On the sizes and formation of moons, Robin Canup pretty well seems to have it wrapped up:

http://www.livestream.com/2011lpsc/video?clipId=pla_5dff90fa-d72b-445c-a33a-4e61773ac83d

18. Joseph G

#5 Brody: It looks like a boob.

I’m glad I’m not the only one who thought that. Still, I think this particularly high level of pareidolia indicates that I really really really need to get laid 😛

19. Jim Baerg

@Kochira
Venus has zero magnetic field & a much thicker atmosphere than Earth. Even if you subtract the CO2 the nitrogen atmosphere of Venus would be a few times thicker than Earth’s atmosphere.

The few data points we have from solar system bodies don’t support the idea that magnetic fields signifcantly hold in atmospheres.

20. Joseph G

Gonçalo Aguiar Says: Incredible how thick Titan’s atmosphere is.

“Rhea was almost two and a half times farther away than her big sister” Dang… I’ve always pictured Titan has a male moon… Btw how do you figure moons’ gender? You play peek-a-… erm

I think all moons are considered more or less feminine? Seeing as how our own has usually been seen as a Goddess in the polytheistic mythologies.

Kochira Says: While the lack of a protective magnetic field would indicate that erosion of ?atmosphere is likely, I don’t think we can make the inverse statement that the presence of a magnetic field ensures the presence of an atmosphere, at least in the case of moons. As a result it does still seem likely that the Saturn’s magnetic field is, at least partially, responsible for the presence and maintenance of atmosphere on Titan.

I was wondering about that – my understanding was that solar wind causes atmospheric erosion. Does this erosion follow the inverse square law with distance, like radiation? Also, does the low temperature and more complex gases (hydrocarbons, etc) also have something to do with it? Might Mercury have a thicker atmosphere if it were as far out as Saturn, even lacking Saturn’s magnetic field to protect it?

21. Joseph G

Jim Baerg: Venus has zero magnetic field & a much thicker atmosphere than Earth. Even if you subtract the CO2 the nitrogen atmosphere of Venus would be a few times thicker than Earth’s atmosphere.

Wow, I never knew that. I always chalked it up to CO2’s density.

Also, the hypothesis I’ve aways heard for the dryness of Mars was that water molecules were broken up by solar charged particles, with the hydrogen then being light enough to get kicked out into space by said particles.
If you’re right, and magnetic fields aren’t important for atmosphere maintainance, this would seem to bode well for long-term terraforming efforts of planets like Mars
(I know, so sue me, I read Kim Stanley Robinson’s Mars Trilogy and now I have terraforming on the brain!)

22. ToSeek (12): I just noticed that and corrected it. Thanks!

23. @Joseph G Hey holla at me! I need to too! lol

24. amphiox

Also, the hypothesis I’ve aways heard for the dryness of Mars was that water molecules were broken up by solar charged particles, with the hydrogen then being light enough to get kicked out into space by said particles.

Atmosphere loss is probably a function of both magnetic field and gravity. The bigger a planet, the more of its atmosphere it’ll be able to retain by gravity alone, and the less it will need to rely on a magnetic field.

The closer it is to it’s parent star, the more intense the solar wind it will face blowing away it’s atmosphere as well.

25. Anchor

It strikes me peculiar that Rhea exhibits an apparently much thinner crescent than Titan does. One would expect them to exhibit a simiilar phase from a particular vantage point…

What I’m saying is that it is interesting – and that I have never previously seen such a discrepancy in previous images.

26. Anchor

“And dangit! I guess I did get a little deep and philosophical here. Ah well, what can I say? Images like this are so pretty and so interesting to look at, they spark all kinds of thought processes in my head. And the more I do that, the more I want to do that. Science is like that: addictive, but in a good way.”

Phil, there is absolutely positively nothing whatsoever wrong with waxing ‘philosophical’ or, to my mind, poetically.

You are expressing your IMPRESSIONS.

That’s a good thing.

More of it* is definitely indicated and desired.

*”it” of course is the simple ability to react as you will. “IT” is ALWAYS cool!!!!

27. JUSTSOMEGUY31167

When it comes to climate change, bad astronomy should be called bad science. More science:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3053361/

28. Pete Jackson

@25 Anchor: If you zoom in on Rhea (cntl+ in your browser?), you will see the illuminated part Rhea extend deep into Titan’s atmosphere – the illuminated extent, i.e. the phase, is the same on both Rhea and Titan. Rhea does exhibit a lot of limb brightening, making it look like a much younger, skinnier phase. Presumably, that part of Rhea at the limb is much brighter, or it appears to be because of the way the surface scatters light. Titan’s atmosphere would scatter light in a diffuse way, more or less the same in all directions.

29. I’ve sometimes wondered about this, but the discrepancy is really in the opposite direction. Jupiter has a much more dramatic size gap between the four Galileans and its smaller moons: the fifth largest moon is Amalthea, which isn’t even round.

Saturn has one moon in the Galilean class, Titan, and a lot of smaller moons as well; but there are also these in-between-sized iceballs, of which Rhea is the largest.

30. 27. JUSTSOMEGUY31167 Says:
When it comes to climate change, bad astronomy should be called bad science.
——
Welp! Can’t argue with that logic!

Coral reefs may be slightly more resilient to temperature change and ocean acidification than we previously thought. Thus, all of climate science is hereby refuted.

Well done, JUSTSOMEGUY31167!

31. justsomeguy31167

…who said that, I pointed to another article done using real science shows another pedestal of the “world is ending tomorrow” paranoia is, well, wrong. Warming is happening, the idea that it is historic and we are going to die is as well founded as the group that called the rapture last month.

32. Anchor

@Pete Jackson #28: yes, I think you’re right, but not necessarily for the reasons you offer. It would not be an effect of light-scatter through Titan’s atmosphere. The phase thickness of Rhea may appear to be truncated in this image because of the way images treat light levels – the placement of Rhea near the diffuse limb of Titan makes it look as IF Rhea has a thinner phase because of the increasing light gradient produced by Titan’s diffuse limb. In other words, we don’t get to see Rhea in a decently soiltary view unbesmirched by the presence of an increasing gradient of brightness supplied by Titan’s fuzzy atmosphere

I could not find the particular image sequence that Phil pulled this image from, but to my mind, there is an incredible diversity of shots that are comparably if not more dramatic, for examples, if one only chooses to look:

http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=238930

http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=236764

http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=232928

http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=238926

…which anyone can easily access and explore here:

http://saturn.jpl.nasa.gov/photos/raw/

EXPLORE!

33. Messier Tidy Upper

Superb work by Cassini yet again – my thanks to them & the BA.

Rhea looks like the mother of all of Titan’s volcanoes there! 😉

Or that it’s just been eaten by Titan which, given the mythology, would be kinda ironic I think! 😉

(Assuming I’m using the word ‘ironic’ right there as apparently it is often misunderstood with many things commonly held as being “ironic” being, well, not-so much.)

34. Messier Tidy Upper

@29. Matt McIrvin :

I’ve sometimes wondered about this, but the discrepancy is really in the opposite direction. Jupiter has a much more dramatic size gap between the four Galileans and its smaller moons: the fifth largest moon is Amalthea, which isn’t even round.

Ken Croswell has an article on the ratio of Jovian versus Saturnian moons here :

http://kencroswell.com/JupiterBigMoons.html

which may be of interest here.

This site :

http://www.spacetoday.org/SolSys/Moons/MoonsSolSys.html

gives the moon and moonlet tally close to this date although it is missing Plutonian moons Nix & Hydra.

http://www.space.com/10468-saturn-rings-remains-ripped-moon.html

via space – dot – com suggests Saturn’s rings may be the result of an earlier Titan-type satellite’s demise.

35. Messier Tidy Upper

@31. justsomeguy31167 :

…who said that, I pointed to another article done using real science shows another pedestal of the “world is ending tomorrow” paranoia is, well, wrong.

See :

No, the evidence compiled by people who have made studying the climate their lives and careers says that the Anthrpogenic Global Warming theory is correct.

Warming is happening, the idea that it is historic and we are going to die is as well founded as the group that called the rapture last month.

Well, of course we’re all going to die eventually. It’s the how and when and why of our deaths that we don’t know.

But see :

http://www.skepticalscience.com/global-warming-positives-negatives-intermediate.htm

for starters on why we have strong reasons to be concerned about the impacts AGW or Human Caused Global OverHeating (HCGOH) as I call it – will have.

Plus see :

http://www.skepticalscience.com/global-warming-1860-1880-and-1910-1940.htm

for how this current Global Warming differs from & is greater than some climate variations of the past.

BTW. The BA has posted several threads on the Global Warming issue before and no doubt will again, I suggest saving this discussion for those more appropriate threads.

36. Messier Tidy Upper

PS. I would also strongly recomend, justsomeguy31167, that you watch this :

Youtube series and also, if you can find a copy, read this book :

http://en.wikipedia.org/wiki/Six_Degrees:_Our_Future_on_a_Hotter_Planet

which I recently finished reading myself which vividly examines the consequences of Global Warming degree by degree based on and citing good scientific studies.

Plus I’d urge you and those of like mind visit the Realclimate website among others. Do some research as I did and you may find you change your mind – as I have done also.

Oh & one last thing :

.. as well founded as the group that called the rapture last month.

Really? *Really?*

Harold Camping was just one old nutter whose erronous (multiple times erroneous actually) predictions were founded on his own idiosyncratic interpretation of the Bible – a collection of sacred writings by a number of sources dating back to the bronze age. The truth or otherwise of Christianity itself is hotly contested and not easily subjected to scientific study – let alone Camping’s tiny sect within it!

The Anthropogenic Global Warming theory OTOH, is supported by well over a century of scientific papers setting forth the evidence (see links in #35 plus more) going back to Svante Arrhenius in 1896 and has hundreds even thousands of scientists and mountains of data in its favour. Ninety-eight percent of climatologists accept the scientific consensus on Global Warming with many noting the IPCC have been conservative and uder-estimated the problem. Contrastingly only Camping and his minuscule group – at least as far as I’m aware – ever accepted his personal version of the “Rapture” idea.

37. Nigel Depledge

Justsomeguy31167 (31) said:

another pedestal of the “world is ending tomorrow” paranoia is, well, wrong. Warming is happening, the idea that it is historic and we are going to die is as well founded as the group that called the rapture last month.

OK, tell me please who exactly is claiming that the world is ending tomorrow, or that we’re all going to die as a result of climate change? With full citations, of course.

38. Anchor

@MTU #33, who says, “Rhea looks like the mother of all of Titan’s volcanoes there!”

[Complete with yet another face…in case nobody understands how sincere he is]

What the FLYING HECK are you talkiing about now?

Oy. How tiresome and downright irksome can a body possibly get?

39. Saturnian? How about just “the Saturn moon Rhea…

Nevertheless a very cool shot.

40. Messier Tidy Upper

@38. Anchor :

@MTU #33, who says, “Rhea looks like the mother of all of Titan’s volcanoes there!”What the FLYING HECK are you talkiing about now?

In case you’re actually curious because it looks like Rhea is a volcanic plume erupting out of the Titanian surface. Think of the early shots of Io’s volcanic eruptions right on that moon’s limb.

I thought that reference was starkly obvious, guess I thought wrong.

[Complete with yet another face…in case nobody understands how sincere he is]

You don’t like emoticons, I do. You say “Tom-ay-to”, I say “To-mart-o”, we have differing styles and views, c’est la vie. 😉

Oy. How tiresome and downright irksome can a body possibly get?

Oy veh!

Dunno, but do you see me being rude about *your* comments here for no apparent reason? Suggestion for ya, Anchor : Get over yourself! 😛

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