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Imaging Diary - Cassini
Imaging Diary - Voyager
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| Craters and Chasms on Tethys - Sep 29, 2005 | | | A recent tweak to Cassini's trajectory allowed an even closer approach of 1,500 kilometers (930 miles) to the Saturnian moon, Tethys, than originally planned, and yielded unrivaled views of the icy moon’s cratered landscape and a look into the gargantuan canyon system called Ithaca Chasma. |
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Alliance Member Comments
Against a Hail of Stars |
Mercury_3488 2008-08-10 07:53:17 | View all member's comments | I agree completely, bruno.thiery,
I've researched what the background sky was, forming a backdrop for Tethys.
This image was obtained back in June on the final day of the Primary Mission of the Cassini Spacecraft, which has now been extended to Wednesday 30th June 2010.
Tethys is seen here, in the shadow of Saturn, but dimly lit by reflected sunlight from Saturn's rings & other moon. Dione is off the frame to the bottom left & I wonder if Dione is responsible for most of the weakillumination of Tethys?
The stars in the background ar within the far north of the constellation of Bootes close to the stars Merez / Rho Bootis & Upsilon Bootis. The red giant star Arcturus / Alpha Bootis is way out of the frame to the lower right.
I really hope we get to see a lot more of these sort of obervations.
Andrew Brown. | |
Enceladus '61EN' Flyby Raw Preview #2 |
Mercury_3488 2008-03-14 06:53:13 | View all member's comments | Cheers anne v.
Yes the left hand 'crescent' lit by Rhea & the right hand 'crescent' lit by Tethys & Dione are quite easy to see, despite the bright 'gibbous' from sunlight reflected of Saturn's rings. So clearly Enceladus is still being lit by reflected sunlight from several objects within the Saturn system & the double northern horns of the opposing crescents is interesting. Really quite eerie & alien.
On a few of the raw images are showing the eclipsed Enceladus in front of the trailed stars of the Camelopardalis / Ursa Minor border, whilst Cassini tracked the eclipsed ice covered moon.
Absolutely fantastic stuff.
Andrew Brown. | |
Enceladus '61EN' Flyby Raw Preview #2 |
anne v 2008-03-13 21:56:16 | View all member's comments | Yes, this image was obtained while Enceladus was indeed in eclipse, illuminated by sunlight reflected first from Saturn's rings then onto the planet and finally onto the moon. Portions of Enceladus are also lit by scattered light from nearby Tethys, Dione, and Rhea, although not as brightly as by Saturn's reflected ringshine. | |
The Rays of Rhea |
Mercury_3488 2008-03-01 20:37:22 | View all member's comments | Hi Red_dragon,
It is so interesting to see how different the individual moons are within a particular system.
With Saturn's system, the difference between Titan & Mimas is most striking. Phoebe almost certainly is an interloper from the KBO, Hyperion is just odd, Enceladus, highly active, Dione also probably moderately active, Tethys, Rhea, & Mimas as dead as door nails.
Iapetus is extremely fascinating for other reasons.
I suspect the Uranian moons had more radiactive elements & it is interesting to see, how much denser Titania & Oberon are to Saturn's similarly sized moons Rhea & Iapetus.
Clearly the Saturn & Uranus systems have evolved very differently.
I like your comparison of the differences between Io & Callisto in the Jovian system. True, Jupiter's immensely gravity & tidal influences have much to do with this.
Even so, Io is poor in volatiles, but heavily differentiated & contains a large amout of heavy materials (Io has the highest density & surface gravity of the solar system's moons). Callisto appears to be a mixture of rock & ice, with a slight concentration of rock towards the centre, but not enough to form a core.
Probably much like Rhea, which appears to be much like a smaller Callisto.
Andrew Brown. | |
The Rays of Rhea |
Mercury_3488 2008-02-24 15:31:10 | View all member's comments | Hi Red_dragon.
I suspect in the case of Rhea, it is lighter less space weathered ice being deposited over older more space weathered ice.
Rhea appears to have had no internal geological activity, the entire surface appears to have been effected by impact events.
I use the word appears, as in the images I have & have seen, can see no evidence for anything else other than impacts.
Many of Saturn's moons appear to have had varying degrees of geological activity to varying degrees.
Enceladus & Titan: extensive.
Dione: moderate overall, extensive in a few locations.
Iapetus: Some, the giant equatorial ridge of the Voyager Mountains.
Tethys: Little, smoother equatorial regions, Ithaca Chasma, little else.
Rhea & Mimas: None as far as I can tell, only impact cratering.
Rhea appears to be a bit of an odd ball in this respect. Rhea is a large object that appears to have had little other than impacts happen to it.
Rhea is similar in size to the Uranus moons Titania & Oberon, as well as Iapetus.
In descending diameter size order below:
Titania: 1,578 KM. Largest moon of Uranus. Density: 1.70 Grammes cm/3
Comments: Giant canyons, smoother subdued craters in places, possibly previous cryovolcanism, possibly very tenuous atmosphere.
Rhea: 1,528 KM. Second largest moon of Saturn. Density: 1.33 Grammes cm/3.
Comments: Little has happened apart from impact cratering.
Oberon: 1,522 KM. Second largest moon of Uranus. Density: 1.64 Grammes cm/3.
Comments. Like Rhea has experienced extensive cratering. Unlike Rhea, has tall mountains & has had experienced cryovolcanism & like Titania possible limited tectonic activity. Possibly a very tenuous atmosphere. Note Oberon's average diameter is only 6 KM less than Rhea's, but is somewhat more massive due to higher density.
Iapetus: 1,436 KM. Third largest moon of Saturn. Density: 1.21 Grammes cm/3.
Comments. Odd ball moon. Dark leading, bright trailling hemispheres. Huge equatorial mountain range, some peaks almost as tall as those on Mars & Io.
I suspect Rhea's lesser density (greater ice to rock ratio) may explain lack of geological activity.
Andrew Brown. | |
Coming to Light |
Mercury_3488 2008-02-17 10:20:16 | View all member's comments | Once again, thank you very much Carolyn.
3488 is strange, I agree completely 100% with you. No secret I'm happy to explain. Due to my troubled youth with my condition (High Functioning Autistic), it was thought that I could never live outside of sheltered housing.
Well I proved them all wrong. Got my own flat & 3488 was my first ever PIN for my bank account, so that number has real significance.
Not only that it is easy to remember & as there is no other 3488 on any message board on the web, I am easy to find.
Back to Sector 6.
I like the idea of placing Cassini inside of the D Ring. A variant if possible, could Cassini be placed into a very close polar orbit around Saturn?
Twice each orbit the rings would be approached very closely, enabling high resolution imaging, both back & front, whilst global weather monitoring would be doable.
A bit like the JUNO mission to Jupiter & would be able to compare JUNO results directly with Saturn.
I still aim for the Northern Summer Solstice though with the mission carrying on as is. Hopefully we will still get many more Titan, Enceladus & Dione encounters in. Shame we cannot revisit Iapetus & Phoebe due to DeltaV issues (it would be scientifically very valuable to encounter ex KBO Phoebe again, but with the opposite side facing Cassini at closest approach).
Are we likely to get a very close encounter with Mimas & / or Janus, Prometheus, the Dione & Tethys co-orbitals?
It is great talking with you.
Andrew Brown. | |
Cosmic Blasting Zone |
Mercury_3488 2008-02-16 07:57:58 | View all member's comments | I've sometimes wondered about this also.
Hyperion is the least dense of the known solid bodies in the entire solar system, only 0.55g cm3. This suggests a rubble pile held together by gravity & / or a more coherent body with huge internal voids.
My bet is the rubble pile. Like polystyrene, it is possible to punch deep but narrow holes in it. We see similar here on Hyperion.
I had seen one suggestion bantered about elsewhere (not my idea), that Titan formed in a heliocentric orbit at one of Saturn's lagrange points & over time, Titan was drawn towards Saturn. Hyperion was suggested to be Titan's former moon.
The mass ratio between Hyperion & Titan is identical to that of Dione with Polydeuces & Helene as well as Tethys with Telesto & Calypso. These facts were part of a thought exercise elsewhere.
Hyperion being a former comet is an interesting idea. Remember that Phoebe is almost certainly a captured comet / KBO.
Also remember the floors of these deep craters are dark, so they absorb the very little solar energy there is & can burn through the ice.
I am sure there are a whole multitude of reasons for Hyperion's weird situation. A most fascinating object. | |
Rebounded Craters |
Red_dragon 2008-01-25 15:52:13 | View all member's comments | Fantastic image. That huge southern basin is interesting; it seems to have a size similar to Odyseeus in Tethys, yet is not "sank" on the moon as that and it SEEMS to have an inner ring of mountains. | |
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