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Iapetus: Saturn's Disappeari
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| Title |
Iapetus: Saturn's Disappearing Moon |
| Explanation |
Iapetus [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/iapetus.html ] has an unusual surface, one half of which is very dark, the other half very light. This caused it's discoverer Cassini [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/help.html#cassini ] to remark that Iapetus [ http://www.c3.lanl.gov/~cjhamil/SolarSystem/iapetus.html ] could only be seen when on one side of Saturn but not the other. The reason for the difference between hemispheres is presently unknown. Iapetus is the third largest of Saturn [ http://antwrp.gsfc.nasa.gov/apod/ap950705.html ]'s moon behind Titan [ http://antwrp.gsfc.nasa.gov/apod/ap950923.html ] and Rhea [ http://antwrp.gsfc.nasa.gov/apod/ap951014.html ]. Iapetus is composed predominantly of water ice. |
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Rhea: Saturn's Second Larges
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| Title |
Rhea: Saturn's Second Largest Moon |
| Explanation |
Rhea [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/rhea.html ] is the second largest moon of Saturn [ http://antwrp.gsfc.nasa.gov/apod/ap950705.html ], behind Titan [ http://antwrp.gsfc.nasa.gov/apod/ap950923.html ], and the largest without an atmosphere. It is composed mostly of water ice, but has a small rocky core. Rhea [ http://www.c3.lanl.gov/~cjhamil/SolarSystem/rhea.html ]'s rotation and orbit are locked together (just like Earth's Moon) so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than its trailing surface. The above photograph was taken with the Voyager 1 spacecraft in 1980. |
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Saturn's Moon Rhea from Cass
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Saturn's Moon Rhea from Cassini |
| Explanation |
Each moon of Saturn seems to come with its own mystery [ http://antwrp.gsfc.nasa.gov/apod/ap050201.html ]. Rhea, Saturn's second largest moon behind Titan [ http://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search?titan ], shows unusual wisps, visible above as light colored streaks. Higher resolution images of similar wisps on Dione indicate that they might be made of long braided fractures. Rhea [ http://www.nineplanets.org/rhea.html ] is composed mostly of water ice [ http://www.mtu.edu/carnival/2005/statues.html ], but likely has a small rocky core [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003PEPI..136..201A ]. Rhea's rotation and orbit are locked together, just like Earth's Moon [ http://antwrp.gsfc.nasa.gov/apod/ap040829.html ], so that one side always faces Saturn [ http://en.wikipedia.org/wiki/Saturn_%28planet%29 ]. A consequence of this is that one side always leads the other. Rhea [ http://antwrp.gsfc.nasa.gov/apod/ap030608.html ]'s leading surface is much more heavily cratered [ http://antwrp.gsfc.nasa.gov/apod/ap990711.html ] than the trailing surface, pictured above. The above image [ http://photojournal.jpl.nasa.gov/catalog/PIA06578 ] in natural color was taken last month by the Cassini robot spacecraft [ http://saturn.jpl.nasa.gov/overview/mission.cfm ] in orbit around Saturn. |
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A Great White Spot on Rhea
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A Great White Spot on Rhea |
| Explanation |
What caused this great white spot on the surface of Saturn's moon Rhea? The spot was first noticed last year by the robot Cassini spacecraft [ http://saturn.jpl.nasa.gov/spacecraft/index.cfm ] now orbiting Saturn [ http://www.solarviews.com/eng/saturn.htm ]. Cassini's flyby of Rhea [ http://www.nineplanets.org/rhea.html ] in April imaged [ http://photojournal.jpl.nasa.gov/catalog/PIA06648 ] in the spot in great detail. Astronomers hypothesize [ http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1526 ] that the light-colored spot is the result of a relatively recent impact [ http://www.spacegrant.hawaii.edu/class_acts/CratersTe.html ] on the surface of the icy moon. The impact that likely created the crater also splashed light-colored material from the interior onto the darker surface. Rhea [ http://en.wikipedia.org/wiki/Rhea_%28mythology%29 ] spans 1,500 kilometers across and is the second largest moon of Saturn after Titan [ http://antwrp.gsfc.nasa.gov/apod/ap050518.html ]. Rhea sports several other light colored surface features [ http://antwrp.gsfc.nasa.gov/apod/ap050215.html ] that are, as yet, not well understood. |
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