A walnut shaped moon circling Saturn?

--------- #1359 - Iapetus, Saturn’s 3^rd Largest Moon

- Attachment: Iapetus

- Iapetus is Saturn’s 3^rd largest moon, behind Titan and Rhea. Saturn has 50 moons, but, this one is the most unusual. Iapetus is shaped like a walnut with a large ridge around 1/4^th of its equator. The ridge is 12 miles high. If it were here you would need a rocket to fly over it. Then ½ of the moon is dark as tar and the other half of the moon is white as snow. What goes on here?

- Iapetus was first discovered as a Saturn moon in 1671. It took another 34 years for astronomers to figure out its orbit because they could only see it when the white side was facing us. When the white side was showing Iapetus is 5 times brighter on the west side of its orbit. Once astronomers figured this out the orbit was calculated at 79.3 days. The rotation was the same, 79.3 days, so the orbit was synchronous always with the same side facing Saturn. In brightness Iapetus went form 12 Magnitude to 10 Magnitude depending on which side we were looking at.

- Voyager spacecraft visited Iapetus in 1980. Cassini spacecraft visited in 2000. What they discovered was that one half of Iapetus was covered in water ice. The other half had a veneer of dark material like the asphalt on a parking lot.

- Astronomer’s think the dark half is carbon-rich dust thrown off by Saturn’s outer moon, Phoebe. Meteors pummeled the moons and the dust from Phoebe was pulled inward to Iapetus’ orbit and coated it 1 foot deep on the forward side of its orbit. The depth is known because meteor impacts on Iapetus expose the ice one foot below the surface.

- The white side is caused by 40 days of sunlight causing ice to sublimate, vaporize, and then getting re-deposited on the cold areas. The temperature only varies from - 229 F to -256 F.

- The raised ridge around Iapetus’ equator rises 8 miles to 12 miles above the surface. The meteor impacts on the ridge indicate an ancient tectonic uplift raised these mountains.

- Iapetus’ orbit is highly inclined to the orbiting plane of Saturn’s rings and other moons.

- Cassini’s last flyby was September 1, 2007 within 1,022 miles of the surface. That is abuot as much as we know about the “walnut” moon until we can get some more explorations. Here is a list of all of Saturn’s moons. Iapetus is 2,210,000 miles away from Saturn towards the outer edge of the group.

-------------------- 11 small inner moons near Saturn’s ring system
------------------- Mimas
------------------- Enceladus - ice balls.
------------------- Tethys
-------------------- Calypso & Telesto - share the orbit with Tethys
-------------------- Dione - a medium sized ice ball
-------------------- Helene & Polydeuces - shares the orbit with Dione.
------------------- Rhea - medium sized ice ball
------------------ Titan - dense atmosphere of methane shrouds the surface ( See review # 1144 to learn about Titan)
----------------- Iherion - The one moon known that does not rotate in a synchronous orbit
---------------- Iapetus - Review # 1359
----------------- Phoebe - very dark, impacts from it may have covered Iapetus.
----------------- 25 irregular moons - captured and highly inclined orbits, some even in backwards orbits.

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707-536-3272, Thursday, December 22, 2011