comic | episodes & e-books | store | about | forum 




Post new topic Reply to topic  [ 2 posts ] 
Iapetus, third-largest moon of Saturn 
Author Message
User avatar

Joined: Fri Mar 13, 2009 4:18 pm
Posts: 4100
    
Iapetus!

Image
(image from NASA)

Iapetus is the third-largest moon of Saturn, and orbits much further out than the planet's other large moons--and on a much more inclined orbit. It is also called "Saturn VIII" (it was originally "V" but then three more moons were discovered :p; this must have been back when the Roman numerals were supposed to represent orbit order, rather than discovery order).

Iapetus is composed primarily of water ice, which means that its 735 km-radius mass is relatively low density.

Image
(image from NASA)

Iapetus has a two-tone color system, in a pattern sort of resembling a tennis ball! One part is dark, the other light, and the difference is so great that the moon's discoverer, Cassini (and the best images of the moon have been taken by the craft named after him), when he found it in 1671, could only see it when it was on one particular side of Saturn, where it was showing its light side; he was finally able to see it on the other side, with its dark side forward, in 1705, using an improved telescope.

Image
(image from NASA)

Iapetus is a crazy-ass moon! It bulges at the equator, squishes at the poles, and has an ancient equatorial ridge running around the dark part, rising up to 20 km above the rest of the surface; the moon has been described as "walnut-shaped." As is not surprising around a planet with such a huge ring system, Iapetus' surface is very heavily impact-scarred. The large crater with the diagonal rise in the middle that you see in many of the images here is the crater Engelier, 504 km in diameter (the largest found so far is Turgis, 580 km in diameter).

Image
(image from NASA)

The dark stuff is a foot-thick layer of what is thought to be mostly residue (called "lag," apparently) from the evaporation of water ice from the surface. Wikipedia says (bolding mine ;): "It contains organic compounds similar to the substances found in primitive meteorites or on the surfaces of comets..." The current theory is that stuff from nearby and opposite-orbiting moon Phoebe--whose large ring of dust was just announced last month, detected by the Spitzer Space Telescope--hit Iapetus, covering half of the moon's surface--this could have happened because Iapetus rotates very slowly: a day on Iapetus is over 79 Earth days. Dark surfaces absorb more heat than light ones, and the color difference caused ice in the dark areas to evaporate more rapidly than in the uncovered, light areas, and that increased evaporation created more dark residue, perpetuating the dark/light surface area division; the dark areas are 15 degrees (C) warmer than the light areas in daylight (128 K vs 113 K)! The estimate is that in 1 billion years, 20 meters of dark surface evaporates away, vs only 10 cm of light surface.

In the upper middle of this shot of some 10-km-high mountains in the dark-side ridge, you can see a bright spot where an impact knocked off the dark residue, exposing the brightly reflecting ice below:

Image
(image from NASA)

Scientists haven't yet agreed on what caused the ridge, but it may be due to effects of heating and cooling when the planet was forming. Its oblate shape suggest that an Iapetian day may have lasted only 10 hours at the early point where it cooled enough for a thick ice crust to form, freezing its shape.


Wed Nov 04, 2009 4:11 am
Profile
User avatar

Joined: Fri Mar 13, 2009 4:18 pm
Posts: 4100
    
A new space.com article describes a new theory of how the moon-ringing ridge could have formed:

Quote:
Now investigators suggest this ridge could be the remains of a dead moon. Their model proposes that a giant impact blasted chunks of debris off Iapetus at the tail end of the planetary growth period more than 4.5 billion years ago. This rubble could have coalesced around Iapetus, making it a "sub-satellite," a moon of a moon.

Under this scenario, the gravitational pull Iapetus exerted on this sub-satellite eventually tore it back into pieces, forming an orbiting ring of debris around the moon. Matter from this debris ring then rained down, building the ridge Iapetus now sports along its equator fairly quickly, "probably on a scale of centuries," Dombard said.

The researchers suggest that, of all the planets and moons in our solar system, only Iapetus has this kind of ridge because of its unique orbit so far away from Saturn. This made it easier to have a moon of its own — if Iapetus was closer in, Saturn might have tugged Iapetus' moon away, Dombard said.


Tue Apr 10, 2012 5:02 am
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 2 posts ] 


Who is online

Users browsing this forum: No registered users and 3 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to:  
cron
Powered by phpBB © phpBB Group.
Modified from the "Hestia" theme designed by Vjacheslav Trushkin.