Strange Moons: Iapetus, A Walnut In Space
Diameter: 1,468 km
Saturn's 3rd largest moon.
Surface temp: Varies from 90 - 130K ( -183 to -143 C )
Iapetus (sometimes referred to as Japetus), was discovered in 1671, by Giovanni Cassini. His discovery of this moon was very interesting because he
could only observe it on the western side of Saturn, but never the eastern side of Saturn.
Turns out the reason was simple: Giovanni Cassini's telescope at that time was not good enough to see it on the eastern side of Saturn, because the
moon's brightness dimmed by an order of 2 magnitudes compared to when it's on the western side of Saturn.
Cassini discovered this when he got a new and improved telescope in 1705. He surmised that Iapetus must have a very bright hemisphere on one side, and
a very dark one on the other side.
This striking contrast stuck out to astronomers over time. Even science fiction writer Arthur C. Clarke mentions it in his book 2001: A Space
, however in his book he used the name Japetus (the book had the space ship Discovery go to Saturn instead of Jupiter as depicted in the
Details of the moon could not be resolved by the most powerful telescopes here on Earth. However in August, 1981, Voyager 2 was able to fly by Iapetus
at 600,000 miles, and gave us this image of it:
Voyager 2's image confirmed that the moon did indeed appear to have two highly contrasted hemispheres.
Iapetus is smaller than our moon, having a radius of only 42% of our moon:
When the space probe Cassini arrived at Saturn, it was able to start taking much higher resolution images of Iapetus. The contrast between the
hemispheres is striking:
Close up image remind me of valleys with snow capped mountains:
So what is it that we are looking at here? Is it ice? If it is ice, why do we have two hemispheres with one being very much covered by that ice, while
the other side is not?
Turns out with the help of Cassini's radar that the dark areas is actually pretty thin and believed to be "lag" (aka residue) from the sublimation of
the water ice on the surface of Iapetus, that becomes darkened even further from exposure to sun light.
Iapetus has a slow rotation of 79 days. It has the warmest day time surface temps and coldest night time temps in the Saturn system (hence the dynamic
surface temperature change from 90 Kelvin to 130 Kelvin).
It means that ice sublimates from the darker area and deposits to the lighter areas of the surface, especially over the poles of the moon.
As interesting as this contrast is, Cassini discovered another physical feature of Iapetus that gave it the nick name of Walnut Moon.
Cassini discovered a equatorial ridge that runs through the middle of the darker region of the moon in December, 2004:
The ridge is 1,300 km long, 20 km wide, and a average height of 13 km.
Some of the peaks reach as high as 20 km, making them some of the highest mountains in the solar system.
When looked at the right angle, the ridge helps give Iapetus a shape like a walnut:
Now for the strangeness of it: Scientist are not sure how it formed.
One problem that is difficult to explain is how the ridge follows the equator almost perfectly. There are about 4 hypothesis that are current, but
none of them explains why the ridge is confined to the dark hemisphere either.
What ever the reason it is there, is sure is cool to look at:
Unfortunately, it is hard for Cassini to explore Iapetus extensively. Iapetus orbits Saturn from far out:
And it has a very inclined orbit around Saturn:
The cause of it's orbit being like this is not known. But it does make it difficult to get the Cassini probe back and forth from it. The last time
Cassini visited Iapetus was in 2007 and there are no more planned visits. However, there might be a opportunity in 2015.
edit on 7-9-2013 by
eriktheawful because: (no reason given)