Thoughts on Iapetus, Part II
My last post about Iapetus was really just a teaser. I wanted to introduce the topic, but didn't feel the motivation to go into an in depth analysis.
This moon really is quite extraordinary. It was discovered by Giovanni Cassini in 1671. It is the second largest moon of Saturn (after Titan) and has extremely distant and inclined (almost 15 degrees) orbit. It is also the outermost major moon. The thing that made this Saturnian moon stand out to Cassini though, was that it appeared extremely bright at one end of its orbital path, and so dark at the other end that it dissapeared entirely.
Cassini concluded that Iapetus must have 1. a very light reflective half and a dark non-reflective half, and 2. that Iapetus must be tidally locked with Saturn (so as to always show the same face to Saturn). Both of these conclusions were confirmed by the 1977 Voyager mission and the current Cassini-Huygens mission.
That was just the beginning of discovery...
The current Cassini mission has captured some very detailed imagery of Iapetus on its first (and to this date only) flyby. Cassini revealed the stark contrast between light and dark sides, as well as another totally stunning feature: the "wall".
This wall traverses the dark side of the moon exactly in line with the equator. It is about 20km wide and about 13km high, and extends 1300km across Cassini Regio. This feature appears to be unexplainable through terrestrial geology.
Another interesting find was the non-spherical shape of Iapetus. Cassini was able to take long exposure images of the night side by Saturn-light. This created an overexposed light side, but allowed Cassini Regio to be imaged with greater contrast, and therefore greater detail. This image is interesting because the overexposed light side limb is shown to be almost polygonal, and definitely not spherical. Some have said this is just image blur due to the spacecraft motion. This is not the case. If you read about how the image was aquired, it becomes clear that this is real.
The composition of Cassini Regio has been a scientific curiosity for as long as we've known about Iapetus. Cassini-Huygens used its Visual and Infrared Mapping Spectrometer (VIMS) to study the composition of the surface. It discovered that the light side was rich in water ice, while the dark side was mainly a substance rich in organic material. This supported the conclusions of an earlier (2001), pre-Cassini, paper that studied Iapetus with Earth-based spectra. This paper also stated that "...observations in this spectral region have not revealed this mix of material on any other object observed thus far...".
A 2004 paper showed that Iapetus also has a much lower radar signature than what would be expected from an icy moon. There is also no significant difference between the radar echo returned from the dark side and that returned by the light side. This effect can possibly be traced to the composition of Cassini Regio. If this were the case, it would mean that the bright half of Iapetus would have to be the same dark material as Cassini Regio with clean ice deposited on top of it.
This brings up an interesting question: is the dark organic material a deposite on top of the icy moon, or is the ice a deposite on top of a rather more mysterious moon?
Hopefully we get at least two more flyby's out of Cassini-Huygens, but I wouldn't count on it. Iapetus is rather hard to get to, and there are a lot of interesting things to see in Saturn's backyard...
This moon really is quite extraordinary. It was discovered by Giovanni Cassini in 1671. It is the second largest moon of Saturn (after Titan) and has extremely distant and inclined (almost 15 degrees) orbit. It is also the outermost major moon. The thing that made this Saturnian moon stand out to Cassini though, was that it appeared extremely bright at one end of its orbital path, and so dark at the other end that it dissapeared entirely.
Cassini concluded that Iapetus must have 1. a very light reflective half and a dark non-reflective half, and 2. that Iapetus must be tidally locked with Saturn (so as to always show the same face to Saturn). Both of these conclusions were confirmed by the 1977 Voyager mission and the current Cassini-Huygens mission.
That was just the beginning of discovery...
The current Cassini mission has captured some very detailed imagery of Iapetus on its first (and to this date only) flyby. Cassini revealed the stark contrast between light and dark sides, as well as another totally stunning feature: the "wall".
This wall traverses the dark side of the moon exactly in line with the equator. It is about 20km wide and about 13km high, and extends 1300km across Cassini Regio. This feature appears to be unexplainable through terrestrial geology.
Another interesting find was the non-spherical shape of Iapetus. Cassini was able to take long exposure images of the night side by Saturn-light. This created an overexposed light side, but allowed Cassini Regio to be imaged with greater contrast, and therefore greater detail. This image is interesting because the overexposed light side limb is shown to be almost polygonal, and definitely not spherical. Some have said this is just image blur due to the spacecraft motion. This is not the case. If you read about how the image was aquired, it becomes clear that this is real.
The composition of Cassini Regio has been a scientific curiosity for as long as we've known about Iapetus. Cassini-Huygens used its Visual and Infrared Mapping Spectrometer (VIMS) to study the composition of the surface. It discovered that the light side was rich in water ice, while the dark side was mainly a substance rich in organic material. This supported the conclusions of an earlier (2001), pre-Cassini, paper that studied Iapetus with Earth-based spectra. This paper also stated that "...observations in this spectral region have not revealed this mix of material on any other object observed thus far...".
A 2004 paper showed that Iapetus also has a much lower radar signature than what would be expected from an icy moon. There is also no significant difference between the radar echo returned from the dark side and that returned by the light side. This effect can possibly be traced to the composition of Cassini Regio. If this were the case, it would mean that the bright half of Iapetus would have to be the same dark material as Cassini Regio with clean ice deposited on top of it.
This brings up an interesting question: is the dark organic material a deposite on top of the icy moon, or is the ice a deposite on top of a rather more mysterious moon?
Hopefully we get at least two more flyby's out of Cassini-Huygens, but I wouldn't count on it. Iapetus is rather hard to get to, and there are a lot of interesting things to see in Saturn's backyard...
posted by Leif at 6:07 AM
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