Date: December 29, 2009
Title: Decoding Iapetus: An Exercise in “Sybil” Engineering
Podcaster: Kevin Grazier
Links: Cassini mission, CICLOPS Cassini image site.
Description: If planetary scientists could take the moons of Saturn and, knowing what we know, re-assign the names already given them, the moon Iapetus would certainly become Janus. Janus was a two-faced god from Roman mythology, and Iapetus is the most two-faced object in the Solar System.
Bio: Dr. Kevin Grazier is the Investigation Scientist and Science Planning Engineer for the Cassini/Huygens Mission to Saturn and Titan. He also performs large-scale computer simulations of Solar System dynamics, evolution, and chaos, and teaches classes in basic astronomy, planetology, cosmology, the search for extraterrestrial life, and the science of science fiction at UCLA and Santa Monica College.
Dr. Grazier also served as the science advisor for the Peabody-award-winning SyFy Channel series Battlestar Galactica, and currently serves that role on Eureka and the NBC animated series The Zula Patrol. He is also co-author of the upcoming book The Science of Battlestar Galactica.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Joseph Brimacombe, a mega-enthusiastic amateur astronomer based at the Coral Towers and Macedon Ranges Observatories in Australia, and the New Mexico Skies observatory in the United States and is dedicated to: James Cameron, whose movie “Avatar” took me to a world I’d imagined as a 10 year old boy peering through a 2 inch refractor; and as a 50 year old man peering through a 20 inch reflector. James, you are a global “life enhancer”. Don’t ever stop. We need you. Please google “Joseph Brimacombe, photostream” for further information.
Transcript:
This time of year, as the current NFL regular season winds down, a common feature of sports magazines and web sites is a hypothetical re-draft. In other words, if NFL teams could have known in advance how rookie players would perform in their first season last April before the draft, how differently might that draft have played out?
In a similar vein, if planetary scientists could take the moons of Saturn and, knowing what we know, re-assign the names already given them, the moon Iapetus would certainly become Janus. Janus was a two-faced god from Roman mythology, and Iapetus is the most two-faced object in the Solar System.
I’m Kevin Grazier, and welcome to the International Year of Astronomy podcast for December 29th, 2009.
Iapetus is the third largest moon of the Saturn system, and the 11th largest in the Solar System. It is one of the more distant moons of Saturn, orbiting at a distance 3,561,300 kilometers – that’s 2,213,000 miles or about 60 Saturn. Seen from Iapetus, gigantic Saturn appears about 4 times larger than the full moon seen from Earth. The density of Iapetus, of just under 1.1 g/cm3 implies it’s composed largely of water ice.
Iapetus was discovered in October 1671 by Giovanni Domenico Cassini, the astronomer for whom the current Cassini mission at Saturn is named. Cassini noted that he could observe Iapetus while it was on one side of Saturn, yet not the other. He concluded, correctly, that this moon must be tidally locked in synchronous rotation with Saturn. That means that, like our moon, the same face of Iapetus permanently faces Saturn. That also means that, as it orbits Saturn, Iapetus has a permanent leading, and permanent trailing, side.
Cassini further concluded that one side of Iapetus is much darker than the other, which was confirmed by subsequent observations using higher powered telescopes. Until recently, the explanation of the two-faced nature of Iapetus was the oldest unsolved mystery in planetary science.
2001 Mission
Science fiction author Arthur C. Clarke used this dichotomy in “2001: A Space Odyssey. In the movie version, a large black monolith of extraterrestrial origin is discovered on the moon. The monolith sends a powerful transmission towards Jupiter, and the spacecraft Discovery and her crew are dispatched to investigate, and they find an enormous monolith, sharing an orbit with the jovian moon Io. In the novelization, however, the monolith was situation on Saturn’s moon Iapetus. This explained Cassini’s observation back in the 17th century: as Iapetus circled Saturn in its 80 orbit, periodically the black monolith came into view.
Voyagers in 1981
As with Clarke’s spacecraft Discovery, NASA spacecraft have investigated this unusual moon as well. In 1981 the twin Voyager spacecraft sailed through the Saturn system. While Voyager 1 was able to view Iapetus only from a very great distance, on August 22 of that year Voyager 2 was able to image Iapetus from a distance of 909,000 km.
What Voyager 2 saw literally looked like a Yin/Yang. One side of Iapetus was nearly black, the other quite bright. We learned that Iapetus is the most “contrasty” body in the Solar System, yet we were not much nearer to answering “why”?
With a density that suggests Iapetus is composed largely of water ice, the nature of the light areas of this moon was no mystery. From where did the dark material come, however? Two hypotheses were suggested as the explanation, the endogenic model vs. the exogenic model. The endogenic model held that some process was forcing dark material from the interior of Iapetus to its surface. The exogenic model, on the other hand, suggested that there may be dark material in the orbit Iapetus that is being swept up by its permanent leading edge.
The hope was that NASA’s Cassini orbiter would shed some light on this long unsolved mystery after its arrival in July 2004.
Cassini Flybys
Iapetus is a distant moon of Saturn, and its orbit is inclined 15.5 degrees to Saturn’s ring plane. This explains why, during its four year nominal mission, Cassini trajectory took it near to Iapetus only twice.
On December 24th 2004 (Pacific Time), the Cassini spacecraft released the European Space Agency’s Huygens probe that had been riding piggyback since launch. It would be another 2 ½ weeks, until January 14 2005, before Huygens would plunge into the atmosphere of, and subsequently land upon, Saturn’s largest moon Titan.
Cassini actually lanched Huygens when both spacecraft were outbound from Saturn. Both spacecraft reached the most distant part of their orbit from Saturn, called perikron, before they plunged back towards Saturn and Titan.
As Cassini neared perikron, it took a brief break from the Huygens mission to observe Iapetus – it passed within 123,400 km of Iapetus, less than 1/7th the distance of Voyager 2. Initially the flyby distance was scheduled to be in the range of 65,000 km, but mission designers, unsure of the mass of Iapetus and its potential gravitational influence on the now-separate Cassini and Huygens probes, chose to give this moon a wider berth. The spacecraft, nevertheless, send back breathtaking images.
After the Voyager mission, scientists described the dark side of Iapetus as being “as dark as freshly laid asphalt”, and the light side “as bright as freshly-fallen snow.” The images sent back by the Cassini spacecraft have confirmed that the dark side is, indeed, very dark… but the light side is more like “Snow… in Detroit… in March.”
Apparent also at the light/dark boundary were streaks – where dark material looked almost windblown across the surface of Iapetus. Only Iapetus has no atmosphere. This seemed to argue in favor of the exogenic model for the source of dark material.
On 10 September 2007 Cassini flew even closer to Iapetus, and came within 1700 km. Careful analysis of the images from this flyby have revealed a third model for the source of Cassini’s dark material, and those results published in a recent issue of the journal Science.
Imagery shows that Iapetus is, indeed sweeping up reddish dust in its orbit around Saturn. Another process though, called thermal segregation, makes the bright areas brighter and the dark areas darker.
Since Iapetus orbits slowly around Saturn, its dark regions are exposed to the sun for long periods of time. The light areas are as well but the light areas, being light, reflect sunlight. The dark material absorbs sunlight and warms. This causes the nearby and underlying ice to evaporate, and it is eventually redeposited on the bright side.
This mystery, nearly 340 years old, seems to be on its way to being solved.
Walnut shape and mountain Ranges
Cassini imagery also revealed that the shape of Iapetus is similar to that of a walnut. The ratio of its polar diameter to its equatorial diameter suggest a body that was spinning very fast, a 17 hour rotation rate, when it formed – not spinning once every 80 days as it is now.
Cassini has also observed a HUGE range of 20 km high mountains that run along the equator of Iapetus. Some scientists have suggested that this is merely a result from when Iapetus spun much more rapidly, while others have suggested that Iapetus may once have had its own ring, that subsequently collapsed to its surface at the equator.
The light/dark nature of Iapetus may be well on its way to being solved, but there are new mysteries arising from this mysterious moon.
Upcoming flybys in XM and XXM.
There were no flybys of Iapetus, even distant ones, scheduled in Cassini’s Extended, or Equinox Mission. There are, however, two distant flybys slated in the trajectory currently proposed for Cassini’s Extended Extended, or Solstice, Mission. On Jun 7 2011 Cassini will pass within
863,000 km of Iapetus. It will pass within 979,000 km on Mar 30 2015. Although these are far more distant than the two flybys during Cassini’s nominal mission, given the spacecraft’s more advanced imaging systems, these should still provide imagery superior to that of Voyager 2. So, we may still learn something new from these encounters and, in the near-term, the book still isn’t closed on Iapetus.
You can learn more about Iapetus, Saturn, the Cassini and Huygens spacecraft, or follow the Cassini mission’s progress at saturn.jpl.nasa.gov. The Cassini Imaging Team site is at www.ciclops.org. Thank you for listening
End of podcast:
365 Days of Astronomy
=====================
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.