Cassini/Huygens: Rewriting the Textbook on Saturn
Date: November 12th, 2009
Podcaster: Kevin Grazier
Description: Cassini has given us a whole new view of Saturn, Titan, and other moons of this amazing and beautiful ringed world. Kevin Grazier from Cassini provides an overview of what has been learned so far from the mission and talks with Bob Mitchell, the Cassini Program Manager, and John Smith, the Cassini tour designer.
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.
Today's sponsor: This episode of “365 Days of Astronomy” is sponsored by Michael Schupbach [pronounced shoe-back] in honor of Bryan, Mom, Dad, Lara, Roxy, Heather, Pamela, and Michael. You all have helped me grow in ways I could never have imagined. Here’s to the future. “To the one with the never-ending and invisible scars; look up, look up; The Stars!” “The Stars” by Patrick Wolf
CASSINI/HUYGENS: REWRITING THE TEXTBOOK AT SATURN
One of the brightest lights in our night sky, the planet Saturn is easily visible with the naked eye and has been observed by humanity for thousands of years. Today a huge spacecraft named Cassini orbits Saturn, and is shedding light on many of Saturn’s darkest secrets.
I’m Kevin Grazier, and welcome to the International Year of Astronomy podcast for November 12th, 2009.
In October 1997 a NASA’s Cassini spacecraft launched from Cape Canaveral with ESA’s Huygens probe riding piggyback. The goal of the mission would be to study Saturn, its rings, its magnetic environment, and its natural satellites – in particular the moon Titan. Appropriately, Cassini was launched aboard a Titan IV rocket.
Because the combined mass of Cassini and Huygens was so great, these spacecraft couldn’t make it to the outer Solar System without a little help from their friends – it took flybys of Venus (twice), Earth, and Jupiter – a technique called gravity assist – to propel Cassini/Huygens out to the orbit of Saturn. After over 6 ½ years enroute, Cassini performed its Saturn orbit insertion, or SOI, maneuver on the first of July, 2004.
While the planet Saturn has been nicknamed the “Gem of the Solar System”, Cassini and Huygens have found the Saturn system to be an entire treasure trove of scientific discoveries.
Almost as if the Cassini spacecraft couldn’t wait to start its investigations, the spacecraft loosed several of its twelve science instruments on ring observations during the SOI maneuver.
During Cassini’s tour of the Saturn system ring scientists have seen the thickness of Saturn’s main rings decrease dramatically. The rings themselves haven’t changed, but our understanding of them has. Prior to Cassini, the rings were estimated to be about a kilometer thick. Over the course of Cassini’s nominal mission, which ran until July 1st 2008, the rings were estimated to be 100 meters thick, then 50 to 60 meters thick, then 30 meters, now we believe that, in places, they are a mere 10 meters thick.
Cassini observations have revealed moonlets within Saturn’s rings, including a thin belt of propeller-shaped moonlets within the A ring. While models of Saturn’s main rings indicate that they are either young, on the order of 100 million years old, or old (Solar System age), the topology of Saturn’s skinny F ring changes continuously. Collisions between moonlets in the F ring occur on a daily basis yielding a ring that changes on scales from hours to years.
All four of the jovian, or Jupiter-like, outer planets have rings, but Saturn’s ring system is the most expansive and brightest of all the planets, thus giving the true Lord of the Rings its unique character. While planetary rings may not be uncommon, the collection of moons with rings, at least for now, has but a lone member. Observations from Cassini have detected a very rarefied ring around one of Saturn’s larger moons Rhea.
Rhea’s ring may be the residue of an impact event in the moon’s distant past – an object may have impacted Rhea, and some of the ejected material may have remained in orbit to form Rhea’s ring.
Another ring-forming moon is tiny Enceladus. Only about 300 miles in diameter, Enceladus was one of the first moons of Saturn discovered, by virtue of this moon being one of the brightest objects in the Solar System. Enceladus reflects an astonishing 99% of the sunlight incident upon it. Voyager 1 was the first spacecraft to flyby and image Enceladus on this day in 1980.
Enceladus orbits within the E-ring. Saturn’s diaphanous E ring was discovered in 1967 by ground observations and its existence confirmed by the Pioneer 11 spacecraft in 1979. Numerical models showed, however, that the E ring is unstable over long time periods, and its estimated lifetime less than a million years, which is a blink of an eye in the lifetime of the Solar System. This fact argued that the E ring must be replenished, there must be a source of material.
When Voyager 1 observations revealed that Enceladus orbits within the portion of the E ring where the particle density was highest, Saturn scientists wondered if there was a link.
What a planetary scientist considers a “rock” depends upon where you are in the Solar System. In the outer Solar System, water ice is considered a rock – it is so cold out at Saturn that ice is as hard as granite, and it is a major component of most of the natural satellites. Voyager images of Enceladus showed craters that where smooth instead of jagged, and areas that looked like icy flows of material. All these suggested melting, and melting suggests heat. Could this little moon have volcanism, a form of volcanism called cryo- or cold-volcanism? Internal heating could explain the smoothed appearance of surface features. Recall that magma is melted rock, and rock here is ice so magma would be… water. Since Enceladus is so small, material ejected fromm volcanoes could easily escape this moon to populate the icy E-ring.
Going into the nominal mission, Cassini scientists knew that they might expect to see icy volcanoes on the surface of Enceladus. Despite that pre-conceived expectation, Enceladus gave scientists a huge surprise nevertheless. In July 2005 Cassini passed over a never-before-seen region of the southern hemisphere of Enceladus. Not only was the southern hemisphere far warmer than expected, but images showed nearly parallel fractures, nicknamed “Tiger Stripes.” Geysers of icy particle streamed from the Tiger stripe regions, thus populating Saturn’s E-ring. Nicknamed “Cold Faithful”, Enecladus is only one of three bodies in the outer Solar System — along with Jupiter’s Io and Neptune’s Triton — where active eruptions have been observed. More recent observations of the icy plumes of Enceladus have detected jets of water vapor within.
Recent images of the Tiger Stripe regions of Enceladus have suggested that these structures may undergo tectonic spreading similar to the mid-ocean ridges on Earth where new oceanic crust is formed.
To learn more about Enceladus, the international Year of Astronomy podcast for this past August 28th, featured Cassini Team Members David Seal, Bob Pappalardo, and Amanda Hendrix discussing Saturn’s Moon Enceladus: Past, Present, and Future.
As with Enceladus, Cassini observations of Saturn’s largest moon Titan have provided no shortage of surprises. Titan is the Solar System’s second largest moon and is larger than the planet Mercury. Restated, if Titan was in orbit around the Sun as opposed to Saturn, there would be no “is Titan a planet” debate, we would consider Titan a planet.
Until Cassini’s exploration of the Saturn system, Titan was a great big orange question mark. Why orange? Although Gerard Kuiper discovered that Titan had an atmosphere all the way back in 1944, it wasn’t until the twin Voyager spacecraft imaged Titan in the early 1980’s that scientists realized that this moon is enshrouded in an opaque orange photochemical haze – a haze through which neither Voyager could see. Titan looks like a big space basketball.
On the 24th of December, Cassini launched the European Space Agency’s Huygens probe towards Titan. After a three week journey, Huygens parachuted through the atmosphere of Titan. Huygens confirmed that the atmosphere of Titan is mostly nitrogen, like Earth’s, along with ethane, methane, as well as some surprisingly complex hydrocarbons. Huygens survived landing, and sent back fascinating images of Titan’s surface.
Why do we care so much about the atmosphere of a distant moon? Many scientists believe that the chemical composition of Titan’s atmosphere may resemble that of Earth’s 3.8 billion years ago. That is when life first arose on Earth. At 9 and ½ times as far from the Sun as Earth, the Saturn system gets only about one percent of the amount of sunlight as Earth – so one term often applied to Titan is that it is like the “early Earth in a deep freeze”.
Although we had hints of the moon’s surface features from Earth-based observations, Titan’s thick atmosphere hid the moon’s surface, giving it the largest unmapped solid surface in the Solar System… until Cassini’s arrival, that is.
Two of Cassini’s optical remote sensing, or ORS, instruments were designed to image through the thick atmosphere of Titan: the Visual and Infrared Mapping Spectrometer, or VIMS, instrument and the Imaging Science Subsystem, or ISS (not to be confused with that other ISS). Further, Cassini’s main communications antenna performs double duty as RADAR emitter. Since Titan’s atmosphere is transparent to certain wavelengths of radio waves, Cassini can transmit radio signals through the atmosphere, and then receive the signal reflection from the surface. Using this technique, Cassini can images “swaths” of Titan’s surface during close flybys, a technique similar to that used by the Magellan spacecraft to map over 98% of the surface of Venus – another body with a thick atmosphere.
In addition to hiding the moon’s surface, in this past summer’s movie “Star Trek”, Titan’s atmosphere had the important job of hiding something else: the starship Enterprise and her crew from the evil Romulan Nero.
Theoretical studies prior to the Cassini-Huygens mission suggested that simple hydrocarbons like ethane and methane, natural gas, could exist in liquid form on Titan’s surface. Cassini images have found numerous lakes of hydrocarbons on Titan’s surface, including one lake nearly the size of the Caspian sea. On Titan’s surface we see huge seas of sand dunes, tectonically-formed mountains as high as the Himalayas, and hints of volcanism. There is also evidence to indicate that Titan may have an ocean of liquid water beneath its icy crust, like that believed to exist underneath the crust of Jupiter’s moon Europa.
Titan’s is not the only atmosphere that Cassini has observed. Because it is a gas giant, Saturn itself is a sphere composed mostly of the gases hydrogen and helium. Saturn can almost be considered one big atmosphere. In 1980 the Voyager spacecraft imaged a giant hexagon-shaped storm at Saturn’s north pole. 24 years later, Cassini observed the very same structure. Additionally, Cassini observed another storm, a gargantuan hurricane, at Saturn’s south pole. The south polar storm is more like terrestrial hurricanes than is the north polar hexagon, except that it is far larger and far stronger.
Astounding discoveries, all of them. With me is Cassini Program Manager Robert T. Mitchell. With Cassini literally rewriting the textbook on Saturn, is there any discovery, any observation, to which you can point as the “most significant”. Is that even possible, there are SO many?
Well no it’s probably not possible to give a single answer to that, at least not in the sense of having several different people agree on a single, most significant, result. I expect the top three contenders for the distinction would be Titan, and the question of whether it had oceans and lakes, the Enceladus plumes, and the dynamics of the rings. I think for me, personally, my choice is Enceladus, because it really came as a surprise.
We knew we were going to learn new things about Titan once we were able to see under its hazy atmosphere and figure out what was down there – particularly on the matter of whether or not there were lakes and oceans of liquid methane.
And we expected to learn more about the ring dynamics, probably not to the extent that we did, but the thing about Enceladus is that it really came as a complete surprise. So for me, personally, that’s my pick for the most exciting, the most significant, discovery we’ve made.
Thank you, Bob.
Cassini’s extended, or Equinox, mission ends on the 1st of July, 2010. With the Cassini spacecraft currently in an excellent state of health, however, mission and science planners are now working hard on an extended extended mission. The Cassini Solstice Mission would run all the way out to 2017.
John Smith is a Cassini Mission planner and trajectory designer. John, what exciting events do we anticipate in the upcoming Cassini Solstice Mission?
The project recently finished a proposal to NASA for a plan governing the rest of the mission. So what do we hope to learn? Saturn, and it’s planet-like moon Titan, have seasons just like the Earth, but each season is more than sever years long. From Saturn’s perspective, we arrived in late Winter, just started Spring, and the goal is to make it to the first day of Saturn Summer in 2017. After all, if you just studied Earth for one season, would you really understand our planet?
So, if all is approved, what do we have to look forward to? 174 orbits about Saturn, 64 Titan and 15 Enceladus close flybys — about half of those will be through the water vapor plume — and many encounters with Saturn’s 59 other moons.
But the coolest part is that we’ll end the mission with a series of orbits threading the gap between the cloud tops of Saturn and its innermost ring, culminating with a suicide plunge into Saturn itself.
Awesome, John, thank you. Since the Solstice Mission nearly doubles the duration that Cassini will be in Saturn orbit, there are certainly more fascinating discoveries in the mission’s future.
You can learn more about Saturn, the Cassini and Huygens spacecraft, or follow the mission’s progress at saturn.jpl.nasa.gov. Thank you for listening
End of podcast:
365 Days of Astronomy
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