QMU – 2012

The first lecture of day was given by Professor Carl Murray. He discussed the Cassini mission to Saturn. He is the only UK member of the imaging team and he described recent results which could impact on our understanding of planetary formation.


Cassini–Huygens is a flagship-class NASAESAASIspacecraft sent to the Saturn system. It has studied the planet and its many natural satellites since arriving there in 2004. The aim of the mission was to put a craft in orbit round Saturn. It was funded by SERP/PParc/STFC/ESA and was a joint mission for NASA/ESA/AST.

The Jupiter flyby took place in December 2000 and Cassini-Huygens arrived at Saturn in July 2004 for a four year tour of the Saturn system. This mission is, in fact, still (2012) ongoing.


This is an artist’s concept of Cassini during the Saturn Orbit Insertion (SOI) manoeuvre (1st July 2004), just after the main engine has begun firing. The spacecraft is moving out of the plane of the page and to the right (firing to reduce its spacecraft velocity with respect to Saturn) and has just crossed the ring plane. The SOI manoeuvre, which was approximately 90 minutes long, allowed Cassini to be captured by Saturn’s gravity into a five-month orbit. Cassini’s close proximity to the planet after the manoeuvre offered a unique opportunity to observe Saturn and its rings at extremely high resolution.

The UK is involved in six out of 12 Cassini instruments and 2 out of 6 Huygen instruments.

The mission is named after Cassini because Cassini was the first astronomer to observe four of Saturn’s moons and a gap between the rings now called the Cassini Division (1675). The probe was named after Huygens because he discovered Titan.

Professor Murray began his lecture by showing some of the images taken during Voyager missions.


Saturn with three moons, Tethys, Dion and Rhea.


Saturn rings with “spoke” features in the B ring.


The full set of rings, imaged as Saturn eclipsed the Sun from the vantage of the Cassini spacecraft on 15 September 2006 (brightness is exaggerated). The “pale blue dot” at the 10 o’clock position, outside the main rings and just inside the G Ring, is Earth.

Saturn has at least 62 moons and Titan is considered to be its biggest.


The Huygens probe was dropped onto Titan’s surface on January 14th 2004. It discovered that the atmosphere was made up of Nitrogen and methane. The atmosphere can be likened to a photochemical smog. Near Infra-red shows variations. Radar images show smooth regions which could be lakes. There are seasonal changes. Glint of sunlight indicates there are liquids.


Enceladus is one of the smallest moons that are spherical in shape and it is believed to be the smallest body in the Solar System that is geologically active. Both Titan and Enceladus are believed to have a methanological cycle (like the water cycle on Earth). Enceladus has smooth regions at the South Pole implying there are magnetic field lines changing it. Ice plumes are seen. This could be a source of liquid water beneath the South Polar Region. This is odd considering how little sunlight reaches it. How is there sufficient heat? Could it be tidal heating? In 2005, the source of the E Ring’s material was determined to be cryovolcanic plumes emanating from the “tiger stripes” of the south polar region of this moon.


Mimas is the smallest and least massive of the inner round moons.


Dione is the second largest inner moon of Saturn. It is 1123km in diameter and nearly spherical.


Iapetus is 1471km in diameter and the third largest of Saturn’s moons. Cassini was able to obtain a lot more information about it. The moon is “locked in” with a leading and trailing hemisphere.


Hyperion has a diameter of 270km. It has a sponge like appearance with a low density. It may be porous. It is locked in with Titan.


Helene is 30km in diameter and it is a Trojan moon. A Trojan body orbits at either the leading L4 or trailing L5 Lagrange point of a much larger object, such as a large moon or planet. Helene is the largest Trojan moon and leads whilst Polydeuces trails.


Methone is 3km in diameter and is very smooth. It is believed to be young because of this.

Saturn has the largest planetary ring system composed of water ice with some impurities (as shown up by spectral analysis). Typical particle size is 1cm. The origin of the rings could be due to the breakup of a giant comet or satellite. The age is about 200 million years. It is one of the wonders of the Solar System, order out of chaos. The rings are extremely thin (10m thick). Each ring is separated by gaps. The Encke gap is a 325km wide gap within the A ring. It is caused by the presence of the small moon Pan (gravity effect). The F ring is the outermost discrete ring of Saturn. Cassini has shown it to consist of one core ring and a spiral strand around it. It is affected by the gravitational pull of Prometheus and Pandora (classed as a shepherd moons. Shepherd moons have the affect of sculpting the rings).

http://www.youtube.com/watch?v=fdUlpeUFfxI&feature=fvwrel Shows Prometheus and Pandora shaping the F ring.

The G ring is very diffuse. It is a very thin, faint ring half way between the F ring and the beginning of the E ring.


Specially designed Cassini orbits place Earth and Cassini on opposite sides of Saturn’s rings, a geometry known as occultation. Cassini conducted the first radio occultation observation of Saturn’s rings on May 3, 2005. Three simultaneous radio signals of 0.94, 3.6, and 13 centimetre wavelength (Ka-, X-, and S-bands) were sent from Cassini through the rings to Earth. The observed change of each signal as Cassini moved behind the rings provided a profile of the distribution of ring material as a function of distance from Saturn, or an optical depth profile. This simulated image was constructed from the measured optical depth profiles. It depicts the observed ring structure at about 10 kilometres (6 miles) in resolution. Colour is used to represent information about ring particle sizes in different regions based on the measured effects of the three radio signals. Purple color indicates regions where there is a lack of particles of size less than 5 centimetres (about 2 inches). Green and blue shades indicate regions where there are particles smaller than 5 centimetres and 1 centimetre (less than one third of one inch). The saturated broad white band near the middle of ring B is the densest region of ring B, over which two of the three radio signals were blocked at 10-kilometer (6-mile) resolution, preventing accurate colour representation over this band. From other evidence in the radio observations, all ring regions appear to be populated by a broad range particle size distribution that extends to boulder sizes (several to many meters across).

The B ring has the greatest optical depth. Spitale and Porco, in a paper written 2010, suggested that the edge of it indicates the structure isn’t fixed.


The central ringlet of the A ring’s Encke Gap coincides with Pan‘s orbit, implying its particles oscillate in horseshoe orbits. As mentioned before moons do affect the ring structure.

A moonlet is only massive enough to clear two small—about 10 km across—partial gaps in the immediate vicinity of the moonlet itself creating a structure shaped like an airplane propeller. The moonlets themselves are tiny, ranging from about 40 to 500 meters in diameter, and are too small to be seen directly.


Daphnis in the Keeler gap.

Professor Heikki Salo has produced a simulation of the form and breakup of the rings. Some of the satellites show simple harmonic motion and resonance is seen. Interparticle collisions cause damping.

References:                                                 http://en.wikipedia.org/wiki/Moons_of_Saturn  http://en.wikipedia.org/wiki/Rings_of_Saturn  http://iopscience.iop.org/2041-8205/718/2/L92/fulltext/apjl_718_2_92.text.html  http://cc.oulu.fi/~hsalo/  http://www.princeton.edu/~rvdb/JAVA/astro/galaxy/StableRings.htm lhttp://www.nasa.gov/mission_pages/cassini/multimedia/pia07873.html http://iopscience.iop.org/1538-3881/140/6/1747/ http://www.lpl.arizona.edu/resources/faculty/faculty.php?nom=Spitale  http://es.wikipedia.org/wiki/Archivo:Cassini_Saturn_Orbit_Insertion.jpg            http://saturn.jpl.nasa.gov/  http://en.wikipedia.org/wiki/Christiaan_Huygens  http://www.nasa.gov/mission_pages/cassini/multimedia/index.html            http://voyager.jpl.nasa.gov/imagesvideo/saturn.html              http://en.wikipedia.org/wiki/Saturn                  http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens  http://en.wikipedia.org/wiki/Giovanni_Domenico_Cassini  http://ciclops.org/

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