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Saturn

Goals of the Lab

• To identify and observe Saturn, its moons, and its ring system;

• To identify and observe the features of Saturn through careful observations.

Required Equipment: a calculator, a compass, a watch, several printouts of the Observation Templates (both generic templates and at least one page of Saturn templates), and printouts of the other instructions linked from this lab (Mapping the Motion of a Planet in the Sky).

Background: Saturn is the second largest Jovian planet, Jupiter being the largest. Its diameter, without its rings, is 9 times greater than the Earth's; its mass is 95 times greater. Saturn is similar to Jupiter but a third as massive and colder. It is a giant, gaseous planet composed mainly of hydrogen and helium, with no solid surface. Astronomers think that there is a small molten core of heavier elements such as rock-forming silicon and iron at the center of the planet. Saturn is about 9 times further away from the Sun than the Earth, resulting in very low temperatures in its outer atmosphere of about 95 Kelvin (-290°F!!). The main constituents of the atmosphere, in addition to hydrogen and helium are methane and ammonia. Ammonia clouds are what can be seen through telescopes.

In 1610 Galileo pointed his telescope at Saturn, he noted that it appeared very flattened, as if it had "ears". In 1655,the Dutch astronomer Christian Huygens resolved and recognized that Saturn is surrounded by rings. As a result of these spectacular rings, Saturn is a favorite object for viewers using small telescopes. In the past twenty years, astronomers have discovered rings around Jupiter, Uranus, and Neptune, albeit not nearly as brilliant as those around Saturn and invisible in small telescopes. The rings around Saturn, of which three are visible in small telescopes, are composed of ice and ice-covered particles ranging in size from grains of sugar to houses. Moving inward from the outer edge, the three rings are known as the A ring, the B ring, and the C ring. Each ring is made up of hundreds of smaller rings known as ringlets. The rings are remarkably thin, starlight from background stars easily shines through them. The rings are only 20 or so meters thick To put this into perspective, if the outer diameter of the rings were 4 km, they would be as thin as a sheet of paper! The origin of the ring system remains a mystery. Some theories suggest that the icy particles are the remains of satellites of Saturn which were destroyed in violent collisions, perhaps with a large comet or asteroid. Other theories maintain that the particles simply condensed out of the protoplanetary disk from which Saturn formed. These particles neither accreted to become part of Saturn, nor came together to form another moon.

Saturn has one large moon Titan, six medium-sized moons, and many smaller moons. Titan is the second largest moon in the solar system, it is larger than the planet Mercury, and the only moon with a thick atmosphere. Titan's atmosphere is hazy and reddish, obscuring its surface. The haze is actually a photochemical smog produced by reactions of methane and other compounds from exposure to sunlight. Based on measurements of temperature and pressure on Titan's surface, researchers believe that the surface may be largely covered by a cold ocean of liquid methane and liquid ethane up to a kilometer in depth. Many of Saturn's other moons are just as unique and as strangely interesting as Titan. The moon Enceladus is one of the shiniest objects in the solar system. It is covered with water ice, which efficiently reflects sunlight. (In 2006, the space probe Cassini observed what appear to be water plumes emerging from Enceladus!) One side of the moon Iapetus is covered with bright white ice, and the other side is as black as tar. For this reason, it is known as the "two-faced" moon.

The space probe Cassini is currently orbiting Saturn, studying the planet and its satellites. It arrived at Saturn in July 2004, and it released the probe Huygens towards Titan on December 25. In mid January, the probe descended through Titan's atmosphere to the surface, relaying data about the composition of the atmosphere, temperature, images of the surface, etc. back to scientists on earth, before succumbing to deadly cold temperatures. To read more and see footage of Cassini's journey, go to the website: .

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Part I: Observing Saturn

Note: All observations should contain the directions N/S and E/W, the time, the date, and the weather condition.

1. Complete one observation of the position of Saturn in the sky, using the method described in Mapping the Motion of a Planet in the Sky.

2. Find Saturn with the 25mm eyepiece. Sketch the field of view using a generic Observation Template. Saturn is so far away that its moons will look like faint background stars in your telescope. Mark the positions of any stars or moons in the field of view. In particular, look for very faint ones near Saturn (which are probably moons).

3. The TAs will give you an appropriate moon identification handout for your lab night. Use this diagram to label which moons you saw in the diagram. If you wish, you can also use this to go back and try to see other moons you may have missed. However, do not draw any moons in your sketch that you do not see yourself in the telescope. Very few people can see all of the moons of Saturn on your handout with these 8" telescopes!

4. Switch to the 10mm eyepiece. Refer to the following figures for features on Saturn which may be visible in your field of view.:

   

   Figure 1: Features of Saturn that may be visible through the telescope.

   

   Figure 2: Representative sketches of Saturn. Your sketches need to include surface detail (bands) and ring detail. The tilt of Saturn's rings, as viewed from the Earth, changes with time so that your sketch could look different from the sketches shown here.

   Sketch Saturn. The template shows an oval for the outline of the disk of the planet and 3 tick marks on either side. The tick marks correspond to the outer boundary of the A ring, the location of Cassini's division that separates the A and B rings, and the inner boundary of the B ring, respectively. These are to help you draw the ring system. See the sketch above for an example. Include as much detail as you can. Scrutinize the planet and the rings for details, shadings, cloud bands, brightness variations, etc. Draw the rings as precisely as you can, paying much attention to how far they extend along the short axis (the long axis being fixed by the tick marks). With reasonable seeing conditions, Cassini's Division in the ring system should be visible as a thin, dark and very sharp line. Note that the inner ring are brighter than the outer rings. You may be able to see the shadow of the planet on the rings as a black gap in the rings next to the planet.  This is visible on the above sketch as a small gap between the planet and rings at the lower right. Remember to indicate the orientation of your drawing (directions of celestial West and North– you learned how to do this in the Telescope Basics lab). If the 10mm eyepiece reveals moons near the planet that you missed in your sketch through the 25mm, go back and add them to your 25mm sketch, making a note of which moons were visible only through the 10mm eyepiece.

   If you observe Saturn on more than one night, note how the moon Titan has moved along its 16-day orbit around the planet.

Part II: Determining the Diameter of Saturn

1. Using the Method of Transit Times, determine the apparent diameter of Saturn as well as that of the long axis of the rings at the position of their greatest extent. You can estimate the position of Saturn on the SC001 star chart and read the declination off the chart. To convert your timing measurement to arc seconds, you need to correct for the declination of Saturn (d) and the tilt of the rings compared to the E-W direction in the field of view. The first correction is described in the Method of Transit Times. The second correction is related to the fact that the length we want to measure is not exactly in the East-West direction (the direction of drift in the telescope) but is inclined at some angle (i). The full correction of your transit time T to the angular dimension is given by:

   Angular size (seconds of arc) = T (seconds)* 15" *cos (d)/cos(i)

   where the angles d and i are measured in degrees.  To get the angle i, simply estimate its value (to the nearest 10°) by comparing the inclination of the ring system to the direction of drift. Here, i=0° means that the length you are measuring is in the E-W direction. To compute the linear dimension of Saturn and the rings, use the small angle formula:

   linear size (km) = angular size (seconds of arc) * distance (km) / 206265

   In March, 2006, the distance from Earth to Saturn is about 8.2 AU (1 AU = 1.496 × 10⁸ km).

   • Measured Transit Time for Saturn [seconds]:
   • Diameter of Saturn [seconds of arc]:
   • Diameter of Saturn [km]:
   
   
   
   • Measured Transit Time for Rings [seconds]:
   • Diameter of Rings [seconds of arc]:
   • Diameter of Rings [km]:

Part III: The rings of Saturn

1. Calculate the opening angle of the rings: The rings appear elliptical as seen from the Earth but if we looked at them from above the North Pole of Saturn, they would appear circular. This is a result of the tilt of Saturn's axis of rotation with respect to the plane of its orbit. If the axis of rotation were perpendicular to the plane of the orbit, then we would observe the rings edge-on all the time. Since they are very thin, they would be barely visible at all! This inclination angle leads to the existence of seasons on Saturn (just like the Earth's tilt of 23° is responsible for the seasons). As the Earth and Saturn orbit the Sun, the relative viewing angle changes and the rings appear more or less tilted. Every 15 years, the rings are oriented so that we see them edge-on and Saturn appears without rings for a few weeks. To visualize this effect, take a CD and tilt it from your line of sight. It appears elliptical. The angle of tilt of the axis relative to the line of sight (t) is simply given by:

   sin t = a/b

   where a and b are the length of the small and long axes of the rings (measured in mm) on the your sketch with the 10mm eyepiece. An angle of t=0° corresponds to the axis being perpendicular to the line of sight (edge-on rings).

2. The shadow of Saturn on its rings: Depending on the date of your observation, you may observe the shadow of the planet on its rings. It appears as a black gap in the rings, right next to the limb of Saturn. Determine in what direction (East or West) the shadow is cast on the rings, using the sketch made in #4 above. Use a compass to draw the orbits of the Earth and Saturn to scale (see the Saturn's motion lab). Assume that the orbits are circular and use orbital radii of 1.00 A.U. and 9.54 A.U. for the Earth and Saturn, respectively. Use your starmap SC-001 to figure out at what angle Earth is in its orbit tonight; draw Earth at the appropriate place on your diagram.

   Based on where you see the shadow of Saturn on its rings, where do you expect Saturn to be in its orbit? Remember that the Sun is the source of the light on Saturn; the location and size of the shadow should tell you how far "off to the side" from the line between the light source (the Sun) and Saturn our vantage point (Earth) is. Explain your reasoning for where you drew Saturn; just a position without a clear explanation in your own words is not sufficient to gain full credit for this section of the lab!

   If you are doing the Saturn's Orbital Motion lab this semester, is your drawing consistent with what you've observed in that lab? Comment.