Saturn
Summary: In this lab you will observe Saturn, determine the diameter of the planet and its ring system, and observe and identify its moons. In doing so, you should gain some perspective of this planet's physical size and characteristics relative to the more familiar characteristics of Earth.
Note: Much of what you should do during lab time is to make observations and sketches and to discuss what you are doing with your lab partner. Any calculations can be done outside of lab time, based on the data you collect during lab. Similarly, you should discuss during lab time how to answer questions posed in this lab, but you should write down your answers outside of lab time. The observations described below must be done by each lab partner (but not necessarily on the same night).
Background: When, in 1610, Galileo pointed his rather crude telescope to observe Saturn, he noted that it appeared very flattened, as if it had "ears." With time, the optics used to make telescopes improved significantly, providing greater image clarity. In 1655, the Dutch astronomer Christian Huygens was able to resolve and recognize that Saturn is surrounded by rings. Because of these spectacular rings, Saturn has enjoyed widespread popularity, and is a favorite object for amateur 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 with typical sizes ranging from that of a ping pong ball to that of a house. Moving inward from th eouter edge, th e three rings are known as the A ring, the B ring and the C ring. Each of these rings is made up of hundreds of smaller rings known as ringlets. The rings are remarkably thin and span only a few tens of meters vertically. To put this into perspective, if the outer diameter of the rings were 4km, 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 passing large comet or steroid. 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 the planet, nor came together to form a another moon.
The moons of Saturn also have given this planet/ring/moon system much notoriety. One good example is Iapetus, or the "two-faced" moon. The hemisphere on the leading side in Iapetus' orbital motion around Saturn is covered with dark, frosty soil the color of reddish tar, while the trailing side is covered with bright, white ice. Titan, Saturn's largest moon, was found by the Voyager space probe to have a hazy, reddish atmosphere. This is the only satellite in the solar system with a substantial atmosphere. It has been shown that this haze is actually a photochemical smog produced by reactions of methane and other compounds when they are exposed to sunlight. Based on measurements of temperature and pressure on Titan's surface, researchers conclude that the surface may be largely covered by a cold ocean of liquid methane and liquid ethane estimated to be a kilometer deep. Still more complicated, gazoline-like compounds may form in the smog and rain out of the hazy clouds. One Voyager scientist had characterized Titan as "a bizarre, murky swamp." These are just two examples of the more than 18 moons orbiting Saturn. Titan is larger than our own Moon and is the brightest of Saturn's moons. The space probe Cassini is on its way to Saturn. Once it reaches the distant planet in 2004, it will start orbiting it and study the planet and its satellites for several years. It will also drop a probe in the atmosphere of Titan, which will relay data about clouds, winds, temperature, composition of the atmosphere, pictures of the surface, etc. for several hours until it succumbs to the frigid temperatures on Titan.
The planet itself 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 (-290oF!!). The main constituents of the atmosphere, besides hydrogen and helium are methane and ammonia. Ammonia is largely condensed to form the cloud tops that can be seen through telescopes. Saturn's telescopic appearance is not nearly as complex as that of Jupiter, perhaps due to some overlying layer of haze, but a persistent pattern of bands is visible.
Procedures
Notes: As always, note the date and time and describe the atmospheric conditions. Is the sky clear, partly cloudy, high cirrus? Is the air moist? Is it windy? Think about how the weather may affect your view of Saturn. Complete one observation of the position of Saturn in the sky.
Find Saturn: Using the 25 mm eyepiece, center Saturn in the field of view of the telescope. Make a sketch of the field of view, including Saturn, using the generic template provided by the TAs. Saturn is far enough away that any of its moons will look very similar to faint background stars in the field of view of your telescope. Mark the positions of any stars or moons in the field of view, including very faint ones that you may see very near Saturn. Indicate the size of the field of view and the orientation (NSEW) of your sketch.
Sketch Saturn:Switch to the 10 mm eyepiece. Refer to the following figures for examples of the features that may be visible on Saturn.:
![[Saturn Figure 1]](saturn_fig1.jpg)
Figure 1: Features of Saturn that may be visible through the telescope.
![[Saturn Figure 2]](saturn_fig2.jpg)
Figure 2: Representative sketches of Saturn. Your sketches should include as much surface detail (bands) and detail about the rings as you are able to see. Note that the tilt of Saturn's rings, as viewed from the Earth, changes with time so that your sketch could look quite different from the sketches below.
Make a sketch of Saturn, using the Saturn template provided by the TAs. 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. However, you may have to look very closely for some time in order to see features in the rings. 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. How can you determine that?). If the 10 mm eyepiece reveals moons near the planet that you missed in your sketch through the 25 mm, go back and add them to your 25 mm sketch, making a note of which moons were visible only through the 10 mm 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.
Diameter of Saturn: 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. Find the declination of Saturn using the dial on the declination axis of the telescope. Calibrate the dial by checking its reading with the declination of a nearby star of known declination (see the list of stars in "Star hopping"). Alternatively, you can plot the position of Saturn on the SC001 star chart and read the declination off the chart. To convert your timing measurement to seconds of arc, 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 has to do with the fact that in general, the length we wish to measure is not exactly in the East-West direction (the direction of drift in the telescope) but is inclined at some angle (i). This should be fairly obvious when you measure the transit time of the rings. 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 10o) by comparing the inclination of the ring system to the direction of drift. Here, i=0o 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
During the Spring 2002 semester, the distance of Saturn will be about 8.13 AU (1 AU = 1.496 X 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]:
Identify the moons of Saturn: Using the corkscrew diagram showing the position of the moons of Saturn , identify the moons that are shown on your sketch. Note that the corkscrew diagram shows time marks in Universal Time (UT). This is the time commonly used by astronomers and is simply the time at the meridian of zero longitude on Earth, the one that goes through Greenwich, England.
To convert from Central Standard Time to UT, add 6 hours.
To convert from Central Daylight Saving Time, add 5 hours.
In either case, be aware that the UT date may be one day ahead of the local date. For example, 7pm CST on Nov 6 would be 1 am UT on Nov 7.
Corckscrew diagrams for Saturn's Moons
- Inner Moons (Mimas, Enceladus, Tethys, Dione)
- Outer Moons (Rhea, Titan, Iapetus)
Convert the time of your observation of the moons of Saturn to UT and consult the corkscrew diagram and the diagram showing the paths of the moons of Saturn around the planet to identify each of the moons on your sketch. Keep in mind that the telescope invert images and compare with the orientation of the corkscrew diagram!! Good luck, this is a more challenging exercise than it may seem.
- Inner Moons (Mimas, Enceladus, Tethys, Dione)
Calculate the opening angle of the rings: The rings appear elliptical as seen from the Earth but if we were to see them from above the North Pole of Saturn, they would appear circular. It is because Saturn's axis of rotation is tilted with respect to the plane of its orbit that we can see the rings at all. If the axis of rotation were perpendicular to the plane of the orbit, then we would see the rings edge on at all times. 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 23o 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 form you line of sight. It appears elliptical. The angle of tilt of the axis of relation 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 sketch you made through the 10 mm eyepiece. An angle of t=0o corresponds to the axis being perpendicular to th eline of sight (edge-on rings).
The shadow of Saturn on its rings: Depending on the date of your observation, you may see 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 #3 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. Using the information gathered in the Saturn's orbital motion lab and a protractor, plot the relative positions of the Sun, the Earth and Saturn along their orbits. From that diagram, determine (approximately) the angle and the direction (E or W) that Saturn's shadow makes with the line of sight to the planet. Does this seem consistent with the way Saturn casts a shadow on its rings, as recorded on your sketch? You do not need to make detailed calculations here, just a qualitative check. Skip this part if you can't see the shadow.