ASTRO 101 Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: Telephone: WWW: Office: Physics 241, hours T TH 3:30-5:00

Homework/Announcements Homework due Tuesday, March 19: Question 6, Chapter 7 (What role did Charon play in enabling astronomers to determine Pluto’s mass?).

Comets! Comet PANSTARRS should be viewable in the western skies starting March 7. It will be close to the crescent Moon on March Comet ISON might be very spectacular in December, 2013, provided it survives its close encounter with the Sun.

Comets!

NEXT:

Recent Events: Mars Rover rover-sees-life-friendly-conditions-in-ancient-mars-rock?litehttp://cosmiclog.nbcnews.com/_news/2013/03/12/ curiosity- rover-sees-life-friendly-conditions-in-ancient-mars-rock?lite

Evidence for Water on Mars These images and others suggest the presence of liquid water on Mars, at least in the past.

Evidence for Water on Mars These images and others suggest the presence of liquid water on Mars, at least in the past.

Life on Mars There is no evidence that there is currently life on Mars. The recent Mars rovers will perform additional experiments along these lines. It is possible that there was primitive life in the early history of Mars, but the only way to be sure would be to go there.

Next: The Giant Planets

The Giant Planets The giant planets are Jupiter, Saturn, Uranus, and Neptune.

The Giant Planets The radii are between about 4 and 11 times that of Earth. The masses are between 14 and 318 times that of Earth. However, the densities are between 0.7 and 1.8 grams/cc, and the albedos are high. The planets are composed of light elements, mostly hydrogen and helium.

The Gas Giants The composition of the giant planets, especially Jupiter, is close to that of the Sun. The internal structures of these planets is completely different from that of the Earth. In particular, there is no hard surface.

Jupiter Jupiter is by far the most massive planet in the solar system (it contains about 2/3 of the solar system mass outside the Sun). It has the largest radius of any solar system planet, and it rotates the fastest (once every 10 hours). It has at least 63 moons. In many categories, Jupiter is the most extreme case.

Jupiter’s Weather Owing to its rapid rotation, Jupiter has a very turbulent atmosphere, which is composed mostly of methane and ammonia (both hydrogen rich).

Jupiter’s Weather Owing to its rapid rotation, Jupiter has a very turbulent atmosphere, which is composed mostly of methane and ammonia (both hydrogen rich).

The Great Red Spot The Spot is at least 150 years old. It is basically a giant hurricane, roughly three times the size of Earth. Smaller features are also evident.

The Great Red Spot

Similar storm systems have been observed to form recently.

Jupiter’s Interior There is no way to directly study the deep interior parts, we rely on computer models. The interior should be hot (35,000 K), with a solid core, possibly surrounded by liquid. The gaseous outer atmosphere surrounds molecular and liquid metallic hydrogen and helium.

Jupiter’s Atmosphere The atmosphere of Jupiter (and Saturn) consists of poisonous gas (ammonia and ammonium hydrosulfide) covering a layer of water vapor.

Jupiter’s Moons Jupiter has 4 fairly large moons, discovered by Galileo in 1610, and at least 59 other moons, all significantly smaller.

Jupiter’s Moons The largest moon is larger than Mercury. Image from Nick Strobel (

Jupiter’s Moons The 4 Galilean moons have small rocky or iron cores. They have similar densities to our Moon, indicating that each contains a thick layer of water and ice.

Io The surface of Io is unlike that of any other solar system body. There are no impact craters, so its surface must be young.

Io There are active volcanoes on Io, the only place in the solar system outside of Earth where this is the case.

Io There are active volcanoes on Io, the only place in the solar system outside of Earth where this is the case.

Volcanoes on Io Io’s surface is constantly altered by volcanoes. On Earth, the energy for geological activity comes from radioactivity. However, Io’s mass is relatively small, so it should have no radioactivity left. The heating source turns out to be Jupiter’s gravity, which compresses Io as its orbits.

Next

Saturn The most obvious feature of Saturn is its large ring system, which has been known for more than 300 years.

Saturn’s Atmosphere The composition of Saturn is similar to that of Jupiter. Saturn also rotates rapidly, once every 10 hours or so. Saturn’s weather is like that on Jupiter, although the contrast between different bands is much less.

Saturn Note the band-like structure in the clouds.

Saturn Note the band-like structure in the clouds.

Saturn The rings are very thin, and gaps are evident, even from the ground. They consist of fragments of ice and ice- covered rocks.

Saturn The rings are very thin, and gaps are evident, even from the ground. They consist of fragments of ice and ice- covered rocks.

Saturn Small satellites can cause some of the rings to ripple or to have braided structure.

Saturn Small satellites can cause some of the rings to ripple or to have braided structure.

Saturn’s Rings From Voyager, we know the rings are composed of hundreds or thousands of “ringlets”.

Saturn’s Moons Saturn has 1 very large moon (Titan), 10 moderate size moons, and a few dozen much smaller ones.

Saturn’s Moons Saturn has 1 very large moon (Titan), 10 moderate size moons, and a few dozen much smaller ones.

Saturn’s Moons Saturn has 1 very large moon (Titan), 10 moderate size moons, and a few dozen much smaller ones. Titan is the only moon with an atmosphere.

Titan Titan’s atmosphere is mostly hydrogen with a substantial amount of methane. There are organic molecules, but it is probably too cold for life to have evolved.

Titan There are lakes of liquid methane, as well as large flat areas.

Uranus and Neptune These distant planets were not known before the invention of the telescope. They are each about 4 times the diameter of the Earth, and about 15 times more massive than the Earth. They are made up mostly of H and He, but with a bit more heavy elements.

Uranus and Neptune Both planets appear blue-green because of the methane gas mixed in with the hydrogen. The weather patterns are much less apparent. Solar heating is not important since they are far from the Sun.

Uranus and Neptune Both planets appear blue-green because of the methane gas mixed in with the hydrogen. The weather patterns are much less apparent. Solar heating is not important since they are far from the Sun.

Uranus and Neptune Both planets are believed to have rocky cores surrounded by compressed water and liquid hydrogen and helium.

Seasons on Uranus The rotation axis of Uranus is inclined by about 90 degrees relative to its orbital plane. This gives rise to highly exaggerated seasons.

Uranus’s Rings Uranus also has rings, which were discovered in 1977 when Uranus passed directly in front of a distant star. They are very dark, reflecting only about 2% of the sunlight.

Uranus’s Rings Uranus also has rings, which were discovered in 1977 when Uranus passed directly in front of a distant star. They are very dark, reflecting only about 2% of the sunlight.

Uranus’s Rings Uranus also has rings, which were discovered in 1977 when Uranus passed directly in front of a distant star. Recent advances in telescopes and cameras allow for a better view than was previously possible.

Uranus’s Rings Recent advances in telescopes and cameras allow for a better view than was previously possible. Note the system of moons that orbit in the equatorial plane. They take their names from Shakespeare’s A Midsummer’s Night Dream and other works.

Neptune’s Rings Neptune also has dark rings, discovered by Voyager in the late 1980s.

Rings All of the gas giants have rings, although the rings of Saturn are by far the largest. They appear to be relatively short-lived (only millions of years) and composed of material from moons that wandered to close to their parent planets.

Neptune’s Moons Neptune has at least 13 moons, most of which appear to be captured asteroids. The largest moon, Triton, was captured into a retrograde orbit, and will soon be destroyed by Neptune’s gravity.

Next: Chapter 7 (Dwarf Planets and Small Solar System Bodies)

The Discovery of the Outer Planets Uranus was found in 1781 by William Herschel. This new planet was barely too faint to see without a telescope.

The Discovery of the Outer Planets Uranus was found in 1781 by William Herschel. This new planet was barely too faint to see without a telescope. After many years of observations, it was apparent that the orbit of Uranus was not quite what was expected.

The Discovery of the Outer Planets Uranus was found in 1781 by William Herschel. This new planet was barely too faint to see without a telescope. After many years of observations, it was apparent that the orbit of Uranus was not quite what was expected. The slight deviations were probably caused by an unseen planet’s gravitational pull.

The Discovery of the Outer Planets After many years of observations, it was apparent that the orbit of Uranus was not quite what was expected. The slight deviations were probably caused by an unseen planet’s gravitational pull. Starting in about 1845, English and French mathematicians worked out where the unseen planet would be by using Newton’s theory of gravity.

The Discovery of the Outer Planets Starting in about 1845, English and French mathematicians worked out where the unseen planet would be by using Newton’s theory of gravity. Neptune was located in 1846.

The Discovery of the Outer Planets Starting in about 1845, English and French mathematicians worked out where the unseen planet would be by using Newton’s theory of gravity. Neptune was located in In the early 1900s, it was thought that there was still another more distant planet (“Planet X”) disturbing Uranus.

Pluto Pluto was discovered in 1930 from Arizona.

Pluto Pluto was discovered in 1930 from Arizona. It is very faint, and appeared nearly point-like on early photographs.

Pluto Essentially nothing was known about Pluto until the 1970s.  It is far from the Sun (more than 40 times the Earth-Sun distance), hence it receives little sunlight. In turn, we see very little light reflected back.  The planet looks nearly pointlike owing to its large distance from us, so it was not possible to see surface features.

Pluto’s Orbit Pluto’s orbit is highly inclined relative to the orbits of the other planets.

Pluto’s Orbit Pluto’s orbit is also more eccentric than the orbits of the other planets. The period is years

Pluto’s First Moon Pluto’s first moon was discovered in The orientation of the moon’s orbit is such that during certain times, Pluto and its moon appear to pass in front of each other every 3.2 days as seen from Earth.

Pluto’s First Moon Pluto and Charon passed in front of each other in the late 1980s.

Pluto’s First Moon Pluto and Charon passed in front of each other in the late 1980s. Studies of these events allowed astronomers to measure the masses and radii of Pluto and Charon. Images from Courtney Seligman (

Pluto’s First Moon The masses can be deduced from Kepler’s Laws. The radii can be deduced by looking at the total brightness over time.

Pluto’s First Moon Pluto’s mass is 1/500 of the Earth’s mass. Pluto’s diameter is 2300 km.

Pluto’s First Moon Pluto’s mass is 1/500 of the Earth’s mass. Pluto’s diameter is 2300 km. This is smaller than Jupiter’s large moons, and also Earth’s moon.

More Moons for Pluto Two additional moons were found using the Hubble Space Telescope in 2005.

Pluto “Demoted”! The definition of a “planet” was changed recently: –Planets: The eight worlds from Mercury to Neptune. Mercury Neptune –Dwarf Planets: Pluto and any other round object that "has not cleared the neighborhood around its orbit, and is not a satellite." –Small Solar System Bodies: All other objects orbiting the Sun. Sun

New Classification Scheme for the Solar System Many catagories overlap. For example, some asteroids are dwarf planets. In most cases, the distinctions between the catagories are not important for us.

Dwarf Planets Ceres was discovered in 1801 by Piazzi. Its orbit lies between Mars and Jupiter. Thousands of asteroids are now known to be in the “asteroid belt”. Ceres was reclassified as a dwarf planet in 2006.

Dwarf Planets Since 2003, three additional dwarf planets have been discovered beyond the orbit of Pluto. Eris, the largest one, is apparently composed largely of water ice. Haumea appears to be slightly oval shaped.

Dwarf Planets Since 2003, three additional dwarf planets have been discovered beyond the orbit of Pluto. Eris has a very eccentric and inclined orbit. The period is 557 years.

Next Small Solar System Bodies