Comet ISON November 17

November 19

Triton

Discussion

Triton, unlike all the other large satellites of the outer planets, orbits retrograde in the opposite direction as the rotation of Neptune. What does this tell you about the origin of Triton?

Kuiper belt

Discussion

What is the effect of tidal friction on a satellite that orbits retrograde?

Discussion

What will happen when Triton reaches Neptune’s Roche limit?

Triton

Discussion

What does the previous picture tell you about the surface of Triton?

Triton

A greenhouse solid?

Nitrogen ice is transparent to visible light, but not to infrared. Thus when the Sun shines on the polar ice cap laid down during the previous winter, ice underneath can heat up, causing fountains or eruptive plumes or geysers.

Discussion

What do you think causes the strong winds on Triton? Hint: The winds are directed away from the south pole.

Cantaloupe terrain

Spitsbergen Island

Mars Pathfinder

Cryovolcanism

Nitrogen geyser on Triton

The Search for planet X

Soon after Neptune’s discovery it was found that it did not seem to be following Kepler’s laws either.

Early calculations indicated a planet 50 to 80 times the mass of Earth must exist beyond the orbit of Neptune.

Percival Lowell – 1905

Discussion

How would you go about searching for a new planet? Where in the sky would you look and why? How could you tell a faint planet from the millions of stars?

Clyde Tombaugh

1929-1930 used an 11-inch telescope to image the zodiacal constellation directly overhead (at the meridian) at midnight. He took two images several days apart and compared them to see if any of the points of light moved between the time the two plates were taken.

Discussion

Why image the point at the zenith at midnight?

Pluto discovery photos

Pluto’s orbit

Highly eccentric orbit, 0.25, is actually closer to the Sun at perihelion than Neptune.

Angle of inclination is 17 degrees, the highest of any of the planets.

3-to-2 resonance with Neptune – Neptune orbits the Sun three times for every two orbits of Pluto.

Pluto never gets close to Neptune

Because of Pluto’s large inclination angle and the 3-2 resonance, Pluto never gets closer to Neptune than 17 AU. On the other hand Pluto gets as close as 11 AU to Uranus.

Discussion

Pluto’s rotation period was measured in 1954 to be 6.4 days. How was this done?

Pluto’s size

Size estimated using its brightness and assumed a reflectivity

Mass estimated using the orbital residuals from Uranus and Neptune

Discussion

Why was Pluto assumed to have a very low reflectivity?

The incredible shrinking planet

As the orbit of Neptune was followed over the years, the size of the residuals got smaller and smaller.

1930 mass estimated at 8-10 Earth masses and

1970 mass estimated to be no bigger than Mars.

Charon discovery photo

Discussion

What is the significance of the discovery that Pluto has a satellite?

Discussion

What is the significance of Charon passing in front of and behind Pluto?

Discussion

How could we get a spectrum of Charon alone, before we could resolve the two?

HST Image

Discussion

How do you think Pluto might have acquired such a large satellite as Charon? What evidence should you look for to either support or contradict your theory?

Discussion

Pluto has an atmosphere, while Charon does not. How do you think we figured this out?

Discussion

Charon doesn’t have an atmosphere. Why do you think it doesn’t?

Discussion

Why is Pluto so bright, with an albedo of 0.6? Why does the methane ice, at least in some spots, stay bright?

Kerberos

Styx