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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