Download Lecture Six (Powerpoint format) - FLASH Center for Computational

Science 3210 001 : Introduction to Astronomy
Lecture 6 : The Outer Planets
Robert Fisher
Items
 Solution set 4 has been posted to the website.
 Reading/Homework set 5 has been posted to the website.
 Lunar eclipse tomorrow!
 Visible from moonrise (at about 5:50 PM) though about 7:15 PM.
 Look just north of due East to see the moon rise.
 Observational project.
Today -- Outer Solar System
 Jupiter
 Saturn
 Uranus
 Neptune
 Pluto (Ok -- not a planet anymore, but a dwarf planet in the
Kuiper belt!)
Comparison of Outer Planets
 A 3D computer rendering of the outer planets, compared with the
inner planets, all to-scale :
Jupiter
 Jupiter, the “king of the planets,” is the largest planet in the solar system.
 It has a mass 320 times that of the Earth.
 It has a radius 10 times that of the Earth, and is visibly oblate due to its
strong rotation.
 Its enormous volume has allowed it to retain the heat left over from its
formation even today -- it radiates away more energy than it takes in
from the sun.
Jupiter
 Jupiter’s cloud layers and red spot.
Giant Red Spot
Moons of Jupiter
 Jupiter has 63 known moons, including 4 major ones which are
large enough to be spherical.
 The four major moons of Jupiter were discovered by Galileo in
1610 and are referred to as the Galilean moons.
Io
Europa
5,000 km
Ganymede
Callisto
Io
 Jupiter’s moon Io is volcanically active, spewing sulfur plumes.
Europa
 Detailed images of the surface of Europa taken with spacecraft
reveal a highly-fractured icy surface, possibly with an underlying
oceanic layer.
Ganymede
 Ganymede, as photographed by Voyager and Galileo, reveals a
complex surface activity similar to Europa.
Jupiter’s Cloud Layer
 Like the Earth, the Coriolis effect due to Jupiter’s rotation divides
the circulation patterns into a number of bands or cells.
 Unlike the Earth, the greater rotation of Jupiter leads to a larger
number of bands.
 The colorations we see are caused when either uprising currents
allow the top layer of ammonia to be visible (as white) or the
lower-lying ammonium hydrosulfide layer to be visible (as
reddish-brown).
Jupiter’s Ring System
 Jupiter has a ring system like Saturn’s, although much less
spectacular.
Saturn
 Saturn is the second-largest planet in the solar system, and is the
only planet in the solar system to have an average density less
than that of liquid water.
 Mass 95 times that of the Earth.
 Radius about 4.5 times that of Earth, and is visibly oblate due to
its strong rotation, similar to Jupiter.
Artist’s Conception of Saturn’s RIngs
Saturn’s Rings Imaged by Voyager
Shepherd Moons
 Two small moons can “shepherd” ring material into a particularly
sharp ring, as is the case with Saturn’s F ring :
Saturn’s Moons
 Saturn has a rich system of satellites -- some 60 in total.
 Of these, seven are major satellites which are large enough to be
spherical.
Titan
 Several years ago, NASA and the European Space Agency (ESA)
sent the Cassini/Huygens mission to Saturn. The Huygens space
probe entered the atmosphere of Saturn and took detailed
measurements of its properties on its descent.
Titan
 Titan is unique in the solar system -- it is a terrestrial-sized world
with an atmosphere containing ethane and methane, both of
which can be solid, liquid, and gaseous, just as water on Earth.
Liquid Ethane Lakes?
Uranus
 Uranus was the first planet to have been discovered, and not
known from antiquity.
 It was discovered by accident by William Herschel
 It has roughly 14 times the mass of the Earth, with a radius about
4 times the radius of the Earth.
Uranus as Photographed by Voyager 2
 In January of 1986, shortly before the Challenger disaster,
Voyager 2 flew by Uranus -- the first space mission to have ever
visited it.
 The atmosphere of the planet consists of a methane haze and
has virtually no distinguishable atmospheric features -- faint cloud
patterns can only be brought out in “false color”.
Moons of Uranus
 Uranus has at least twenty-seven known moons, including five
major ones.
 The names of the moons are chosen not from classical mythology
but rather Shakespeare.
Miranda
Arial
Umbriel
Titania
Oberon
1500 km
The Surprising Miranda Moon
 On its flyby, Voyager 2 approached the moon Miranda the closest.
 Given the distance from the Sun and the extremely cold
temperatures (-335 degrees Fahrenheit), scientists expected a
cold dead world.
 Instead, they were stunned to find a surprising world with a
remarkably complex surface.
Miranda as Imaged by Voyager 2
A Closeup of the Surface of Miranda
Neptune
 Neptune is similar in more similar to Uranus than it is to either
Jupiter and Saturn.
 Mass of 17 times the Earth.
 Radius of about 4 times that of Earth.
The Story of the Discovery of Neptune
 If Uranus was the first planet to be discovered, Neptune was the first
(and actually only) planet to be predicted.
 By the early 1840s, it became evident that the actual observed motion of
Uranus deviated substantially from its predicted motion -- a so-called
orbital anomaly.
 British scientist John Couch Adams first inferred a predicted location for
the new planet in 1843. Adams sent the British Royal Astronomer Sir
George Airy the prediction. The British observational effort, however,
floundered.
 Independently, the French Scientist Urbain Leverrier predicted a similar
location for a new planet in 1846. Using the prediction of Leverrier, the
German astronomer Johann Gottfriend Galle was the first to actually
observe Neptune, on September 23, 1846.
Neptune
 The Voyager 2 encounter with Neptune on August 25, 1989 was
its last major encounter with a planet before leaving the solar
system.
Dark Spot
Moons of Neptune
 Neptune has 13 known moons, including the major moon Triton,
which is unique among the major moons in the solar system for
its retrograde motion.
Triton as Imaged by Voyager 2
 Like Miranda, Triton revealed a remarkably complex, geologicallyactive surface.
“Canteloupe”
Triton as a Kuiper Belt Object ?
 The unusual orbital and surface properties of Triton have led
some astronomers to suggest that in fact it is a captured Kuiper
Belt Object, like Pluto.
 The most common model suggests that Triton may have had its
own satellite prior to the encounter, which was lost during the
capture. Other “primordial moons” of Neptune would also have
been lost in the process.
T
T
Pluto / Kuiper Belt Objects
 Pluto (now recognized by the International Astronomical Union as
a “dwarf planet” and a Kuiper Belt Object) was discovered by
Clyde Tombaugh in 1931.
The Saga of Planet X
 Neptune had been discovered by Adams and Leverrier by the
careful analysis of unexplained anomalies in the orbit of Uranus.
 By about 1900, the orbit of Neptune began to exhibit apparent
anomalies in its orbit, much like Uranus had nearly in the early
part of the 19th century.
 Some astronomers believed that apparent anomalies in the orbit
of Neptune could be explained by the existence of a ninth, yetundiscovered planet -- Planet X.
Percival Lowell (1855 - 1916)
 Percival Lowell was an astronomer, mathematician, and writer
born to an elite Boston clan. He is perhaps one of the best and
one of the last examples of a Renaissance scholar who funded
his own research efforts and constructed his own observatory.
And Planet X Inspired Bad Science Fiction…
Clyde Tombaugh (1906 - 1997)
 Tombaugh was born on a farm in Streator, Illinois. His family later
moved to Kansas, where he built his own telescope.
 After sending his astronomical drawings to the Lowell observatory
in Arizona, he was offered a position as an observer -- with just a
high school diploma!
 He discovered Pluto in 1930 while searching for the mysterious
“Planet X” which had been predicted by Percival Lowell almost 30
years earlier.
 After discovering Pluto, he went on to attend college and
graduate school!
Clyde Tombaugh with Homemade Telescope -c. 1930
Blink Comparator
 By careful comparison of two photographic plates of the same
portion of the sky taken at different times, Tombaugh found Pluto - initially thought to be the mysterious Planet X.
Pluto Discovery Plates
 After inspection of the discovery plates in January, 1930, it
became clear that Pluto had been seen on other plates taken by
other more senior professional astronomers at Lowell, but
missed.
Pluto’s Orbit
 The orbit of Pluto is highly unusual in comparison to the other
planets in the solar system -- it is both highly eccentric and
inclined with respect to the plane of the ecliptic.
Pluto Compared with the Earth
 Pluto is a tiny world, much smaller than any of the other planets in
the Solar System.
Pluto as Compared to Some Other Major Moons
in Solar System
 Pluto is much smaller even than most major moons in the solar
system -- its mass is equivalent to 0.2 times the mass of our our
moon.
Planet X ?
 Eventually it became clear that Pluto was far too small to account
for the anomalies of the much-larger planet of Neptune.
 The Planet X hypothesis remained for decades to come -- though
now it was hypothesized that Planet X was a tenth planet outside
Pluto.
 It was only years later, in the 1990s, that astronomers finally
understood that the “anomalies” in Neptune’s orbit were actually
the direct consequence of a few early inaccurate measurements
of Neptune’s orbit.
Next Week -- New Planetary Systems, and the
Formation of the Sun and the Planets
 Next week we will discuss the radical new detections of planets
outside our solar system that has emerged in the last ten years -revolutionizing our understanding of how planets form.
 We will also discuss how stars form, and explore some of the
riddles of star formation that scientists are exploring today.