Download Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Announcements
• Homework #3: Class average=39.
• Homework #4: Class average=45.
• Overall course average: 311/400 = 78%
• Mid-term exam#2: Tuesday 11/07/06
• New materials on-line: lectures through today, and ASTR
330 Spring 2004 Mid-Term Exam #2 and solutions.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 19:
Uranus and Neptune
Picture credit: solarviews.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Planets We Cannot See
• All the planets we have discussed so far were and are visible to the unaided eye, and therefore have been known since ancient times.
• Two thousand years after the five ‘wandering stars’ were named by
classical civilizations of the Mediterranean, the first new planet was
discovered: Uranus.
• Uranus led astronomers directly to Neptune, and thence to Pluto: about
one ‘planet’ a century.
• Since the first in 1992, a flood of Edgeworth-Kuiper Belt Objects have
now been discovered culminating in 2003 with the largest to date: Eris,
and object bigger than Pluto.
• But for now let’s look at the discovery of Uranus and Neptune.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Herschel and Uranus
• Uranus was discovered by a professional musician and amateur
astronomer, William Herschel (1738-1822), in Bath, England on the night
of March 13, 1781. (What was happening in North America at this time?)
• Herschel with his 6 in. home-made telescope had noticed a strange
object during his methodical charting of the skies: a star which appeared
circular, rather than point-like.
• Herschel was knighted by King George III for
his discovery, and went on to become a great
astronomer: discovering 2 moons of Saturn and
2 of Uranus, the true nature of binary stars, and
the disk of the Milky Way, plus 1000s of galaxies.
• Herschel wanted to dedicate the star to his
patron, but fortunately was over-ruled. Uranus
was father to Saturn in classical mythology: who
in turn was father to Jupiter – a nice touch.
Picture credit: Royal Astronomical Society
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The quest for planet #8
• Astronomers began to keenly observe the new planet, but soon noticed
that it did not hold to its expected movement on the sky.
• By 1830, Uranus was 0.004 degrees off course: more than 4 times the
apparent size of the planet on the sky.
• The path of Uranus could not at all be fit by an elliptical orbit, a fact
which seemed to defy Newton’s laws (and Kepler’s).
• As there was no reason to expect all planets except Uranus to obey
Newton’s gravity, there must be something else going on.
• The most likely explanation was a unseen, eighth planet having a
gravitational effect on Uranus.
• Using a guessed distance of 39 AU (from numerology) several
mathematicians began the arduous task of computing the eighth planet’s
whereabouts.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
A missed opportunity
• Neptune alone of all the planets was tracked down by mathematics, not
by observation.
• The first person to finish the calculation was John Couch Adams (18191892), in September 1845 not long after his graduation in mathematics
from Cambridge University.
• Adams gave his calculations to the director of
Cambridge Observatory, who didn’t follow
through with the needed observations.
• Adams went on to become a distinguished
mathematical astronomer, and is also renowned
for his calculation that the Leonids meteor shower
was due to the remains of a comet.
Picture credit: St Andrews
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Found At Last
• Meanwhile in Europe, Urbain J J Le Verrier (below left) published his
own calculations in June 1846, and followed up in August that year by
asking the German astronomer Johann Galle in Berlin to look for the new
planet.
• Galle, armed with an accurate set of
celestial tables, was able to find the quarry
within one hour of searching, on the very first
attempt, on September 23, 1846.
• The disk of the planet was too small to be
resolved, but its motion as a wanderer with
respect to the fixed stars was apparent.
• After some discussion, the planet was
eventually named after Neptune, the Roman
god of the ocean, which is well fitted by
Neptune’s color.
Picture credit: St Andrews
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Other Searches
• The successful hunt for Neptune by analyzing the orbit of Uranus led in
turn to a detailed study of the orbit of Neptune.
• Neptune’s orbit too seemed to show perturbations, which inspired
Percival Lowell, the Martian canal sketcher in Arizona, to devote much
energy and telescope time to hunting for planet #9.
• Eventually, a 9th planet was discovered, as a result of Lowell’s
persistent campaigning, and at Lowell Observatory in 1930, Pluto was
first observed. In fact, the discovery was fortuitous, and the supposed
perturbations in the orbits of Neptune and Uranus were never there!
• In France, Le Verrier spent much of the remainder of his life analyzing
perturbations in the orbit of Mercury, in the hope of locating an innermost planet, unseen against the glare of the Sun.
• The Mercurian motions in fact were real, but eventually explained by
Einstein’s General Relativity, not the hypothesized ‘Vulcan’.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Giant planets comparison
• This figure shows the planets to scale. Neptune and Uranus, although
considered gas giants due to density and composition, are really
intermediate worlds in terms of size, between J&S and the terrestrials.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Facts and Figures
• Uranus and Neptune are very similar
sizes: Uranus is slightly larger, but
Neptune is heavier, due to greater
density.
• Their spin period is intermediate
between Jupiter and Saturn which are
about 7 hrs less, and Mars and Earth
which are about 7 hrs more.
• However, Uranus rotates backwards
compared to most other planets, with a
spin axis inclined at 98° to the plane of
the solar system – i.e. it is lying on its
side.
Picture credit: NASA
Uranus Neptune
Diameter (km)
51118
49528
Mass (Earth=1)
14
17
Density (g/cm 3)
1.3
1.6
19.2
30.1
Orbital Period (yrs)
83.75
163.72
Spin Period (hrs)
17.3r
16.1
27
13
Semi-major axis (AU)
Number of Moons
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Density and Composition
• Once again, let us consider planetary density as
an insight into composition.
• Uranus and Neptune are smaller than Jupiter and
Saturn, so if they had the same composition, they
should be less compressed, and have lower
densities.
• In fact, Uranus has the same density as Jupiter,
but Neptune is even higher. This tells us straight
away that Uranus and Neptune are likely to contain
greater proportions of heavy elements than Jupiter
and Saturn.
• We believe that much of these planets is water ice
and rock (oxygen and silicon being common
elements). They are surrounded by a relatively thin
layer of liquid and gaseous hydrogen, mixed with
some other gases.
Picture credit: Kaufmann and Comins
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Interior Comparison
• Model interiors of Neptune and Uranus are depicted below, in
comparison with Saturn and Jupiter.
• Note that neither Uranus or Neptune is massive enough to have a
liquid metallic hydrogen layer, unlike their larger counterparts.
Picture credit: Bennett et al
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Appearance: Voyager 2
• The Voyager 2 passes of Uranus (1986) and Neptune (1989) were the
first detailed views we had of these worlds. Uranus was proved to be
featureless at first glance, whereas Neptune had visible spots like Jupiter.
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheres
• Both planets appear green to blue in color. The high, white clouds we
see on Saturn and Jupiter are mostly absent.
• Therefore, sunlight penetrates further into the atmosphere, before
being back-scattered to space. Absorption by methane gives the
characteristic blue-green color.
• The spectra of both planets show much stronger methane features than
at J&S as expected: we expect a higher proportion of heavy elements
and ices.
• Hence, N&U are depleted in hydrogen and helium relative to J&S. The
molar gas abundances are about: H2 (84%), He (14%) and CH4 (2%) for
both N&U.
• The He/H ratio is more similar to the Sun for these worlds than for J&S:
the lack of a metallic hydrogen core means that the helium ‘rain’ effect
does not exist, to deplete the He concentration.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Internal Heat Source
• At Jupiter and Saturn, we found very substantial heat excesses: the fact
that the planets were radiating much more heat in the infrared than they
were absorbing in the visible.
• This led us to the conclusion that Jupiter and Saturn are generating
heat internally somehow: which turned out to be from helium
precipitation.
• What about Uranus and Neptune? In fact, Uranus has no heat excess,
but Neptune, further from the Sun, does have an excess. This was
noticed when the two planets showed the same temperature at 25
micron wavelength in the infrared, when Neptune should be colder.
• The conclusion is that Neptune, being slightly larger, is still radiating
primordial heat from its formation. The high proportion of rock and ice to
overall mass has led to a very slow cooling.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Planetesimals and Formation
•
The hydrogen-helium atmospheres of N&U are the natural product
of the two-stage accretion process we have discussed before.
1. The core forms, of ice and rock, about 10-15 Earth masses.
2. Hydrogen and helium are captured as a secondary
atmosphere, from the remaining gases in the solar nebula.
• At the same time as the capture (2), there is also outgassing from
the core, mostly of N2, CO and CH4. However, the amounts are
much smaller than the H2 and He captured, so the atmospheres
will be dominated by these lighter gases.
• Apparently, Neptune and Uranus were able to attract much less H2
and He than Jupiter and Saturn.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheric temperatures
• The atmospheres of N&U are much colder than J&S: about 73K (-200°C)
at the 1 bar level, so ammonia and water are completely frozen out. Also
note that Neptune has a much stronger temperature inversion.
Picture credit: Eric Weisstein
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Composition and Spectroscopy
• On planets with a tropopause – a temperature inversion between the
troposphere and stratosphere - we see emission lines of various gases
in the infrared: methane and ethane for example.
• Neptune does have such an inversion, and hence allows detection of
these gases spectroscopically. But Uranus lacks a significant inversion,
and its IR spectrum is essentially blank.
• Note that the tropopause regions of both planets are so cold (55 K) that
only H2, He and Ne will not condense, although CO, N2 and CH4 can
remain partially in the vapor state.
• All these gases have been detected on Neptune so far except Ne
(which is hard to detect), and additionally HCN has been found.
• It is interesting that nitrogen and carbon are found as N2 and CO: i.e.
they are not completely hydrogenated to NH3 and CH4, as on J&S. A lack
of suitable catalyst for the reactions is probably to blame.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Radio Temperatures
• At Uranus and Neptune, we do not see the decametric (10s of meters) or
decimetric (10s of cm) wavelength non-thermal radio emission which
Jupiter and Saturn produce.
• However, at shorter wavelengths we are able to probe the atmosphere at
depth, and measure the temperature. As the wavelength increases, we
see deeper into the atmosphere, so we can build up an idea of the
temperature profile.
• Normally, atmospheres get warmer as we go deeper into the
troposphere. This is true of Jupiter, Saturn and Neptune as confirmed by
radio observations.
• But Uranus does not show an increasing temperature with depth. This is
attributed to the fact that Uranus does not have an internal energy source,
unlike the other three worlds, and so there is no significant convection
taking place. This is comparable to the deep oceans on Earth.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
General Circulation of the Atmospheres
• On Neptune, rotating with a axial tilt of 27° similar to the Earth, seasons
occur as on Earth, except 165 times as long!
• But what about Uranus, rotating on its side? For 42 years it has one
pole sunlit, and then for 42 years the other pole is sunlit.
• We expect some sort of global Hadley cell to arise, with warm gas
rising at one pole and streaming to the other pole.
• However, this type of
circulation does not in fact
arise. The rapid rotation of
Uranus dominates the global
circulation, and so Uranus
exhibits a banded pattern
parallel to latitude lines, like
Saturn and Jupiter.
Picture credit: NASA. Voyager 2 false color image showing banding.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Uranus Clouds
• This false color image
of Uranus was taken in
1998 with the HST in
three near-infrared
wavelengths.
• The orange-colored
clouds near the bright
band circle the planet at
about 500 km/h.
• The rings of Uranus
and 10 satellites are
also visible.
Picture credit: HST/Arizona
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Weather On Neptune
• Unlike Uranus, Neptune has welldefined high altitude white clouds.
• Neptune also has dark clouds
which mark the lower limit of the
visible atmosphere.
• The Great Dark Spot (upper right)
was a huge eddy the size of the
Earth, and similar to Jupiter’s GRS.
• As with the GRS, the GDS is a
southern hemisphere anticyclone,
with counterclockwise winds
blowing around a high-pressure
region. Recent images show that
the GDS has disappeared.
Picture credit: NASA/HST-APL/Nanjing Univ.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Neptune cloud compositions
• The white clouds are probably some form of ice crystals,
most likely methane, which is the main volatile in the
atmosphere.
• Clouds of ice crystals are called cirrus clouds.
• The composition of the dark lower clouds is even less
certain: possibly methane droplets, or H2S ice crystals.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Winds
•
To measure wind speeds, again we need to know two things:
1. How long a cloud feature takes to circle the planet.
2. The System III rotation period, by measuring the magnetic field, which
gives us the rotation speed of the planet interior.
•
For example, clouds on Uranus were observed to circle in 16 hours, and the
System III period was measured to be 17.2 hours. Taking the difference gives
us a measurement of how fast the clouds are moving relative to the interior.
•
Figure 14.11 in the textbook shows the variation of wind speeds with latitude
on Uranus and Neptune.
•
Note that Voyager measurements indicate that both the illuminated and
unilluminated poles are at the same temperature, showing that heat is rapidly
redistributed around the planet.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Magnetic Field of Uranus
• Our initial expectation, from our experience elsewhere in the solar
system, was that magnetic fields are generally aligned close to the
rotation axis of the planet.
• However, at Uranus,
the magnetic field
was found not only to
be inclined at 60° to
the rotation axis, but
also to be offset from
the rotational axis by
one third of the
planet’s radius.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Magnetosphere
• Uranus’s magnetosphere is similar in size to Saturn’s, but simpler in
composition. It is composed almost entirely of electrons and protons
derived from hydrogen escaping from the planet.
• The magnetotail stretches out 10s of planetary radii behind the planet,
and also rotates like a corkscrew due to the inclination between
magnetic and rotation axes.
• On the sunlit side of Uranus, there is an ultraviolet glow (the
‘electroglow’) emitted by escaping hydrogen atoms.
• Uranus also has aurorae like the other planets, produced by the
collision of magnetospheric electrons and ions with the upper
atmosphere.
• Due to the inclination difference, the aurorae occur near the equator.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Neptune: magnetic field
• If we thought that perhaps Uranus’s offset and inclined magnetic field
was perhaps due to the planet’s own inclined rotation axis, we would be
wrong. The magnetic field of Neptune is in fact quite similar to Uranus.
• Neptune’s
magnetic field is
offset from center
by half the planet’s
radius, and
inclined at 47° to
the rotation axis,
with a field
strength about half
that of Uranus.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Magnetic Field: Origins
• What causes these strange magnetic fields? We believe that an
electrically conducting fluid is required, but it cannot be molten rock
(as on the Earth) or liquid metallic hydrogen (as in Saturn and
Jupiter).
• Our best guess for the conducting fluid is some sort of pressureionized ‘ice’: compounds of C, H, O and N ionized by high
pressures.
• This could perhaps also explain the offset of the fields from the
planet centers: as we expect the molten ice layer to be outside the
rocky cores.
• Much more research is needed to obtain a better understanding of
these fields and the planet interiors.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Magnetic Fields: Comparison
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Discover Of Pluto and Charon
• Pluto was discovered on February 18th 1930 at
Lowell Observatory by Clyde Tombaugh, a young
Kansan.
• It was named for the Greek god of the
underworld, but coincidently, the first letters also
honor Percival Lowell who first pursued it.
• Pluto’s moon Charon was discovered in 1978,
and named after the boatman who conveyed the
dead across the Styx and into Hades.
• Charon is 1/8 the mass and 1/2 the diameter of
its parent. Due this closeness in size, Pluto and
Charon are sometimes considered to be a double
or binary planet system.
Picture credit: Univ. Northern Iowa
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pluto
• Very, very little is known about Pluto, due to its distance from the
Sun (semi-major axis 39.48 AU), small size (2302 km diameter, 2/3
the size of the Moon), and the fact that no spacecraft has visited it.
• Pluto has other anomalies. It rotates in about 6.4 Earth days,
longer than all planets except Venus and Mercury. Also, its elliptical
orbit crosses Neptune’s.
• Pluto’s axial inclination of 112° also means that it rotates
backwards, like Venus and Uranus.
• Was Pluto originally a moon of Neptune which somehow escaped?
In fact, Pluto and Neptune are in a resonance which prevents them
from getting closer than 17 AU, making this possibility unlikely.
• Also, the fact that Pluto has its own moon argues for an
independent formation.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pluto-Charon System
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pluto-Charon System contd
• From 1985 to 1991 Pluto and Charon lined up as an eclipsing binary
system, as seen from Earth. This enabled us to better determine their
masses and sizes.
• Charon orbits Pluto at just 20,000 km, and both planets are tidally
locked, presently the same face to each other at all times.
• Pluto’s mass was
uncertain until Charon was
discovered, when Kepler’s
laws could be applied.
• We now know that its
density is 2.1 g/cm3,
similar to Neptune’s moon
Triton.
Picture credit: NASA/APL/HST
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pluto-Charon Surface and Atmosphere
• Pluto’s brightness was observed to change as it rotates, from about
0.3 to 0.5 in reflectivity.
• Infrared spectroscopy showed first the presence of methane ice, and
then CO and N2 ices as well, on Pluto’s surface.
• Charon is different, covered in water ice. Perhaps the energy of its
formation event was sufficient to drive off more volatile gases.
• Pluto’s atmosphere was first observed in 1988, as a dimming before
disappearance during a stellar occultation.
• Calculations suggest that the atmosphere is probably 1-20x10-6 bar of
N2, at a surface temperature of 35-40 K.
• Due to Pluto’s eccentric orbit around the Sun (30-50 AU), the amount
of solar heating changes by a factor 3 over its year, and hence the
atmosphere will soon grow much colder and freeze out on the surface.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
1. Briefly describe how Neptune, Uranus and Pluto were discovered.
Which of the three was not found by accident?
2. What are the similarities and differences between Uranus and
Neptune, in terms of mass, size, rotation and orbit?
3. Are the interiors of Uranus and Neptune similar to Jupiter and Saturn?
4. Which of the two outer gas giants has more visible features? Which
have internal heat sources?
5. What is the main difference in the formation of Uranus and Neptune
which led to a different composition from Saturn and Jupiter?
6. What gases are found in the atmospheres of U&N apart from H2 and
He: why are they not fully hydrogenated?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
7. Is the GDS on Neptune similar to the GRS on Jupiter? In what ways?
8. Which planet does not have a pronounced tropopause? Why?
9. Describe the magnetic fields of Uranus and Neptune. In what ways
were they unexpected?
10. What could cause these magnetic fields?
11. Describe the orbital properties of the Pluto-Charon system.
12. What sort of surface and atmosphere might we expect to find on these
very outer worlds (P&Ch).
13. Which planets rotate ‘backwards’ relative to most of the solar system?
Dr Conor Nixon Fall 2006