Transcript ASTR 330: The Solar System

ASTR 330: The Solar System
Announcements
•
Dr Conor Nixon Spring 2004
ASTR 330: The Solar System
Lecture 19.5:
Review #2
For
Mid-term exam #2
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 12: The Moon
• Seen from the Earth, visible hemisphere has dark ‘seas’ (maria).
Map: Jim Loy
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 12: Lunar Characteristics
• The Moon has never had liquid water, but the water names persist: e.g.
mare (sea), oceanus (ocean), lacus (lake), sinus (bay) and swamp
(pallus).
• The ‘land’ features also have latin names: planitia (plain), vallis (valley),
dorsa (ridge), mons (mountain), and catena (crater chain).
• Physical:
Diameter around 3500 km
Mass 1% of Earth.
Density is 3.3 g/cm3 - 2/3 of Earth (no iron core).
Albedo: 8% (maria) 15% (bright highlands).
• Craters originally though to be volcanoes. Evidence in favor: (1)
volcanoes known on Earth; (2) no large meteorites known; (3)
volcanoes have craters.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 12: Craters
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End of 19th century: impact theory begins to gain acceptance. Main
argument: craters lower than surrounding plains.
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Problem with impacts is no oval craters, as expected from oblique
impacts. Finally resolved when it was realized that impacts cause
explosion.
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Crater formation:
1. Impact, meteorite gouges hole, heats up and vaporizes.
2. Ejection of debris into circular ‘blanket’ around impact.
3. Later re-adjustment: slumping, in-filling by lava, uplift of central peak etc.
•
Different crater sizes have different characteristics:
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Simple craters (<15 km). Bowl-shaped depressions with wellformed sides, and depth to diameter ratios of 20%.
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Complex (>20 km). Have central uplifts, flat floors, slump blocks,
and terraces.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 12: Craters and Crust
• In addition, complex craters:
• 20-175 km typically have central peaks.
• 175-300 km have ring-shaped central uplifts.
• >300 km are called impact basins: affect regional geology.
• Stratigraphy gives relative time information: e.g. Imbrium Basin and
associated Fra Mauro material.
• Crater densities between highland and lowland do not give good age
estimates.
• Crater retention age: time since last lava flow or crater re-covering.
• The primary material of the highland crust are igneous silicate rocks
called anorthosites: contain oxides of Si, Al, Ca, and Mg, but are depleted
in lava volatiles: elements which can be evaporated below 1000 C
including N, C, S, Cl and K, and of course water. Anorthosites tell us that
the Moon differentiated early on, before the crust solidified.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 13: Tides
• Tides are caused by differential gravity, the fact that the side of a planet
towards a companion is more strongly pulled than the side away from the
companion.
• The Earth has two tides per day, caused by the Moon. On a frictionless
Earth, one tidal bulge would be on the side towards the Moon and one on
the opposite side.
• However, friction causes the tides to be delayed somewhat from their
expected position.
• The friction of the tides causes the Earth to slow down its rotation.
• By conservation of angular momentum, the Moon recedes from the Earth
over time.
• Tides of the Earth on the Moon have caused it to slow its spinning so
that it is now face-locked to the Earth.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L13: Rotation of
Mercury
• Mercury orbits the Sun in 88 days,
but takes only 59 days to spin once
on its axis, a 3:2 ratio. Why?
• Tidal forces from the Sun have acted
on Mercury, attempting to force it into
synchronous rotation.
• Mercury’s elliptical orbit made it
impossible to settle into synchronous
rotation for the whole orbit, so it spins
as though it was face-locked at
perihelion (closest approach to Sun).
• Its solar day is 176 days - 2 ‘years’.
Figure credit: ESO Education and Outreach Office
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L13: Geology of Mercury
• Mercury has been explored by just one spacecraft in detail: Mariner 10.
(Why is it difficult to observe Mercury well from the Earth?).
• The surface of Mercury resembles the lunar highlands, but there is no
parallel to the lunar maria. Hence, Mercury is brighter than the Moon.
• Polar ices have been detected on both Mercury and the Moon. These
ices must have come from cometary impacts since formation.
• Caloris is the only large impact basin on Mercury, and is not filled with
lava like the lunar basins. Why is it called Caloris?
• Mercury does not show volcanic features (flow fronts, sinuous rilles
etc). The most distinctive geological features are the compression
scarps, due to contraction of the planet.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L13: Formation of Moon and Mercury
• Densities: Earth 5.5, Moon 3.3, Mercury 5.4 g/cm3. Uncompressed
densities would be: Earth 4.5, Moon 3.3 and Mercury 5.2 g/cm3. Mercury
therefore has the highest proportion of iron and nickel.
• The Moon is very depleted in silicates compared to the Earth, why?
• Three traditional scenarios of lunar formation: (i) ‘Daughter’ (ii) ‘Sister’,
(iii) ‘Capture’. None of these explain the data.
• Currently preferred theory is the ‘Giant
Impact’ of a Mars-sized body: which
explains oxygen isotope ratios, volatile
inventories, lunar depletion of heavy
elements.
• Mercury may have formed from a giant
impact which left only the core of an
originally differentiated larger planet.
Figure: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L14: Above the Earth’s Surface
1. MAGNETOSPHERE: region
of charged particles above
the atmosphere, from 200 km
to 100,000 km above the
surface.
2. ATMOSPHERE: gas layer,
from surface to 200 km
altitude. Composed of 78%
N2, 21% O2, 1% Ar, plus
variable H20, CO2.
Atmosphere is divided into:
•
•
•
•
Troposphere
Stratosphere
Mesosphere
Thermosphere.
Figure credit: Michael Pidwirny, Okanagan Univ. Coll.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L14: Greenhouse effect
• The atmosphere is mostly
transparent to visible sunlight.
• When sunlight strikes the
surface, it is absorbed, and reemitted again as heat: infrared
radiation.
• However, the atmosphere is
much more opaque to IR light:
it can’t very easily escape. It
tends to get trapped in the
atmosphere.
• The planet warms up more
than it would have without an
atmosphere: this is called the
‘greenhouse effect’.
Figure credit: IPCC 1990
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L14: Earth Surface
3. HYDROSPHERE: or ocean, including ice caps (seasonally varying),
which covers 5/8 of the surface.
• Earth is the only planet with surface liquid H2O.
• Oceans contain salts (NaCl, KCl), dissolved CO2, O2 gases.
• Calcium is removed by the formation of limestone.
4. CRUST: the solid surface of the Earth. Comes in two types:
a) Oceanic crust: thinner (6 km) and younger (~100 Myr) type.
Formed at rifts (e.g. mid-Atlantic ridge). Almost entirely igneous,
mainly basalts. Some parallels with lunar maria.
b) Continental crust: thicker (20-30 km) and older (up to 3.8 Gyr).
Composed of igneous rocks such as granites which formed at
depth. Also sedimentary, metamorphic rocks.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L14: Summary of Tectonics
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L14: Earth Interior
• We can probe the interior structure using seismology: P (compression)
and S (shear) waves tell us about liquid and solid layers.
5. MANTLE: the solid but plastic rock layer extending to 2900 km below
the surface. Three regions exist:
a) Upper mantle: 100 km thick, attached to crust (both are called the
lithosphere). Moves over the middle mantle as plates move.
b) Middle or convective mantle: 100-700 km from surface. Heat
transport from core by convection. Same composition, different
mechanical properties to upper mantle.
c) Lower mantle: solid, no convection.
6. CORE: metal sphere, divided into outer (liquid) and inner (solid).
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The core is different composition from the mantle: Fe (S, Ni) rather
than rock (Si, O). Millions of bars pressure, 1000s of degrees K.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L14: Earth Surface Processes
• Surface processes include erosion, chemical weathering,
sedimentation, and impacts (e.g. Tunguska, Chicxulub).
• Volcanism: occurs when heat within the mantle drives magma through
the lithosphere to the surface, where it may erupt in several ways:
• Lava plains (e.g. Deccan Plains, India) - vast outflows similar to the
lunar maria.
• Volcanoes. If the lava is low viscosity, get shield volcano (e.g.
Mauna Loa), if more viscous, get composite (e.g. Mt. St. Helens).
• Plate tectonics: deduced from the evidence of continental drift.
• In the past, the continents were all together (Pangaea).
• When plates collide we get boundaries such as: rifts, subduction
zones, faults and folds.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L15: Atmosphere of Venus
• Venus is similar size to the
Earth, but completely
shrouded in thick clouds.
• Atmosphere suffers a
runaway greenhouse effect,
trapping heat and raising
the surface temperature to
750 K, 90 bars.
• Atmosphere is 96.5% CO2
and 3.5% N2. Clouds
composed of sulfuric acid.
• Unlike Earth, Venus has
lost most of its water.
Figure credit: Greg Holmes, Embry-Riddle Univ.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L15: Venus Surface
• Pioneer Venus surface radar map shows ‘continents’ (highlands, 10%)
and ‘oceans’ (lowlands, 90%) similar to other terrestrial planets.
• Equatorial continent is Aphrodite, northern continent is Ishtar.
• We see craters, but no large impact basins. Must have been tectonic
activity since LHB period.
• Soviet Venera 15 & 16
improved mapping down to 2
km (1983).
• Magellan spacecraft (1990)
improved mapping still further,
down to 100 m resolution:
good enough to resolve
craters in detail.
Figure credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L15: Cratering and Topography
• Venus has no small craters: massive atmosphere shielded from objects
smaller than 500 m in size.
• Crater floors are smooth; craters are surrounded by outflows. Impacts
probably caused surface melting and lava flows.
• Craters do not erode: remain ‘fresh’ until wiped out suddenly.
• Lowlands are about 500 Myr old,
from crater densities - younger than
lunar maria. Highlands younger still.
• Tectonics (compression) have
created highlands such Lakhshmi, with
11 km mountains. However, no plate
movements like the Earth.
Figure credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L15: Volcanism on Venus
•
Major types are:
1. Lava plains: cover 80% of surface. Appear to have been created
in a massive outpouring event 500 Myr ago.
2. Volcanoes: shield type, e.g. Sapas Mons (below right).
3. Pancake Domes: eruptions of viscous lava (below left).
4. Coronae: concentric circular cracks, with central dome: probably
where a magma plume has failed to break the surface.
Figure credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L16: Martian Surface Features
• The surface of Mars was
first explored in detail by
Mariner 9, which discovered:
• The huge canyon
system now known as
the Valles Marineris.
• Impact basins, such as
Hellas.
• The 3 great Tharsis
volcanoes, and the
Olympus Mons, which
stands alone.
Figure credit: NASA/USGS
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L16: Mars Interior and Surface
• The density of Mars (3.8 g/cm3), is less
than the Earth, so we expect a lower iron
to silicate ratio.
• Mars has an iron core (solid) but mixed
with sulfur: lowering the density compared
to the Earth’s Fe-Ni core.
• We also expect no magnetic field. Why?
• There is in fact a small magnetic field, due to rocks which were
magnetized in the past.
• The surface also can be divided into northern plains and southern
highlands. The north is younger, lower, smoother and brighter.
• At the boundary, is the Tharsis uplift, a bulge the size of North America.
Figure credit: Albert T Hsui, Univ. Ill
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L16: Canyons and Craters
• Canyons, such as the Candor
Chasma (right, part of the Val. Mar.)
were produced originally by tectonics.
• Later widening occurred by water
erosion: undercutting and sapping
causing landslides.
• Impact craters are seen, with similar
shapes to Mercury and the Moon.
• The ejecta blankets are quite different
however, showing a fluid boundary (left).
• We believe this indicates that crustal ices
were melted and the surface became liquid.
Figure credit: NASA/JPL. NASA ARC/CMEX
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L16: Water on Mars
• There are three distinct types of water features (not including the
canyons, which are tectonic in origin):
• Run-off channels: (right): think of
rainwater running down the side of the road
after a storm. Very old: 4 Gyr on Mars.
• Outflow channels: think of floods. Much
bigger than run-off channels: 10 km wide.
Probably due to natural dam bursts. Show
teardrop islands, terraces, sandbars.
• Gullies: evidence for present day liquid water on Mars, at least for
brief periods. Seen in crater walls etc, these tiny channels are probably
carved by sub-surface ice deposits becoming heated (by magma) and
then releasing explosively in an outburst.
Image credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L17: Martian Atmosphere
• The atmosphere is thin compared to the
Earth’s: around 0.008 bar (8 millibar) and
largely composed of CO2 (95%), like Venus.
Most of the remainder is N2 (3%), Ar (1.5%).
• Temperatures range from a high of -30°C to a
low of -100°C. Pressure may vary by 20% (like
going to the top of high mountain on Earth).
• Massive wind-blown dust storms occur which
last for months, covering the entire planet.
• Sand dunes show wind-blown ripples: may
cover and uncover dark areas (originally
thought to be seasonal vegetation).
Figure credit: MSSS/NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L18: Visible Appearance of J&S
• Jupiter’s
appearance
shows
pronounced
zones
(bright) and
belts (dark).
• Saturn’s bands are relatively
less pronounced in visible
images, than Jupiter. In this
image, different near-infrared
wavelengths show the banding.
Figure credit: James Schombert, U. Oregon; NASA Voyager 1; HST/Arizona
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L18: Hydrogen on Gas Giants
•At the very outer layers, Jupiter has a
hydrogen gaseous atmosphere of H2
molecules – very thin: not shown here.
•Below this, the outer 25% by radius of
Jupiter is liquid molecular hydrogen (H2).
•Most of the inner 75% is hydrogen metal,
a liquid state where the single electron
and proton are separated, and is hence
electrically conducting.
•In the very core resides some denser
compounds, made of silicon, oxygen,
heavy volatiles and metals.
•Saturn is similar to Jupiter, but the
metallic part is a smaller fraction of the
planet.
Picture credit: Kaufmann and Comins (book)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L18: Winds on J&S
• Winds are measured relative to the ‘System III’ rotation of the planets;
the rotation period of the radio emission. Right is Saturn. Both planets
show strong eastward equatorial winds, and reversals with latitude.
Picture credit: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L18: Cloud Layers
• Both giant planets are expected to have the same three cloud layers
(NH3, NH4SH and H2O), although Saturn’s atmosphere is colder, with
thicker, more opaque clouds.
Figure: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L18: Jovian Banding
• Darker belts on Jupiter are hence seen as cloud-free areas of down
welling (bright in IR), whereas zones are cloudy regions of upwelling.
• The Galileo probe seems to have gone through an unusually dry, cloudfree region: an infrared hot-spot.
Picture credit: Nanjing University; NASA Galileo
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L19: Neptune, Uranus & Pluto
• The outermost three planets were discovered in the last 250 years.
Uranus (1781) and Pluto (1930) by observation, Neptune (1846) by
mathematics.
• Uranus and Neptune are called gas giants, but really are halfway in
size between the terrestrials and the true giants.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L19: Interiors of U&N
• Uranus is slightly larger than Neptune, while
Neptune is somewhat more massive.
• Both rotate in similar periods, although Uranus at
a 98° tilt while Neptune is a more ‘normal’ 27º.
Neptune takes twice as long to orbit the Sun, being
50% further away than Uranus.
• Both have low densities, thought not so low as we
would expect if they were composed of H, He.
• Therefore they have a substantial rocky core,
surrounded by a mantle of condensed volatiles
(mainly H2O), and finally some liquid H2 and He.
• Neither has a liquid metallic hydrogen layer.
Picture credit: Kaufmann and Comins
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
L19: Atmospheres of U&N
• The atmospheres are colder than J&S, and have a higher proportion of
‘heavy’ elements compared to H and He. There is no helium ‘rain’ effect,
and the He/H ratio is more like the Sun, than for J&S.
• N2, CO and CH4 are the principal minor species in the atmosphere of
Neptune, which also has small amounts of cloud, probably CH4.
• The atmospheres are therefore much more transparent than J&S, so
we see deeper. The blue color comes from methane absorption of red
light.
• The incomplete reduction of CO and N2 to CH4 and NH3 requires
explaining: it may be that there is a lack of a suitable catalyst.
• Uranus does not have a pronounced tropopause. Also, the temperature
does not appear to increase uniformly with depth, probably due to lack o
convection. (Remember: Uranus has no internal heat source).
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
L19: Pluto and Charon
• The outermost ‘double planet’ system is dim and distant, and no
spacecraft has yet visited the pair.
• Pluto is 2300 km across, and Charon half that size. Both are more
typically the size of large moons.
• Pluto rotates at inclination
112º: similar to Uranus. Its
elliptical orbit crosses that of
Neptune.
• Charon and Pluto both face
each other at all times, tidally
face-locked.
• Pluto shows surface ices of
CH4, N2 and CO, while Charon
has H2O ice.
Picture credit: NASA
Dr Conor Nixon Fall 2006