ASTR 330: The Solar System

ASTR 330: The Solar System

Announcements

Dr Conor Nixon Spring 2004

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Lecture 19.5:

Review #2For

Mid-term exam #2

Dr Conor Nixon Fall 2006


Lecture 12: The Moon


• Seen from the Earth, visible hemisphere has dark ‘seas’ (maria).

Map: Jim Loy


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 kmMass 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.


Lecture 12: Craters


• End of 19th century: impact theory begins to gain acceptance. Main argument: craters lower than surrounding plains.

• Problem with impacts is no oval craters, as expected from oblique impacts. Finally resolved when it was realized that impacts cause explosion.

• 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:

• Simple craters (<15 km). Bowl-shaped depressions with well-formed sides, and depth to diameter ratios of 20%.

• Complex (>20 km). Have central uplifts, flat floors, slump blocks, and terraces.


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.


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.


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


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.


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.

Figure: Nanjing University

• 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.


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.


L14: Greenhouse effect

Dr Conor Nixon Fall 2006Figure credit: IPCC 1990

• The atmosphere is mostly transparent to visible sunlight.

• When sunlight strikes the surface, it is absorbed, and re-emitted 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’.


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.


L14: Summary of Tectonics



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).

• The core is different composition from the mantle: Fe (S, Ni) rather than rock (Si, O). Millions of bars pressure, 1000s of degrees K.


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.


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.


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.

Figure credit: NASA/NSSDC

• 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.


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.

Figure credit: NASA/JPL

• 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.


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


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


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?

Figure credit: Albert T Hsui, Univ. Ill

• 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.


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.

Figure credit: NASA/JPL. NASA ARC/CMEX

• 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.


L16: Water on Mars


• There are three distinct types of water features (not including the canyons, which are tectonic in origin):

Image credit: NASA/JPL

• 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.


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


L18: Visible Appearance of J&S


• Jupiter’s appearance shows pronounced zones (bright) and belts (dark).

Figure credit: James Schombert, U. Oregon; NASA Voyager 1; HST/Arizona

• Saturn’s bands are relatively less pronounced in visible images, than Jupiter. In this image, different near-infrared wavelengths show the banding.


L18: Hydrogen on Gas Giants

Dr Conor Nixon Fall 2006Picture credit: Kaufmann and Comins (book)

•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.


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


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


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, cloud-free region: an infrared hot-spot.

Picture credit: Nanjing University; NASA Galileo


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


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


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).


Magnetic Fields: Comparison

Dr Conor Nixon Fall 2006Picture credit: NASA


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.

Picture credit: NASA

• 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.

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ASTR 330: The Solar System

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