ASTR 330: The Solar System Lecture 10.5: Review 1 for Mid-Term Exam 1 Dr Conor Nixon Fall 06

ASTR 330: The Solar System

Lecture 10.5:

Review 1

for

Mid-Term Exam 1

Dr Conor Nixon Fall 06


Review 1: What have we learned?


• Lecture 1: Introduction to course: dates, times, advice, rules etc.

• Lecture 2: Our place in space: planetary motion, eclipses.

• Lecture 3: The Sun: nuclear energy, solar lines, solar activity.

• Lecture 4: The planets: densities, atmospheres, spectroscopy.

• Lecture 5: Spectral windows, albedos, telescopes, future technology.

• Lecture 6: Solar System Formation: nebula collapse, planetesimals.

• Lecture 7 & 8: Meteorites. Types, radioactivity, half-life.

• Lecture 9: Asteroids:

• Lecture 10: Comets


Lecture 2: History and Perspective


• Early ideas about the planets: “wandering stars”.

• The ecliptic: path of the Sun across the sky, through the zodiac.

Picture credit: wikipedia.org

• The seasons; equinoxes, solstices.

• Phases of the Moon, and what causes them; eclipses.

• Geocentric universe; Aristotle.

• Retrograde motion of the planets; epicycles, Ptolemy.

• Heliocentric universe; Copernicus.


Lecture 2: Eclipses


• What is a total solar eclipse?

•Terminology:

• Umbra and penumbra

• Totality

• Bailey’s beads

• Diamond ring

• What is a lunar eclipse?

• Kepler’s three laws of planetary motion:

1. Law of Orbits: Each planet moves in an elliptical orbit about the sun, with the Sun at one focus of the ellipse.

2. Law of Areas: An imaginary line connecting the Sun with a planet sweeps out equal areas in equal times as the planet moves about the sun.

3. The Law of Periods: The square of the period of any planet is proportional to the cube of the semi-major axis of orbit. T2 = K a3

• Galileo’s discoveries.

• Newton’s ideas on gravity: inverse square law of universal gravitation.

• Escape velocity.


Lecture 2: Laws of Planetary Motion


221

R

MGMF =


Lecture 3: Definitions


• Basic definitions: parallax, Kelvin temperature scale (=celsius +273).

• Matter:

• Atoms and elements.

• Atomic structure: the nucleus; protons and neutrons; electrons.

• Hydrogen and Helium.

• Molecules and compounds.

• Light: photons, waves and particles, speed of light, mass of light.

• EM spectrum: types of EM radiation, from Radio to Gamma.


Lecture 3: The Sun


• Parts of the Sun:

• Core: where nuclear fusion occurs; temperature 15 million K. Energy is released as gamma radiation.

• Radiation zone: the next layer of the Sun out from the core: gamma ray photons are repeatedly absorbed and re-emitted.

• Convection zone: the level immediately below the visible surface; globs of solar liquid rise and fall by convection, transporting energy.

• Photosphere: the visible surface of the Sun: 5800 K.

• Chromosphere: the layer immediately above the photosphere: visible during eclipses as a pink color.

• Corona: the very outer part of the Sun, merging with the solar wind. Very hot 4 million K. Gas in ionized.


Lecture 3: Solar Lifetime & Activity


• The Sun consumes 4x106 tons of matter/second, converting H to He with a small fraction lost to pure energy. This is the Sun’s power source.

• Einstein’s formula: E = mc2 gives the relation between a small amount of mass and a large amount of energy. The fuel source will last an estimated 10 billion years or more.

• The Sun exhibits several types of activity:

• Sunspots: ‘dark’ patches on the photosphere, which are in fact only 1000 K cooler than the rest of the photosphere. Caused by material trapped in magnetic fields. 11-year cycle. Migrate to equator.

• Flares: huge eruptions from the surface, associated with the breaking of magnetic field lines. Energy is released from radio to X-ray.

• CMEs: huge ejections of matter from the corona into the solar wind, which are related to solar flares.


Lecture 4: The Planets


• Sizes and densities: we categorize the planets into 3 groups:

1. Terrestrial: small size, high density: Mercury, Venus, Earth, Moon, Mars.

2. Giant: large size, low density: Jupiter, Saturn, Neptune, Uranus.

3. Icy/KBO: small size, low density: Pluto and Charon, KBOs.

• Composition clues:

• Mercury is same density as Earth, should be less (smaller gravity) if same protosolar proportions of the elements.

• Giant planets are less dense than Earth, should be more (more gravity).

• We solve this riddle by concluding that planets have different compositions. Closer to the sun, more heavy elements.


Lecture 4: Forms of Matter


• For the planets, it is sometimes useful to think of four principle components: gas, ice, rock and metal.

• Rocks and minerals:

• A rock is an assembly of one or (usually) more different minerals.

• Minerals may be elemental (e.g. Gold, Graphite) or compound (SiO2).

• Rock types: igneous (e.g. basalt), sedimentary (e.g. limestone), metamorphic (e.g. marble).

• Atmospheres: ways for a planet to acquire an atmosphere:

1. Direct capture from original solar nebula (priordial or primary).

2. Outgassed from rocks after differentiation (secondary).

3. From later cometary impacts (secondary).


Lecture 4: Atmospheres


• Ways to lose an atmosphere:

1. Thermal escape from exosphere: the thermal energy of the molecule is enough to surpass the escape velocity of the planet.

2. Impacts: comet or asteroid impacts may blast atmosphere into space.

3. Ablation by solar wind particles.

• What affects which molecules escape? (i) mass of planet (ii) mass of molecule (iii) temperature.


Lecture 5: Planetary Astronomy


• Remote sensing vs in situ investigations.

• Spectroscopy: visible (solar) and infrared (planetary).

• Spectral windows: absorption and transmission in the Earth’s atmosphere.

• Meaning of albedo: percentage of light reflected.

• Infrared spectral lines: give information on:

Temperature Composition Pressure


Lecture 5: Telescopes


• Telescopes:

Size - affects both spatial resolution, and amount of light collected, (or faintness of objects which can be detected).

Sites - high and dry sites are best: Antartica, Atacama desert in Chile, Mauna Kea in Hawaii, space!

• Future techniques and technology:

Segmented giant mirrors (e.g. Keck I and II). Adaptive optics, to correct for atmospheric twinkling. Optical interferometry, to improve spatial resolution dramtically

without having to build an impractically large single mirror. Space telescopes - best ‘seeing’ of all sites.


Lecture 6: Solar System Formation


• Facts we can use: composition:

i. the Sun is mostly H and He, most of the mass is in the Sun, therefore the nebula was mostly H and He.

ii. The inner planets do not now have much in the way of volatiles – probably never had much.

• Other facts: orbits and rotations:

i. Planets mostly orbit in the same plane, near-circular orbits.ii. Planets orbit Sun in same direction.iii. Sun rotates in same direction as planets orbit.

• Nebular theory of Laplace.



Figure: Universities Corp. For Atmospheric Research (UCAR)


Planetesimals to Planets, Proto-sun to Sun


Picture credit: James Schimbert, U. Oregon, Eugene

Picture: NASA


Left-overs: asteroids, comets, EKOs/KBOs

Dr Conor Nixon Fall 06Graphic: SWRIPicture: Johns Hopkins University

Picture credit: NASA GSFC


Lecture 7: Meteorites


• Major mineralogical types: (1) Stony (2) Iron (3) Stony-Iron.

• Types by origin/history: (1) Primitive (chondrite) (2) Differentiated (achondrite) (3) Breccias.

• Primitive meteorites are all stones: either (i) carbonaceous, or (ii) other stones.

• Differentiated meteorites can be any of the mineralogical types.

• Breccias are typically crustal material from asteroids or planets. May be primitive or differentiated.

• Types by landing/detection: (1) Falls (2) Finds.

• Finds have a skewed distribution towards iron or stony-iron meteorites.


Lecture 7: Finding Meteorites


• Famous meteorite falls include:

• L’Aigle Fall (1803, France): the fall which first drew scientific attention to meteorites.

• Allende Fall (1969, Mexico): the first big fall since the space race began. Dated to be very old (4.56 Gyr).

• Murchison Fall (1969, Australia): a large fall of carbonaceous chondrite material, in which was found organics, inc. amino acids.

• The Antarctic is a very happy hunting ground for meteorites, not only because they are easily spotted on the ice, but also because movements of the ice sheets tend to concentrate meteorites against natural barriers.


Lecture 7: Primitives etc


• We can determine meteorite origins by photographing their path across the sky from several different locations, and calculating the trajectory. E.g. Peekskill meteorite. They turn out to be from the main asteroid belt.

• Primitive meteorites are old (4.5+ Gyrs) with chondrules, possibly iron grains (H, L, LL) and quite often are breccias.

• Carbonaceous chondrites are a special class of primitive stones, which are less dense, contain more volatiles and carbon.

• Tagish Lake and Murchison are famous C.C. meteorites, which have been found to contain amino acids.

• We know these molecules are from space by their chirality.

• IPD or Brownlee particles come from comets.


Lecture 8: Differentiated Meteorites


• Differentiated meteorites lack chondrites; hence are called achondrites.

• Iron meteorites contain up to 10% nickel. When polished they show a crisscross Widmanstatten pattern due to slow cooling (below right).

• These meteorites come from bodies (typically asteroids) which have undergone differentiation: a fractional separation under heat and gravity.

Graphic: SaharaMet, R&R Pellison Photo: NEMS


Lecture 8: Stony-Irons, Basalts


• Stony-iron meteorites are thought to come from the boundary layer between the iron core and stony mantle of differentiated parent bodies.

• Stony-irons can be dated due to the their silicate (stony) component, and hence an age for similar iron meteorites can be determined.

• Ages for differentiated bodies are 4.4 to 4.5 Gyr, not much younger than the solar system; hence differentiation took place early on.

• Eucrites are meteorites which have been found to come from the crust of the asteroid Vesta.

• Basalts tend to be lighter rocks from the asteroid crust. Some basalts however have been determined to some from Mars.


Lecture 8: Radioactivity


• Radioactivity (mostly) refers to the process of nuclear transformation, from one unstable ‘parent’ isotope to a daughter isotope which is usually more stable.

• Alpha decay occurs when a helium nucleus is emitted, moving the nucleus 2 steps up the periodic table.

• Beta decay takes place when a neutron is transformed into a proton and an electron, moving the nucleus 1 step down the period table.

• Gamma radioactivity is not an isotope transformation at all, rather the emission of a high-energy EM photon by the nucleus.

Picture: www.impcas.ac.cn


Lecture 8: Dating Meteorites


• Radioactive dating uses the observation that the parent to daughter nuclear ratio decreases over time. By measuring the ratio in a sample, and using a sister isotope to estimate the original amount of the daughter product, we can estimate the age of a sample.

•The half-life of a radioactive isotope is the time it takes for half of the parent nuclei to spontaneously decay to the daughter product.

• The solidification age is the time since the sample was last molten.

• The gas retention age is the time since the sample was last mechanically disturbed, e.g. by a shock or impact.

Picture: Univ. of South Carolina/CSE


Lecture 9: Asteroids


• First asteroid discovered in 1801: Ceres (still the largest) in the ‘gap’ region between the orbits of Mars and Jupiter.

• Missing planet found… but Juno, Pallas and Vesta detected soon after. Exploded planet?

• Asteroid population follows a 1/D2 population (number decreases with increasing size) rather than the expected 1/D3 population.

• Hence, we know most of the mass in the main asteroid belt: less than 1/20 mass of Moon.

• Measuring asteroid orbits (by astrometry) and rotation rates (by light curves) was relatively easy. Measuring actual sizes and masses was harder. Could use either occultation, or spectroscopy to get sizes.


Lecture 9: Asteroid Orbits


• Asteroids orbit the sun between 2.2 and 3.3 AU, with periods of 3.3 to 6 years. Collisions are rare (several per 10,000 years).

• Gaps in the distribution of asteroid semi-major axes are known as resonance or Kirkwood gaps.

• The gaps are caused by Jupiter’s gravity. Asteroids with a certain SMA have a fixed period by Kepler (3).

• Orbits which cause the asteroid to repeatedly pass Jupiter in the same place are unstable.

Picture: JPL/SSD Alan B. Chamberlain


Lecture 9: Families and Classes


• Asteroids which have similar spectroscopic properties and similar orbits are said to form a family.

• We also note that some asteroids are bright with silicate features in the spectrum, while others are dark with water signature, hence 3 types:

• C-TYPE: carbonaceous; dark, with water; primitive (e.g. Ceres)

• S-TYPE: stony, with silicates; primitive (e.g. Eros)

• M-TYPE: (rare) metallic; radar-bright (e.g. Psyche)

• C-type (75%) are outer edge of main belt, S-type (25%) inner edge.

• Densities are not useful indicators of composition, e.g. Psyche.


Lecture 9: Asteroids contd.


• Asteroids are also found:

• In the Lagrangian L4 and L5 points of Jupiter’s (and Mar’s) orbit, called Trojans.

• In Earth crossing orbits: NEAs/NEOs.

• In elliptical orbits: Centaurs.

• Trojans and centaurs are redder than main-belt asteroids.

• Visits:

1. Gaspra (1991) by Galileo. Found irregular object, craters.2. Ida (1993) by Galileo. Found moon (Dactyl), more cratered than Gaspra.3. NEAR-Shoemaker at Mathilde: found very dark, slow rotating object.4. NEAR-Shoemaker at Eros: orbited and landed. Found flat-bottomed

craters filled with dust.


Lecture 10: Comets


• Comets are small primitive bodies from the outer solar system.

• Whereas asteroids are mainly rock and metal, comets are mainly ice.

• When comets approach the inner solar system their volatiles evaporate forming a temporary atmosphere (coma) and tail.

• Plasma or ion tail is ionized volatile material.

• Dust tail is dust particles released by vaporization of the volatiles.

Figure credit: thursdaysclassroom.com


Lecture 10: Comets - contributions


• Tycho Brahe made important contributions to cometary science in the 16th century. By parallax he showed that the comet was further than the Moon, hence not in Earth’s atmosphere. Also showed that the comet head is as big as the Earth.

• Halley was the first to find convincing proof of cometary re-occurrence. He successfully predicted the return of his namesake in 1758.

• Comets follow elliptical orbits, and these are often inclined with respect to the planet orbits.

• Long (>200 yrs) and intermediate (30-200 yrs) period comets come from Oort cloud. Short period comets are from Kuiper belt.

Diagram: Tim Stauffer


Lecture 10: Composition etc


• Largest component of comets is water ice. Also perhaps 10% CO, plus CH4, NH3, CO2 and others.

• Molecules quickly ionized or dissociated (broken up) by solar radiation after leaving the nucleus, see H2O+, CH, CH2, OH, NH etc.

• Tail colors:

• Hydrogen envelope is blue due to H emission.• Plasma tail is blue due to CO+ fluorescence (straight tail).• Dust tail is yellow due to reflected sunlight (curved tail).

Picture: Nanjing University Astr.


Lecture 10: Comet missions and fates


• Comet close-up missions:

• Giotto/Vega missions to Halley (1986). Found a dark (4% albedo) small nucleus (16x8x8 km).

• Deep Space 1: passed Borelly in 2001: darker than Halley.• Stardust: passed Wild-2 in Jan 2004.

• Also note S-L 9 impact with Jupiter.

• Possible fates of a comet:

• Total evaporation• Dead comet • Collision• Gravitational ejection


Lecture 1-10 Quiz


1. What is meant by a geocentric universe? A heliocentric one?

2. Give a major contribution to astronomy by each of the following: Aristotle, Ptolemy, Copernicus, Kepler, Galileo, Newton.

3. How old is the Sun, and what is the name of the mechanism which powers it?

4. Name the three visible layers of the Sun’s atmosphere, and give an approximate temperature for each.

5. What information can we tell about a planet from infrared spectral lines?

6. Give three examples of volatile species in the solar system, and say where they could be found.


Lecture 1-10 Quiz


7. Why did the gas cloud collapse to a disk and not a point; why did everything not fall into the Sun?

8. Why are the inner planets volatile-poor while the outer planets are volatile-rich?

9. Name three famous meteorites, and say what they are famous for.

10. Why are the Allan Hills in Antarctica a good place to hunt for meteorites?

11. What is the meaning of (a) parent nucleus (b) daughter nucleus (c) half-life?

12. Basaltic meteorites are another type of differentiated meteorite. What does the term mean? From what part of the parent body do basalts come?


Lecture 1-10 Quiz


13. What are the three main asteroid types?

14. What are resonance gaps, and why do they occur?

15. Give the general properties of comet orbits; are their orbits similar to those of the planets?

16. It has been stated that comets are actually very dark. How come we can see them?

Documents

ASTR 330: The Solar System Lecture 10.5: Review 1 for Mid-Term Exam 1 Dr Conor Nixon Fall 06