2000 January 18, SPS 1020 (Introduction to Space Sciences) - Read TNSS Ch. 9 (Planet Earth) by Tuesday, January 25. - Read TNSS Ch. 10 (The Earth's Moon) by Thursday, January 27. - Finish the Basics of Spaceflight (online) by Thursday, January 27. - We will have a reading quiz on Thursday, January 27. ------------------------------------------------------------------------- A Little More Intro. Then we're off to explore the planets! ------------------- My Very Elegant Mother Just Showed Us Nine Planets. Mercury Venus Earth Mars Juputer Saturn Uranus Neptune Pluto Asteroid belt is between Mars and Jupiter. Comets: come from Kuiper Belt and Oort Cloud, beyond Pluto. The Solar System is made up of several distinctive kinds of objects: Terrestrial planets (Terra = Earth): small and rocky Jovian planets (Jove = Jupiter): large and gaseous Jupiter has 11x the diameter of Earth. More like "failed stars" than Earth-like planets: no solid surfaces Satellites, or natural satellites, or moons: Definition: they orbit a planet (which orbits the Sun). Nature: as diverse as the planets, or more so. "There's no such thing as a boring moon, if you look at it in enough detail." Rocky in inner Solar System (e.g. Luna), icy in outer Solar System. Small bodies: Asteroids (mostlly rocky) and comets (mostly icy) Distinction between asteroids and comets is getting blurred! Asteroids not only in belt: Near-Earth Objects (NEOs), others all over. Pluto: Distinction between planets and KBOs blurring. (Recent silly argument) The Origin of the Solar System (TNSS, Chapter 2): ------------------------------------------------- The Cosmic Calendar (abbreviated version) ------------------- (The full version is at: http://www.astro.fit.edu/ringwald/calendar.txt) Suppose the origin of the Universe to have occurred at the stroke of midnight, December 31 / January 1.0. Let now to be represented by the next stroke of midnight, December 31 / January 1.0---one year later. There is still some uncertainty in the absolute age of the Universe, about 13 +/- 2 billion years, so don't take this model too literally. Still, with time proportioned this way, the following events in cosmic history would occur at the following times on the Cosmic Calendar: (*) January 1, 12:00 a.m. (The Beginning): The Universe begins in a hot, dense, expanding fireball called the Big Bang. Most of the H and He in the Universe is formed. (We are reasonably sure the Big Bang happened, since we observe that the Universe is expanding. For more detail, see Chapters 28 and 29 of Universe 5th ed., by Kaufmann and Freedman, the text for SPS 1010 last semester.) (*) Before April 23 (9.5 billion years ago): The disk of our Galaxy was formed. Star formation, throughout the Universe, was probably more active at early times than now. The Sun formed *after* the epoch of maximum star formation. The heavy elements were formed in star formation, all heavier than iron in supernovae. (*) August 23 (4.55 billion years ago): Formation of the Solar System, in the Solar nebula. The Sun, in the center of this disk-shaped cloud (nebula = cloud [Latin]) formed in about 100 million years. The disk became the planets. Process not understood well. In contrast, the Appalachian mountains are 300 million years old. " Rockies " 60-80 " " " Grand Tetons " 9 " " " The planets formed within 10-100 million years. (*) August 26 (100 million years later): The Sun and the planets were mostly formed. Much debris was left over. The planets therefore had much planetesimal bombardment---in other words, they were often hit by *big* meteorites. Planets formed by _accretion_, the gravitational aggregation of matter. (*) September 16 (3.8 billion years ago): Planetesimal bombardment mostly ended, in the Late Heavy Bombardment. Most features on the Earth's Moon are at least this old. Earth's surface cools enough to solidify. Last impacts powerful enough to vaporize Earth's oceans occur. (*) Before September 25 (3.5 billion years ago): Fossil stromatolites, earliest signs of life on Earth. (!) (But there's even earlier evidence, from C isotopes) (*) December 11 (700 million years ago): First multicellular organisms. (*) December 20 (400 million years ago): First fish. Probable first emergence of vertebrates onto land. (*) December 25 (225 million years ago): First dinosaurs (*) December 29 (65 million years ago): Dinosaurs become extinct, probably because of a giant impact. (*) December 31, 9:18 p.m. (4.2 million years ago): First hominid ancestor, Australopithecus, essentially an ape that walked upright Most of these dates come from _radioactive dating_: When a rock crystallizes, the ratios of different isotopes of radioactive elements are fixed. As time passes, any isotopes (forms of the same element, having different atomic masses, from different numbers of neutrons in their atomic nuclei) that are unstable undergo radioactive decay. Radioactive decay occurs exponentially, but different isotopes decay at different rates. Therefore, chemical analysis to measure the different isotope ratios can reveal the relative ages of rocks, meteorites, Moon rocks, formerly living material (from C14), etc. Solar System Formation (also called cosmogony) - ---------------------------------------------- We do not understand star formation well. Gravity obviously involved. One problem: Stars are spinning, so how do stars shed enough angular momentum to collapse, by gravity? But what is angular momentum? Turntable demo: in absence of friction, angular momentum is conserved. Pull arms in => go faster (like ice skater). Angular momentum for an orbiting point mass: L = m v r m = mass, v = rotation velocity, r = radius (For a rotating sphere, e.g. a star or planet, L = 2 m r / 5 v.) BUT: if rotation is too fast, accreting object in center will break up! Star must lose L, but how? Unsolved, but jets possibly important. => Spinning star extrudes a _protoplanetary disk_, or _proplyd_. For the Solar System, this has a name: the Solar Nebula. More constraints on (perhaps clues to?) Cosmogony: 1) Planets all lie nearly in same plane, the ecliptic (defined by Earth's orbit). 2) All planets orbit the Sun in same direction (direct = CCW, as viewed from N, vs. retrograde, CW). 3) Rotation of the Sun is in same direction, and nearly in the plane of the ecliptic. 4) Most planets rotate nearly in the ecliptic plane. 5) Planet orbits are nearly circular (except Pluto). 6) Most planetary satellites also have these properties. --- 7) Titius-Bode law? May be a coincidence, but planetary satellites (of Uranus and Saturn, at least) appear also to follow this rule. Recent simulations of the long-term stability of the terrestrial planets show that the inner Solar System is packed as tightly as it can be, however: if there were another large terrestrial planet, it would soon be ejected from the Solar System. Observed: T Tauri and stars like it (the T Tau stars) are similar to the Sun, only much younger, about 1 million years old. T Tau stars have infrared excess (more infrared light than the Sun does) => a warm disk, surrounding the star Hubble Space Telescope: discovered _proplyds_ in the Orion Nebula (interstellar gas cloud in the constellation Orion). The rough sequence of events: Gravitational collapse of interstellar nebula => (Badly understood; gravity involved, but how to lose angular momentum?) => Formation of disk-shaped Solar Nebula (for Sun) or proplyd => Grains condense in disk (Badly understood; how to make grains stick? Not massive enough for gravity to hold together) => Planetestimal accretion (reasonably well understood from computer simulations) => Planet formation. Grain condensation and planetesimal accretion: Refractory materials (rocks, e.g., silicates, metal oxides) can condense at high T (> 2000 K), *or low T,* NOT ONLY at high T => Are therefore found everywhere in the Solar System. Volatile materials (e.g. ices, H and He gas) can condense *ONLY* at low T. => Are therefore found mostly in the outer Solar System We therefore used to think we understood why the small, rocky Terrestrial planets (Mercury, Venus, Earth, Mars; Terra = Earth) are in the inner Solar System, and the large, gaseous Jovian planets (Jupiter, Saturn, Uranus, Neptune; Jove = Jupiter) are in the outer Solar System. PROBLEM: Since 1995, it's become possible to detect Jovian planets around other stars. Many are HOT JUPITERS: closer to parent star than Mercury, but more massive than Jupiter! (Now no doubt they're Jupiters: new tranisting system.) Do Jovian planets move around? Probably the best evidence that it really does happen, and that cosmogony doesn't need a fundamental revision, is that some of the >29 newly found extrasolar planets are eccentric Jupiters. These are comparable in mass to Jupiter, but have orbits like those of comets, much more eccentric than those of any Solar System planets. A massive planet having such an eccentric orbit must be a short-lived situation: the gravitational forces from other planets in any stellar system tend to circularize planetary orbits, over billions of years. Eccentric Jupiters are therefore probably planets caught in the process of moving around. That several of the first 20 extrasolar planets found have such eccentric orbits suggests they might be common. Are stable systems, like the Solar System, rare? How do hot or eccentric Jupiters start moving around? No one really knows, but if there is more than one planet in any stellar system, their mutual, periodic gravitational tugs on each other might eventually add up---especially if there's a resonance involved (e.g., if one planet's orbital period is almost exactly twice that of the other). This may eventually amplify, to where one planet may be perturbed so much, it's kicked into an eccentric orbit---perhaps plunging near, or into, its parent star, or perhaps being kicked completely out of the stellar system. Perhaps this might happen in our own Solar System, at any time. Yet another thing to worry about...