2000 March 30, SPS 1020 (Introduction to Space Sciences) - Reading: today was TNSS Chs. 25 and 26 (Asteroids and Meteorites). - Catch up on the reading! --------------- Asteroids and Meteorites ------------------------ Astra = star (Latin); asteroids are "starlike", in that they're almost never resolved in ground-based telescopes. On 1801 January 1 (1st day of the 19th century), Giuseppe Piazzi discovered the first known asteroid, or _minor planet_: _Ceres_ Distance from Sun = 2.8 AU, predicted by Bode's law 6 German astronomers ("The Celestial Police") found 3 more: Pallas 1802; Juno 1804; Vesta 1807; Next 1845 by 1890 -> 300 known 1923 -> 1,000 known 1984 -> 3,000 known 1990 -> 5,000 known 2000 -> 10,000 known 2001 -> 150,000! Over 10,000 have been sighted, but lost: not observed enough to measure orbits well enough for long-term predictions. Asteroids with known orbits get assigned "Minor Planet numbers" by the International Astronomical Union (IAU). The recent flap over Pluto's status was when Brian Marsden suggested that Pluto be given Minor Planet Number 10,000 (see current Sky & Telescope). Numbered asteroids are often named after people, as an honor. (Ran out of Greek & Roman deities long ago; most other mythologies of the world also exhausted. => Mr. Spock, David Levy's cat; every one of the Beatles; frankzappa and jerrygarcia.) Basic properties: ---------------- Most are 2.2 - 3.3 AU from Sun => the _Main Belt_ => 3.3 < Porb(yr) < 6.0 Diameter of Ceres: 1000 km (comparable to Tethys & Dione) Nearly all others: much smaller, most known D ~ 10 km, some as small as 100 m. Total mass in asteroid belt ~ that of Earth's Moon. Population: number of objects falls off with 1/r^2 (a power law) Extrapolating, there are probably ~ 100,000 with D > 1 km Orbits: ------ e < 0.3, i < 20 degrees for most: like the major planets (not like comets, with e = 0.1 - 0.9) Kirkwood gaps: gaps in Main Belt, cleared by orbital resonances with Jupiter and Mars; much like in planetary rings. Origin: ------ Probably from a terrestrial planet that never formed, because of the constant gravitational perturbations (or disturbances) from Jupiter. Almost certainly *not* from "a planet that exploded". (What could do that?) Most have undergone many collisions and now are not solid rock: more like "flying rubble piles" - not unlike more primitive comets, without ice. Recent result with Phobos, moon of Mars, from Mars Global Surveyor: it's covered in regolith! Regolith = a very fine powder of rock fragments and dust. Some asteroids (e.g. Vesta) are big enough to be chemically differentiated. => Iron meteorites, made of nearly pure iron, came from the cores of differentiated asteroids shattered in collisions. Rotation is also a clue to collisional history (why?); typically hours. Asteroid Families: ----------------- Groups with similar orbits and optical/IR spectra suggesting a common origin => parent body. Today ~ 50 % are members of families; 10 % are members of 3 families Similar spectra => similar surface minerals Albedos either between 3-5 % or 15-25 % Dark asteroids: spectra have no major absorption features, are gray Lighter asteroids: reddish in color, show spectral features of silicate materials Dark => C-type => like Carbonaceous meteorites (primitive material) Light => S-type => Silicate, like Stony meteorites (like Earth rocks) Also rare M-type => Metallic: like iron meteorites More properly, Nickel-iron meteorites: composition of about 95-97% Fe and 3-5% Ni, a high-grade stainless steel used since prehistoric times. Vesta: a rarity, a volcanic asteroid ----- - Among the largest and the most reflective: albedo ~ 30 % - Spectra show signs of igneous rocks, basalt especially (like lunar maria or Earth's ocean floors). => Spectrum changes as the asteroid rotates. Surface composition is not uniform! - Spectrum like _Eucrite meteorites_ Asteroids not in the Main Belt: ------------------------------ - Near-Earth asteroids: ~ 200 known, Most S-type, a few C-types Probably ~ 300,000 down to 100 m in diameter Perturbed into inner Solar System by Jupiter's & other planets' gravity. - Trojan asteroids: in Sun-Jupiter L4/L5; mostly C-type - Cruithne: Asteroid 3753, "Earth's other moon", in a horseshoe orbit around Earth-Sun L4/L5. d ~ 5 km. - Asteroids beyond Jupiter, e.g. Chiron: origin a mystery; definitely icy Asteroids studied during spacecraft flybys: ------------------------------------------ Phobos and Deimos, moons of Mars (Mariner 9, Viking 1 and 2, Phobos, ----------------- Mars Global Surveyor) Gaspra (by Galileo in 1991) S-type ------ 18 x 11 x 9 km; mostly small craters, unlike Mathilde (see below) => Young surface? Or made of strong material, mostly metal? Showed differences in color, "space weathering" reddened surfaces (micrometeoroid impacts, radiation over ages) Ida (by Galileo in 1993) S-type, 56 x 15 km; confirmed space weathering --- Has small moon (!), Dactyl (Sagan, on PBD p. 161 calls it "To be named") ------ 1.5 km diameter Asteroid moons are becoming common; now that we know what to recognize, many are turning up in ground-based observations. Many known to be double, or dumb-bell shaped, from their _light curves_ (how their brightness varies over time, as they rotate). Dactyl's orbit around Ida => Ida's density is only 2.0-3.1 g/cm^3 => S types probably mostly like ordinary C-types inside, the surfaces of which have been "weathered", which drives off the volatiles and makes them stony (since it leaves the refractories). Mathilde and Eros (by NEAR, Near-Earth Asteroid Rendezvous, 1997 & 1998) ----------------- Mathilde: C-type main-belt asteroid (most common type and subtype) Mostly craters, BIG craters! => Confirms "flying rubble pile" theory Eros: a near-Earth asteroid Sadly, NEAR missed its opportunity to rendezvous (meet) with Eros, so it only flew by; it will get another chance in February 2000. Radar studies of near-Earth asteroids: -------------------------------------- Castalia in 1989; Toutatis in 1992; Geographos in 1997: cigar-shaped! (Why not use radar on Main Belt asteroids?) Also, Hubble Space Telescope studies, e.g. of Vesta. A great way to study asteroids: METEORITES! ----------- Remember: meteoroids vs. meteors vs. meteorites ------------------------------------- Meteoroids are the ones that are still in space. ---------- Typically 0.1 - 10 cm across; Like asteroids, distribution of sizes drops off exponentially. _Zodiacal light_ (or "false dawn") caused by sunlight reflecting off interplanetary dust; also the _Gegenschein_ ("counterglow"), opposite the Sun. Meteors are what they become when they enter Earth's atmosphere. ------- Typical speeds: 1-20 km/s (What is Earth's, around Sun?) Meteor showers: occur when Earth passes paths of comets. Because of perspective (as with parallel railroad tracks), meteors appear to come from one point in the sky: the _radiant_. A shower is named after the constellation its radiant appears in. Typical showers have 10-20 meteors/hour; fun to observe while sitting in a lawn chair and binoculars, or better, _no_ optical aid. Good showers (e.g., the Perseids, in August) are more like 60/hour; but often don't impress the uninitiated much. The best I ever saw, the 1998 Leonids, had 900/hour, by some reports! _Sporadic meteors_ (not associated with any shower) appear at a rate of about 6/hour. I see them all the time at dark sites, like Kitt Peak. => City lights (light pollution) is ruinous to observing meteors, though. Modern research on meteors uses radar; amateurs increasingly use radio. Micrometeors = microscopic particles; by far most common type of meteors. ~ 10^5 tons/yr fall to Earth. Meteorites are the ones that hit the ground. Remnants of creation! ---------- Fusion layer: The thin black crust on the surface of most meteorites, caused by atmospheric heating Breccias: rocks welded together by impacts, as in Moon rocks; found in all types of meteorites. Falls: a witnessed meteorite descent; four were photographed well enough to trace orbits to Main Belt! - Iron meteorites are the most common found on Earth (> 75%). since they look unusual and don't weather much. density > 7 g/cm^3 (Most often found in Antarctica: black rock on a white ice sheet, also concentrated by glaciers) M-type asteroids are the least common in space. - Stony: resemble terrestrial rocks, sometimes even with hydated minerals (which once contained water, unlike Moon rocks) - Carbonaceous asteroids are the most common in space (> 75%). Carbonaceous meteorites are the least common found on Earth, since they look like ordinary Earth rocks (to the untrained eye), and do weather easily. Origin: ------- - Carbonaceous ("primitive"): chemistry representative of early Solar System; stony made on surfaces of carbonaceous by space weathering. as opposed to: - Iron and Stony-Iron: from differentiated asteroids, since shattered; Irons solidified out of a molten state. Primitive meteorites: called Chondrites => -------------------- Have "chondrules": small (~ 1 mm), solidified drops of rock, embedded in matrix of other, carbonaceous material. Origin of the chondrules: a major mystery. (From flares from Solar Nebula, shooting up like Solar prominences? Solar Nebula was an accretion disk.) Carbonaceous meteorites: silicates, carbon compounds, & water (unlike all known Moon rocks, with NO water). => Volatile and refractory materials mixed together (undifferentiated) density ~ 2.5 g/cm^3 The most famous carbonaceous meteorite: The Allende Meteorite Fell in 1969 Feb, JUST AS the Lunar Receiving Lab in Houston opened! (First Moon rocks wouldn't arrive until July.) The chondrules in carbonaceous chondrite meteorites are inclusions of refractory minerals, apparently incorporated as the meteorite accreted. => Pieces of the first clumps that condensed from the Solar Nebula! Radioactive dating => 4.566 +/- 0.002 Gyr old (Clock set when rock crystallizes; proportions of parent to daugter products show age.) => And now for the big one: IT CONTAINED AMINO ACIDS! Differentiated meteorites: -------------------------- - Iron Meteorites - Stony-Iron (mixture; rare, <1% of meteorites like this) - Basaltic (sub-group of Stony: from basaltic lava on parent body) => Eucrites (have no chondrules): basalts, some match spectra from Vesta - Lunar meteorites: 14 known; minerals & isotopes similar to Moon rocks - SNC meteorites ("snicks"): from Mars, 13 known Related chemically to eucrites Argon isotopes match those in Mars atmosphere (sampled by Viking landers); Oxygen isotopes match expected compositon of anything that far from Sun.