Today in Astronomy 111: asteroids and meteorites Classification of asteroids by composition The interiors of asteroids Special features • Asteroids with moons • Orbit families and collisional fragmentation • Near-Earth-orbiters Meteorites • Composition, 243 Ida and its satellite, classification, and age Dactyl (Galileo/JPL/NASA) • Origins in planets and the asteroid belt 13 October 2011 Astronomy 111, Fall 2011 1 The bad news Exam #1 takes place here next Thusday. To the test bring only a writing instrument, a calculator, and one 8.5”×11” sheet on which you have written all the formulas and constants that you want to have at hand. • No computers, no access to internet or to electronic notes or stored constants in calculator. The best way to study is to work problems like those in homework and recitation, understand the solutions and reviews we distributed, refer to the lecture notes when you get stuck, and make up your cheat sheet as you go along. Try the Practice Exam on the web site. Under realistic conditions, of course. 13 October 2011 Astronomy 111, Fall 2011 2 Asteroid taxonomy The composition of the surface of an asteroid can be determined by reflectance spectroscopy at ultraviolet, visible and infrared wavelengths. Broad classes (Bus & Binzel 2002): C group – carbonaceous, low albedo (< 0.1). 1 Ceres 3 Juno S group – silicaceous (stony), moderate albedo (0.1-0.25). 2 Pallas 4 Vesta X group – metallic, usually The first four asteroids discovered, moderate to large albedo. shown on the same scale as Earth and Moon (NASA). Together they comprise And several “assorted” groups. 2/3 of the mass of the asteroid belt. 13 October 2011 Astronomy 111, Fall 2011 3 C-group asteroids C-type asteroids are the largest population: at least 40% of all asteroids. They lie toward the outer part of the main belt. Dark, with albedo ~ 0.05; flat spectrum at red visible wavelengths. Reflectance spectra generally similar to carbonaceous chondrite meteorites (see below). A few show additional absorption at UV wavelengths and are given by some the classification G-type. D-type asteroids currently appear to comprise about 5% of the total. 1 Ceres, a C- (or G-) type Like Cs they are concentrated in the asteroid (HST/STScI/ outer main belt but are seen further NASA), the largest and out, too; e.g. among Jupiter’s Trojan third brightest of the asteroids. asteroids. 13 October 2011 Astronomy 111, Fall 2011 4 C-group asteroids (continued) Ds are very dark – on average even darker than Cs – and red, with featureless spectra: hard to identify composition. Distant + dark = hard to detect small ones. Thus we may currently underestimate the size of this population. Recently a meteoritic analog of the Ds was found, with the result that they appear even more primitive than Cs. B-type asteroids are much rarer, until recently counting only 2 Pallas as a member (then called “U-type”). Though carbonaceous, Bs have higher albedo and bluer color than Cs and Ds. 624 Hektor, perhaps the best known D-type asteroid (HST image by Storrs et al. 2005) 13 October 2011 Astronomy 111, Fall 2011 5 S-group asteroids S-type (stony) asteroids are the second most numerous type: about 30% of all asteroids. Concentrated toward inner part of main belt, with large albedos (~ 0.20); thus we may be overestimating their fraction of the total. Reflection bands in the infrared similar to those from pyroxenes and olivines. They are either thermally processed and crystallized (like igneous rocks) or have been “space weathered” by impacts and UV. Adaptive-optical images and artist’s conception of 3 Juno, the second-largest S-type asteroid (Harvard-Smithsonian Center for Astrophysics) 13 October 2011 Astronomy 111, Fall 2011 6 S-group asteroids (continued) Other S-group asteroids are rare, but some are still notable. They differ from S-type by having much stronger mineral absorption features near 1 µm wavelength. A-type: olivine Q-type: pyroxene and olivine R-type: pyroxene, olivine and plagioclase V-type: pyroxene; relative mineral abundances closely resemble those of basaltic lavas (!). Until fairly recently the only member of the V type was its eponym, 4 Vesta, which was more conventionally accounted under the “U-type” (unclassifiable, or unique). • Now there are a few more but all are tiny, and all are members of Vesta’s orbital family (Vesta fragments?). 13 October 2011 Astronomy 111, Fall 2011 7 Points of historical interest: 4 Vesta Discovered in 1807 by Heinrich Olbers. • Olbers is famous for the theory (since refuted) that the asteroids are remnants of a destroyed planet, and for his paradox about the darkness of the night sky in an infinite Universe. Since he had already discovered and named 2 Pallas, Olbers left it to his bright young grad student, Carl Friedrich Gauss – yes, that Gauss – to name #4. • In keeping with the Roman goddess theme, and considering its location in Virgo when discovered, Gauss chose Vesta, goddess of the hearth, whose sacred fire was attended by the “Vestal Virgins.” NASA’s Dawn satellite is currently orbiting Vesta. 13 October 2011 Astronomy 111, Fall 2011 8 X-group asteroids M-type (metal) asteroids comprise about 10% of asteroids. They’re shiny and relatively blue, with albedo ~ 0.20, but lacking in silicate spectral features, so they’re probably rich in metallic elements. Live mostly in the center of the main belt. Artificially-sharpened Arecibo radar images of 216 Kleopatra, not the largest M- type but probably the most famous (Steve Ostro, JPL). 13 October 2011 Astronomy 111, Fall 2011 9 X-group asteroids (continued) P-type asteroids comprise about 5% of the total. Dark (albedos in the C-type range) and concentrated in the outer main belt, but otherwise similar to M-type. E-type asteroids are rare but prominent, as the observational biases are all in their favor. Highest albedos among asteroids (0.2-0.5) but otherwise spectrally similar to Ms. Concentrated on the inner rim of the main belt. 2867 Steins, an E-type asteroid (Rosetta/ESA). 13 October 2011 Astronomy 111, Fall 2011 10 Asteroid interiors Not much is known for sure about asteroid interiors. A few of them are probably differentiated… • Several S-group asteroids have bulk densities which exceed the densities of the minerals which dominate their surfaces. • One is 4 Vesta, which has a basaltic-lava surface. …but the only spherical one, 1 Ceres, doesn’t seem to be. Many are so low in density that they must be quite porous, or not really be very solid (rubble piles). • This is consistent with the appearance of the craters in planetary-probe flyby pictures of small asteroids: they tend to look soft-edged, as if made in sand. 13 October 2011 Astronomy 111, Fall 2011 11 Typical small asteroids Clockwise from right: 951 Gaspra (by Galileo), 253 Mathilde (by NEAR), and 25143 Itokawa (by Hayabusa) (JPL/NASA and JAXA). 13 October 2011 Astronomy 111, Fall 2011 12 10 9 C Iron meteorites Asteroid bulk ) 8 S 3 - 7 X density and 6 Ordinary- porosity 5 chondrite grains 4 3 Many fairly large Bulk density (gm cm (gm density Bulk 2 Carbonaceous- asteroids, and 1 chondrite grains 0 most of the 1 1.E+20 1.E+21 1.E+22 1.E+23 1.E+24 0.9 smaller ones, are 0.8 Mass (gm) rubble piles. 0.7 0.6 Rubble piles Rubble piles can 0.5 be any spectral 0.4 Bulk porosity 0.3 type, though the 0.2 tendency is 0.1 strongest in Cs. 0 1.E+20 1.E+21 1.E+22 1.E+23 1.E+24 Mass (gm) Data from Baer et al. 2011. 13 October 2011 Astronomy 111, Fall 2011 13 Special features: asteroids with moons 131 asteroids have been observed to have satellites, since the first one in 1993 (Ida/Dactyl, by Galileo). Five of them have two moons. 90 Antiope is a double asteroid, with nearly equal-mass components. The moons have provided a means Infrared AO image of by which to measure masses of 90 Antiope (Bill asteroids, and thus to provide much Merline et al., SWRI), more accurate average densities. (Not whence was determined the as accurate as we’d like, because the separation of 160 km asteroids tend to be so irregular that and density 0.6 gm cm-3 it’s hard to estimate their volume.) (!). 13 October 2011 Astronomy 111, Fall 2011 14 Special features: dynamical families and fragmentation In 1918, the first great Japanese astronomer, Kiyotsugu Hirayama, discovered three “families” of asteroids, the members of which have very similar orbits, much too similar for the agreement to be a result of random chance. Koronis, Eos and Themis are his three original groups. Same reasoning that Olbers tried to apply to all asteroids. Hirayama concluded that each of these groups consists of fragments of a larger asteroid that broke apart, that subsequently were entrained into their similar orbits by the influence of Jupiter. With typical asteroid sizes and speeds, most collisions should be explosive, so we expect such groups. Family members turn out all to have very similar spectra and composition. 13 October 2011 Astronomy 111, Fall 2011 15 Special features: near-Earth-orbiting asteroids What happens to asteroids kicked into eccentric, orbit- crossing paths? Most get scattered out of the solar system by Earth or Mars, or get swept up by these planets. But some stabilize “briefly” in lower-eccentricity inner orbits following weaker interactions with planets. These comprise the near-Earth-orbiting asteroids (NEAs). Most of these should only last in their orbits for 1-10 Myr, til they get ejected or swept up in an encounter with Earth.