Asteroids & Meteorites

Transcription

1 Asteroids & Meteorites MIDTERM #1 ON TUESDAY! - PRACTICE EXAM AVAILABLE ONLINE 12 October 2017 ASTRONOMY 111 FALL Midterm Exam #1 Takes place here on Tuesday Make sure to bring only a writing instrument, a calculator, and one 8.5 x11 sheet on which you have written all the formulae and constants that you want to have at hand. No computers, no access to the internet or to electronic notes or stored constants in your calculator The best way to study is to work through problems like those on the homework and recitations, understand the solutions distributed, refer to the lecture notes when you get stuck, and make up your equation sheet as you go along. Try the practice exam on the website. Ideally, under realistic conditions (especially the time!). 12 October 2017 ASTRONOMY 111 FALL

2 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, classification, and age Origins in planets and the asteroid belt 243 Ida and its satellite, Dactyl (Galileo/JPL/NASA) 12 October 2017 ASTRONOMY 111 FALL 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) S group silicaceous (stony), moderate albedo ( ) X group metallic, usually moderate to large albedo And several assorted groups The first four asteroids discovered, shown on the same scale as Earth and the Moon (NASA). Together, they comprise 2/3 of the mass of the asteroid belt. 12 October 2017 ASTRONOMY 111 FALL

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 A few show additional absorption at UV wavelengths and are given by some the classification G-type. 1 Ceres, a C- (or G-) type asteroid (HST/STScI/NASA), the largest and third brightest of the asteroids 12 October 2017 ASTRONOMY 111 FALL C-group asteroids: D-types and B-types D-type asteroids currently appear to comprise about 5% of the total. Like Cs, they are concentrated in the outer main belt but are seen further out as well; e.g. among Jupiter s Trojan asteroids Ds are very dark on average, even darker than Cs and red, with featureless spectra: hard to identify their composition Distant + dark = hard to detect small ones. Thus, we may currently underestimate the size of this population. A meteoritic analog of the Ds was recently 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) 12 October 2017 ASTRONOMY 111 FALL

4 S-group asteroids S-type (stony) asteroids are the secondmost numerous type: about 30% of all asteroids. Concentrated toward the inner part of the main belt, with large albedos (~0.20); thus we may be overestimating their fraction of the total. Reflection bands in the infrared are 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 (CfA) 12 October 2017 ASTRONOMY 111 FALL S-group asteroids other types 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 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 are members of Vesta s orbital family (Vesta fragments?) 12 October 2017 ASTRONOMY 111 FALL

5 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 graduate 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 was in orbit around Vesta for fourteen months (mid-2011 through late 2012) before moving on to Ceres. This was the first time we sent a satellite to visit Vesta. 12 October 2017 ASTRONOMY 111 FALL X-group asteroids: M-types M-type (metal) asteroids comprise about 10% of asteroids. They are shiny and relatively blue, with an albedo ~0.20, but lacking in silicate spectral features, so they are probably rich in metallic elements. Live mostly in the center of the main belt. Artificially-sharpened Aricebo radar images of 216 Kleopatra, not the largest M-type but probably the most famous (Steve Ostro, JPL) 12 October 2017 ASTRONOMY 111 FALL

6 X-group asteroids: P-types & E-types 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 ( ) but otherwise spectrally similar to Ms. Concentrated on the inner rim of the main belt Steins, an E-type asteroid (Rosetta/ESA) 12 October 2017 ASTRONOMY 111 FALL Asteroid interiors Not much is definitively known 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, does not 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. 12 October 2017 ASTRONOMY 111 FALL

7 Typical small asteroids From left to right: 951 Gaspra (Galileo), 253 Mathilde (NEAR), and Itokawa (Hayabusa) (JPL/NASA and JAXA) 12 October 2017 ASTRONOMY 111 FALL Asteroid bulk density and porosity Many fairly large asteroids, and most of the smaller ones, are rubble piles. Rubble piles can be any spectral type, though the tendency is strongest in Cs. Data from Baer et al. (2011) 10 9 C Iron meteorites ) 8 S -3 7 c m X m 6 (g Ordinarychondrite grains 5 sity n e 4 d lk 3 u B 2 Carbonaceouschondrite 1 grains E E+21 1.E+22 1.E+23 1.E Mass (gm) Porosity Bulk density [cm -3 ] 0.7 sity 0.6 ro o 0.5 p lk u 0.4 B Rubble piles 1.E+20 1.E+21 1.E+22 1.E+23 1.E+24 Mass (gm) 12 October 2017 ASTRONOMY 111 FALL

8 Asteroids with moons 145 asteroids have been observed to have satellites since the first one in 1993 (Ida/Dactyl, by Galileo). Eight of them have two moons. 90 Antiope is a double asteroid, with nearly equal-mass components. The moons have provided a means by which to measure masses of asteroids and thus to provide more accurate average densities. (Not as accurate as we would like because the asteroids tend to be so irregular that it is difficult to estimate their volume.) Infrared AO image of 90 Antiope (Bill Merline et al., SWRI), from which a separation of 160 km and a density of 0.6 g cm -3 were determined. 12 October 2017 ASTRONOMY 111 FALL 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 and were subsequently entrained into their similar orbits by the influence of Jupiter. With typical asteroid sizes and speeds, most collisions should be explosive, so we would expect such groups to exist. Family members all turn out to have very similar spectra and composition. 12 October 2017 ASTRONOMY 111 FALL

9 Near-Earth-orbiting asteroids What happens to asteroids which get 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. Some stabilize briefly into 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, until they get ejected or swept up in an encounter with Earth. Such sweeping-up involves damage similar to 1 megaton 10,000 gigaton nuclear detonations. 12 October 2017 ASTRONOMY 111 FALL Size* Examples Most recent Planetary effects Effects on life Super-colossal R > 2000 km Colossal R > 700 km Mammoth R > 200 km Jumbo R > 70 km Extra large R > 30 km Large R 10 km Medium R > 2 km Small R > 500 m Midget R > 200 m Peewee R > 30 m Moon-forming event Mars Pluto largest few KBOs 4 Vesta 3 other asteroids 8 Flora 90 other asteroids Comet Hale-Bopp 464 asteroids KT impactor 433 Eros (large NEA) 1211 other asteroids 4.45x10 9 yr ago Melts planet Impact of the impacts *Based on USDA classification for olives, pecan halves, and eggs. Drives off volatiles Wipes out life on planet 4.3x10 9 yr ago Melts crust Wipes out life on planet ~3.9x10 9 yr ago Vaporizes oceans Life may survive below the surface 3.8x10 9 yr ago Vaporizes upper 100 m of oceans Pressure-cooks troposphere May wipe out photosynthesis ~2x10 9 yr ago Heats atmosphere and surface to ~1000 K Continents cauterized 6.5x10 7 yr ago 1620 Geographos ~5x10 6 yr ago ~1000 NEOs Lake Bosumtwi ~500,000 yrago Fires, dust, darkness Atmospheric/oceanic chemical changes, large temperature swings Optically thick dust, substantial cooling Ozone layer threatened High altitude dust for months Some cooling Half of species extinct Photosynthesis interrupted Significant extinction Massive crop failures Civilization threatened Apophus ~50,000 yr ago Tsunamis Coastal damage Tunguska event Meteor crater 30 June 1908 Major local events Minor hemispheric dust Newspaper headlines Romantic sunsets increase birth rate 12 October 2017 ASTRONOMY 111 FALL

10 Meteors and meteorites A meteorite is a rock that has fallen from the sky. It is called a meteor while it is falling through the sky. If a rock associated with a meteor is found, it is called a fall; otherwise, it is called a find. Only after falls were well observed and documented by Chladni (1794) and Biot (1803) did it become accepted that they did actually fall from the sky. Before that, the educated considered the idea to be as crazy as sightings of spacecraft piloted by extraterrestrials are today. And the latter is demonstrated to be crazy, of course. Meteorites turn out to come mostly from asteroids and are relatively un-thermally-processed, so they preserve a record of the state of solid matter in the early Solar System. 12 October 2017 ASTRONOMY 111 FALL Meteorite classification: Irons Guess what these are made of Meteorites without a lot of iron are known as stones. There are also stony-irons like pallasites, which consist of gem-quality olivine crystals embedded in a nickel-iron matrix. Irons contain nickel and iron along with siderophile elements (those that alloy well with iron, like gold and silver). Irons and stony-irons come from differentiated parent bodies. Section of an iron meteorite, polished and lightly acid-etched, in the New England Meteoritical Services collection. Note the distinctive Thomson-Widmanstätten patterns. 12 October 2017 ASTRONOMY 111 FALL

11 10/11/17 Example pallasites (Left) Part of the Krasnojarsk meteorite, the first pallasite found (Pallas, 1776) (Right) Thin, polished section of the Esquel pallasite 12 October ASTRONOMY 111 FALL 2017 Meteorite classification: Achondrites These are rocky, nonmetallic pieces resembling the Earth s crust, and lacking chondrules. Mostly composed of silicates and iron-nickel oxides. Enriched in lithophile (easily incorporated in silicates) or chalcophile (alloying well with copper) elements. Significantly depleted in iron and siderophile elements. Must also come from differentiated parent bodies. 12 October 2017 ASTRONOMY 111 FALL 2017 Microscopic image of a thin section of a eucrite achondrite containing mostly plagioclase and (more birefringent) pyroxenes. By J.M. Derochette 22 11

12 Meteorite classification: Chondrites Structurally, the most primitive of all meteorites. They are called chondrites because they contain chondrules. They have never completely melted, though in some cases they have been modified by aqueous and/or thermal processes, and have igneous silicate and metal inclusions in close proximity. Have abundances of elements that belong to nonvolatile molecules which are precisely the same as those of the Sun, in stark contrast to irons and achondrites. Thus, chondrites come from non-differentiated bodies. Two fragments of the Allende meteorite. Photo by Brian Mason, Smithsonian National Museum of Natural History 12 October 2017 ASTRONOMY 111 FALL Classes of chondrites Volatile-rich ones, containing several percent of carbon, are called carbonaceous chondrites. Of this type, there are slight differences in composition, which leads to subtypes such as CI, CM, CO, and CV. Allende is a CV carbonaceous chondrite. The extra letter refers to the meteorite that was the first or best example; e.g. Murchison for the CMs. Ordinary chondrites: classified based on their Fe/Si ratio (H = high Fe, L = low Fe, LL = low Fe, low metal, mostly oxidized metal). Enstatite chondrites are dominated by that mineral. Classified based on iron abundance into subclasses EL and EH. 12 October 2017 ASTRONOMY 111 FALL

13 CAI Chondrules Depleted in meteorite Depleted in the Sun Matrix Carbonaceous chondrite: Allende Left: cross section (NASA/JSC) Right: Element abundances, compared to those in the Sun 12 October 2017 ASTRONOMY 111 FALL Chondrule Ordinary chondrites Chondrite H5 Sahara 97095, by J.M. Derochette 12 October 2017 ASTRONOMY 111 FALL

14 Type Falls Finds, N Finds N % Antarctica Elsewhere % Ordinary chondrites Carbonaceous chondrites Enstatite chondrites Asteroidal achondrites Martian meteorites Lunar meteorites Pallasites Irons Falls and finds, by type Totals as of 26 September 2017, from the Meteoritical Bulletin Database: 57,071 validated meteorites, of which 86.2% are ordinary chondrites. The vast majority of these reside in museums and research labs; private collectors account for a large additional total. 12 October 2017 ASTRONOMY 111 FALL Chondrules Chondrules are mm diameter, spherical, often glassy igneous inclusions which required high temperatures (T = K) to form. Because some are glassy, they probably melted and cooled very fast: minutes to hours. There is a correlation between chondrule size and composition, suggesting that they were not well mixed before incorporation into larger bodies. Olivine chondrule, mostly surrounded by carbonaceous material; again by J.M. Derochette. 12 October 2017 ASTRONOMY 111 FALL

15 CAIs Chondrites also have calcium-aluminum-rich inclusions (CAIs) which form at higher temperatures than chondrules. Ca-Al rich anorthite, melilite, perovskite, and forsterite, mostly. Though to be the oldest solids in the Solar System: ~1.7 Myr older than chondrules in the same chondrite (Amelin et al. 2002, Connelly et al. 2008). 12 October 2017 ASTRONOMY 111 FALL Meteorite recover Many meteorites are found well preserved and concentrated in Antarctica. Some deserts provide good samples, too. Suppose you were walking around the plains of Antarctica and came upon a rock laying on the surface. What were its options for getting there? Same holds for desert plains, like deep in the Sahara. If running water could not have brought the rock there, it might be a meteorite. 12 October 2017 ASTRONOMY 111 FALL

16 Source regions: large bodies Some meteorites are exactly the same as luna rrocks (anorthosite breccias); they must be from the Moon. The SNC class includes three types that come from Mars: The most convincing evidence is the noble gas abundances, which are distinctive and the same as those measured by the Viking landers. One, ALH84001, became infamous: a 4.5 billion year old Martian achondrite meteorite recovered from Antarctica, with magnetite which has been interpreted as evidence for life on Mars. Impacts on rocky Solar System bodies can eject rocks which can travel to Earth, particularly from Mars and the Moon because of their lower surface gravity. 12 October 2017 ASTRONOMY 111 FALL Source regions: smaller bodies 99.4% of meteorites are from bodies smaller than the terrestrial planets. Reflectance spectra of classes of meteorites match reflectance spectra of classes of asteroids well. Comets and asteroids are the two major classes of parent body populations for chondrites. Of these, the C-group asteroids dominate by a wide margin, but the dividing line is somewhat indistinct. Achondrites and irons clearly come from the asteroid belt (Ss and Xs). 63% of achondrites the H-E-D classes are from 4 Vesta alone! Morrison & Owen (1996) 12 October 2017 ASTRONOMY 111 FALL

17 Ages of meteorites Because they commonly contain silicate minerals, meteorites can be radioactively dated, just like rocks. Result: they all turn out to be very old even older than Moon rocks and similar in age. Example: the CAIs in the Allende meteorite (a CV3) are ±0.0009x10 9 years old (Connelly et al. 2008). This pretty much determines the age of the Solar System. CAIs are the oldest solids found; it is thought that the pre-solar nebula itself formed only years earlier. Moon rocks are younger (3 4.45x10 9 ), so they have melted since then. Terrestrial rocks are all less than 4x10 9 years old. Differences in composition tell us about where they formed (mass fractionation), nuclear decay, processing by melting and water and cosmic rays. 12 October 2017 ASTRONOMY 111 FALL Ages of meteorites (cont.) Ages of chondrules and CAIs in Allende, derived from U-Pb radioisotope dating (Connelly et al. 2008). U-Pb is the isotope currently favored for use on the oldest meteorites, as Rb-Sr is for the oldest terrestrial and lunar rocks. Note the significant difference in the ages of chondrules and CAIs. 12 October 2017 ASTRONOMY 111 FALL