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Today in Astronomy 111: and

 Classification of asteroids by composition  The interiors of asteroids  Special features • Asteroids with • Orbit families and collisional fragmentation • Near--orbiters  Meteorites • Composition, and its satellite, classification, and age Dactyl (/JPL/NASA) • Origins in planets and the 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 at ultraviolet, visible and infrared wavelengths. Broad classes (Bus & Binzel 2002):  C group – carbonaceous, low (< 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 (NASA). Together they comprise And several “assorted” groups. 2/3 of the of the . 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 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 ’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 (~ 0.20); thus we may be overestimating their fraction of the total.  Reflection bands in the infrared similar to those from pyroxenes and .  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 , 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:  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 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 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 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 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: (by Galileo), (by NEAR), and 25143 Itokawa (by Hayabusa) (JPL/NASA and JAXA).

13 October 2011 Astronomy 111, Fall 2011 12 10 9 C 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 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 by Earth or , 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. • Such sweeping-up involves damage similar to 1 megaton - 10000 gigaton nuclear detonations.

13 October 2011 Astronomy 111, Fall 2011 16 Impact of impacts

Size * Examples Most recent Planetary effects Effects on life

Super-colossal Moon-forming event 4.45×109 yr Melts planet Drives off volatiles R > 2000 km Mars ago Wipes out life on planet Colossal Pluto, largest few  4.3×109 yr Melts crust Wipes out life on planet R > 700 km KBOs ago Jumbo 4 Vesta, 3 other ~ 4.0×109 yr Vaporizes oceans Life may survive below R > 200 km asteroids ago surface Extra large 8 Flora; 90 other ~ 3.8×109 yr Vaporizes upper 100 m of oceans Pressure-cooks troposphere R > 70 km asteroids ago May wipe out photosynthesis Large Hale-Bopp, ~ 2×109 yr ago Heats atmosphere and surface to Continents cauterized R > 30 km 464 asteroids ~1000 K Medium KT impactor 6.5×106 yr ago Fires, dust, darkness; Half of species extinct R > 10 km 433 Eros (large NEA) atmospheric/oceanic chemical 1211 other asteroids changes, large temperature swings Small About 650 NEAs ~ 300,000 yr Global dusty atmosphere for months Photosynthesis interrupted; R > 1 km ago few species die; civilization threatened Peewee Tunguska event 103 yr ago Major local effects Newspaper headlines R > 100 m Minor hemispheric dust Romantic sunsets increase birth rate

* Based on USDA classification for olives and eggs. From de Pater & Lissauer 2010, Lissauer 1999, and Zahnle & Sleep 1997, updated.

13 October 2011 Astronomy 111, Fall 2011 17 Meteors and meteorites

A is a rock that has fallen from the sky.  It’s called a meteor while it’s falling through the sky. If a rock associated with a meteor is found it is called a fall; otherwise, it’s 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 idea was considered by the educated 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. Meteors from the 2004 Perseid shower, by Fred Bruenjes.

13 October 2011 Astronomy 111, Fall 2011 18 Meteorite classification

Irons  Guess what those are made of.  Meteorites without a lot of iron are known as stones. There are also stony-... • …like pallasites, which consist of gem-quality olivine crystals embedded in a nickel-iron matrix. Section of an iron  Irons contain nickel and iron along meteorite, polished and lightly acid-etched, in the with siderophile elements (those that New England Meteoritical alloy well with iron, like gold and Services collection. Note silver.) the distinctive Thomson-  Irons and stony-irons come from Widmanstätten patterns. differentiated parent bodies. 13 October 2011 Astronomy 111, Fall 2011 19 Meteorite classification (continued)

Thin, polished section of the Esquel pallasite (Wikimedia Commons). Part of the Krasnoyarsk meteorite, the first pallasite found (Pallas, 1776). From MeteoriteCollector.org.

13 October 2011 Astronomy 111, Fall 2011 20 Meteorite classification (continued)

Achondrites  These are rocky, nonmetallic pieces resembling the earth’s crust, and lacking chondrules (see below).  Mostly composed of silicates and iron-nickel oxides.  Enriched in lithophile (easily incorporated in silicates) or chalcophile (alloying well with copper) elements. Microscopic image of a thin  Significantly depleted in iron and section of a eucrite achondrite siderophile elements. containing mostly plagioclase and (more birefringent) pyroxenes. By  Must also come from differentiated J.M. Derochette. parent bodies.

13 October 2011 Astronomy 111, Fall 2011 21 Meteorite classification (continued)

Chondrites  Structurally most primitive of meteorites.  They are called chondrites because they contain chondrules, (for which, see below).  Have never completely melted, though they have been modified in some cases 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 Two fragments of the Allende precisely the same as those of the Sun, in meteorite. Photo by stark contrast to irons and achondrites. Brian Mason, • Thus chondrites come from non- Smithsonian National Museum differentiated parent bodies. of Natural History.

13 October 2011 Astronomy 111, Fall 2011 22 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.

13 October 2011 Astronomy 111, Fall 2011 23 Carbonaceous chondrites

CAI Chondrules

Depleted in meteorite

Depleted in the Sun Matrix

The Allende meteorite. Left: cross section (NASA/JSC). Right: Element abundances, compared to those in the Sun (de Pater & Lissauer 2010).

13 October 2011 Astronomy 111, Fall 2011 24 Ordinary chondrites

Chon- drule

Chondrite H5 Sahara 97095, by J.M. Derochette.

13 October 2011 Astronomy 111, Fall 2011 25 Falls and finds, by type

Type Falls Finds, N Finds N % Antarctica Elsewhere % Ordinary chondrites 853 79.9 26655 9687 87.4 Carbonaceous chondrites 43 4.0 921 464 3.3 Other chondrites 18 1.7 407 214 1.5 Asteroidal achondrites 89 8.3 776 888 4.0 Martian meteorites 4 0.4 25 74 0.2 Lunar meteorites 0 0.0 33 113 0.4 Stony-irons 11 1.0 82 177 0.6 Irons 49 4.6 144 911 2.5 Totals as of 6 October 2011, from the Meteoritical Bulletin Database: 42,638 validated meteorites, of which 87.2% are ordinary chondrites.  The vast majority of these reside in museums and research labs; private collectors account for a large additional total. 13 October 2011 Astronomy 111, Fall 2011 26 Chondrules

Chondrules are 0.1-2 mm diameter, spherical, often glassy igneous inclusions which required high temperatures (T = 1500-1900 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 Olivine chondrule, mostly composition, suggesting that surrounded by carbonaceous they were not well mixed matrix; again by J.M. Derochette. before incorporation into larger bodies.

13 October 2011 Astronomy 111, Fall 2011 27 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.  Thought 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). (Sasha Krot, U. Hawaii)

13 October 2011 Astronomy 111, Fall 2011 28 Meteorite recovery

Lots of meteorites are found, well preserved and concen- trated, in Antarctica. Some deserts provide good samples too.  Suppose you were walking around in 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 couldn’t have brought the rock there, it might be a meteorite.

13 October 2011 Astronomy 111, Fall 2011 29 Source regions: large bodies

Whence come the meteorites?  Some meteorites are exactly the same as lunar rocks (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. 13 October 2011 Astronomy 111, Fall 2011 30 Source regions: smaller bodies

But 99.4% of meteorites are from bodies smaller than the terrestrial planets. Morrison & Owen 1996  Reflectance spectra of classes of meteorites match reflectance spectra of classes of asteroids well.  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 (!).

13 October 2011 Astronomy 111, Fall 2011 31 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 4.5677±0.0009×109 years old (Connelly et al. 2008).  This pretty much determines the age of the solar system. • CAIs are oldest solids found; it is thought that the pre- solar nebula itself formed only 104-105 years earlier.  Moon rocks are younger (3-4.45×109), so have melted since then. Terrestrial rocks are all less than 4×109 years old.  Differences in composition tell us about where they formed (mass fractionation), nuclear decay, processing by melting and water and cosmic rays. 13 October 2011 Astronomy 111, Fall 2011 32 Ages of meteorites (continued)

Ages of chondrules and CAIs in Allende, derived from U- Pb radioisotope dating (Connelly et al. 2008). U-Pb is the isotope system 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.

13 October 2011 Astronomy 111, Fall 2011 33