13 October 2011 Astronomy 111, Fall 2011 1

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,

classification, and age• Origins in planets and the

asteroid belt

243 Ida and its satellite, Dactyl (Galileo/JPL/NASA)

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.

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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 (0.1-0.25). X group – metallic, usually

moderate to large albedo.And several “assorted” groups.13 October 2011 Astronomy 111, Fall 2011 3

1 Ceres

2 Pallas

3 Juno

4 Vesta

The first four asteroids discovered, shown on the same scale as Earth and Moon (NASA). Together they comprise 2/3 of the mass of the asteroid belt.

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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 chondritemeteorites (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. Like Cs they are concentrated in the

outer main belt but are seen further out, too; e.g. among Jupiter’s Trojan asteroids.

1 Ceres, a C- (or G-) type asteroid (HST/STScI/ NASA), the largest and third brightest of the asteroids.

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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)

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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)

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: olivineQ-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

13 October 2011 Astronomy 111, Fall 2011 9

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).

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.

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2867 Steins, an E-type asteroid (Rosetta/ESA).

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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.

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Typical small asteroids

Clockwise from right: 951 Gaspra (by Galileo), 253 Mathilde (by NEAR), and 25143 Itokawa (by Hayabusa) (JPL/NASA and JAXA).

Asteroid bulk density and

porosityMany 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. 2011.13 October 2011 Astronomy 111, Fall 2011 13

0

1

2

3

4

5

6

7

8

9

10

1.E+20 1.E+21 1.E+22 1.E+23 1.E+24

Bulk

den

sity

(gm

cm

-3)

Mass (gm)

C

S

X

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.E+20 1.E+21 1.E+22 1.E+23 1.E+24

Bulk

por

osity

Mass (gm)

Rubble piles

Ordinary-chondrite grains

Carbonaceous-chondrite grains

Iron meteorites

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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

by which to measure masses of asteroids, and thus to provide much more accurate average densities. (Not as accurate as we’d like, because the asteroids tend to be so irregular that it’s hard to estimate their volume.)

Infrared AO image of 90 Antiope (Bill Merline et al., SWRI), whence was determined the separation of 160 km and density 0.6 gm cm-3

(!).

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Special features: dynamical families and fragmentation

In 1918, the first great Japanese astronomer, KiyotsuguHirayama, 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.

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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. • Such sweeping-up involves damage similar to 1

megaton - 10000 gigaton nuclear detonations.

Impact of impacts

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Size * Examples Most recent Planetary effects Effects on life

Super-colossalR > 2000 km

Moon-forming eventMars

4.45×109 yr ago

Melts planet Drives off volatilesWipes out life on planet

ColossalR > 700 km

Pluto, largest few KBOs

4.3×109 yr ago

Melts crust Wipes out life on planet

JumboR > 200 km

4 Vesta, 3 other asteroids

~ 4.0×109 yr ago

Vaporizes oceans Life may survive below surface

Extra largeR > 70 km

8 Flora; 90 otherasteroids

~ 3.8×109 yr ago

Vaporizes upper 100 m of oceans Pressure-cooks troposphereMay wipe out photosynthesis

LargeR > 30 km

Comet Hale-Bopp,464 asteroids

~ 2×109 yr ago Heats atmosphere and surface to ~1000 K

Continents cauterized

MediumR > 10 km

KT impactor433 Eros (large NEA)1211 other asteroids

6.5×106 yr ago Fires, dust, darkness; atmospheric/oceanic chemical changes, large temperature swings

Half of species extinct

SmallR > 1 km

About 650 NEAs ~ 300,000 yr ago

Global dusty atmosphere for months Photosynthesis interrupted; few species die; civilization threatened

PeeweeR > 100 m

Tunguska event 103 yr ago Major local effectsMinor hemispheric dust

Newspaper headlinesRomantic sunsets increase birth rate

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

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Meteors and meteorites

A meteorite 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.

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Meteorite classification

Irons Guess what those 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 MeteoriticalServices collection. Note the distinctive Thomson-Widmanstätten patterns.

Meteorite classification (continued)

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Part of the Krasnoyarsk meteorite, the first pallasite found (Pallas, 1776). From MeteoriteCollector.org.

Thin, polished section of the Esquel pallasite(Wikimedia Commons).

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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.

Significantly depleted in iron and siderophile elements.

Must also come from differentiated parent bodies.

Microscopic image of a thin section of a eucrite achondritecontaining mostly plagioclase and (more birefringent) pyroxenes. By J.M. Derochette.

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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 belongto nonvolatile molecules which areprecisely the same as those of the Sun, in stark contrast to irons and achondrites.• Thus chondrites come from non-

differentiated parent bodies.

Two fragments of the Allendemeteorite. Photo by Brian Mason, Smithsonian National Museum of Natural History.

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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.

Carbonaceous chondrites

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

Depleted in meteorite

Depleted in the Sun

ChondrulesCAI

Matrix

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Ordinary chondrites

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

Chon-drule

Falls and finds, by type

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

Type Falls Finds, N FindsN % Antarctica Elsewhere %

Ordinary chondrites 853 79.9 26655 9687 87.4Carbonaceous chondrites 43 4.0 921 464 3.3Other chondrites 18 1.7 407 214 1.5Asteroidal achondrites 89 8.3 776 888 4.0Martian meteorites 4 0.4 25 74 0.2Lunar meteorites 0 0.0 33 113 0.4Stony-irons 11 1.0 82 177 0.6Irons 49 4.6 144 911 2.5

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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 composition, suggesting that they were not well mixed before incorporation into larger bodies.

Olivine chondrule, mostly surrounded by carbonaceous matrix; again by J.M. Derochette.

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).

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(Sasha Krot, U. Hawaii)

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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.

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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.

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Source regions: smaller bodies

But 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

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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.

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.

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