Asteroids and Meteorites

AST 111 Lecture 8?

Spherical chondrule made of glass and feldspaths.

High interference color olivine crystals. Microscope images from JM Derochette

Hudson and Ostro used radar images obtained with the Deep Space Network Goldstone radar antenna in California and the Arecibo telescope in Puerto Rico in 1992, when Toutatis passed to within a little more than 2 million miles of the Earth.

Spatial distribution

The Asteroid Belt, Orbits

InclinationsHungaria

Phocaea

Eos

Koronis

Trojans

Distribution of Orbital Elements

• Most asteroids are located in the “main belt” between 2.1 and 3.3 AU.

• The spread in eccentricities has an exponential tail, suggesting some kind of equilibrium.

• Mean eccentricity is 0.14. • Mean inclination is 15o. • Many asteroids are on orbits that will not last more

than a billion years. • NEO (Near Earth Object) orbits are unstable on

timescale of 107 years.

End states• NEO’s will wind up in the Sun or ejected from the Solar

system, with smaller percentages hitting other objects (like Earth or other planets).

• The end states of NEOs are governed by resonances with high eccentricity regions which may be chaotic (3:1 and ν6).

• Almost all short period comets are eventually ejected from the solar system. Some small percentage can be trapped in the inner solar system.

• There are some objects in the asteroid belt which have shown cometary behavior. For example Chiron has a Saturn crossing orbit and has exhibited extreme variations in brightness.

• Asteroids on highly eccentric or inclined orbits might be dead comet remains.

Impact of impacts

Size * Examples Most recent Planetary effects Effects on life

Super-colossal R > 2000 km

Moon-forming event Mars

4.45×109 yr ago Melts planet Drives off volatiles Wipes out life on planet

Colossal R > 700 km

Pluto, largest few KBOs

t 4.3×109 yr ago Melts crust Wipes out life on planet

Jumbo R > 200 km

4 Vesta, 3 other asteroids

~ 4.0×109 yr ago

Vaporizes oceans Life may survive below surface

Extra large R > 70 km

8 Flora; 90 other asteroids

~ 3.8×109 yr ago

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

Large R > 30 km

Comet Hale-Bopp, 464 asteroids

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

Continents cauterized

Medium R > 10 km

KT impactor 433 Eros (large NEA) 1211 other asteroids

6.5×107 yr ago Fires, dust, darkness; atmospheric/oceanic chemical changes, large temperature swings

Half of species extinct

Small R > 1 km

About 650 NEAs ~ 300,000 yr ago

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

Peewee R > 100 m

Tunguska event 103 yr ago Major local effects Minor hemispheric dust

Newspaper headlines Romantic sunsets

From de Pater & Lissauer 2010, Lissauer 1999, and Zahnle & Sleep 1997.

Asteroid taxonomy

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.

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.

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. ❑ 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.

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)

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: 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?).

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

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

differentiated. ❑ 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.

Typical small asteroids

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

Asteroid Taxonomy• Largest class (40%) are called C-type for carbonaceous. Very dark,

albedos ~0.04, flat spectrum in the red. UV absorption bands (hydrated silicates). Similar in reflectance spectra to carbonaceous chondrites.

• S-type (stony) (30%). Brighter with albedo ~ 0.15. Absorption bands in the infrared similar to those you might expect from iron and magnesium bearing silicates (pyroxene and olivine). They are either igneous (crystallized) or have been space weathered.

• D and P-types (5-10%) dark with spectra that are relatively featureless. Reddish. Could be very primitive bodies with no proposed meteoritic analog.

• M-type (M=metal). Albedo ~ 0.15, lacking silicate spectral features so are probably irons. Include a W subclass with hydrate IR featuers.

• U-type. Weird and unclassified. • E-type. Enstatite asteroids, near inner edge of belt. Very high albedo.

Asteroid type distribution• No class of asteroids resembles ordinary chondrites. Is it

space weathering that has changed their expected spectrum? Or do they tend to be small? Could be that S-type asteroids include ordinary chondrites as well as stony asteroids.

• Taxonomy is correlated with spatial distribution suggesting that some of the differences in asteroids are primordial. C-types tend to be in the outer asteroid belt. S-type tend to be in the inner belt. D,P only in the extreme outer part and as Trojans. M-type in the central regions.

Example asteroids• Phobos and Deimos. These

are Mars’s moons, however, their spectral properties are very similar to C-type asteroids.

• They are small enough that they are not round.

• Phobos is heavily cratered and has cracks, possibly caused by a large impact.

• Deimos is less cratered, possibly because it is a rubble pile with a lot of sediment.

Strength

Pressure at core Pc~GM2/R4 If core pressure exceeds material strength then the body deforms until it is spherical Material strength of ice ~ 3GPa Material strength of rock ~ 50GPa

Gaspra: Flora family. S-type, heavily cratered. Grooves and albedo variations. Regolith?

Ida and Dactyl: Koronis family. Ida is an S-type asteroid with a very small moon (Dactyl). Spectral differences, Dactyl has more pyroxene. Degraded craters, suggesting old age. Dactyl would be disrupted in 108 years from an impact. It could be reformed during collisions.

Mathilde: C-type. Very low density suggesting that it is porous. Smooth albedo, suggesting that it is homogeneous and undifferentiated. No evidence for hydrates.

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 (!).

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.

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.

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 Meteoritical Services collection. Note the distinctive Thomson-Widmanstätten patterns.

Meteorite classification (continued)

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

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

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 achondrite containing mostly plagioclase and (more birefringent) pyroxenes. By J.M. Derochette.

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 precisely 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 Allende meteorite. Photo by Brian Mason, Smithsonian National Museum of Natural History.

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

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

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.

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

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). (Sasha Krot, U. Hawaii)

Meteorite recovery

Lots of meteorites are found, well preserved and concentrated, 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.

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.

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

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.

Basic classification-Irons

• Most people are familiar with chunks of metal. These meteorite are known as irons.

• Those without a lot of iron are known as stones. There are also stony-irons (a mix).

• Irons contain nickel and iron along with siderophile elements (those that like to combine with iron like gold and silver.)

• Irons and stony-irons come from differentiated bodies.

Achondrites

• Differentiated rocky non metallic pieces resembling the earth’s crust are called achondrites.

• Silicate and oxide composition • Enriched in lithophile or chalcophile elements

rather than siderophile elements. • Significantly depleted in iron and siderophile

elements compared to Solar composition.

Achondrites Millbillillie Eucrite

On this thin section, two main minerals are present: low relief and low birefringence plagioclase and high relief and higher birefringence pyroxene. Again microscopy by JM Derochette.

Primitive meteorites• Also known as chondrites. • Have never melted. • Modified in some cases by aqueous processes and by

thermal processes. • Silicates and metals in close proximity. • The are called chondrites because they tend to contain

chondrules. • Composition provides the best estimates of average solar

system composition of elements. • As far as we know, the abundances of non gas elements in

chondrites is exactly the same as that of the Sun.

Chondrites L6 chondrite JALU (Libya)

Polished section of a chondrite H5 with an oblique fiber optics illumination. The opaque minerals appear dark on this image.

Nice chondrule at the top left. Chondrite H5 Sahara 97095.

Chondrules

• Chondrules are spherical glassy igneous inclusions.

• Because they are glassy cooled quickly. • There is a correlation between chondrule

size and the types of crystals in them suggesting that the cooling rate is somehow related to the size of the globule.

Classes of chondrites

• Volatile rich containing several percent of carbon “carbonaceous chondrites”. Of this type there are slight differences in composition CI, CM, CO and CV types.

• 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 which are dominated by that mineral. Classified based on iron abundance EL and EH.

Processing of chondrites

• They are primitive but could have suffered metamorphisms, shocks, brecciation (breaking up and reassembly), chemical processing such as by water.

• Given a petrographic type: Range from volatile rich type 1 to volatile poor type 6.

• Since type 1,2 are processed chemically by water, they are more altered than type 3.

• Type 7 have undergone partial melting.

Brenham : PALLASITE: olivine crystals cemented in a kamacite and taenite matrix. Analogous terrestrial rocks are not known, but are possible. (JPEG, 458 K) (Ward's catalogue, Rochester, NY)

Iron meteorites are classified according to their nickel and trace elements of germanium and gallium.

Compositional differences are correlated with crystalline properties.

Pallasites are igneous and formed at the interface of molten metal and rock where olivine can crystallize in the metal.

Source regions-Large bodies• Some meteorites are exactly the same as lunar rocks

(breccias). • SNC class believed to be Martian. • The most convincing evidence is the noble gas abundance

which is the same as that measured by the Viking landers. • ALH84001 infamous 4.5 billion year old Martian meteorite

recovered from Antarctica, with magnetite which has been interpreted as evidence for life on Mars.

• Impacts can eject rocks which can travel to Earth particularly from Mars and the Moon because of their lower surface gravity.

Source regions-smaller bodies• 99% of meteorites are from smaller bodies. • Comets and asteroids are the two major classes of expected

parent body populations. • This dividing line is now more obscure because of the

different classes of comets and the Kuiper belt. And there could be extinct comets.

• Achondrites and irons are clearly from the asteroid belt. • Spectral classifications match reasonably well. In other

words, spectral information observed from asteroids are consistent with what you would expect from meteor composition.

Micrometeorites• In the 1950s, Dr. Fred L. Whipple predicted that very small

particles from comets could survive entry into the Earth's atmosphere. These fragments are less than 2/1000ths of an inch across, and gradually descend through the atmosphere to land on Earth. Such particles were first collected using high altitude balloons, but because the particles are so rare in the upper atmosphere, only 10 samples were collected.

• Beginning in the 1970s, Lockheed U-2 aircraft were used to collect samples of the upper atmosphere. Plates coated with silicone oil were mounted in airtight containers on the wing tips, and were exposed to the air stream when the plane reached a cruising altitude of 60,000 feet.

Dust Photos courtesy of Donald

Brownlee, Univ. of Washington. Because the particles are so small, their surfaces can only be studied in detail using electron microscopy. This scanning electron microscope photograph shows a grain about 10 microns (.0004 in) wide, which has a very open structure, presumably once filled by icy comet material. As these particles are ejected from comets, the ice immediately sublimes (turns from solid to gas), leaving a framework of dust similar in comparison to ancient carbon-rich meteorites.

Fall Phenomena

• For the Earth impact velocities range from 11 to 73 km/s.

• Interactions with upper atmosphere can slow down small particles.

• Micrometeorites are able to reradiate the heat. (smaller than 100 microns).

• They have been recovered both on the ground and from jets.

Fall Phenomenon-larger objects• The surface of the meteor is heated by an atmospheric shock front. • It cannot radiate away the energy if it is above a particular size and

velocity. • It gets hotter than 2000K at which point rock melts. • Liquid evaporates off the surface. “Ablation” cools the object. • Heat of ablation is Q~5 ×1010 erg g-1 for stones and irons.

2 23

2

12

"heat transfer coefficient" read fudge factor ~0.1 density of air.

surface area of object velocity of object. ~ 3km/s, is the velocity below which

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dE dm v vQ C Avdt dt vC

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ablation isn't important.

Atmospheric Entry

• The outer part of the meteor gets hot enough to melt rock.

• However it falls so fast (and conductivity is fairly slow) that the rock only melts to a depth of a few mm (for stones) ~1cm for irons which have higher conductivity.

• If the object survives ablation then when it hits the ground it is cold.

• The crust is still altered (becoming magnetized for irons) but the interior may be undisturbed.

Velocity of the meteorite

2

The meteor feels a pressure due to drag of~ / 2 where is the velocity,

is the atmospheric density and ~1 a drag coefici

D atmos

atmos

D

P C v v

C

ρρ

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ent If the pressure exceeds the tensile strength of the materialthe meteor can fragment resulting in a strewn field.Force is times area

/ 2

How do we figure out when changes siD atmos

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m C v A mgdt

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gnificantly?1When ~ where is the transit time. ~ .

~ / 2 ~ mass of atmospheric column.

As a guideline, if the meteor encounters it mass in the atmosphereit sweeps up,

D atmos atmos

dv v vtdt t t h

vm C v A m Ahh

ρ ρ→ =

it will loose a substantial fraction of its incoming velocity.

P ⇠ CD

⇢atmos

v2/2The meteor feels a pressure due to drag

𝜌atmos atmospheric density CD ~1 drag coefficient 𝑣 relative velocity

If the pressure causes a stress that exceeds strength of material then the meteor fragments resulting in a strewn field

Force is pressure times area A

mdv

dt= �C

D

⇢atmos

v2A/2 +mg

If the meteor sweeps up its mass in the atmosphere then it looses a large fraction of its incoming velocity

Asteroid 2008 TC3 first asteroid predicted to impact Earth prior to impact

Map of the Nubian Desert of northern Sudan with the ground-projected approach path of the asteroid 2008 TC3 and the location of the recovered meteorites. Credit: P. Jenniskens, et. al Asteroid exploded in the air, leaving a trail of debris

Explosions/Fireballs• Bolides (fireball, then bright explosion at the end of a streak) • Famous Tunguska explosion 1908 • Mechanical and Thermal stresses (pressure difference on

different sides of body, pressure from heated volatiles) • Brighter it is in the sky, the less likely pieces get to the

ground. • Small cometary icy materials apparently common, more

likely to explode cause fireballs than leave falls

Review

❑Classification of asteroids ❑Classification of meteors ❑Chondrites, Achondrites and Irons. ❑Micrometeorites ❑Fall phenomena