Meteorites

AS3141 Benda Kecil dalam Tata SuryaProdi Astronomi 2007/2008

Budi Dermawan

Falls and finds (1)

• Meteorite find: typically, a farmer finds a strange rocky/metallic object when ploughing his field

(most common in the museums)

• Meteorite fall: the fireball of the falling meteorite is observed, and the freshly fallen pieces are found on the ground

(useful for statistics of different types)

Falls and finds (2)

• The meteorites usually fragment during flight; the largest fragments travel furthest along an oblique in fall path

• The Antarctic ice forms accumulation sites for meteorites; these have been explored recently

Meteorite find in the Lybian desert

Spectra

Classification

Meteorite types• Chondrites ~85% of falls- formed in the solar nebula

• Achondrites ~8% of falls- formed by igneous processes near the surface of major or

minor planets

• Stony irons ~1% of falls• Irons ~6% of falls- formed by fragmentation of core-mantle differentiated

asteroids

Meteorites that are finds are likely to be iron, because these are obviously different from Earth’s rocks. Whereas the stony meteorites can blend in with other rocks when viewed by untrained eye

} stony

Origin of Meteorites Radioactive dating puts ages at 4.6 Byr Meteorites originate in silicon and metal rich

meteoroids (asteroids), not the icy cometary material that would burn up in the atmosphere

Iron meteorites suggest molten cores. The heat source would not have lasted long, and this is consistent with a picture where the meteorites formed early in the history of the solar nebula

Interactions with cosmic rays from the solar wind alter or age the meteorites, but there isn’t that much aging apparent, suggesting that the meteorites must have been protected under layers or rock until recently

Meteorites originated relatively recently (<1 Byr) in collisions between asteroids or planetesimals

Iron Meteorites Rare Interior generally shows complex structure

called Widmanstatten patterns formed from iron-nickel alloys and the very high degree of order requires that the molten metal must have cooled extremely slowly (~20 K every Myr)

Must originate in the cores of meteoroids large enough to be molten (to support differentiation) and large enough to have a significant insulating layer that leads to very slow cooling of the molten core

Stony Meteorites

Rich in silicates or stony materials The most common type is chondrite (from

the glassy inclusions called chondrules), which have the same composition as the Sun with all volatile gasses (H, He) missing

Expected to be original samples of material that condensed in the solar nebula

Glassy chondrules are bits of melted rocks that cooled too quickly to form ordered crystalline structures

Chemical classes of chondrites

CI (Ivuna)

CM (Murchison)

CO (Ornans)

CV (Vigarano)

carbonaceous ~4% of falls

H (high iron)

L (low iron)

LL (low-low)

ordinary ~79% of falls

EH (high iron)

EL (low iron)

enstatite ~2% of falls

Structure of chondrites

• Matrix: dark, fine-grained background

• Chondrules: nearly spherical “droplets”, typically of mm-size

• CAI (Calcium-Aluminum-rich Inclusions) are whitish, irregularly shaped

Meteoritic compounds

• Chemical equilibrium reaction network of solids in the solar nebula

• Each mineral is marked at the temperature where it condenses or sublimates

Chondrite formation

• Separation of high-and low-temperature materials

• CAIs may result from extreme heating in the early, active nebula

• Chondrules were made by rapid, less extreme heating whose nature is not understood

• Volatile depletion of matrix remains to be explained

Chondrites as chronometers of solar system formation

• Allende CAIs have Pb-Pb ages of 4560 Myr• Whole-rock Pb-Pb ages of chondrites cluster around

4555 Myr (207Pb enrichment due to U decay)

• Suggestion: CAIs formed during the early collapse phase; chondrites were assembled a few Myr later in a quiescent nebula

12C/13C ratio in meteorites (1)

• Solar System average = 89.9• The gas in the presolar cloud (mainly CO) was

homogenized• The grains in the presolar cloud retained very

different ratios, reflecting various formation environments

• Did such grains survive until they were incorporated into chondrites?

12C/13C ratio in meteorites (2)

• The answer is YES!• The SiC grains are presolar and may be much older than the

Solar System• Organic grains in 1P/Halley were found to range from 0.01 to

60, a still much wider range: presolar

Extinct radionuclides

Radio-nuclide T1/2 (Myr) Daughter species

26Al53Mn107Pd129I146Sm244Pu

0.7

3.7

6.5

16

103

82

26Mg53Cr107Ag129Xe142Nd

fission Xe

Achondrites / parent bodies

• SNC meteorites (Shergotty, Nakhla, Chassigny) come from Mars

• Lunar meteorites• HED meteorites (Howardites, Eucrites,

Diogenites) come from (4) Vesta• Ureilites come from a large carbonaceous

asteroid that is likely collisionally disrupted

Recent Results: Marchi et al. 2005 (1)

Flux of Meteoroid Impacts on Mercury

drdrhrfdrdr )(),(),(Φ

Model:

1. Meteoroid flux (radius r & impact velocity ):

2. Delivery routes from MBAs are 3:1 & 6 resonances (Morbidelli & Gladman 1998, Bottke et al. 2002)

(,r) differential fluxf (,r) differential normalized impact velocity distributionh (r) number of impacts

= 1

= 5

Mercury Earth

Recent Results: Marchi et al. 2005 (2)

is the ratio between 3:1 & 6 resonances

has only a little influence

Recent Results: Marchi et al. 2005 (3)

• Impacts on Mercury occur from15 to 80 km s-1 (Earth 50 km s-1)

• Impacts at perihelion happen at considerably greater velocity than averaged over Mercury’s entire orbit (47%, 43%, 33% for r = 10,000, 100, 1 cm)

Recent Results: Marchi et al. 2005 (4)

Impacts at aphelion have a symmetric distribution (am/pm = 1) for r = 270 cm, while at aphelion is always am/pm > 1

c is catastrophic collisional half-time of meteoroids that are crossing the MBAs (r in cm)

(Wetherill 1985, Farinella et al. 1998)

Myr 4.1)( rrc

Recent Results: Bottke et al. 2006 (1)

Iron meteorites as remnants of planetesimals formed in the terrestrial planet region

Scattered into the main-belt zone.

Once there the objects are dynamically indistinguishable from the rest of the main-belt population

o Enter the main-belt zone through a combination of resonant interactions and close encounters with planetary embryos

o Much of the particles is delivered to the inner main-belt, where most meteoroids are dynamically most likely to reach Earth

Recent Results: Bottke et al. 2006 (2)

Recent Results: Bottke et al. 2006 (3)

Inner solar system planetesimals experienced significantly more heating than S- and C-type asteroids, with the most plausible planetesimal heat source being radionuclides like 26Al and 60Fe

If main-belt interlopers are derived from regions closer to the Sun, their shorter accretion times would lead to more internal heating and thus they would probably look like heavily metamorphosed or differentiated asteroids

Recent Results: Bottke et al. 2006 (4)

Delivery efficiency of test bodies from various main-belt resonances striking the Earth

Recent Results: Domokos et al. 2007

• Meteoroid flux at Mars: <4.410-6 meteoroids km-2 h-1,

Masses > 4 g• Flux at Earth: 10-6

meteoroids km-2 h-1 (Grün et al. 1985)

New mechanism of triggering meteorite delivery to Earth

Yarkovsky thermal forces on Veritas family

The End

iron meteorite with shiny fusion crust (width ca. 25 cm)www.kosmochemie.de