Meteorites AS3141 Benda Kecil dalam Tata Surya Budi Dermawan Prodi Astronomi 2006/2007

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 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 sillicates 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

Petrologic types of chondrites Reflect the state of alteration - either aqueous alteration (carbonaceous) or thermal metamorphism (other classes)

Primitive and differentiated material in meteorites

Structure of chondrites Matrix : dark, fine- grained background Chondrules : nearly spherical “droplets”, typically of mm-size CAI are whitish, irregularly shaped, calcium-aluminum-rich inclusions

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 ( 207 Pb enrichment due to U decay) Suggestion: CAIs formed during the early collapse phase; chondrites were assembled a few Myr later in a quiescent nebula

12 C/ 13 C 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?

12 C/ 13 C 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-nuclideT 1/2 (Myr)Daughter species 26 Al 53 Mn 107 Pd 129 I 146 Sm 244 Pu Mg 53 Cr 107 Ag 129 Xe 142 Nd 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 (1) Flux of Meteoroid Impacts on Mercury 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 flux f ( , r )  differential normalized impact velocity distribution h ( r )  number of impacts

 = 1  = 5 Mercury Earth Recent Results: Marchi et al (2)  is the ratio between 3:1 & 6 resonances  has only a little influence

Recent Results: Marchi et al (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 (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)

Recent Results: Bottke et al (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

oEnter the main-belt zone through a combination of resonant interactions and close encounters with planetary embryos oMuch 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 (2)

Recent Results: Bottke et al (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 26 Al and 60 Fe  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 (4) Delivery efficiency of test bodies from various main-belt resonances striking the Earth

New mechanism of triggering meteorite delivery to Earth Yarkovsky thermal forces on Veritas family