Meteorites: What We Know, and Don’T Know

Meteorites: What we know, and don’t know

Outline:

Meteorites Meteorite parent bodies Meteorite diversity Organic synthesis Pre-solar grains Fiery rain Short-lived nuclides Rock swapping © A.Ruzicka Meteorites Meteorites

What is a meteorite? • On Earth, all extraterrestrial rocks • Any rock that did not form on the body which it is found

• Mostly 4.56 b.y. old (exceptions)

Leonid meteor shower, 1998 (European Fireball Network Image) Meteoroid

Meteor (fireball)

<< Unclassified meteorite from Meteorite Northwest Africa © A.Ruzicka Meteorites 1992 Peekskill fireball video clips

(How to turn a $300 car into one worth $10,000.) Meteorites © A.Ruzicka Meteorite parent bodies What we know – Parent bodies

Most meteorites were derived from parent bodies in the asteroid belt.

Meteoroid orbits © A.Ruzicka Meteorite parent bodies

Asteroids as the main source of meteorites © A.Ruzicka Meteorite parent bodies

How do meteorites get from asteroids to the Earth?

(1) Gravitational perturbations by Jupiter & Mars can put asteroidal material into asteroid-crossing orbits.

(2) Collisions between asteroids fragment material into smaller pieces.

(3) The Yarkovsky Effect can cause rotating m-sized objects to spiral inwards to (or outwards from) the sun.

Cosmic-ray exposure (CRE) ages of meteorites (~1 Ma to ~0.5 Ga) give travel time needed for m-sized object-- consistent with Yarkovsky Effect © A.Ruzicka Meteorite parent bodies 4-Vesta: probable parent body of HED meteorites

H = howardite, E = eucrite, D = diogenite diameter = 540 km

albedo = 0.38

Prot = 5.3 hr

spectral class = V (nearly unique match to HED meteorites)

density = 3.4 g/cm3 a = 2.36 AU

giant south polar basin howardite (NWA 2060) © A.Ruzicka Meteorite parent bodies

What we don’t know:

1. Which asteroids (besides Vesta) supplied our meteorites?

2. Did they form there, or move in from elsewhere?

3. How were materials assembled and processed in small bodies? © A.Ruzicka Meteorite parent bodies

Was collisional disruption common?

Rubble pile

Break-up Reassembly © A.Ruzicka Meteorite diversity

What we know – Diversity

Meteorites are highly variable in their properties.

• Include both melted & unmelted types

• Unmelted meteorites (chondrites) formed in unique environment: the solar nebula

• Melted meteorites formed in differentiated bodies © A.Ruzicka Meteorite diversity

Meteorites: different types

Designation Type of rock

Chondrite agglomerate-- never melted (stony)

All else igneous; impact breccias-- (stony, stony- melted at least once iron, iron) © A.Ruzicka solar nebula: our proplyd Chondrite formation setting: protoplanetary disks (proplyds) around young stellar objects (YSOs)

W.K. Hartmann © A.Ruzicka

note light elements— variable amounts in different chondrites

Chondrites have “solar composition” for most elements © A.Ruzicka Meteorite diversity Different chondrite groups

16 chondrite groups recognized © A.Ruzicka Meteorite diversity Melted Gibeon (IVA iron) Millbillillie (eucrite) (differentiated) meteorites • achondrites • irons • stony irons

DAG 485 (ureilite) © A.Ruzicka Meteorite diversity

What we don’t know

What is the exact relationship between chondrites and melted (igneous) meteorites?

It’s assumed that igneous meteorites were derived from chondritic parent bodies that were melted

However, dating suggests that some chondrites formed after igneous meteorites

How were different chondrites and igneous meteorites produced? © A.Ruzicka Organic synthesis

What we know – Organics

Pre-biotic organic synthesis occurred in solar system building blocks.

• Organic compounds found in interstellar medium (ISM)-- molecular clouds-- and in carbonaceous chondrite meteorites

• Solar system formed by collapse of molecular cloud; chondrites formed in the early solar system © A.Ruzicka Organic synthesis Molecular clouds cold, dense areas in interstellar medium (ISM)

Horsehead Nebula

Mainly molecular H2, also dust, T ~ 10s of K © A.Ruzicka Organic synthesis

Carbonaceous chondrite— Rich in organic material © A.Ruzicka Organic synthesis

Many organic compounds in carbonaceous chondrites

Include: macromolecular (kerogen-like) carbon, carboxylic acids, dicarboxylic acids, amino acids, lower alkanes, higher alkanes, aromatic hydrocarbons, N-compounds

Synthesis possible in different ways, environments:

• in molecular clouds

• in our solar system-- within parent bodies, maybe in dispersed grains within the solar nebula © A.Ruzicka Organic synthesis

What we don’t know

1. How much and what type of pre-biotic organic synthesis occurred via different mechanisms?

2. Were these pre-biotic compounds used to help jump-start life on Earth? © A.Ruzicka Pre-solar grains What we know – Pre solar grains

Pre-solar grains were incorporated & preserved in chondritic meteorites.

<< contains microscopic pre-solar grains, found by acid dissolution, gas extraction, or isotope mapping © A.Ruzicka Pre-solar grains Pre-solar material in meteorites material suggested astrophysical site

Ne-E exploding nova S-Xe Red Giant or Supergiant Xe-HL supernovae Macromolecular C low-T ISM

SiC C-rich AGB stars, supernovae Corundum AGB stars Nanodiamond supernovae

Graphite, Si3N4 supernovae

These materials are released into the ISM when stars die. © A.Ruzicka Pre-solar grains Supernova remnants

Planetary nebulas

Note: planetary nebula have nothing to do with planets! © A.Ruzicka Pre-solar grains

What we don’t know

1. How many different pre-solar stars contributed matter to our solar system?

2. Besides contributing matter, did shock waves from dying stars help trigger the formation of our solar system? © A.Ruzicka Fiery rain

What we know – Fiery rain

A substantial amount of dust in the early solar system was processed by intense heating events to make chondrules & CAIs (Ca-Al-rich inclusions).

• Chondrules formed as free-floating melt droplets (“fiery rain”) in early solar system, accreted to form chondrites. Chondrites accreted to form other bodies (including planets).

• CAIs formed by an approach to equilibrium at high temperatures, either as vaporization residues or condensates. Most were molten. © A.Ruzicka NWA 2697 (CV3 chondrite)

Ca-Al-rich inclusions (CAIs)

chondrules

matrix © A.Ruzicka Fiery rain Chondrule textures in thin-section

<< barred olivine, almost completely remelted

<< microporphyritic olivine >> mostly remelted

radial pyroxene & microporphyritic pyroxene , completely or partly remelted >> © A.Ruzicka Fiery rain

What we don’t know

1. What was the nature of the heating events that formed chondrules and CAIs? Many possibilities.

2. How did these heating events chemically and isotopically modify the objects?

3. What is the relationship of chondrules & CAIs to one another & to other meteorite components?

4. What do these components have to tell us about the evolution of the solar nebula & how planets formed? © A.Ruzicka Short-lived nuclides

What we know - Short lived nuclides

The decay of short-lived radioactive nuclides was an important heat source in the early solar system.

• Evidence for many short-lived nuclides found in various meteorites, can be used as relative chronometers

• Many meteorite parent bodies melted, and short-lived radioactive decay most promising heat source © A.Ruzicka Short-lived nuclides

Radionuclide Half-life (Ma) Daughter Ratio measured

26Al 0.73 26Mg 26Mg/24Mg 60Fe 1.5 60Ni 60Ni/58Ni 53Mn 3.7 53Cr 53Cr/52Cr 129I 15.7 129Xe 129Xe/130Xe + others

HED meteorite parent body melted & differentiated while 53Mn present slope proportional to 53Mn/55Mn

Proportional to 53Cr/52Cr

What we don’t know

1. What were the most important heat sources for asteroidal differentiation? (leading candidate: 26Al)

2. Can various short-lived decay schemes be reconciled to give a coherent timescale of early solar system evolution?

3. What do short-lived chronometers tell us about how long it took to form the solar system? © A.Ruzicka Rock swapping What we know - Rock swapping

Planetary rock-swapping has occurred throughout solar system history.

• ~150 martian meteorites, ~150 lunar meteorites (as of 2019) recognized on Earth; younger than 4.56 b.y. • Impact-blasted off surfaces; brought to Earth in last ~0.1-10 m.y. probably many more at earlier times • Now finding meteorites on the Moon and Mars

Ancient terrain on farside of Moon—

Impact battered

<< Mars meteorite found in Northwest Africa

NWA 773

<< Mars meteorite EETA 79001

C1, C2, C3 = EETA79001 glass

A, B = Zagami glass

Normal = Zagami

What we don’t know

1. How much swapping occurred in early solar system?

2. Did Earth receive samples from planets other than Mars?

Meteorites present major interdisciplinary problems

for progress, will require increased collaboration from scientists from different fields--

Questions?