Chapter 6: Meteor ages and origins

1. Chapter 6: Meteor ages and origins

2. Review • From the velocity and deceleration of a meteor, we can estimate its mass: • Meteors can be either entirely broken up, or gradually ablated, during their passage through the atmosphere. • Only the slower-moving meteors will survive to the ground • Heating is restricted to the outer layers; inner regions remain cool. • Change of state (gas, liquid solid) depends on temperature, pressure • Sublimation is the process of solid transforming directly to a gas • Solids are classified as ices or rocks depending on their condensation temperature; rocks remain solid at higher T.

3. Carbonaceous Chondrites • High abundance of carbon, mostly in the form of graphite grains, silicon carbide and mixtures of organic molecules • Lowest temperature condensates – formed at low temperature and have not been altered since. • Unusually high concentration of volatiles and organic compounds (which would boil off at high T) • Low densities • Contain heavier elements in nearly original proportions • No evidence for heating above 500 K • Only account for 5% of falls, but they are more common in space: • Easily broken up in atmosphere • Dominant type of meteorite in lunar soil • Probably come from outer asteroid belt or comets (>2.5 AU) based on spectral analysis of distant bodies

4. Carbonaceous Chondrites • CM and CV • 2-16% bound water • Breccias are common • CI • Closest to solar composition • Higher volatile content (up to 22% water bound to other minerals) • Low density, only 2200 kg/m3 • No heating, but high brecciation • Actually: have no chondrules! • CO • Only about 1% bound water • Breccias are rare

5. Ordinary Chondrites • Most numerous meteorites • Similar chemical composition to CCs; also have not been melted • Do not have the carbon and water-bearing matrix • Slightly more processed than CCs • Probably formed among terrestrial planets • Spectra of inner asteroids indicate they have similar compositions • Same proportions of O isotopes in Earth, Moon and Mars rocks

6. Parent bodies of chondrites • Minor heating did affect some chondrites • Cooling rate can be deduced from the properties of individual crystals in the metal particles • In a large body, the outer layers act as insulation: the deeper inside you go, the longer the cooling time. • Chondritic material was typically insulated by overlying material up to about 50 km thick. • Fits the theory that chondrites are fragments of asteroids

7. Achondrites • Coarse crystal structure, which indicates slow cooling in insulated surroundings • Most similar to terrestrial igneous rocks. • Non-solar chemical compositions • Iron and other metals is purely metallic • Produced when parent material (probably a chondrite) melted • Melting would destroy chondrules • Iron would drain away leaving the silicate material typical of achondrites

8. Eucrites • A type of achondrite that is lavalike, basaltic igneous rock • Formed from the solidification of molten material • Probably from the asteroid 4 Vesta, which is the only asteroid whose spectrum shows it has a eucritelike lava surface • HST found evidence for a huge crater on 4Vesta, supporting this theory

9. Iron meteorites • Iron meteorites commonly present large-sized crystals, being compounds of two iron-nickel alloy varieties. • The large-sized crystals indicate that they cooled more slowly – probably deep inside a larger, parent body.

10. Falls and Finds • Falls are meteoroids seen in their flight through the atmosphere and located on Earth by following that trajectory. • Finds are meteorites discovered serendipitously. • Note that stones represent a much larger % of falls than finds; presumably this is because stones are more likely to be eroded by wind, water etc. and they are also more similar in appearance to normal Earth rocks.

11. Impact Rates • Estimate of total meteoroid flux range from 107-109 kg/yr. • i.e. At least ~1 kg every second ! • Most are very small, micrometeorites that do not hit the ground • Objects large enough to hit the ground subsonically and form craters occur about once per year • Giant explosions about once per century • Most (6/7) falls occur over oceans or poles so go unnoticed.

12. Impact Rate

13. Primeval impact rates • Analysis of lunar surface: compare dates of surface rock to the number of craters to determine how impact rate changes with time. • Little information about conditions <4 Gyr ago, before the oldest surfaces were formed.

14. Break

15. Radioactive decay age measurement • Many elements have several isotopic forms, some of which are unstable and decay into other elements. • Radioactive decay obeys a simple law: the probability that a given isotope will decay into its “daughter” isotope is constant, independent of time and the original number of atoms. • Mathematically: dn/dt = -λn where λ is the decay constant (units=#/sec). • Integrating this from t=0 to t=t gives a classical exponential relation: n(t) = n(0)e-λt. • In a given sample we can measure n(t) and we know λ for a given decay process; if we can somehow determine n(0) we can find t.

16. Radioactive decay age measurement • Consider two isotopes r (the radiogenic/unstable) and s (the stable decay product). • Initially (t=0) the sample will start with some atoms of the unstable isotope, r0, and some of the stable, s0. • When we measure the sample at some later time (t) it contains fewer atoms of the unstable isotope: • and more of the stable:

17. Radioactive decay age measurement • If we measure s and r for different pieces of a given meteorite, we could make a plot which has (hopefully) a linear slope given by elt-1 • However, we cannot be sure that r0 and s0 were the same throughout the sample. • So compare the abundances to a stable isotope of the daughter (s), call it s.

18. Rubidium-Strontium System • One common method uses isotopes of Rubidium and Strontium • 87Rb is a radioactive element that decays into 87Sr with a half-life of 48.8 Gyr • Measure abundances relative to the stable isotope 86Sr

19. Half-life • Decay constants are usually given in terms of the half-life, the time it takes for the sample to decay to half its initial mass. What is the relationship between half-life t and the decay constant l?

20. Example • 1993 observations of chondrules in the Allende meteorite, which fell as a 2 ton fireball in Mexico, 1969. • Analysis of the whole rock indicates an age of 4.5 Gyr. • Suggests in this case the chondrules were disturbed by a later event.

21. Ages • Ages for the oldest meteorites are found to be 4.566±0.002 Gyr. • There is a significant difference between the oldest ordinary chondrites at 4.563±0.001 Gyr and the oldest achondrites at 4.558±0.001 Gyr. • Thus the formation of planetesimals began within a few million years of the earliest grains condensing out of the protosolar nebula and the planetesimals themselves formed over a period of ~10Myr.

22. Short-halflife isotopes • The elements most useful for SS age dating are those with long half-lives of around 1 Gyr. • But radiogenic dating on shorter time scales is also useful. One example is 26Al (t1/2=720,000yr) → 26Mg. • In many meteorites we observe a correlation between the abundance of aluminum and an excess of 26Mg/24Mg; but in these samples we also see that the abundance of 25Mg/24Mg (both non-radiogenic isotopes) is normal. • This is strong evidence that 26Al was present in the early SS nebula, requiring a short time between the nucleosynthesis reactions producing it and the formation of solid bodies – on the order of a few million years or less. • This strongly suggests that a supernova occurred in our vicinity ≤106yr before formation of the Sun and SS. • Another possible explanation for the presence of 26Al in the protoplanetary disk is bombardment of stable 26Mg by energetic photons associated with powerful flares from the early Sun.

23. Where do they come from? • There are few places in the Solar System where small bodies could have survived for so long. • Even on circular orbits, in between large planets, most small bodies will be perturbed onto planet-crossing orbits.

24. Where do they come from? • There are few places in the Solar System where small bodies could have survived for so long. • Asteroid belt • Between Mars and Jupiter • Trojans • Lagrangian points along Jupiter’s orbit • Kuiper belt • Outside Neptune’s orbit • Oort cloud • Huge reservoir of comets outside heliosphere

25. Origins of meteorites • Orbits of Earth-approaching meteors have been measured for some using networks of automatic cameras • All are found to have aphelia in or near asteroid belt

26. Orbit reconstruction • The fireball producing the Tagish Lake meteorite on 18 Jan 2000 was witnessed at dawn in the Yukon and NWT. • 70 eyewitnesses interviewed • 24 still photos and 5 videos were obtained; a subset of these had sufficient foreground structure to permit angular measurements • From a synthesis of various data, the orbital parameters could be measured:

27. Collisions in the asteroid belt • How do meteors get out of the asteroid belt? • What is the typical collision time between asteroids? • Requires 3 stages • Initial collision ejects fragments • Fragments 0.1-10 m diameter would drift due to Yarkovsky effect • Sunlight warms one side of a larger body. • The warm side rotates away from the Sun and radiates thermal energy as photons which provide a “thrust” • This can move particles either in or out • Hit an orbital resonance which send them into orbits intersecting Earth’s

28. Lunar meteorites • Found in Antarctica, just following the American and Russian trips to the moon • Subsequently found in hot deserts: Australia, Africa, Oman • Mostly originate from the far side of the moon, and other regions we have not directly sampled

29. Martian meteorites • Lavalike types: shergotites, nakhlites, chassignites (SNC) • Most examples are basaltic and only 1.3 Gyr old • Oxygen isotopes ratios show they are not from Earth or moon. Asteroids cooled too early to produce such young lava. • Contain N and noble gases matching those found on Mars by the Viking lander • Crater counts on Mars indicate widespread basaltic lava flows 1.3 Gyr ago • Simulations show 8% of particles knocked off Mars in a big collision would impact Earth.

30. Next Lecture: Asteroids • Spatial and size distribution • Shapes, rotation and composition • Heating and cooling