Chapter 6: Meteor ages and origins

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.

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

Carbonaceous Chondrites CI Closest to solar composition Higher volatile content (up to 22% water bound to other minerals) Low density, only 2200 kg/m 3 No heating, but high brecciation Actually: have no chondrules! CM and CV 2-16% bound water Breccias are common CO Only about 1% bound water Breccias are rare

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

Enstatite chondrites Rare type of chondrite Composition close to that for early-condensed material high in the mineral enstatite and most of their iron is in the form of metal or sulfide rather than taken up as oxides in silicates.enstatite  This implies that E chondrites formed in an area of the solar nebula that was very poor in oxygen, possibly inside the orbit of Mercury.

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

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

Achondrites Achondrites come from differentiated parent bodies and have gone through a phase in which they were mainly molten, allowing heavier compounds to sink to the centre. Their overall chemical compositions are distinctly not solar; moreover, the iron and other metals in these is generally purely metallic.

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

Stony-iron meteorites Mesosiderites are among the strangest of all meteorites. They are a breccia of an approximately equal mixture of silicates and metal that is indicative of multiple and repeated impacts.breccia Pallasites are principally olivine, contained within a matrix of nickel- iron. very scarce and believed to have formed on differentiated bodies in the transition area between the metal-rich core and the olivine-rich mantle where the olivine could cool slowly enough to form relatively large crystals.

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.

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. Meteorite Type Falls%FallsFinds%FindsTotal%Total Irons 425%68127%72322% Stony-Irons 91%592%682% Stones 78194%174171%252276% Total % % % Of the Stones: Chondrites 71291%166796%237994% Achondrites 699%744%1436% 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.

Impact Rates Estimate of total meteoroid flux range from 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.

Impact Rate

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.

Break

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.

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, r 0, and some of the stable, s 0. When we measure the sample at some later time (t) it contains fewer atoms of the unstable isotope: and more of the stable:

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 e t -1  However, we cannot be sure that r 0 and s 0 were the same throughout the sample.  So compare the abundances to a stable isotope of the daughter (s), call it s.

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

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  and the decay constant ?

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

Meteorite ages Gas retention ages:  The time since the last degassing event (usually the last major impact)  Measure the gaseous daughter elements (e.g. 40 K → 40 Ar) that accumulate in crystal-lattice structure  Concentrated at Myr ago, but with ~30% at various, more recent times  Most chondrites have ages very close to one another, between 4560 and 4571 Myr ago  This is most likely the age of the solar system, and meteoritic material went from condition of dispersed dust to incorporation within solid bodies in only ~10 Myr.

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.

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 26 Al (t 1/2 =720,000yr) → 26 Mg.  In many meteorites we observe a correlation between the abundance of aluminum and an excess of 26 Mg/ 24 Mg; but in these samples we also see that the abundance of 25 Mg/ 24 Mg (both non-radiogenic isotopes) is normal.  This is strong evidence that 26 Al 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 ≤10 6 yr before formation of the Sun and SS.  Another possible explanation for the presence of 26 Al in the protoplanetary disk is bombardment of stable 26 Mg by energetic photons associated with powerful flares from the early Sun.

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.

Where do they come from? There are few places in the Solar System where small bodies could have survived for so long. 1.Asteroid belt  Between Mars and Jupiter 2.Trojans  Lagrangian points along Jupiter’s orbit 3.Kuiper belt  Outside Neptune’s orbit 4.Oort cloud  Huge reservoir of comets outside heliosphere

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

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: semimajor axis2.1 AU eccentricity0.57 perihelion distance0.891 aphelion distance3.3 AU inclination1.4 degrees orbital period1072 days entry velocity15.8 km/s

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 1.Initial collision ejects fragments 2.Fragments 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 3.Hit an orbital resonance which send them into orbits intersecting Earth’s

CR exposure ages Recall that stones have lower CR exposure ages than irons  Stones drift faster by the Yarkovsky effect  Stones are weaker than irons  Micrometeorites continually erode them  Larger impacts shatter them Thus stones must be ejected close enough to a resonance to be sent on an Earth-crossing orbit within a few tens of Myr.

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

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.

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