2  Meteorites & Asteroids

Part 1: Structure and Texture of Meteorites and Asteroids (2.1 - 2.5)
Meteorites are fragments from asteroids. Few asteroids have not be changed since they formed. In this case, the material in meteorites is primitive, unprocessed material from the protoplanetary disk. However, many asteroids were heated or collided with other asteroids. Learn, how asteroids can be structured as a result of their individual evolution. Further, learn which general meteorite types exist with respect to their parent body evolution, and how these meteorites look like today.
Learning Goals
Describe the possible structures of asteroids. Explain why asteroids can be heated, and what this does to an asteroid. Explain the various secondary processes that can change an asteroid, and what macro- and microscopic changes these cause. Explain the connection between asteroids and the various meteorite types. Describe how meteorite melt crusts appear, and how and when these are formed.

Part 2: Origin and Find Locations of Meteorites (2.6 - 2.10)
Meteoroids – rocks, travelling through the Solar System – continuously collide with Earth, cross Earth’s as meteors, and finally land on the Earth’s surface as meteorites. Find out and describe from where meteorites originate, how we know this, what process ultimately deflects meteorites towards Earth, how long meteorites travel through space – but also, where meteorites are most easily found.
Learning Goals
Illustrate how we know meteorites originate from the asteroid belt. Explain how meteorites arrive at Earth crossing orbits. Recall the various travelling times of meteorites from asteroids to Earth. Summarise the meteorite find locations and state why meteorites are primarily found there.

Bonus 1: Asteroids and their Origin (2.11 - 2.15)
Asteroids are small planetary bodies that formed the planets. Meteorites originate from asteroids. This means, asteroids are older than planets, and meteorites witnessed the earliest events in our Solar System. Here, we solely focus asteroids. Describe, where asteroids are located in the Solar System, how they look like, how many there are, where and how they orbit the Sun, and how we know from which asteroids which meteorites originate. Learning Goals
Show the general location and orbits of asteroids in our Solar System. Describe the overall appearances of asteroids. Relate spectral features of asteroids to meteorites. Ability to detail the difference between un/differentiated asteroids. Vocabulary like element partititon, siderophile/lithophile elements, source of (not) primitive meteorites, chronology. For example: (1) differences: petrology, petrography, chemical composition, age, size, thermal history, core, mantle, primitve. (2) causes: heating, short lived radionuclides, differentiation, liquid immiscibility. (3) similarities: age, accretion from precursors.

Bonus 2: Asteroid Cooling Rates (2.16 - 2.19)
Differentiated, but a als some undifferentiated asteroids were heated to high, but also intermediate temperatures, respectively. The cooling history of asteroids can be reconstructed using their chronology or structures in iron meteorites. Learn about cooling rates and times of asteroids, and recall and describe these and how these are determined. Learning Goals
Recall and describe cooling rates and times of Asteroids and iron meteorites. Recall the important phase diagram for iron meteorites, including its parameters, and be able to describe a typical cooling path in this phase diagram.

2.1 Possible Structures of Parent Bodies

The undifferentiated asteroids were thermally altered to various degrees, and were thereby metamorphically changed. If there was a thermal gradient from the inside to the surface, the various petrologic types could have formed in the same asteroid. The maximum petrologic type in the center would depend on the peak temperature. The structure of such an asteroid resembles an onion, with shells of various petrologic types. Asteroids were particularly destroyed again in the early solar system, their debris mixed and then re-accreted into new asteroids. Various lithologies might then occur in direct contact to each other. The structure of such asteroids resemble breccias, and are called ›rubble piles‹

The maximum temperature in undifferentiated planetesimal was insufficient to melt the planetesimal, i.e., the peak temperature was below the solidus temperature of planetesimal.

The onion-shell type consists of concentric layers, each of a different petrologic type, reflecting a temperature gradient from the center of the planetesimal outwards. In the rubble pile type, fragments of various petrologic types are in direct contact with each other. These breccias formed from disrupted onion-shell type planetesimals, and/or from planetesimals of a uniform petrologic type.

✓True

✗False

✓True

✗False

✗True

✓False

2.2 Differentiation of Planetesimals

Planetesimals have been heated to various degrees by collision and/or the decay of short-lived radio-nuclides such as 26Al or 60Fe. Depending on the peak temperatures reached in the planetesimals, these were either primitive, i.e., unaltered, experienced metamorphic alteration, which is the expressed in the petrologic type, or were molten and differentiated. Primitive planetesimals contain unaltered material that formed directly in the protoplanetary disk. Differentaited planetesimals contain a silicate as well as a metal- and/or sulphide-phase, e.g., as core. The lithophile (e.g., Mg, Si, Al, Ca) and siderophile/chalkophile (e.g., Fe, Ni, S, PGE) elements are concentrated in the silicate mannte and metallic/sulphidic core, respectively.

2.3 Hands on Meteorites

When we hold a meteorite in our hands, we are holding something that is older than Earth. Meteorites are characterise by their fusion crust, which they develop during the passage through the Earth’s atmosphere. When we find meteorites in the field or on rock shows, it becomes clear why their popular classification is into stone, stone-iron and iron meteorite: this reflects their overall appearance. Iron-meteorites are quite heavy, resulting from their high density of almost 8 g/cm3. Quite contrary, chondrites – having densities of maybe 2.5 g/cm3 – can be lighter than average terrestrial rocks, with densities of typically somewhat higher than 3 g/cm3.

Fusion crust forms during the passage of the meteoroid through the Earth’s atmosphere. The outer layer of the meteorite is heated by friction and melts. Droplets of melt can peel off. The melt then solidifies as a dark, often almost black fusion crust similar to basalt, as the rocky meteorites generally have mafic to ultra-mafic compositions. Terrestrial weathering oxidises the reduced Fe, and the fusion crust turns into a rusty red over time.

Stone, stone-iron, and iron meteorites are 3 popular meteorite classes that is not really used in the official meteorite classification scheme.

✗… younger than Earth

✗… about the same age as Earth

✓… older than Earth

✗… generally heavier than terrestrial rocks

✗… about similar in weight as terrestrial rocks

✗… generally lighter than terrestrial rocks

✓… sometimes heavier and sometimes lighter than terrestrial rocks

✓… olivine

✗… pyroxene

✗… feldspar

✗… kamacite

2.4 Images of Meteorite Hand Specimens

The various meteorite types are quite distinctive in hand species, i.e., sections of those. Chondrites are conglomerates and are visibly dominated by about mm-small, roundish grains – the chondrules. Differentiated Meteorites from an asteroids mantle have magmatic or brecciated structures, and can easily be mistaken for a terrestrial rock. Iron meteorites originate from the core of asteroids are easily identified by their reflectance and Widmanstätten exsolution patterns. Pallasites were likely formed at the core-mantle boundary of asteroids, and consists of metal and silicate, typically olivine.

Meteorites originate from differentiated or undifferentiated asteroids. The latter are the chondrites, primarily consisting of chondrules and matrix. The roundish chondrules are already visible with the naked eye. Differentiated meteorites are iron meteorites from asteroid cores and meteorites with magmatic or brecciated structures, originated from asteroid mantles or the crust of Mars or Moon. Pallasites are mixtures of metal and silicate – mostly olivine – from the core-mantle boundary of asteroids.

Meteorites from the mantle of asteroids or the crust of Moon and Mars, as these have magmatic or brecciated structures that are frequently found in terrestrial rocks.

✗6 t

✓60 t

✗600 t

✗In asteroids

✗In molecular clouds

✗In other stars

✓In the protoplanetary disk

✓True

✗False

2.5 Meteorite Shock Classification

Meteorites can experience shock when they are expelled from their parent body after a collision. Such a shock changes the appearance and/or mineralogy of the host meteorite. These changes are used to define 6 shock stages/classes from unshocked to highly shocked, which can reach shock pressures of up to 90 GPa.

2.6 Triangulated Meteorite Trajectories

The orbit of a number of ordinary chondrites could be determined using triangulation. All these meteorites have their origin in the asteroid belt. This is one direct piece of evidence that the meteorites indeed originate from the asteroid belt.

The orbits of a few meteorites that fell on Earth are known. These orbits have their origin in the asteroid belt, and are therefore direct evidence that meteorites originate from the asteroid belt. There is, of course, additional evidence, e.g., the similar reflection spectra of meteorites and asteroids.

✗At least 2

✗At least 3

✗At least 4

✓At least 5

✗Biangulation

✗Trigonometry

✓Triangulation

2.7 Kirkwood Gaps

Jupiter and an asteroid are in resonance when their closest encounter is always at the same position during their orbit around the sun. This is the case when e.g., Jupiter orbits the sun once, and the, in the same time, orbits the sun twice. Or, when Jupiter orbits the sun once, and the asteroid, in the same time, orbits the sun three times, and so on. Each time Jupiter and the asteroid encounter each other at the same position, the asteroid is pulled out of its current orbit by Jupiter’s gravitational pull. Over time, the orbit of the asteroid becomes very different, and no longer where it initially was. Thereby, the orbit where such asteroids previously lived, e.g, at the 1/2 resonance, becomes empty over time, which is then the Kirkwood Gap. Also, the objects orbit might become successively more elliptic, and eventually Earth or other planet orbit crossing. These can then become meteorites.

The orbital period of an asteroid can be faster than Jupiter by exact such an amount that the shortest distance between both is always at the same position. The quotients of the orbital periods between Jupiter and asteroids that cause these repeating constellations are called ›resonances‹. Jupiter, with its large gravitation, pulls an asteroid at these resonant positions perpetually further towards to it. Over time, the asteroid orbit becomes more and more elliptic, until it can cross Earth’s orbit. It is then, when an asteroid, or smaller fragment – i.e., a meteoroid – can collide with Earth. This mechanism constantly supplies meteorites to Earth: asteroids collide, some of their fragments might end up in a resonance, and are from there deflected toward and Earth crossing orbit.

5 AU

✗2:1

✓1:3

✓1:2

✗3:1

✓True

✗False

✓True

✗False

2.8 Cosmic Ray Exposure Ages of Meteorites

The time a meteoroid travelled from its source asteroid to Earth’s surface can be determined. This is done using radiogenic isotopes that were produced in the meteoroid from cosmic rays during this travel. This travel time is called cosmic ray exposure (CRE) age. Stone meteorites generally have much shorter CRE ages (millions to tens of millions of years) than iron meteorites (hundreds to in cases thousands of millions of years). Stone meteorites are likely more easily eroded and destroyed by space weathering – by collisions with smaller grains and exposure to irradiation – during their travel through space. The HED meteorites are likely all from the asteroid Vesta. The HEDs have two peaks in their CRE ages. It is like these peaks represent specific material ejection events from the HED parent body and in response to individual impacts. If true, the peaks observed for H- and LL-chondrites likely also represent individual ejection events. This would mean, many of the ordinary chondrites in our collections are from the same parent body.

Maybe – this is not yet entirely clear. What is clear, is that the CRE ages of many OC are similar. Therefore, many OC might come from the same parent body, and were ejected from this during a single impact event. The abundant OC in our collections could therefore also simply reflect a small number of meteorite ejection events that delivered a large number of meteorites from the same class to Earth.

Cosmic nuclides are, of course, also formed on the meteorite parent bodies – but only on the surface of the parent body. Material that is only somewhat deeper will be shielded from cosmic rays by the overlaying material. If this is the excavated and sent of to its travel through the Solar System, it is exposed to cosmic rays, and cosmic nuclides start to form.

✓multi-modal

✗bi-modal

✗uni-modal

✗log-normal

✗homogenous

✗up to ca. 8 Ma

✓up to ca. 80 Ma

✗up to ca. 800 Ma

✗up to ca. 8000 Ma

✓OC

✓HED

✓CI

✗iron

✓martian

2.9 Find Locations of Meteorites

Meteorites are primarily searched for on flat, bright surfaces in hot and cold deserts. The dark meteorites stand out on the bright surfaces and can be easily spotted. Meteorites can lie on Earth for ten thousands of years, and are altered during this time, e.g., the metal starts to rust, producing a red-brownish meteorite surface. Of course meteorites fall everywhere on Earth, i.e., mostly into water and down to the ocean floor, but also into the woods where they are hard to find a quickly rot away. Meteorite densities can be up to several tens per square-km.

2.10 Meteorite Concentration during Antarctic Ice Movement

The density of meteorite falls in Antarctica is as low as anywhere else on Earth. However, meteorites can melt through the upper layers of snow into the underlying ice layer. The antarctic ice sheets are constantly moving. When such ice sheets collide with mountain ranges, the ice – together with the enclosed meteorites – are pushed upwards along the slopes of the mountains. The surface of this ice is constantly removed, and the meteorites subsequently exposed, producing regions of unusual high meteorite densities. These are ideal for meteorite search expeditions.

In ablation zones close to mountain ranges. The ice movement pushes up the ice, which ablates and exposes enclosed material, such as for example meteorites. This process exposes an increasing number of meteorites over long periods of time.

Meteorite weathering is much slower in deserts, compared to e.g., forests and in general regions with plenty of rain. Further, the dark meteorites are more easily spotted on bright surfaces. Weiter können die dunklen Meteorite auf hellen Böden besser gefunden werden. Finally, the large planes in many deserts allow for a ideal search conditions by foot, car or Ski-Doo.

✗True

✓False

✗Multiple years to hundreds of years

✗Multiple hundreds to thousands of years

✓Multiple thousands to ten thousands of years

✗Multiple ten thousands to hundred thousands of years

✗True

✓False

2.11 The Asteroid Belt

The asteroid belt with likely more than 1 million asteroids is located between Mars and Jupiter, stretching from about 2 to 4 AU. Asteroids are, however, found througout the entire Solar System.

Mostly in the asteroid belt between about 2-4 AU from the Sun. However, asteroids can occur throughout the entire Solar System. The trojans, for example, are located in the Lagrange points behind and ahead of Jupiter, i.e., in the same orbit as Jupiter.

Between Mars and Jupiter.

✗

50,000

✗

100,000

✗

250,000

✓

500,000

✗

500,000

✓

1,000,000

✗

5,000,000

✗True

✓False

2.12 Distribution of Asteroid Types

Each of the various asteroid types is distinctly distributed throughout the asteroid belt with respect to their diestances from the Sun. The origin of this distribution is unknown.

The reflectance spectra of an asteroid as observed from Earth using telescopes.

Their distributions are very different vor the various asteroid classes. Some are unimodal with a narrow peak, while others are more or less unimodal with a very wide peak. Other classes might be multi-modal.

✓CC

✗OC

✗EC

✗R & K chondrites

✗CC

✓OC

✗EC

✗R & K chondrites

✓S-Type

✗C-Type

2.13 Asteroid Orbital Inclinations

Asteroid orbits are typically inclinded against the ecliptic, between about 0 and ±40º. The inclinations are not homogeneously distributed, individual groups can be identified.

The plane in wich the Earth orbits around the Sun.

The angle of a planetary body orbital plane and the ecliptic.

✗±5º

✗±10º

✗±20º

✗±30º

✓±40º

✗±50º

✗True

✓False

✗True

✓False

2.14 Image Examples of Asteroids

Asteroids occur in numerous shapes. In many cases – e.g., Itokawa – they have a potato shape. The largest, such as Ceres, are round. Curious asteroids – e.g., Kleopatra – are shaped like a dog bone and likely the result of an asteroid-asteroid collision. Others have a captured moon, like Ida (20 km) and Dactyl (1 km). These are unstable, temporary constellations. The surfaces of asteroids are as well highly variable, in cases even on individual asteroids. Surfaces are mainly characterised by boulders, pebbles, planes, craters, or bright spots.

Potato shaped, spherical, binary, i.e., with a moon, odd shaped, collisional (›dog bone‹)

m to max. ~950 km in diameter.

✓boulders

✓rocks

✗river beds

✗mountains

✓craters

✗volcanoes

✓dust

✓True

✗False

✗DEH

✓HED

✗HDE

✗EHD

2.15 Reflection-Spectra of Asteroids and Meteorites

Asteroids are classified based on their relfectance spectra obtained using telescopes. Similar reflectance spectra are obtained from meteorites in the laboratory. The comparison of asteroid and meteorite reflectance sepctra allwo to assign specific meteorite groups to specific asteroid groups.

2.16 Chronology of Parent Body Cooling

The cooling path of individual chondrites can be determined with a set of mineral thermometers that can also be dated. This means, a certain mineral forms at a certain temperature and time, and both can be determined. This is done for a set of minerals and for chondrites of various petrologic types. The slope of the resulting cooling path further represents the cooling rate of each chondrite. These cooling rates vary from a few Ma per 100 K for H4 chondrites up to ~10 Ma per 100 K for H6 chondrites. Hence, H6 chondrites cool slower than H4 chondrites. This is in accordance with the onion shell model for asteroids, in which H6 chondrites form in the center, where heat conductivity to the surface is slow, and therefore the cooling rate is slow, when compared to H4 chondrites, which form closer to the surface of an asteroid, where heat conductivity is faster.

2.17 Fe-Ni Phase Diagram

Fe and Ni are the two most abundant siderophile elements. The relative, solar abundances of Fe and Ni are at first order about 10:1. Meteoritic metal therefore contains about 90 wt% Fe and 10 wt% Ni. Metal in meteorites occurs as one of the phases: Ni-poor kamacite and Ni-rich taenite. This can be directly understood from the Fe-Ni phase diagram: all metal is liquid above ca. 1500ºC. A solid solution crystallises below this temperature – gamma-Fe. This gamma-Fe decomposes below ca. 500-700ºC and depending on Ni-concentration into alpha-Fe (kamacite) and gamma-Fe (taenite). Below ca. 360ºC gamma-Fe is replaced by a new phase, FeNi3. This transformation is at this low temperature, however, normally to slow, and only kamacite and taenite is found. The decomposition structure of kamacite and taenite is used to determine the cooling times of iron meteorites.

The range between about 5-50 at% Ni. Although Fe is about 10x more abundant than Ni, Ni-rich metal forms during the exsolution of gamma-Fe into kamacite and taenite. Therefore, also the areas with high Ni are relevant.

Increasing pressure changes the locations of the phase boundaries. The pressure inside asteroids is typically far below 1 GPa, which is why the metal phases are usually exclusively kamacite (Ni-poor) and taenite (Ni-rich).

✓Taenite

✗Kamacite

✗<600-800ºC

✗<400-700ºC

✓<500-700ºC

✓Below 0.1 GPa

✗Below 1 GPa

✗Below 1000 Pa

✗Below 100 Bar

✗Below 0.1 kBar

✓Below 1 kBar

2.18 Overview of Iron Meteorite Cooling Rates

Iron meteorites have in cases highly variable cooling rates, ranging from about 1 to more than 3000 K/Ma. These variations are most likely due to the formation depths, but also the size of the parent body. The cooling rate is certainly much more slower deep inside a large parent body, compared to more shallower regions of a smaller parent body.

0.1 to 4000 K/Ma.

✓True

✗False

✓<100 ka to <100 Ma

✗<100 a to <1 Ma

✗<10 Ma to <100 Ma

✗<100 Ma to <100 Ga

2.19 Iron Meteorite Cooling Rates Using Island Widths

Various methods exist to determine the cooling rate of iron meteorites. One method measures the micro-micro-structure of taenite (Ni-rich metal). This taenite decomposes on the µm-scale into various, Ni-rich metals. This produces a cloudy structure with islands set in the centres of a honeycomb structure. The size of these islands correlate negatively with the cooling rate. This means, smaller islands form at high cooling rates, while larger island form at low cooling rates. This method can further be used for other meteorite groups that contain metal.

We can learn about the evolution of the parent body, e.g., its size and cooling time.

0.5 to 500 K/Ma.

✓10-500 nm

✗10-500 µm

✗10-500 mm

✗True

✓False