Meteorites, asteroids, comets, planet formation
Meteorites
Meteorites are our hard evidence for clues on how the early solar system formed, and how the Earth became an abode for life. We have very little evidence from our rocks on the Earth to tell us what was happening in the first 100 million years of Earth history. Meteorites hold many of the keys to understanding how life on Earth may have developed.
3 main types of meteorites: Stony, Stony-Iron, and Iron.
Stony-Consists of Chondrites and Achondrites. Chondrites are unmelted (mostly) rocks that still have their solar system complement of metal. This means that the asteroidal (or in rare cases cometary) parent body did not undergo igneous differentiation, wholescale planetary melting, or core formation. Achondrites are basalts, igneous rocks like Hawaii. They do not have their metal, and come from asteroids that have undergone core formation.
Irons-Dense meteorites made of Fe metal and Fe,Ni metal. They are from the cores of asteroid parent bodies. They are generally devoid of silicate materials (though some iron meteorites have inclusions of silicates).
Stony-irons-Mixture of metal and silicates or sulfides. Come from the core/mantle boundary of asteroidal parent bodies.
See class handout on Properties of Meteorites for more description.
The latest Classification of Meteorites (ca. 2002)
Number of meteorite falls and total amounts currently known (ca. 2002)
Carbonaceous Chondrites (like Allende)-These are special type of chondritic meteorite because they have Calcium-Aluminum Inclusions (CAIs), highly refractory materials, also contain presolar grains, organic molecules, and one type of carbonaceous chondrite (called CI) has a similar composition to that of the non-volatile elements of the sun.
The Standard Abundance Distribution (or SAD) shows the match between CI and the solar photosphere.
The abundances of elements in CI chondrites Pt. 1 and 2.
The CAIs and chondrules in the chondrites are also evidence of a separation of material in the solar nebula, as the CAIs contain some of the most refractory (least volatile) elements in the solar system. See the cosmochemical classification of the elements based on their volatility in space.
Solar System Chronology
The solar system formed from a collapsing molecular cloud of gas and dust. Evidence from presolar grains and the meteorites suggest that there were several other stars and at least one supernovae in our near region of space. A supernova may have triggered the cloud collapse. The collapsing cloud gave way to a protosun with a protoplanetary disk. During the time between cloud collapse and planet formation (~1-10 Ma), the chondrules and CAIs formed, the main components of the chondritic meteorites (see Properties of meteorites). The chondrules are ~1mm spherical silicates that formed as free-floating objects in the solar nebula by energetic events that caused multiple episodes of melting and evaporation. The CAIs (Calcium-Aluminum Inclusions) formed before the chondrules (~1-3 Ma before chondrules). The CAIs are time zero in dating the solar system, as they are the oldest objects we have here on Earth. The CAIs probably formed within the first million years after initiation of cloud collapse.
There was also a separation of chemical elements based on their volatility in the solar system. Ices are more dominant than rocky materials in the outer parts of the solar system, probably due to a temperature gradient from the newly formed star outward. This is why there are no Icy planets in the inner solar system.
Meteorite-Asteroid Connection
Most meteorites come from the asteroid belt (between Mars and Jupiter). How do we know this? We have tracked the fireball networks of several meteorites to determine their orbits, which show they come from the asteroid belt. How do meteorites leave their home in the asteroid belt and end up in earth-crossing orbits? Jupiter's gravity has influenced the asteroid belt from the beginning, when it likely disrupted any planet formation in the asteroid belt, and instead led to many small planetesimals forming. Now Jupiter can have resonances with the orbits of asteroids (a 1/3 resonance means that for every revolution Jupiter makes around the sun, the asteroid makes 3 revolutions; thus every 11.86 years the two bodies are in the same position, and Jupiter can apply a small push to it which can lead to an Earth-crossing orbit). Resonances in the asteroid belt are depleted in asteroids due to this effect and are called Kirkwood Gaps.
The reflectance spectra of asteroids are also very interesting when compared to the lab spectra of meteorites. Some meteorites have excellent matches with asteroid spectra suggesting that these are the parent bodies of these meteorites. For example, the eucrite meteorites are a class of achondrite that show an excellent match with the large asteroid Vesta, and many scientists consider Vesta the home of the eucrites.
One problem in the meteorite-asteroid connection is that the number of asteroids with a certain meteorite signature does not match up well with the abundance of those meteorites here on Earth. We apparently do not sample the asteroid belt in a random fashion. Some asteroids are not represented in the meteorites we have on Earth. The asteroid spectral classes also vary with distance.
Meteorite-Comet connection
Comets could also be a source of meteorites. Carbonaceous chondrites are among the rarest meteorites, and some believe they come from comets. IDPs (Interplanetary Dust Particles) are raining down on the Earth every day. They are very small, and some people believe they are remnants of comets. The most compelling evidence for a comet-meteorite connection is the Comet Shoemaker-Levy 9 that impacted Jupiter. Spectra of the comet matched up very well with Murchison, a carbonaceous chondrite. Comets come from either the Kuiper belt (short-period comets) or the Oort cloud (long-period comets).
Current Space Missions to further our understanding of the early solar system
Genesis Mission-collect the solar wind (crashed in desert, but will still be ok), to determine what the the solar wind is.
Deep Impact Mission-comet rendevous on July 4, 2005. To learn more about the nature of comets.
Planet Formation
The chondritic meteorites still have their solar system complement of metal, suggesting strongly that their parent bodies did not undergo core formation (separation of Fe metal and siderophilic elements from the silicates). The achondrites did undergo core formation. They are planetary basalts. If you melt a chondrite, and separate out the metal, you make a basalt. This is why basalt is the most common rock in the solar system (covers the Moon, Earth, Mars, Vesta, and Venus).
If the parent body is >100 km in size, it can undergo wholescale melting due to its size being able to retain its heat from accretion, core formation, and radiogenic elements. Less than 100 km in size, the heat is lost to space efficiently, and the parent body does not melt.
A model of a growing planetesimal >100 km in size.
Various sources of planetary heating.
