Institute of Physics lecture
Throughout the year the Institute of Physics puts on lectures about different aspects of physics. On the 31st of October the lecture was about meteorites and what they can tell us about how the universe, stars and planets can form.
Astronomy by Microscope Professor Monica Grady Department of Physical Sciences at the Open University
We usually rely on telescopes collecting the different radiations emitted and reflected from astronomical bodies to find out about them but we can also use microscopes to investigate the structure of meteorites and this information also gives us information about astronomical bodies like the Sun.
What can we learn from meteorites? They can give us information about the origin and evolution of our Solar System and other planetary systems (exoplanets); Stellar evolution; Galactic evolution; Formation and evolution of our Moon; Formation and evolution of Mars; Origin of life (the universe and everything).
Laboratory-based Astronomy Precise and accurate data from meteorites provide “ground truth” for astronomical observations and astrophysical models. Comparisons between laboratory measurements and astronomical observations can be made. Techniques are: Microscopy; Optical observations allow us to look at the texture, electron microscopy allows us to look at the composition and X-ray diffraction allows us to look at the structure. Spectroscopy; Different wavelengths (UV/visible to Mid IR; Raman) are used to investigate the chemical composition of the meteorites and mass spectroscopy (Radiogenic and stable isotopes as well as organic compound investigation) is used also used to investigate the the chemical composition as well as investigating isotopes. Directanalysis of astrophysically-significant materials; Presolar grains were condensed in the expanding envelopes of dying stars and were captured into primitive meteorites.
Star Formation Cycle Meteorites contain materials that provide evidence for the life cycle of stars.
Star formation is the process by which dense regions within molecular clouds in interstellar space collapse into spheres of plasma to form stars. Interstellar dust, containing organic and inorganic matter, is found in galaxies such as our Milky Way and consists of about 0.1 to 1 particles per cm^3 and is typically composed of roughly 70% hydrogen by mass, with most of the remaining gas consisting of helium. This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place. In contrast to spirals, an elliptical galaxy loses the cold component of its interstellar medium within roughly a billion years, which hinders the galaxy from forming diffuse nebulae except through mergers with other galaxies. In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H2) form, so these nebulae are called molecular clouds. Star formation occurs when enough gas and dust has collected into a giant ball. At the centre of this ball (protostar) when the temperature (from all the gas and dust bumping into each other under the great pressure of the surrounding material) reaches about 15 million degrees nuclear fusion begins and the ball starts to glow. Stellar evolution depends on the mass of the star and stars of different masses are thought to form by slightly different mechanisms. The theory of low-mass star formation, which is well-supported by a plethora of observations, suggests that low-mass stars form by the gravitational collapse of rotating density enhancements within molecular clouds. The collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk through which matter is channelled onto a central protostar. For stars with masses higher than about 8 solar masses, however, the mechanism of star formation is not well understood. The most prominent theory is of competitive accretion, which suggests that massive protostars are “seeded” by low-mass protostars which compete with other protostars to draw in matter from the entire parent molecular cloud, instead of simply from a small local region. Another theory of massive star formation suggests that massive stars may form by the coalescence of two or more stars of lower mass.
The Orion Nebula is an archetypical example of star formation, from the massive, young stars that are shaping the nebula to the pillars of dense gas that may be the homes of budding stars. If the star is at least five times bigger than our Sun it could form a supernova at the end of its life. After the outer layers of the star have swollen into a red supergiant (i.e., a very big red giant), the core begins to yield to gravity and starts to shrink. As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begin to occur, temporarily halting the collapse of the core. However, when the core becomes essentially just iron, it has nothing left to fuse (because of iron’s nuclear structure, it does not permit its atoms to fuse into heavier elements) and fusion ceases. In less than a second, the star begins the final phase of its gravitational collapse. The core temperature rises to over 100 billion degrees as the iron atoms are crushed together. The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in an explosive shock wave. As the shock encounters material in the star’s outer layers, the material is heated, fusing to form new elements and radioactive isotopes. In one of the most spectacular events in the Universe, the shock propels the material away from the star in a tremendous explosion called a supernova. The material spews off into interstellar space — perhaps to collide with other cosmic debris and form new stars, perhaps to form planets and moons, perhaps to act as the seeds for an infinite variety of living things.
Planet formation Meteorites also contain materials that provide evidence for planet formation.
Planet formation was first discussed by Immanuel Kant in 1755. Kant proposed that a nebulae, which is a huge cloud of dust and gas, was pulled together by gravity so that it collapsed into a flat, rotating disk. The disk eventually coalesced into the Sun and planets. Kant also stated that because a similar process occurs around other stars, our Solar System is not alone in the universe. After faults were found with the nebular hypothesis, other explanations of planet formation were sought. After many failures, such as the encounter theory, astronomers returned to the nebular hypothesis to find improvements during the mid 1900s. A modern version of the nebular hypothesis, called the protoplanet hypothesis, was formed independently by Carl von Weizsacker and Gerard Kuiper. The steps in planet formation theorized by the protoplanet hypothesis are shown in the diagram below. (A) The solar system begins to form as a rotating cloud, or nebulae, collapses. (B) Instabilities in the nebulae cause dust particles to stick together. The dust particles accrete into billions of planetesimals with diameters of about 10 meters. The planetesimals then collide and form protoplanets. Meanwhile, the protosun in the centre of the nebular disk becomes massive and hot enough to “turn on” by fusing hydrogen. (C) The Sun begins to radiate energy and vaporize dust in the inner part of the Solar System. The remaining gas is blown away by solar winds.
Despite the protoplanet theory’s success in correcting problems with the nebular hypothesis, it did not provide an explanation for the distribution of angular momentum in the Solar System. To explain the transfer of angular momentum from the Sun to the planets, scientists proposed a braking action caused by the Sun’s magnetic forces. The magnetic lines of force from the Sun transferred angular momentum from the spinning Sun to the planetary disk. With the addition of this proposal, the protoplanet hypothesis became free of any known faults.
Asteroids and meteorites Asteroids are leftovers (clumps of dust and metal) from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today. That is why most asteroids lie in a vast ring there and are given the name the asteroid belt. This main belt holds more than 200 asteroids larger than 60 miles (100 kilometres) in diameter. Scientists estimate it also contains more than 750,000 asteroids larger than three-fifths of a mile (1 kilometre) in diameter and millions of smaller ones. Not everything in the main belt is an asteroid — for instance, comets have recently been discovered there, and Ceres, once thought of only as an asteroid, is now also considered a dwarf planet. Many asteroids lie outside the main belt. For instance, a number of asteroids called Trojans lie along Jupiter’s orbital path (stable orbits influenced by Jupiter and resonance orbits that cause the asteroids to be flung out). Three groups — Atens, Amors, and Apollos — known as near-Earth asteroids orbit in the inner solar system and sometimes cross the path of Mars and Earth. Many are found in the Keiper belt at the edge of the Solar System (comets seem to originate form the Oort cloud also found at the edge of the Solar System). Asteroids can reach as large as Ceres, which is 940 kilometres (about 583 miles) across and is also considered a dwarf planet. On the other hand, one of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across. Nearly all asteroids are irregularly shaped, although a few are nearly spherical, such as Ceres. They are often pitted or cratered — for instance, Vesta has a giant crater some 285 miles (460 km) in diameter. As asteroids revolve around the Sun in elliptical orbits, they rotate, sometimes tumbling quite erratically. More than 150 asteroids are also known to have a small companion moon, with some having two moons. Binary or double asteroids also exist, in which two asteroids of roughly equal size orbit each other, and triple asteroid systems are known as well. Many asteroids seemingly have been captured by a planet’s gravity and become moons — likely candidates include among Mars’ moons Phobos and Deimos and most of the distant outer moons of Jupiter, Saturn, Uranus and Neptune. The average temperature of the surface of a typical asteroid is minus 100 degrees F (minus 73 degrees C). Asteroids have stayed mostly unchanged for billions of years — as such, research into them could reveal a great deal about the early solar system. In addition to classifications of asteroids based on their orbits, most asteroids fall into three classes based on composition. The C-type or carbonaceous are greyish in colour and are the most common, including more than 75 per cent of known asteroids. They probably consist of clay and stony silicate rocks, and inhabit the main belt’s outer regions. The S-type or silicaceous asteroids are greenish to reddish in colour, account for about 17 per cent of known asteroids, and dominate the inner asteroid belt. They appear to be made of silicate materials and nickel-iron. The M-type or metallic asteroids are reddish in colour, make up most of the rest of the asteroids, and dwell in the middle region of the main belt. They seem to be made up of nickle-iron. There are many other rare types based on composition as well — for instance, V-type asteroids typified by Vesta have a basaltic, volcanic crust. Ever since Earth formed about 4.5 billion years ago, asteroids and comets have routinely slammed into the planet. The most dangerous asteroids are extremely rare, according to NASA. An asteroid capable of global disaster would have to be more than a quarter-mile wide. Researchers have estimated that such an impact would raise enough dust into the atmosphere to effectively create a “nuclear winter,” severely disrupting agriculture around the world. Asteroids that large strike Earth only once every 1,000 centuries on average, NASA officials say. Smaller asteroids that are believed to strike Earth every 1,000 to 10,000 years could destroy a city or cause devastating tsunamis. Dozens of asteroids have been classified as “potentially hazardous” by the scientists who track them. Some of these, whose orbits come close enough to Earth, could potentially be perturbed in the distant future and sent on a collision course with our planet. Scientists point out that if an asteroid is found to be on a collision course with Earth 30 or 40 years down the road, there is time to react. Though the technology would have to be developed, possibilities include exploding the object or diverting it. For every known asteroid, however, there are many that have not been spotted, and shorter reaction times could prove more threatening. When an asteroid, or a part of it, crashes into Earth, it’s called a meteorite. Here are typical compositions:
Iron Meteorites: •Iron 91% •Nickel 8.5% •Cobalt 0.6% Stony Meteorites: •Oxygen 36% •Iron 26% •Silicon 18% •Magnesium 14% •Aluminium 1.5% •Nickel 1.4% •Calcium 1.3%
In 1801, while making a star map, Italian priest and astronomer Giuseppe Piazzi accidentally discovered the first and largest asteroid, Ceres, orbiting between Mars and Jupiter. Ceres accounts for a quarter of all the mass of all the thousands of known asteroids in or near the main asteroid belt. Since the International Astronomical Union is less strict on how asteroids are named when compared to other bodies, there are asteroids named after Mr. Spock of “Star Trek” and rock musician Frank Zappa as well as more solemn tributes, such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003. Naming asteroids after pets is no longer allowed. Asteroids are also given numbers — for example, 99942 Apophis. The first spacecraft to take close-up images of asteroids was NASA’s Galileo in 1991, which also discovered the first moon to orbit an asteroid in 1994. In 2001, after NASA’s NEAR spacecraft intensely studied the near-earth asteroid Eros for more than a year from orbit, mission controllers decided to try and land the spacecraft. Although it wasn’t designed for landing, NEAR successfully touched down, setting the record as the first to successfully land on an asteroid. In 2006, Japan’s Hayabusa became the first spacecraft to land on and take off from an asteroid. It returned to Earth in June 2010, and the samples it recovered are currently under study. NASA’s Dawn mission, launched in 2007, began exploring Vesta in 2011 and is slated to explore Ceres in 2015 and will be the first spacecraft to visit either body. In 2012, a company called Planetary Resources, Inc. announced plans to eventually send a mission to a space rock to extract water and mine the asteroid for precious metals.
The above picture are different views of 433 Eros. A near-Earth S type asteroid (NEA) discovered in 1898. As mentioned above a meteorite is a natural object originating in outer space that survives impact with the Earth’s surface and may derive from an asteroid. It may have a browner outer surface due to melting as it experienced frictional forces when passing through the atmosphere. Friction slows down the “fire-ball”. Eventually the outer surface solidifies again but the heat is never transferred inside. It is not homogenous.
There are three main types of meteorite and you can trace them back to asteroids. Most meteorites are stony meteorites, classed as chondrites and achondrites. Only 6% of meteorites are iron meteorites or a blend of rock and metal, the stony-iron meteorites. Modern classification of meteorites is complex, the review paper of Krot et al. (2007) summarizes modern meteorite taxonomy. About 86% of the meteorites that fall on Earth are chondrites, which are named for the small, round particles they contain. These particles, or chondrules, are composed mostly of silicate minerals that appear to have been melted while they were free-floating objects in space. Certain types of chondrites also contain small amounts of organic matter, including amino acids, and presolar grains. Chondrites are typically about 4.55 billion years old and are thought to represent material from the asteroid belt that never formed into large bodies. Like comets, chondritic asteroids are some of the oldest and most primitive materials in the solar system. Chondrites are often considered to be “the building blocks of the planets”. They show a chain of evidence for the formation and provide a keystone for the understanding of the evolution of the Solar System by the study of the minerals in them. For instance Calcium-Aluminium (CAls) rich inclusions in chondrite meteorites are the most primitive and oldest objects formed in the Solar System. The picture below left is a cut section, 30 microns thick and polished. Plane polarised light is passed through it and the amount of refracted light tells you information about the composition and texture by the “frothy” look and irregular colours. A melt at very high temperatures (1800K) produces a very refractive material.
Chondrules were formed in the protoplanetary disk about 4.5 billion years ago. Friction caused the dust to melt. It was then quenched and the surface tension gave roundness with different textures. About 8% of the meteorites that fall on Earth are achondrites (meaning they do not contain chondrules), some of which are similar to terrestrial mafic igneous rocks. Most achondrites are also ancient rocks, and are thought to represent crustal material of asteroids. One large family of achondrites (the HED meteorites) may have originated on the asteroid 4 Vesta. Others derive from different asteroids. Two small groups of achondrites are special, as they are younger and do not appear to come from the asteroid belt. One of these groups comes from the Moon, and includes rocks similar to those brought back to Earth by Apollo and Luna programs. The other group is almost certainly from Mars and are the only materials from other planets ever recovered by man. About 5% of meteorites that fall are iron meteorites with intergrowths of iron-nickel alloys, such as kamacite and taenite. Most iron meteorites are thought to come from the core of a number of asteroids that were once molten. As on Earth, the denser metal separated from silicate material and sank toward the centre of the asteroid, forming a core. After the asteroid solidified, it broke up in a collision with another asteroid. Due to the low abundance of irons in collection areas such as Antarctica, where most of the meteoric material that has fallen can be recovered, it is possible that the actual percentage of iron-meteorite falls is lower than 5%. Stony-iron meteorites constitute the remaining 1%. They are a mixture of iron-nickel metal and silicate minerals. One type, called pallasites, is thought to have originated in the boundary zone above the core regions where iron meteorites originated. The other major type of stony-iron meteorites is the mesosiderites. Tektites (from Greek tektos, molten) are not themselves meteorites, but are rather natural glass objects up to a few centimetres in size which were formed—according to most scientists—by the impacts of large meteorites on Earth’s surface. A few researchers have favored Tektites originating from the Moon as volcanic ejecta, but this theory has lost much of its support over the last few decades.
Radiogenic Isotopes These are produced by radioactive decay and are important tools in geology. They are used in two principal ways: 1) In comparison with the quantity of the radioactive ‘parent isotope’ in a system, the quantity of the radiogenic ‘daughter product’ is used as a radiometric dating tool (e.g. uranium-lead geochronology); 2) In comparison with the quantity of a non-radiogenic isotope of the same element, the quantity of the radiogenic isotope is used as an isotopic tracer (e.g. 206Pb/204Pb). Parent (P) decays to Daughter (D) with a characteristic half-life i.e. uranium decaying to lead. We don’t need to know how much uranium there was originally as we are just measuring the ratio. Lead-lead system (using relationships between 204, 206 and 207 isotopes) gives an absolute age. Fractionating (e.g. during melting) separates parent from daughter. You can determine age intervals relative to lead-lead age. Different isotope systems date different processes.
26Al decaying to 26Mg happens relatively rapidly. 26Al was present when the grain was formed. The isotope was formed by high neutron flux e.g. during a supernova.
Nebula evolution Primitive materials in the solar system, found in both chondritic meteorites and comets, record distinct chemical environments that existed within the solar nebula. These environments would have been shaped by the dynamic evolution of the solar nebula, which would have affected the local pressure, temperature, radiation flux, and available abundances of chemical reactants. Short lived radionuclides allow us to investigate these processes (half lives of less than 1 Myr are required such as 26Al, 41Ca and 60Fe). The presence of daughte isotopes implies short times between production of the radioisotope and incorporated into solid material. The traditional view allows this to define a chronology that predates chondrules by about 1Myr.
Melted stony meteorites record the earliest processes of melting and igneous rock formation on asteroids. Different meteorites record the extent, timing and duration of melting and later magmatic activity of their parents, indicating the short timescale on which parent bodies aggregated, then differentiated. Broadly speaking, an achronite is a stony meteorite that formed from a melt on its parent body; it resembles basalts produced by igneous processes on the Earth. Therefore achrondites have differentiated compositions, having lost a large fraction of their primordial metal content, and generally do not contain chondrules. There are many different groups of achrondites. One of the largest groups is thought to come from asteroid 4 Vesta, or asteroids similar to Vesta.
4 Vesta above left and a Vesta meteorite on the right.
These meteorites form in a similar manner to iron processing. The slaggy stuff forms on the crust then stony iron and finally iron in the core. Therefore they are evidence for planetary evolution. They are also very rare. Stony-iron meteorites are a combination of silicate, or stone materials, in an iron matrix. They are quite rare and only account for about 1% of all meteorites. There are two different types of stony-iron meteorites: pallasites and mesosiderites. Pallasites contain green or golden olivine crystals embedded in a nickel-iron matrix. like iron meteorites, they display the Widmanstatten pattern in the nickel-iron matrix when polished and etched. Scientists believe that these meteorites formed when a planet was forming. The material from the molten metal core of the planet mixed with the silicate magma, and the olivine crystallized out of the silicate as it cooled. These crystals were then forced into the metal “mold” where the mass solidified. Mesosiderites consist of metal and fragments of rock. While pyroxene is the main stone element, no single crystals are found in the matrix. Instead, these pyroxene crystals are fragmented and scattered throughout the metal. One theory suggested that the silicate and metal portions were “smashed” together when they were partially molten. Tektites, which are often mistaken for meteorites, are silica-rich, impact-generated glass objects that are believed to have formed as a result of a meteorite impact. Some show signs of ablation, the unique melting action that is caused by friction that occurs when an object enters our atmosphere. Tektites may be formed when a large meteorite crashes into Earth and throws particles of terrestrial materials into space, where they re-melt as they descend through the atmosphere.
More evidence for planetary evolution. Hf likes to be with the stony mantle. Tungsten found in the metal core.
Presolar grains These are minor constituents of chondrites and they were formed before the Solar System originated. They are recognised on the basis of unusual stable isotope compositions. They are inorganic consisting of graphite, diamond SiC, corundum, Noble gases (Xe, Ne), carbon 13/12 and nitrogen 14/15. They are interstellar, circumstellar and thrown out of supernovae.
The study of presolar grains provides information about nucleosynthesis and stellar evolution. Grains bearing the isotopic signature of rapid-process and alpha capture nucleosynthesis are useful in testing models of supernovae explosions. Xenon is found after the S-process.
Silicon Carbide is not easy to find. Where the purple and blue regions merge in the above picture there are 40 to 50 astronomical processes.
A cloud of diamond crystallites made by more than one astronomical process. The diamonds were implanted with xenon outside the Solar System.
Meteorite studies have changed astronomers’ minds about galactic evolution (Silicon-carbide quantities do not follow the model and grains were formed at different times). Contrary to their opinion that nearby galaxies had achieved their present state 8 billion years ago, observations suggest galaxies were changing steadily during this time period. They found that the most distant blue galaxies exhibit disorganized motions in multiple directions, much different from disk-shaped galaxies where rotation dominates over other internal motions, such as the Andromeda Galaxy or the Milky Way. Gradually, disorganized motions dissipate and rotation speeds increase, whereby galaxies settle into organized disks. Susan Kassin, an astronomer at NASA’s Goddard Space Flight Centre in Greenbelt, Md., and the study’s lead researcher said, “Astronomers thought disk galaxies in the nearby universe had settled into their present form by about 8 billion years ago, with little additional development since. The trend we’ve observed instead shows the opposite, that galaxies were steadily changing over this time period.” Blue coloured galaxies where stars are forming, show less disorganized motions and ever-faster rotation speeds the closer they are observed to the present. This trend holds true for galaxies of all masses, but the most massive systems always show the highest level of organization. Researchers say the distant blue galaxies they studied are gradually transforming into rotating disk galaxies like our own Milky Way. Co-author Benjamin Weiner, an astronomer at the University of Arizona in Tucson said, “Previous studies removed galaxies that did not look like the well-ordered rotating disks now common in the universe today. By neglecting them, these studies examined only those rare galaxies in the distant universe that are well-behaved and concluded that galaxies didn’t change.”
Organic Interstellar Material This occurs as a variety of species: Aliphatic (straight and branch chains) and aromatic. Enriched with deuterium, Carbon 13 and Nitrogen 15, which act as tracers for ion-molecule reactions on grains in the interstellar material (astrochemistry). These show the evolution of molecular clouds and therefore the evolution of stars and planets. During star formation, interstellar molecules and dust become the building blocks for protostellar disks, from which planets, comets, asteroids and other macroscopic bodies such as meteorites form. Over a century ago, it was established that some meteorites contain carbonaceous material. These carbonaceous chondrites contain a few per cent of carbon and some of them exhibit a large variety of organic compounds. Isotopic analysis shows that grains found in primitive meteorites are formed in stellar atmospheres and thus represent samples of ancient stardust. Small bodies in the solar system, such as comets, asteroids and their fragments (e.g. meteorites, interplanetary dust particles) carry pristine material left over from the solar system formation process, thus sampling the molecular cloud material out of which the sun and planets formed. To search for organic matter in the different space environments therefore allows us to address the basic questions of our existence and to determine the nature of the material which impacted the young planets. External delivery of organic material is now widely accepted as an alternative or additional pathway to the internal production of such material on the early Earth and it may have contributed to the start of life.
The stardust mission (launched on February 7, 1999) refuted the general expectation that the particles collected from comet Wild 2 would be mainly dust that formed around other stars, dust that was older than the Sun. Such particles are called stardust or pre-solar grains and this was the main reason why the mission was named Stardust. What was found was remarkable! Instead of rocky materials that formed around previous generations of stars it was found that most of the comet’s rocky matter formed inside our solar system at extremely high temperatures. In great contrast to its ice, the comet’s rocky material had formed under white-hot conditions. Even though it was confirmed that Comets are ancient bodies with an abundance of ice, some of which formed a few tens of degrees above absolute zero at the edge of the solar system, we now know that comets are really a mix of materials made by conditions of both “fire and ice”. Comet ice formed in cold regions beyond the planet Neptune but the rocks, probably the bulk of any comet’s mass, formed much closer to the Sun in regions hot enough to evaporate bricks. The materials that was collected from comet Wild 2 do contain pre-solar “stardust” grains, identified on the basis of their unusual isotopic composition, but these grains are very, very rare. Among the high temperature materials some are already well known components of primitive meteorites; rocks from asteroids that formed between Mars and Jupiter. These include odd rounded particles called chondrules and white irregular particles known as Calcium Aluminium Inclusions (CAIs). Chondrules are the dominant material in many primitive meteorites and they are rounded droplets of rocks that melted and then quickly cooled as they orbited the Sun. CAIs are much rarer than chondrules and are distinguished by their unusual chemical and isotopic composition. They are also the oldest solar system materials and are composed of exotic minerals that form at the very high temperatures. It was very exciting to find that pieces of CAIs and chondrules in the comet and the scientific implications of this were profound. The discovery of chondrules and CAIs proves that matter abundantly formed in the inner solar system was somehow transported to the edge of the young solar system where comets formed. There are some theories that suggest that CAI’s formed just a few radii from the surface of the Sun, 4.567 billion years ago. The finding that inner solar system materials, formed at very high temperature, were transported all the way to the edge of the Solar System to the region where Pluto is one of the major scientific findings of Stardust. In other words, instead of being dominated by particles formed around other stars, our comet’s rocks were predominantly formed close to the Sun. Thus, these comet sample studies have provided a direct look at the nature and origin of the building blocks of planets, materials that were sprayed all over the young solar system and must have been incorporated into all planets, moons and meteorites. This does indicate that some meteorites may have originated from comets.
Comets have often been described as dirty snowballs but as the ice may actually be hidden beneath their crust they are also known as dirty balls. The tail of a comet has constituents similar to asteroids. When the Solar System was young there were more impacts of meteorites on the planets. On Earth this might have had two effects. The meteorites may have delivered the building blocks of life or they may have resulted in the extinction of life.
Rosetta space mission Rosetta is a robotic spacecraft of the European Space Agency on a mission to study the comet 67P/Churyumov–Gerasimenko. The probe was named after the Rosetta Stone, as it is hoped the mission will help form an idea of how the solar system looked before planets formed. In May 2014, the Rosetta craft will enter a slow orbit around the comet and gradually slow down in preparation for releasing a lander that will make contact with the comet itself. The lander, named “Philae”, will approach Churyumov–Gerasimenko at relative speed around 1 m/s and on contact with the surface, two harpoons will be fired into the comet to prevent the lander from bouncing off. Additional drills are used to further secure the lander on the comet. Once attached to the comet, expected to take place in November 2014, the lander will begin its science mission: Characterisation of the nucleus; Determination of the chemical compounds present, including enantiomers; Study of comet activities and developments over time. On March 16 2010 Rosetta made observation of the dust tail of the asteroid P/2010 A2. Together with observations of Hubble space telescope it could be confirmed that the P/2010 A2 is not a comet but an asteroid and the tail most likely consists of particles from an impact of a smaller asteroid. Rosetta was also able to measure the quantities of hydrogen, carbon and nitrogen isotopes.
In January 1982, John Schutt, leading an expedition in Antarctica for the ANSMET program, found a meteorite that he recognized to be unusual. Shortly thereafter, the meteorite now called Allan Hills 81005 was sent to Washington, DC, where Smithsonian Institution geochemist Brian Mason recognized that the sample was unlike any other known meteorite and resembled some rocks brought back from the Moon by the Apollo program. Several years later, Japanese scientists recognized that they had also collected a lunar meteorite, Yamato 791197, during the 1979 field season in Antarctica. About 134 lunar meteorites have been discovered so far (as of October, 2010), perhaps representing more than 50 separate meteorite falls (i.e., many of the stones are “paired” fragments of the same meteoroid). The total mass is more than 46 kg. All lunar meteorites have been found in deserts; most have been found in Antarctica, northern Africa, and the Sultanate of Oman. None have yet been found in North America, South America, or Europe. Lunar origin is established by comparing the mineralogy, the chemical composition, and the isotopic composition between meteorites and samples from the Moon collected by Apollo missions. Most lunar meteorites are launched from the Moon by impacts making lunar craters of a few kilometres in diameter or less. No source crater of lunar meteorites has been positively identified, although there is speculation that the highly anomalous lunar meteorite Sayh al Uhaymir 169 derives from the Lalande impact crater on the lunar nearside. Cosmic ray exposure history established with noble gas measurements have shown that all lunar meteorites were ejected from the Moon in the past 20 million years. Most left the Moon in the past 100,000 years. After leaving the Moon, most lunar meteoroids go into orbit around Earth and eventually succumb to Earth’s gravity. Some meteoroids ejected from the Moon get launched into orbits around the sun. These meteoroids remain in space longer but eventually intersect the Earth’s orbit and land. All six of the Apollo missions on which samples were collected landed in the central nearside of the Moon, an area that has subsequently been shown to be geochemically anomalous by the Lunar Prospector mission. In contrast, the numerous lunar meteorites are random samples of the Moon and consequently provide a more representative sampling of the lunar surface than the Apollo samples. Half the lunar meteorites, for example, likely sample material from the far side of the Moon.
A martian meteorite is a rock that formed on the planet Mars, was ejected from Mars by the impact of an asteroid or comet, and landed on the Earth. Of over 53,000 meteorites that have been found on Earth, 99 were identified as martian (as of July 2011). These meteorites are thought to be from Mars because they have elemental and isotopic compositions that are similar to rocks and atmosphere gases analysed by spacecraft on Mars. By the early 1980s, it was obvious that the SNC group of meteorites (Shergottites, Nakhlites, Chassignites) were significantly different from most other meteorite types. Among these differences were younger formation ages, a different oxygen isotopic composition, the presence of aqueous weathering products, and some similarity in chemical composition to analyses of the martian surface rocks in 1976 by the Viking landers. Several workers suggested these characteristics implied the origin of SNC meteorites from a relatively large parent body, possibly Mars (e.g., Smith et al. and Treiman et al.). Then in 1983, various trapped gases were reported in impact-formed glass of the EET79001 shergottite, gases which closely resembled those in the martian atmosphere as analysed by Viking. These trapped gases provided direct evidence for a martian origin. In 2000, an article by Treiman, Gleason and Bogard gave a survey of all the arguments used to conclude the SNC meteorites (of which 14 had been found at the time) were from Mars. They wrote, “There seems little likelihood that the SNCs are not from Mars. If they were from another planetary body, it would have to be substantially identical to Mars as it now is understood.” As of July 2011, 98 of the 99 Martian meteorites are divided into three rare groups of achondritic (stony) meteorites: shergottites (83), nakhlites (13), and chassignites (2), with the oddball meteorite Allan Hills 84001 usually placed within a specific “OPX group”. Consequently, Martian meteorites as a whole are sometimes referred to as the SNC group. They have isotope ratios that are said to be consistent with each other and inconsistent with the Earth. The names derive from the location of where the first meteorite of their type was discovered.
The famous specimen Allan Hills 84001 (seen above) has a different rock type than other martian meteorites: it is an orthopyroxenite (an igneous rock dominantly composed of orthopyroxene). For this reason it is classified within its own group, the “OPX martian meteorites”. This meteorite received much attention after an electron microscope revealed structures that were considered to be the fossilized remains of bacteria-like life forms. As of 2005, scientific consensus was that the microfossils were not indicative of Martian life, but of contamination by earthly biofilms. However, in 2009, new analyses ruled out earthly and non-biological origins, presenting strong evidence of life on Mars. ALH 84001 is as old as the basaltic and intermediate shergottite groups — i.e., 4.1 billion years old. The majority of SNC meteorites are quite young compared to most other meteorites and seem to imply that volcanic activity was present on Mars only a few hundred million years ago. The young formation ages of martian meteorites was one of the early recognized characteristics that suggested their origin from a planetary body such as Mars. Among martian meteorites, only ALH 84001 has a radiometric age older than about 1400 Ma (Ma = million years).
Several Martian meteorites have been found to contain what some think is evidence for fossilized Martian life forms. The most significant of these is a meteorite found in the Allan Hills of Antarctica (ALH 84001). Ejection from Mars seems to have taken place about 16 million years ago. Arrival on Earth was about 13 000 years ago. Cracks in the rock appear to have filled with carbonate materials (implying groundwater was present, i.e. warm fizzy water) between 4 and 3.6 billion-years-ago.
Question and answer session Somebody asked whether any progress in understanding what happened in Tunguska over one hundred years ago and Professor Grady said no. See http://en.wikipedia.org/wiki/Tunguska_event Somebody asked if our Sun had a predecessor and Professor Grady suggested that a supernova caused a molecular cloud to collapse and start the formation of our Solar System (and others).
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