1 Chapter 12 The Origin of the Solar System - Bolides - Meteorites - Age determination of solar system bodies -The story of planet formation: - From the

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Chapter 12The Origin of the Solar System

- Bolides- Meteorites

- Age determination of solar system bodies-The story of planet formation:

- From the beginning to planetesimal formation- Accumulation into protoplanets

- Further collisional evolution

ASTA01 @ UTSC – Lecture 14

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Bolides = biggest meteors

• 22 April 2012, over central California and Nevada• Minivan-sized• ~70 ton mass, 5 kton TNT explosion• ~1/yr

http://www.youtube.com/watch?v=RrL-cWaYdno

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Meteoroids, Meteors, and Meteorites

• Most meteoroids are specks of dust, grains of sand, or tiny pebbles. • Almost all the meteors you see in the sky are produced

by meteoroids that weigh less than 1 g. • Only rarely is one massive enough and strong enough to

survive its plunge, and reach Earth’s surface.• Such a rock is called a meteorite (this one was found in

Canada)

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• Meteorites can be divided into three broad categories.

• Iron meteorites are solid chunks of iron and nickel. • Stony meteorites are silicate masses that resemble

Earth rocks. • Stony-iron meteorites are iron-stone mixtures.

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• One type of stony meteorite called carbonaceous chondrites has a chemical composition that resembles a cooled lump of the Sun gas with the hydrogen and helium removed.

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• These meteorites generally contain abundant volatile compounds including significant amounts of carbon and water.• They may have similar composition to comet

nuclei.

• Allende meteorite

found in Mexico

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• Heating would have modified and driven off these fragile compounds.• So, carbonaceous chondrites must not have been

heated since they formed. • Astronomers conclude that carbonaceous chondrites, unlike

the planets, have not evolved and thus give direct information about the early solar system.

• Ca+Al inclusion in a chondrite• Sign of original heating:• Lightnings in the protoplanetary

disk?

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• You can find evidence of the origin of meteors through one of the most pleasant observations in astronomy: a meteor shower, a display of meteors that are clearly related by a common origin.

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• For example, the Perseid meteor shower occurs each year in August.

• Orionids have a peak on 21 Oct (2012)• During the height of the shower, you might

see 25-40 meteors per hour. • The showers are so named because all their meteors

appear to come from a point in the constellation Perseus and Orion, respectively• The fall rate is ONLY <1 /min, so don’t expect to see

many meteors at the same time

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• Meteor showers are seen when Earth passes near the orbit of an active or a ‘dead’ comet.

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• The meteors in meteor showers are produced by dust and debris released from the icy head of the comet.

• The Orionids, as well as Eta Aquarids meteor shower (best viewed from the southern hemisphere) is caused by the Earth passing through dust released by Halley’s Comet.

• Orbits of many meteorites have been calculated to lead back into the asteroid belt.

• Asteroid collisions are the 2nd major source of meteoroids and account for the background of meteors: in a shower the number of meteors is typically only 4 times higher than during other times.

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• Meteorites provide a specific clue concerning the solar nebula: meteorites can reveal the age of the solar system and the relative ages of its components.• The challenge for modern planetary

astronomers is to compare the characteristics of the solar system with the solar nebula theory and tell the story of how the planets formed. They also need to explain the diverse features of exoplanetary systems (extrasolar systems), that is solar systems other than our own.

Empirical determination of ages

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The Age of the Solar System components

• The most accurate way to find the age of a rocky body is to bring a sample into the laboratory and determine its age by analyzing the radioactive elements it contains.• When a rock solidifies, the process of cooling

causes it to incorporate known proportions of the chemical elements.

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The Age of the Solar System

• A few of those elements are radioactive and can decay into other elements, called daughter elements or isotopes.• The half-life of a radioactive element is the

time it takes for half of the radioactive atoms to decay into the daughter elements.

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• For example, potassium-40 decays into daughter isotopes calcium-40 and argon-40 with a half-life of 1.3 billion years. • Also, uranium-238

decays with a half-life of 4.5 billion years to lead-206 and other isotopes.

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• As time passes, the abundance of a radioactive element in a rock gradually decreases, and the abundances of the daughter elements gradually increase.

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• Measuring the present abundances of the parent and daughter elements allows you to find the age of the rock. • This works best if you have several

radioactive element “clocks” that can be used as independent checks on each other.

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Itokawa

• Asteroid Itokawa dust

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Haybusa returns from asteroid Itokawa, June 2010

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• Hayabusa mission during 2003-2010 to asteroid Itokawa brought back dust samples.

• Reentry videos resemble a bolide • http://www.youtube.com/watch?v=ZCdGQhYLo30• http://www.youtube.com/watch?v=NoSpRoGNr5M

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Itokawa asteroid dust: battered by nono-impacts

• Asteroid Itokawa dust

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Stardust mission (1999-2006) to comet Wild 2

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Comet Wild 2 surprises

• The material of the comet formed in the coldest parts of the solar system

• …from particles which were predominantly not pristine stardust (extrasolar particles) but from highly heated particles of solar materials (olivines, pyroxenes etc.)

• How did the hot stuff end up in the outer solar system?

• Perhaps the solar nebula (protoplanetary disk) was initially very hot everywhere

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• The oldest Earth rocks so far discovered and dated are tiny zircon crystals from Australia, 4.4 billion years old.

• The surface of Earth is active, and the crust is continually destroyed and reformed from material welling up from beneath the crust.• The age of these oldest rocks informs you

only that Earth is at least 4.4 billion years old.

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• Unlike Earth’s surface, the Moon’s surface is not being recycled by constant geologic activity.• So, you can guess that more of it might have

survived unaltered since early in the history of the solar system.

• The oldest rocks brought back by the Apollo astronauts are 4.48 billion years old. The Moon must be at least that old.

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• Although no one has yet been to Mars, over a dozen meteorites found on Earth have been identified by their chemical composition as having come from Mars. • The oldest has an age of approximately 4.5

billion years.• Mars must be at least that old.

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• The most important source for determining the age of the solar system is meteorites. • Carbonaceous chondrite meteorites have

compositions indicating that they have not been heated much or otherwise altered since they formed.

• They have a range of ages with a consistent and precise upper limit of 4.56 billion years.

• The spread of ages between different meteorite groups is a few million years, less than +-0.01 billion years

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• That is in agreement with the age of the Sun, which is estimated to be (5 +-1.5) Gyr• This has been calculated using mathematical

models of the sun’s interior that are completely independent of meteorite radioactive ages.• Apparently, all the bodies of the solar system formed at

about the same time, some 4.56 billion years ago.

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The Origins of the Solar System

• According to the solar nebula theory, the planets should be about the same age as the Sun. Here is the brief timeline of events:

• The primordial gas cloud collapses in about 105 yrs, forming a rotating disk/nebula

• The nebula cools down sufficiently for silicate rocks to condense in the form of dust in ~105 yrs

• Turbulence in the disk dies down sufficiently to allow settling of dust into a thin layer in the midplane of the disk in several times 105 yrs (Remember Leukippos and Democritos? They’ve predicted both the rotating nebula and that process! Of course not the time scale.)

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• During the settling, the dust agglomerates into sand and pebbles by collisions and electrostatic forces.

• Next, the sub-layer undergoes instability: gravity of the thin layer of dust and stones fragments it into dense chunks which soon shrink to become km-size planetesimals. Less than 1 Myr (1 million years passed at this time from the beginning of the stellar formation process).

• The nebula enters a period of slow evolution lasting 1-3 Myr, in most cases (although we have observations of disks which still have a substantial amount of primordial hydrogen+helium gas while 10 Myr old; one of them is called TW Hydrae).

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• During these several millions of years, terrestrial planets and solid cores of giant planets assemble in mutual collisions of smaller solid bodies, planetesimals. Since a large portion of the nebula is at low temperatures, ices as well as silicates dominate the chemical composition of planetesimals.

• After the largest bodies reach the size > 10 km, their gravity is substantial enough to speed up their buildup in a “runaway” fashion. Isolated protoplanets grow in such a way, out of reach of each other’s perturbing gravity force.

• The growth of cores is curbed by the lack of material located on close enough orbits, which can be destabilized and accreted (meaning: absorbed) by a protoplanet.

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• After several Myr, planets in the inner solar system exhaust the supply of material and stop growing.

• At the same time, giant planet cores grow to the mass ~10 Earth masses. At that point, their massive hydrogen & helium atmospheres become unstable and in ~0.1 Myr they acquire a very massive gaseous envelope.

• After the lifetime of the disks expires (3-10 Myr), they are dispersed, but giant planets are already gas-rich.

• Active planet formation is over after 10-30 Myr • The next long stage is the removal of Kuiper belt bodies,

mainly comets in the region beyond Jupiter; it can last up to 500 Myr.

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Chemical Composition of the Solar Nebula

• Everything astronomers know about the solar system and star formation suggests that the solar nebula was a fragment of an interstellar gas cloud. • Such a cloud would have been mostly

hydrogen (75% mass) with some helium (23%) and minor traces of the heavier elements (1.22-1.94)%.

• The lower value is the recently revised average solar composition.

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• Of course, in the sun nuclear reactions have fused some hydrogen into helium.• This, however, happens in the core and has

not affected its surface composition. • Thus, the composition revealed in its spectrum is

essentially the same composition of the solar nebula gases from which it formed.

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• You can see that same solar nebula composition is reflected in the chemical compositions of the planets.

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• The composition of the Jovian planets resembles the composition of the Sun.• Furthermore, if you allowed low-density gases

to escape from a blob of sun-stuff, the remaining heavier elements would resemble the composition of the other terrestrial planets – as well as meteorites.

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• The key to understanding the process that converted the nebular gas into solid matter is the observed variation in density among solar system objects.

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Condensation of Solids

• The four inner planets are high-density, terrestrial bodies.• The outer, Jupiter-like planets are low-density,

giant planets.• This division is due to the different ways gases are

condensed into solids in the inner and outer regions of the solar nebula.

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• Even among the terrestrial planets, you find a pattern of slight differences in density. • The uncompressed densities – the densities

the planets would have if their gravity did not compress them – can be calculated from the actual densities and masses of each planet.

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• In general, the closer a planet is to the Sun, the higher is its uncompressed density.• This density variation is understood to have

originated when the solar system first formed solid grains.• The kind of matter that is condensed in a particular

region would depend on the temperature of the gas there.

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• In the inner regions, the temperature seems to have been 1500 K or so.• The only materials that can form grains at this

temperature are compounds with high melting

points, such as metal oxides and pure metals.• These are very dense, corresponding to the

composition of Mercury.

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• Farther out in the nebula, it was cooler.• Silicates (rocky material) could condense.• These are less dense than metal oxides and

metals, corresponding more to the compositions of Venus, Earth, and Mars.

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• Somewhere further from the Sun, there was a boundary called the ice line – beyond which the water vapour could freeze to form ice.

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• The sequence in which the different materials condense from the gas as you move away from the Sun is called the condensation sequence.

• It suggests that the planets, forming at different distances from the Sun, accumulated from different kinds of materials.

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• The original chemical composition of the solar nebula should have been roughly the same throughout the nebula.

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• Astronomers have recently found that Jupiter is rather poor in Oxygen but overabundant in Carbon. Isotopic and elemental abundances (mass ratios of elements and different sub-species of elements) in Jupitr are like those in asteroids, not in comets, which contradicts the importance of the ice line to inner-vs-outer planet division• Other reasons for relative unimportance of ice line are:• Migration and thus mixing of solids in the solar nebula • Roughly 1:1 mass ratio of ice and rock in comets – not

important enough for locating Jupiter at 5.2 AU from sun.

Documents

1 Chapter 12 The Origin of the Solar System - Bolides - Meteorites - Age determination of solar system bodies -The story of planet formation: - From the