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Cosmic Samples Cosmic Samples & & the the Origin of the Origin of the

Solar SystemSolar System

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Meteors (1)Meteors (1)When comets approach the Sun, the ices in them are heated and evaporate, spraying millions of tons of dust and rock into the inner solar system

The Earth is surrounded by this material

When one of the larger dust or rock particles enters the Earth’s atmosphere, it creates a brief fiery tail known as a meteor

It is popularly called a shooting star, although it has no connection to a real star

Meteors may also come from other interplanetary debris

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Meteors (2)Meteors (2)If the particle that produces a meteor is large enough (say, the size of a golf ball), a much brighter trail will be produced, called a fireballFriction with the air vaporizes meteors at altitudes between 80 and 130 kmOver the entire Earth, the total number of meteors bright enough to be visible is estimated to be about 25 million per day

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Meteor ShowersMeteor ShowersMany dust particles from a given comet retain approximately the orbit of their parent, continuing to move together through spaceWhen the Earth crosses such a dust stream, a sudden burst of meteor activity, called a meteor shower, is produced

Such a meteor activity can last for several hours

From the ground, the paths of the shower meteors appear to diverge from a place in the sky called a radiant

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Meteorites: Stones from Heaven Meteorites: Stones from Heaven (1)(1)

Any fragment of interplanetary debris that survives its fiery plunge through Earth’s atmosphere is called a meteoriteTheir extraterrestrial (not from Earth) origin was not accepted by scientists until the beginning of the 19th centuryMeteorites are found in two ways:

Someone tracking a meteor fireball to the ground (a meteorite fall)Someone finding an unusual looking rock (a meteorite find)

A variety of meteoritesA variety of meteorites

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Meteorites: Stones from Heaven Meteorites: Stones from Heaven (2)(2)Antarctica is now a major source

of meteoritesThey tend to stand out in contrast to the ice

Astronomers believe that meteorites carry some clues about the early history of the solar system

Radioactive dating of most meteorites has produced ages of about 4.5 billion years

Meteorites have been grouped into 3 classesThe irons, composed of nearly pure metallic nickel ironThe stones, composed of silicate or rockThe stony-irons, made of mixtures of stone and metallic iron

Antarctic meteoriteAntarctic meteorite

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MeteoritesMeteoritesThe irons and stony-irons are the most obviously extraterrestrial

Pure iron almost never occurs naturally on EarthAny chunk of metallic iron found on Earth is sure to be either man-made or a meteorite

Frequency of occurrence of meteorites

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Formation of the Solar SystemFormation of the Solar System

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Observational Constraints Observational Constraints Any theory of the formation of the solar system must be able to explain certain basic properties of the solar system

These include some of the information scientists have accumulated about the Sun, planets, moons, rings, asteroids, and comets

There are three types of constraints that a theory must satisfy:

Motional constraintsChemical constraintsAge constraints

A full theory must also be prepared to deal with the irregularities (exceptions to the general trends) in the solar system

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Motional ConstraintsMotional ConstraintsMotional constraints

All the planets revolve around the Sun in the same direction and approximately in the plane of the Sun’s own rotationMost of the planets spin in the same direction as they revolveMost of the satellites also rotate and revolve in the same direction (counterclockwise when seen from the north)There are exceptions that the theory must handle, like Venus’ retrograde rotation

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Chemical ConstraintsChemical ConstraintsChemical constraints

Jupiter and Saturn are similar in composition (mostly hydrogen and helium) to the Sun and other starsThe other planets are lacking in hydrogen and heliumThe inner planets are metal rich, then farther out are rocky objects, and furthest out are icy bodiesThe general chemical pattern can be interpreted as a temperature sequence: hot near the Sun and cooler as one moves farther away from itThe exceptions to the general trends include the presence of water on Earth and Mars

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Age ConstraintsAge ConstraintsAge constraints (from radioactive dating)

Some rocks on Earth’s surface have ages of at least 3.8 billion yearsCertain lunar samples are 4.4 billion years oldSome meteorites have ages of ~4.5 billion yearsThe similarity of the measured ages suggests to astronomers that the planets formed, and their crusts cooled, relatively “rapidly”

within a few hundred million years of the beginning of the solar system

Examination of primitive meteorites indicates that they are made mostly from material that condensed or coagulated out of a hot gas

Few identifiable fragments appear to have survived from before the formation of the solar system

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Solar Nebula Model (1)Solar Nebula Model (1)All the constraints are consistent with the general idea that the solar system formed 4.5 billion years ago out of a rotating cloud of hot vapor and dust called the solar nebula

The nebula is similar in composition to the Sun today

As the cloud collapsed under its own gravity, material fell toward the center, where things became more and more dense and hot At the same time, the collapsing nebula began to rotate faster (because of angular-momentum conservation) and take the shape of a disk

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Solar Nebula Model (2)Solar Nebula Model (2)At the end of the collapse phase, when the nebula was at its hottest, with no more gravitational energy to heat it, most of it began to coolBut the material at the center, where it was hottest and densest, formed a star, the Sun, that was able to keep high temperatures in its immediate neighborhood by producing its own energyMaterial away from the center began to condense, forming solid grains which quickly joined into larger and larger chunks leading to the formation of planetesimals, which are the precursors of the planetsSome planetesimals were large enough to join their neighbors gravitationally and thus grew by accretion into protoplanets Finally, protoplanets grew, also by accretion, into planets

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Geological Stages of Terrestrial PlanetsGeological Stages of Terrestrial Planets

The smaller the planet, the more quickly it passes through these stages

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Elevation DifferencesElevation DifferencesMountains on the terrestrial planets owe their origins to different processesOn the Moon and Mercury, the major mountains are ejecta thrown up by large crater-forming impactsThe large mountains on Mars are volcanoesOn Earth and Venus, the highest mountains are the result of compression and uplift of the surface

Highest mountains on Mars, Earth, and VenusHighest mountains on Mars, Earth, and Venus

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MountainsMountainsWhy does Mars have the highest mountain in the solar system? Possible reasons:

Mars does not have plate tectonics that can impede the growth of a large volcano

In contrast, the continual motion of the Pacific plate has led to the creation of multiple Hawaiian islands

Mars has lower surface gravity than Earth or VenusUnderlying material can more easily support the weight of the mountain above (the mountain “weighs” less)

Mars has a thin atmosphere and little erosion to reduce the height over a very long time

Olympus MonsOlympus Mons

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AtmospheresAtmospheresThe atmospheres of the planets may have been formed by a combination of gas escaping from their interior and the impacts of volatile-rich debris from the outer solar systemIt is likely that all the terrestrial planets originally had similar atmospheres

Mercury and the Moon were apparently too small to retain their atmospheresVenus seemed to have experienced a runaway greenhouse effectMars probably underwent some kind of runaway refrigerator effectEarth . . . was lucky?

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Assessing the Solar Nebula ModelAssessing the Solar Nebula ModelThe solar nebula model attempts to explain how the solar system may have formed

The model is still “evolving” and many of its details are yet to be worked outPowerful computers are used for simulations

The model continues to be evaluated and refined by confronting it with observations of our solar system and of other planetary systems

For the past decade, astronomers have discovered more than one hundred giant “planets” near other stars

Computer simulationComputer simulation

HST image

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ConclusionConclusionThere is still much to learn about the origin and evolution of the solar system

New findings sometimes surprise or contradict long-held theories and viewpoints

Space probes (spacecraft & advanced telescopes) continue to add to our understandingFor the last 10 years or so, astronomers have found more than 100 “planets” orbiting other stars

Perhaps studies of these distant “planetary” systems will yield better understanding of our own