Download Cosmic Samples & Origin of Solar System

Cosmic Samples
&
the Origin of the Solar System
1 March 2005
AST 2010: Chapter 13
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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)
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 fireball
Friction with the air vaporizes meteors at
altitudes between 80 and 130 km
Over 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 Showers
Many dust particles from a given comet retain
approximately the orbit of their parent, continuing to
move together through space
When the Earth crosses such a dust stream, a sudden
burst of meteor activity, called a meteor shower, is
produced
Such 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 the
radiant
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Meteorites: Stones from Heaven (1)
Any fragment of interplanetary debris that
survives its fiery plunge through Earth’s
atmosphere is called a meteorite
Their extraterrestrial (not from Earth) origin
was not accepted by scientists until the
beginning of the 19th century
Meteorites 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 meteorites
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Meteorites: Stones from Heaven (2)
Antarctica is now a major
source of meteorites
Astronomers believe that
meteorites carry a record
of the formation and early
history of the solar system
Radioactive dating of most
meteorites has produced ages
of about 4.5 billion years
Antarctic meteorite
Meteorites have been grouped into 3 classes
The irons, composed of nearly pure metallic nickel iron
The stones, composed of silicate or rock
The stony-irons, made of mixtures of stone and metallic
iron
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Formation of the Solar System
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Observational Constraints (1)
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 constraints
Chemical constraints
Age 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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Observational Constraints (2)
Motional constraints
All the planets revolve around the Sun in the same direction
and approximately in the plane of the Sun’s own rotation
Most of the planets spin in the same direction as they revolve
Most 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
Chemical constraints
Jupiter and Saturn are similar in composition (mostly
hydrogen and helium) to the Sun and other stars
The other planets are lacking in hydrogen and helium
The inner planets are metal rich, then farther out are rocky
objects, and furthest out are icy bodies
The general chemical pattern can be interpreted as a
temperature sequence: hot near the Sun and cooler as one
moves farther away from it
The exceptions to the general trends include the presence of
water on Earth and Mars
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Observational Constraints (3)
Age constraints (from radioactive dating)
Some rocks on Earth’s surface have ages of at
least 3.8 billion years
Certain lunar samples are 4.4 billion years old
Some meteorites have ages of about 4.5 billion
years
The 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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The 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 composition of the nebula is
similar to the Sun’s composition
today
As the cloud collapsed under its own
gravity, material fell toward the
center, where things became more
and more dense and hot
As material falls inward and the
collapsing nebula became smaller in
size, it began to rotate faster
(because of angular-momentum
conservation) and take the shape of
a disk
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The Solar Nebula Model (2)
At the end of the collapse phase, with no more
gravitational energy to heat it,
most of the nebula began to cool
But 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 energy
Material 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 planets
Some 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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Assessing the Solar Nebula Model
The 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 out
Powerful computers are used for
simulations
Computer simulation
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
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HST image
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Planetary Evolution
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Elevation Differences
Mountains on the terrestrial planets owe their
origins to different processes
On the Moon and Mercury, the major mountains
are ejecta thrown up by large crater-forming
impacts
The large mountains on Mars are volcanoes
On Earth and Venus, the highest mountains are
the result of compression and uplift of the surface
Highest mountains on Mars, Earth, and Venus
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Olympus Mons
Mountains
Why does Mars have
the highest mountain
in the solar system?
Possible reasons:
Mars does not have plate
tectonics that can
impede large volcanoes
There are multiple Hawaiian
islands because the Pacific
plate is moving over the hot spot beneath
Mars has lower surface gravity than Earth or Venus
Underlying 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
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Atmospheres
The atmospheres of the planets may have
been formed by a combination of gas escaping
from their interior and the impacts of volatilerich debris from the outer solar system
It is likely that all the terrestrial planets
originally had similar atmospheres
Mercury and the Moon were apparently too small to
retain their atmospheres
Venus seemed to have experienced a runaway
greenhouse effect
Mars probably underwent some kind of runaway
refrigerator effect
Earth . . . was lucky?
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Conclusion
There is still much to learn about the origin
and evolution of the solar system
Space probes (spacecraft & advanced
telescopes) continue to add to our
understanding
In the last 10 years, astronomers found more
than 100 “planets” orbiting other stars
Perhaps studies of these distant “planetary”
systems will yield better understanding of our own
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