Download Origin of the Solar System – Notes Rings encircle Jupiter, Saturn

Origin of the Solar System – Notes
Rings encircle Jupiter, Saturn, Uranus, and Neptune; and impact craters dot the surfaces of Mercury, Venus,
Earth, and Mars, showing that all of these planets have been bombarded by interplanetary debris.
The orbits of the four inner planets (Mercury, Venus, Earth, and Mars)
are crowded in close to the Sun. In contrast, the orbits of the next four
planets (Jupiter, Saturn, Uranus, and Neptune) are widely spaced at great
distances from the Sun.
Most of the planets have orbits that are nearly circular, with the notable
exception of Mercury.
If you could observe the solar system from a point several astronomical
units (AU) above Earth’s north pole, you would see that all the planets
orbit the Sun in the same counter clockwise direction. Furthermore, the
orbits of the eight planets all lie in nearly the same plane.
Until the late 1990s, Pluto was generally regarded as the ninth planet. But in light of recent discoveries many
astronomers now consider Pluto to be simply one member of a large collection of trans-Neptunian objects that
orbit far from the Sun. These objects all orbit the Sun in the same direction as the planets, though many of them
have orbits that are steeply inclined to the plane of the ecliptic and have high eccentricities (that is, the orbits are
quite elongated and noncircular).
When we compare the physical properties of the planets, we find that they fall naturally into two classes—four
small inner planets and four large outer ones. The four small inner planets are called terrestrial planets because
they resemble Earth (in Latin, terra). They all have hard, rocky surfaces with mountains, craters, valleys, and
volcanoes. You could stand on the surface of any one of them, although you would need a protective spacesuit
on Mercury, Venus, or Mars. The four large outer planets are called Jovian planets because they resemble
Jupiter. (Jove was another name for the Roman god Jupiter.) An attempt to land a spacecraft on the surface of
any of the Jovian planets would be futile, because the materials of which these planets are made are mostly
gaseous or liquid. The visible “surface” features of a Jovian planet are actually cloud formations in the planet’s
atmosphere.
The most apparent difference between the terrestrial and
Jovian planets is their diameters. The Jovian planets are much
larger than the terrestrial planets.
The masses of the terrestrial and Jovian planets are also
dramatically different. Astronomers have found that the four
Jovian planets have masses that are tens or hundreds of times
greater than the mass of any of the terrestrial planets.
Once we know the diameter and mass of a planet, we can
learn something about what that planet is made of. The trick
is to calculate the planet’s average density, or mass divided
by volume.
The four inner, terrestrial planets have very high average densities with dense iron cores. Chemical analysis of
soil samples from Venus, Earth, and Mars demonstrate that the terrestrial planets are made mostly of heavier
elements, such as iron, oxygen, silicon, magnesium, nickel, and sulphur.
The outer, Jovian planets have quite low densities. The outer layers of the Jovian planets are composed
primarily of the lightest gases, hydrogen and helium.
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Temperature plays a major role in determining whether the materials of which planets are made exist as solids,
liquids, or gases. Hydrogen and helium are gaseous except at extremely low temperatures. By contrast, rockforming substances such as iron and silicon are solids except at temperatures well above 700°C.
The Jovian planets are sometimes called “gas giants.” It is true that their primary constituents, including
hydrogen, helium, ammonia, and methane, are gases under normal conditions on Earth. But in the interiors of
these planets, pressures are so high that these substances are liquids, not gases. The Jovian planets might be
better described as “liquid giants”!
As you might expect, a planet’s surface temperature is
related to its distance from the Sun. The four inner
planets are quite warm. The outer planets, which receive
much less solar radiation, are cooler.
All the planets except Mercury and Venus have moons.
More than 160 moons are known: Earth has one (the
Moon), Mars has two, Jupiter has at least 63, Saturn at
least 61, Uranus at least 27, and Neptune at least 13.
Like the terrestrial planets, all of the moons of the
planets have solid surfaces.
In addition to the eight planets, many smaller objects orbit the Sun. Asteroids and meteoroids are rocky or
metallic and are found in the inner solar system, between the Sun and Jupiter. Asteroids are generally considered
to be larger than 1 metre-wide while meteoroids are smaller than 1 metre-wide. Trans-Neptunian objects are
found beyond Neptune in the outer solar system and contain rock and ice. Comets are mixtures of rock and ice
that originate in the outer solar system, beyond Neptune, but can venture close to the Sun.
Hundreds of thousands of kilometre-sized asteroids are known, and there are probably hundreds of thousands
more asteroids and meteoroids that are boulder-sized or smaller. All of these objects orbit the Sun in the same
direction as the planets. Most asteroids and meteoroids orbit the Sun at distances of 2 to 3.5 AU. This region of
the solar system between the orbits of Mars and Jupiter is called the asteroid belt.
Trans-Neptunian objects are small bodies whose orbits lie beyond the orbit of Neptune. The first of these to be
discovered (1930) was Pluto. Pluto is larger than any asteroid, but smaller than any planet. Pluto’s density is
only 2000 kg/m3. Hence, its composition is thought to be a mixture of about 70% rock and 30% ice.
Like asteroids and meteoroids, all trans-Neptunian objects orbit the Sun in the same direction as the planets. A
handful of trans-Neptunian objects are comparable in size to Pluto; at least one, Eris, is even larger than Pluto.
Just as most asteroids lie in the asteroid belt, most transNeptunian objects orbit within a band called the Kuiper belt
(pronounced “ki-per”) that extends from 30 AU to 50 AU from
the Sun. Astronomers estimate that there are 35,000 or more
trans-Neptunian objects with diameters greater than 100 km.
Like asteroids, trans-Neptunian objects are thought to be debris
left over from the formation of the solar system. In the inner
regions of the solar system, rocky and metallic fragments have
been able to endure continuous exposure to the Sun’s heat, but
any ice originally present would have evaporated. Far from the
Sun, ice has survived for billions of years. Thus, debris in the
solar system naturally divides into two families (asteroids and
meteoroids, and trans-Neptunian objects), which can be arranged
according to distance from the Sun.
2.
Two objects in the Kuiper belt can collide if their orbits cross each other.
When this happens, a fragment a few kilometres across can be knocked off
one of the colliding objects and be diverted into an elongated orbit that
brings it close to the Sun. Such small objects, each a combination of rock
and ice, are called comets. When a comet comes close enough to the Sun, the
Sun’s radiation vaporizes some of the comet’s ices, producing long flowing
tails of gas and dust particles. Some comets appear to originate from
locations far beyond the Kuiper belt. The source of these is thought to be a
swarm of comets that forms a spherical “halo” around the solar system
called the Oort comet cloud (also known as Oort cloud). This “halo’ extends to 30,000 AU from the Sun (about
one-fifth of the way to the nearest other star).
When an asteroid, meteoroid, or comet enters Earth’s atmosphere, it becomes a meteor, generating light. This
phenomenon is commonly referred to as a “shooting star”. If a meteor reaches Earth’s surface, it is referred to as
a meteorite.
We can gather important clues about the interiors of terrestrial planets and
satellites by studying the extent to which their surfaces are covered with craters.
The planets orbit the Sun in roughly circular orbits. But many asteroids and
comets are in more elongated orbits. Such an elongated orbit can put these small
objects on a collision course with a planet or satellite. If the object collides with a
Jovian planet, it is swallowed up by the planet’s thick atmosphere. But if the
object collides with the solid surface of a terrestrial planet or a satellite, the result
is an impact crater.
The easiest way to view impact craters is to examine the Moon through a telescope or binoculars. Some 30,000
lunar craters are visible from Earth; with diameters ranging from 1 km to several hundred kilometres. Close-up
photographs from lunar orbit have revealed millions of craters too small to be seen from Earth.
Nearly all craters are circular. If craters were merely gouged out by high-speed rocks, a rock striking the Moon
in any direction except straight downward would have created a noncircular crater. An asteroid or meteoroid
colliding with the Moon generates a shock wave in the lunar surface that spreads out from the point of impact.
Such a shock wave produces a circular crater no matter what direction the asteroid meteoroid was moving.
Many of the larger lunar craters also have a central peak, which is characteristic of a high-speed impact.
The Moon is heavily cratered over its entire surface, with craters on top of craters. On Earth, by contrast, craters
are very rare. Geologists have identified fewer than 200 impact craters on our planet.
The Earth and the Moon formed at nearly the same time and have been bombarded at comparable rates over
their histories. But Earth is a geologically active planet: the continents slowly change their positions over eons,
new material flows onto the surface from the interior (as occurs in a volcanic eruption), and old surface material
is pushed back into the interior (as occurs off the coast of Chile, where the ocean bottom is slowly being pushed
beneath the South American continent). These processes, coupled with erosion from wind and water, cause
craters on Earth to be erased over time. The few craters found on Earth today must be relatively recent, since
there has not yet been time to erase them.
The Moon, by contrast, is geologically inactive. There are no volcanoes and no motion of continents (and,
indeed, no continents). Furthermore, the Moon has neither oceans nor an atmosphere, so there is no erosion, as
we know it on Earth. With none of the processes that tend to erase craters on Earth, the Moon’s surface remains
pockmarked with the scars of billions of years of impacts.
In order for a planet to be geologically active, its interior must be at least partially molten. This partially molten
state is necessary so that continents can slide around on the underlying molten material and so that molten lava
can come to the surface, as in a volcanic eruption. Hence, geologically inactive (and hence heavily cratered)
worlds like the Moon have less molten material in their interiors than does Earth.
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We have uncovered a general rule for worlds with solid surfaces: The smaller the terrestrial world, the less
internal heat it is likely to have retained, and, thus, the less geologic activity it will display on its surface. The
less geologically active the world, the older and hence more heavily cratered its surface.
An important exception to our rule is Jupiter’s satellite Io, which, despite its small size, is the most volcanic
world in the solar system. The energy comes from Jupiter, which exerts powerful tidal forces on Io as it moves
in a relatively small orbit around its planet. These tidal forces cause Io to flex like a ball of clay being kneaded
between your fingers, and this flexing heats up the satellite’s interior.
The solar system is believed to have begun forming about 4.6 billion years ago from a gigantic cloud of gas and
dust called the solar nebula.
Over millions of years it collapsed into a flat, spinning disk, with a hot dense central region. The central part of
the disk eventually became the Sun. The planets and everything else formed from a portion of the remaining
material.
No one knows for certain what caused the solar nebula to start to collapse. What is certain is that gravity
somehow overcame the forces associated with gas pressure that would otherwise have kept it expanded.
The disk began to rotate faster as it contracted. The central region also became hotter and denser.
In the parts of the disk closest to this hot central region, only rocky particles and metals could remain in solid
form. These rocky and metallic particles gradually came together to form planetesimals and then the inner rocky
planets. A planetesimal is an object in the protoplanetary disk that will either eventually clump together with
other planetesimals to form a planet or remain isolated as an asteroid or meteoroid. In the cooler outer regions a
similar process occurred, but the solid particles that came together to form planetesimals contained large
amounts of various ices as well as rock. These eventually became the cores of the gas-giants.
After tens of millions of years of planetesimal formation, the final stages of planet construction are thought to
have happened relatively quickly, about 4.56 billion years ago.
Once the planetesimals were a few miles in diameter their gravity was strong enough to attract more and more
material. Many planetismals came together to form Moon-sized bodies called protoplanets, which underwent a
series of dramatic collisions to form the rocky planets and the cores of the outer gas-giants. Many of the leftover
planetesimals are thought to have become comets, asteroids and meteoroids.
In total, the solar system is about 15 trillion kilometres across. The planets occupy a zone just 6 billion
kilometres from the Sun.
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