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NASA (larger image and
other side)
The Planet
The third planet from the Sun, Earth is the fifth
largest with a diameter of 12,756 kilometers (7,928 miles). It is the
densest major body in the solar system and has an unusually large
satellite for its size, as Earth's Moon has about a quarter of its
diameter.
Many astronomers now believe that the Earth was formed from the
collision of a Mars-sized protoplanet with the primordial Earth. Much
of the protoplanet's core merged with the Earth's own, while the
lighter materials of the collision reformed as the Moon
(see schematic
view of the collision). The gravitational interaction of the Earth
with its Moon slows the Earth's rotation by about two milliseconds per
century, so that about 900 million years ago, Earth's "year" was
comprised of 481 "days" that lasted only 18 hours long.
NASA -- larger
image
The primordial Earth probably had much more carbon dioxide, but
virtually all of it has been incorporated into carbonate rocks,
dissolved into the oceans, or incorporated into living plants.
However, the tiny amount of carbon dioxide still in the atmosphere
at any time warms the Earth's average surface temperature by about
35 °C (95 °F) above what it would otherwise be (from a frigid
-21 °C/-17 °F to a comfortable 15 °C/59 °F) via the greenhouse effect.
Without this improved retention of solar heat, the oceans would freeze.
On the other hand, the activities of Earth's dominant lifeform (Humans)
is increasing the level of carbon dioxide in Earth's atmosphere which
appears to be promoting a rising trend in global temperatures.
Earth is the only planet on which water can exist in liquid form on the
surface, and most (71 percent) of the planet's surface is indeed
covered with water. The heat-absorbing capacity of Earth's oceans is
crucial to keeping the planet's temperature relatively stable. Liquid
water is also responsible for most of the erosion and weathering of the
Earth's continents, a process that is unique in the Solar System today,
although it may have occurred on Mars in the past).
Most of Earth's surface is very young and active. Within an
astronomically short period (500 million years or so), erosion and
tectonic processes destroy and recreate most of its surface, and so
evidence of earlier geologic history including even large impact
craters is quickly erased. Thus, while the Earth is about 4.56
billion years old, the oldest known rocks are about four billion years
old, and rocks older than 3 billion years are rare. The oldest fossils
of living organisms are less than 3.9 billion years old, and no record
of the critical period when life was first
getting started has yet been detected and recognized.
Unlike the other three terrestrial planets, Earth's crust is divided
into eight large and 20 small, solid "plates" which float around
independently on top of the semi-fluid mantle below. Under the modern
the theory of plate tectonics, some plates move away from each other
and new crust is created by upwelling magma from below in a process
called "spreading." Other plates are being destroyed through the
process of "subduction," where plates collide and the edge of one
dives beneath the other to be melted in the hot mantle below.
Once in a while, a truly massive plume of hot rock from the Earth's
mantle can erupt through the crust for centuries or even millenia,
producing acid rains, destruction of the ozone layer from emissions of
chlorine-bearing compounds, and a chill down in climate from the
resulting increase in atmospheric dust and sulphur-based aerosols.
The last such eruption occurred some 65 million years ago
(press
release, Basu et al, 1993), created India's
Deccan
Traps, and -- in combination with the Chicxulub meteorite impact
-- contributed to the
extinction
of the dinosaurs. An earlier and even larger event in Siberia
in combination with
an
impact by a meteorite that may have been slightly larger than
the one associated with the Dinosaur extinction event -- that is,
3.7 to 7.4 miles wide (six to 12 km) and equivalent to a
magnitude-12 earthquake -- may have caused the
Permian-Triassic
extinction 251 million years ago, when 90 to 95 percent of all
species were wiped out. The researchers
(U.R.
and U.W.
press releases, Becker et al, 2001) suggest that perhaps in both
extinctions, a plume of lava may have already risen close to the
surface, but a massive meteorite impact turned a small eruption into a
colossal one. As a result, within less than a million years, enough
lava oozed out of the ground in Siberia to cover the entire planet
10 feet (three meters) deep. (One Permian-Triassic crater candidate
is the 125-mile/200-km or more diameter
Bedout
Structure off the coast of northwestern Australia. For more
discussion of geologic evidence, see:
Asteroid
and Comet Impact Craters and Mass Extinctions; and Luann Becker,
Scientific American, March 2002.)
NASA Total
Ozone Mapping
By mass, the Earth's is composed of mostly iron (35 percent), oxygen
(30 percent), silicon (15 percent), and magnesium (13 percent). It is
made of distinct layers: a thin crust, upper mantle, lower mantle,
outer core, and inner core, as well as transition zones. The crust is
thinner under the oceans but thicker under the continents.
While the inner core and crust are solid, the outer core and mantle
layers are semi-fluid.
NASA -- larger
image
Most of the mass of the Earth is in the mantle,
which is mostly made of the minerals olivene and pyroxene (iron and
magnesium silicates). The elements calcium and aluminum are abundant
in the upper mantle and while silicon, magnesium, and oxygen are major
components of the lower mantle. The core is probably composed
mostly of iron (perhaps with some nickel), where the temperature may be
as high as 7227 °C (13,040 °F) at the center -- which is hotter than
the surface of the Sun (See further discussion of the Earth's
interior
structure ).
Earth has a modest magnetic field produced by electric currents
generated in the metal core by its rapid spin, which is distorted
into a tear-drop shape by the solar wind. Along with the
atmosphere, the magnetic field shields life on Earth's surface
from most harmful solar and cosmic radiation. This field traps
radiation in a pair of doughnut-shaped rings of ionized gas (plasma)
in orbit around the Earth called the Van Allen radiation belts.
The outer belt stretches from 19,000 to 41,000 km (11,800 to 25,500
miles) in altitude, while the inner belt lies between 7,600 to
13,000 km (4,700 to 8,100 miles) in altitude.
According to the geological record, Earth's magnetic field dwindles
down to nothing for about a hundred years every few hundred
thousand years. Then, it gradually reappears but with the north
and south poles flipped. The last reversal of the poles occurred
about 780,000 years ago, and the strength of the magnetic field has
diminished by about five percent over the past century. Hence, the
Earth may be overdue for this cyclical event. Given the
anticipated loss of the magnetic field's shielding against the
Sun's energetic subatiomic radiation and more ultraviolet radiation
from the consequent erosion of the ozone layer, however, a magnetic
reversal may cause serious ecological disruption to all surface
lifeforms on Earth as well as threaten human well-being.
Viewed from space, Earth's classic image is that of a bluish ball with
shifting white clouds, the result of an atmosphere actively sustained
by its widespread life. Its atmosphere helps to shield its surface
from meteors, most of which burn up before they can strike the surface.
The atmosphere is composed of mostly nitrogen (78 percent) and oxygen
(21 percent), with traces of argon, carbon dioxide, and water. Its
presence of free oxygen is quite unusual because oxygen is a very
reactive gas. Without the constant and massive respiration of plants
worldwide, oxygen would quickly combine with other elements, and there
would be little free oxygen, as has happened on
Venus and Mars.
NASA
Although smaller than the
Chicxulub impact
structure that may have contributed to the demise of the Dinosaurs,
Manicouagan,
a 214-million-year-old crater in eastern Canada (Quebec) that has
been eroded nearly flat by glaciers, is easily seen from space because
of its huge 62-mile (100-km) diameter. It was made by the largest
known fragment of an object that also created: the 25-mile (40-km)
diameter, Saint Martin crater in western Canada (Manitoba); the
15-mile (25-km) Rochechouart crater in the Massif Central of France;
the 9.3 mile (15-km) Obolon' impact structure in the Ukraine; the
5.6-mile (9-km) Red Wing crater in the western U.S.; and possibly
also the 7.4 mile (12-km) Wells Creek, Tennessee impact structure
and the 2-mile (3-km) Newporte, North Dakota crater -- both in the
U.S.
U.S.
Geological Survey
From the single supercontinent
Pangea to today, in about 250
million years
(more
information).
©
Christopher
R. Scotese (Artwork from
PALEOMAP Project,
used with permission -- larger image)
In the coming 250 million years, tectonic movements
will cause today's continents to slowly
converge and form another mega-continent. (For more
information and images of Earth over
its geologic past and future, see Dr. Scotese's
PALEOMAP Project and a
recent
NASA
article.)
NASA
Earth is a geologically active planet. Some geological processes such as
plate tectonics are always slowly moving the planet's mass around
(more),
other processes such as water -- or ice -- flows may shift masses faster
(more).
Spectrometer (TOMS)
More
images.
Mostly Human-made, record-sized
ozone hole over Antarctica on
September
9-10, 2000 allowed
intense ultraviolet radiation
to damage tissues and DNA of
surface lifeforms on land and
in water, leading to severe
sunburns, blindness, skin cancers,
and death. For the first time,
the hole extended over a major
Human population -- the 120,000
residents of
Punta
Arenas, a city
in southern Chile.
Gravitational bumpiness exaggerated.
While Earth is a fairly uniform sphere,
its gravitational field does vary by small
fractions of a percent from place to place.
Not only is the planet bumpy as the result
of slow geological processes such as
tectonic plate movements or polar ice cap
changes, but these processes also create
mass density variations that affect the
local gravitational field
(more from
NASA
and
Astronomy
Picture of the Day).
NASA -- larger image
Many charged particles from the Sun's Solar Wind are trapped by Earth's
magnetic
field
to form its
magnetosphere,
deflecting the Wind from a head-on collision with its atmosphere.
NASA -- larger
ultraviolet image
Although the relatively cold plasma
captured by the Earth's magnetic field
is usually transparent, its ultraviolet
glow can be seen with NASA's Imager
for Magnetopause to Aurora Global
Exploration
(IMAGE)
spacecraft.
NASA -- larger
false color image
Neutral atoms glowing from relatively hot
plasma within the Earth's magnetosphere
can be seen with IMAGE's High Energy
Neutral Atom
(HENA)
instrument.
NASA -- larger
image
The Moon lies 384,403 km (238,857 miles) distant from the Earth. While its diameter at 3,476 kilometers (2,160 miles) is just under half that of Earth's, its gravity is only one-sixth as strong as the Earth's. Both the rotation of the Moon and its revolution around the Earth takes 27 days, 7 hours, and 43 minutes. This synchronous rotation is caused by an unsymmetrical distribution of mass in the Moon, which has allowed Earth's gravity to keep one lunar hemisphere permanently turned towards it. (See an animation of the Moon's orbit around the Earth, with a table of orbital and physical characteristics.)
The Moon was heavily bombarded early in its history, which thoroughly mixed, melted, buried, or obliterated much its original crust. The thinning and cracking of the crust allowed molten basalt from the interior to reach the surface and form its large dark lava plains ("maria"). The bombardment of large impacts ended about 3.5 billion years ago, and most lunar volcanism end about two billion years ago. Because the Moon does not have a significant atmosphere or surface water, the lunar surface has not weathered chemically as it would have on Earth. While there are still occasional large impacts, the Moon is considered to be geologically dead today.
NASA --
larger image
Huge boulders at Taurus-Littrow,
apparently dislodged from bedrock
farther up the slopes of the upland
hills near the Apollo 17 landing site,
were sampled by astronauts such as
Harrison Schmitt.
The Moon's surface has roughly the
same
land area as the continent of Africa. It is comprised primarily of
the heavily cratered and very old uplands ("highlands") and the
relatively smooth and younger maria. The maria (which comprise about
16 percent of the Moon's surface) are huge impact craters that were
later flooded by molten lava. Most of the surface is covered with
regolith, a mixture of fine dust and rocky debris produced by eons
of meteorite impacts.
Courtesy Jet Propulsion Laboratory. Copyright (c) California
Institute of Technology, Pasadena, CA. All rights reserved.
The Moon's crust averages 68 km (42 miles) thick and varies from a thin layer under Mare Crisium to 107 km (66.5 miles) north of the crater Korolev on the lunar far side. Below the crust is a mantle and possibly a small core of about 340 km (210 miles) in radius with two percent of the Moon's mass. Unlike the Earth's mantle, however, the Moon's is only partially molten. Hence, the Moon has no global magnetic field, but some of its surface rocks still exhibit residual magnetism, indicating that there may have been a global magnetic field early in the Moon's history. With no significant atmosphere and no magnetic field, the Moon's surface is exposed directly to the Solar wind and cosmic rays like Mercury.
Other Information
Information and images about proposals to settle and exploit the Moon can be found at Sol Station.
More images of Earth and the Moon are available at NASA's Planetary Photojournal. Fact sheets on the Earth and Moon Venus is also available from NASA's National Space Science Data Center.
David Seal (a mission planner and engineer at NASA's Jet Propulsion Laboratory at CalTech) has a web site that generates simulated images of the Sun, planets, and major moons from different perspectives and at different times of the year. Try his Solar System Simulator.
For more information about the Solar System, go to William A. Arnett's website on "The Nine Planets", or to Calvin J. Hamilton's web page on "Earth".
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