Play and Learn 2/19941.ZIP/19941/EDUCNOMY/ACE1/DESCRIP.DAT

Text File | 1994-02-04 | 11.3 KB | 196 lines

SUN
The sun (Sol) is the central body of the solar system and nearest star to
earth.  The sun is a representative yellow dwarf.  It is the only star whose
surface and outer layers can be examined in detail.

The sun is composed predominantly of hydrogen and helium (relative abundance by
mass approximately 3:1), with about 1% of heavier elements.  It generates its
energy by nuclear fusion processes.  From the center to the surface of the sun,
the temperature falls from around 15 000 00 K to 6 000 K.

The surface of the sun, i.e., the photosphere, represents the boundary between
the opaque convective zone and the transparent solar atmosphere.  A permanent
feature of the photosphere is the granulation, which gives it a mottled
appearance.  More striking are the sunspots and their associated faculae.
Immediately above the photosphere is the chromosphere.  Between this and the
exceedingly rarified corona is the transition region.  The corona itself
extends into the interplanetary medium, where the solar wind carries a stream
of atomic particles to the depths of the solar system.

The sun is thought to possess a weak general magnetic field.
$
MERCURY
At just over one-third earth's distance from the sun, Mercury is the solar
system's innermost planet and the only one known to be almost entirely without
an atmosphere.  Mercury is a small world only 6% as large as earth by volume--
barely larger than our moon.  One hemisphere of Mercury is extremely heavily
cratered, in many respects identical in appearance to the far side of earth's
moon.  Mercury's orbit is the most elliptical of any planet except Pluto's.
Mercury's sidereal rotation period is the longest of any planet.

Mercury has a tight orbit that constrains the planet to a small zone on either
side of the sun as viewed from earth.  When Mercury is east of the sun, we may
see it as an evening star low in the west just after sunset.  When it is west
of the sun, we might view Mercury as a morning star in the east before sunrise.
Because of celestial geometry involving the tilt of earth's axis and Mercury's
orbit, we get much better views of Mercury at certain times of the year.  The
best time to see the planet in the evening is in the spring, and in the
morning in the fall (from the northern hemisphere).
$
VENUS
Venus is the only world in the solar system that closely resembles earth in
size and mass.  It also comes nearer to earth than to any other planet, at
times approaching as close as 40 million kilometres.  Despite the fundamental
similiarity, surface conditions on earth and Venus differ greatly.  The chief
disparity is that Venus's surface temperature varies only a few degrees from a
mean of 728 K on both day and night sides of the planet.  The high temperature
is due to the dense carbon dioxide atmosphere of Venus that, when combined
with small amounts of water vapor and other gases known to be present, has the
special property of allowing sunlight to penetrate the planet's surface, but
does not permit the resulting heat to escape.  This process is commonly known
as the greenhouse effect.  The clouds and haze that cloak the planet,
consisting chiefly of droplets of sulphuric acid, are highly reflective.

Because its orbit is within that of earth's, Venus is never separated from the
sun by an angle greater than 47o.  When Venus is far from us (near the other
side of its orbit), we see the planet nearly fully illuminated, but because of
its distance it appears small.  As Venus moves closer to earth, the phase
decreases (we see less of the illuminated portion of the planet).  It takes
Venus several months to move from one of these extremes to the other.
$
EARTH
The earth (Terra) has a substantial atmosphere, mainly of nitrogen and oxygen,
and a magnetosphere linked to a magnetic field.  Two-thirds of the planet is
covered by water.  On average the earth's surface transfers to the atmosphere
an amount of energy equal to that it absorbs.

The earth consists of three main internal layers:  the crust, mantle, and core.
The crust consists largely of sedimentary rocks, such as limestone and
sandstone, resting on a base of igneous rocks, such as granite and basalt.  The
mantle's composition is thought to contain a high proportion of silicate rocks.
The core is composed predominantly of iron with several, possibly many,
additional components.

It is believed that the pressure at the earth's center is about 400
gigapascals, whereas the internal temperature may exceed 5 000 K at the center.
The heat required to maintain these temperatures is derived from the natural
radioactivity of the earth's constituent rocks.  After 4.6 billion years the
earth's internal heat is still a source of mechanical power, producing
earthquakes and volcanic eruptions, raising mountains, and moving continent-
sized blocks about its surface.
$
MARS
The landscapes of Mars are basically desert vistas strewn with rocks ranging up
to several metres wide.  Judging by their texture and color and chemistry
analysis, the rocks are fragments of lava flows.  The soil composition
resembles that of basaltic lavas on earth and our moon.  About 1% of the soil
is water, chemically bound in the crystal structure of the rock and soil
particles.  Some planetary scienticsts speculate that water in the form of
permafrost exists a few metres below the surface.  Water was once abundant
enough on Mars to leave major structures on the planet resembling riverbeds,
likely carved during the planet's early history.

The red planet's thin atmosphere has an average surface pressure only 0.7% of
earth's and consists of 95% carbon dioxide, 2.7% nitrogen, 1.6% argon, 0.6%
carbon monoxide, 0.15% oxygen, and 0.03% water vapor.  Winds in the Martian
atmosphere reach speeds exceeding 300 km/h and, in so doing, raise vast
amounts of dust that can envelop the planet for weeks at a time.
$
JUPITER
Jupiter, the solar system's largest planet, is a colossal ball of hydrogen and
helium without any solid surface comparable to land masses on earth.  In some
respects Jupiter is more like a star than a planet.  Jupiter likely has a small
rocky core encased in a thick mantle of metallic hydrogen, which is enveloped
by a massive atmospheric cloak topped by a quilt of multi-colored clouds.

The windswept visible surface of Jupiter is constantly changing.  Vast dark
belts merge with one another or sometimes fade to insignificance.  Brighter
zones--actually smeard bands of ammonia clouds--vary in intensity and
frequently are carved up with dark rifts or loops called festoons.

Jupiter's rapid rotation makes the great globe markedly oval so that it
appears about 7% "squashed" at the poles.  There is a ring of dust-sized
particles around the giant planet's equator.  The ring apparently extends
from the Jovian clouds out to 59 000 km.

The Great Red Spot, a salmon-colored oval votex whose hue may possible be due
to organic-like compounds that are constantly spewed from some heated
atmospheric source below, is the longest-lived structure on the visible
surface of Jupter.
$
SATURN
Basically, Saturn has a belt system like Jupiter's, but it is much less active,
and the contrast is reduced.  Very rarely a spot among the Saturnian clouds
will appear unexpectedly, but less than a dozen notable spots have been
recorded since telescopic observation of Saturn commenced in the 17th century.

Saturn's rings consist of billions of particles that range in size from
microscopic specks to flying mountains kilometres across.  The reason "rings"
is plural and not singular is that gaps and brightness differences define
hundreds of distinct rings.  However, from earth only the three most prominent
components--known simply as Rings A, B, and C--can be distinguished visually.
Cassini's Division, a gap between rings A and B discovered in 1675, is a region
less densely populated with ring particles than adjacent rings.

Titan, the largest of Saturn's satellites, is the only satellite in the solar
system with a substantial atmosphere, now known to be primarily nitrogen and
4.6 times as massive as earth's, with a surface pressure of 1.6 earth
atmospheres.
$
URANUS
Uranus was apparently unknown until 1781, when it was accidentally discovered
William Herschel with a 150-mm reflecting telescope.

Jupiter, Saturn, Uranus, and Neptune are rather similar in the sense that their
interiors consist mainly of hydrogen and helium and their atmospheres consist
of these same elements and simple compounds of hydrogen.  Unlike the three other
giant planets, the axis of Uranus is tipped almost parallel to the plane of the
solar system.  This means that we can view Uranus nearly pole-on at certain
points in its 84-year orbit of the sun.

Uranus has a substantial magnetic field tilted at a remarkably large angle of
some 60o to the rotation axis and nine main slender, dark rings with a tenuous
structure within the ring system.
$
NEPTUNE
The discovery of Neptune in 1846, after its existence in the sky had been
predicted from independent calculations by Leverrier in France and Adams in
England, was regarded as the crowning achievement of Newton's theory of
universal gravitation.  Actually Neptune had been seen--but mistaken for a
star--several times before its "discovery."

Jupiter, Saturn, Uranus, and Neptune are rather similar in the sense that their
interiors consist mainly of hydrogen and helium and their atmospheres consist
of these same elements and simple compounds of hydrogen.

Neptune's large moon Triton is smaller than earth's moon.  Spectral studies
indicate that the surface of Triton may be rocky, with methane glaciers and
a shallow seal of liquid nitrogen.
$
PLUTO
Pluto, the most distant known planet, was discovered at the Lowell Observatory
in 1930 as a result of an extensive search started two decades earlier by
Percival Lowell.  The faint star-like image was first detected by Clyde
Tombaugh by comparing photographs taken on different dates.

In 1978 James W. Christy detected an elongation of Pluto's image on some of the
photographs that has been confirmed as a large satellite revolving identically
to the planet's rotation period.  This means that the moon is visible only from
one hemisphere of Pluto.  The moon too would likely have one side constantly
turned to Pluto, forming a unique double-planet system.  Pluto and Charon are
almost certainly balls of ice, most likely water, methane, and ammonia.
However, since Pluto's surface gravity is too feeble to retain a primordial
methane atmosphere, it is probable that as the planet nears perihelion, the sun
is evaporating its frosty surface.

Besides being the solar system's smallest planet, Pluto is different from the
other eight in almost every respect.  Its unique characteristics include its
orbit, which is relatively higher inclined.  Just where such a freak fits into
the solar system's origin and evolution is unknown.  Perhaps Pluto is the
largest member of a group of small, icy, comet-like structrues beyond Neptune.
$
Compiled from OBSERVER'S HANDBOOK 1987, pp. 106-118.

Copyright 1984, 1985, 1986 AstroSoft, Inc.  Excerpts from copyrighted material
are included by permission of The Royal Astronomical Society of Canada.