THE GENERAL PHYSICAL PROPERTIES OF THE PLANETARY BODIES
The
major planets are the largest satellites of the Sun, with a few exceptions.
Some of the moons are larger
than
some of the planets.
Jupiter,
Saturn, Uranus, and Neptune are classified as giant planets or Jovian planets.
The other planets are
classified
as terrestrial planets.
The giant planets are physically very different than the terrestrial planets.
1.
The giant planets have extensive atmospheres with a small solid core, whereas
the terrestrial planets have
shallow atomospehres and the bulk of the planet is solid.
2.
The giant planets form a more homogeneous class in terms of their physical
properties than do the terrestrial
planets. For example, there is a big difference between Pluto and
the Earth.
3. The gaint planets have many more moons than the terrestrial planets.
A. PHYSICAL CHARACTERISTICS OF THE PLANETARY BODIES
Terrestrial-like Planets (Mercury,
Venus, Earth, Mars, The Moon, some of Jupiter's moons
and most of the minor planets. Some
astronomers include the TNOs):
1. Relatively near the Sun, therefore relatively warm surfaces and atmospheres.
2. Somewhat small masses, and therefore, weak surface gravities.
3. Comparatively small diameters
4. High average density (approximately 3 gms/cm3).
5. Thin, shallow atmospheres of CO2, NO2, 02, N2, H2O (oxidized state).
6. Bulk of planet is rock (silicate material) and a metallic or rocky core
Giant or Jovian Planets (Jupiter, Saturn, Uranus, Neptune)
1. Far from the Sun and therefore, colder atmospheres (< -100C)
2. Large in mass with strong gravities
3. Large overall diameters
4. Low average densities ( approximately 1 gm/cm3)
5. Deep, dense atmospheres of H2, He, CH4, NH3, and other H compounds (reduced state).
6.
Most of planet comprised of various layers of gas, liquid, and ice. A rocky
core may exist.
For Jupiter, the core may be solid hydrogen.
Icy Planetary-like Bodies (Pluto,
Eris, & most of the moons of planets beyond 5.0 AU,
and the TNOs)
Comets are not considered to be planetary bodies, though they have many of the characteristics below.
1. Very far from the Sun, and therefore, very cold.
2 Small mass
3. Small size
4. Low density
5. No atmosphere or very shallow atmosphere
6. Composed of a great amount of ice (frozen gases such as H2O, CO2, and NH3) and a core made of rock.
B. COMPARATIVE CHEMICAL COMPOSITIONS
The stars,
including the Sun, have chemical compositions that are 70% H, 28% He, and
all the other elements
amount to about 2%.
On the other hand, the terrestrial major and minor planets are composed mostly of O, Si, Mg, Al, and Fe.
Pluto,
its moon Charon, Eris, and the TNOs, have relatively large amounts of various
ices (H2O, CO2,
and NH3) in their
structure. Comets also consist of a relatively large amount
of various frozen gases or ices,
but they are not planetary-like
bodies.
The giant
or Jovian planets (Jupiter, Saturn, Uranus, and Neptune) have chemical
compositions that are
similar to that of the Sun.
But they are not stars, because they have insuffient masses to generate
energy by TNF.
Actually brown dwarfs that have masses greater than 13 - 17 Jupiter
masses may innitiate some
limited amount of TNF for a short time. However, this does not
qualify them to be true
stars in the usual sense.
The IAU considers bodies with masses less than 13 Jupiter masses to be massive planets.
C. Exoplanets
Exoplnets are planetary bodies that have been found revolving in orbit
around another star
other
than the Sun. More than 1000 exoplanets have been discovered
so far. Most of these exoplanets
are very massive planets.
At first, only very massive exoplanets were discovered by detecting small
changes
in the motion of a star
in its orbit around the center of the galaxy. These small changes
were produced by
the gravitational interaction
between the star and the planet. Now exoplanets are being detected
by
photometric means.
That is, as a planet happens to transit its host star, it bloicks out some
of the light of the
star. Using very sensitive
photodetectors, such very small dips in the star's light curve may be interpreted
to reveal the presence of
an orbiting planet. The satellite telescope Kepler has been in orbit
around the Earth
now for about 2 years doing
just that. To date it has discovered hundreds of exoplanets.
D. THE PLANETARY SATELLITES OR MOONS
Below is a table of the number of satellites for each of the major planets:
Planet No. of Satellites
Mercury
0
Venus
0
Earth
1
Mars
2
Jupiter
64 (>250 very small moons)
Saturn
61
Uranus
27
Neptune
13
Pluto
3
Total number of good sized moons: 170 as of 2011
Source: Observer's Handbook 2012, pg. 24, University of Toronto Press.
The total
number of moons may actually be greater than several hundred if we include
some of the newly
discovered small objects
orbiting the giant planets, especially Jupiter.
At least two minor planets are also known to have a satellite.
Two planetary
satellites are larger than the major planet Mercury, and 7 planetary satellites
are larger than
Pluto. See the
diagram below.
The satellites
of Earth, Mars and some of those belonging to Jupiter and Saturn consist
mostly of rocky
material. Most of the larger
moons of the outer planets contain a large amount of ice in their structure.
This
is because of the very low
temperatures found far from the Sun.
Most of the planetary satellites (moons) belong to the giant planets.
All the
giant planets possess ring systems that are comprised of small bodies moving
in orbits very close
together around their host
planet. Each of these small bodies is actually a satellite also or a very
small moon.
The rings of Saturn are
the most conspicuous because a large number of the bodies are covered with
ice.
E. PLANETARY SURFACE
FEATURES (TERMINOLOGY)
1. Craters: Most are the result of meteorite impacts, although a few are volcanic in origin.
2. Caldera: A large opening in the top of a volcano that may contain several vents
3. Plain: A relatively flat region of large expanse. The maria of the Moon are lava plains.
4. Ridge.: A pressure fold in a lava plain caused by cooling and shrinking.
5. Rays: Bright streaks emanating from an
impact crater. In reality, they are radial bands of small,
secondary craters produced by material ejected from a larger impact site
falling back on
the surface.
6. Rills: Channel like grooves forming complex
patterns on a planetary surface. They appear to be
similar to meandering river gorges and may actually have formed by running
water a long
time ago. Some are chains of collapse craters caused
by sinking along a fault line.
7. Regolith: A granular or powder like
material forming a layer on the surface of a planetary body.
This material is produced by numerous small meteorites striking the hills,
mountians,
and othr outcroppings of rock on the surface.
F. THE LUNAR SURFACE (Read corresponding chapter in the textbook)
The
surface of the Moon consists of apparent dark and bright regions.
However, the far side of the Moon
has fewer and less distinct
dark areas, as shown in the image below. The far side of the Moon was first
imaged
by a Soviet Union spacecraft
in 1961.
Early
telescopic observers saw that the dark regions were relatively smooth looking
with no or few
craters. Hence they
concluded these regions were bodies of water which they named the Maria
(plural)
of the Moon or seas.
One sea is called a Mare, such as Mare Serenitatis.
We now
know there is no water on the Moon and the maria are actually basins filled
with solidified lava.
The maria are dark because
they are smooth. The basins were formed by large objects colliding
with the
Moon about 3.9 billion years
ago. Notice on a photograph of the Moon that the maria are
roughly circular
in outline with some overlapping
one another.
The ages
of the maria range from 3.1 to 3.9 billion years, as determined from radioactive
dating of the
samples of rock obtained
from the maria by the Apollo Astronauts during the 1970s.
The bright
regions of the Moon are called the Highlands. These regions are actually
higher in elevation
than the maria. The
highlands are bright because they are heavily cratered from an intense
meteorite
bombardment that was in
progress as the surface of the Moon cooled to form a solid crust.
Lunar rocks retrieved from the Highlands range in age from 4.0 billion to 4.6 billion years.
The heavily
cratered highlands of the Moon, which have not been covered with lava,
indicate that
the Moon was subject to
an intense meteorite bombardment prior to 4.0 billion years ago.
Since
there is much less cratering of the maria than the highlands and the maria
are about a billion years
younger than the highlands,
means the intense meteorite bombardment that produced the cratering of
the
highlands had ended prior
to the formation of the maria. However, there is evidence to
indicate the Moon
and other planets were subjected
to what is being called "the Late Heavy Bombardmen" at about 3.9 billion
years ago. It was
during this interval that several large bodies bombarded the Moon to produce
the Mare
basins and produced the
lava flows that filled these basins thereby destroying any older craters
that were
on these areas of the Moon's
surface.
The Moon's
surface has essentially remained unchanged for the last billion years or
more.
Photo of the Lunar Excursion Module that carried
two astronauts down
to the surface from the command module (courtesy
of NASA).
The Moon probably formed
by the accretion of material ejected into orbit around the Earth as a result
of a
collision between the very
young Earth and another planetary body about the size of Mars. This
happened
about 4.55 billion years
ago. A computer similation of this event is shown below.
The diagram below displays in graphical form the chronology of the evolution
of the Lunar surface.
In this figure, the intensity of the cratering rate is plotted versus
time since the Moon formed.
A meteorite colliding with the surface cannot result in a crater visible
today unless the Lunar surface
has cooled and become solid. The meteoroid bombardent abates
simply because material is being
consumed. The more massive meteoroids are used up faster the
less massive ones so that only the
smaller meteoroids are striking the Lunar surace after a sufficiently
long time. Though not shown in
the diagram, the age of the crater Kepler is about 780 million years.
Diagram courtesy
of Addison-Wesley