A.    DETAILS OF THE MAJOR PLANETS (Read corresponding chapters
        in the textbook.)

MERCURY

Atmosphere:  Essentially none.

Surface: Similar to that of the Moon: solidified lava plains, numerous craters, ridges, rills, and
                mountains. The surface temperature on the sunlit side is > 600 F.

Density: Highest of the planets.

Interior: Relatively cool; no molten rock; large dense core of iron and other heavy metals.  Since
              Mercury appears to have a weak magnetic field, their must be a central kernal in the core
              that is molten metal.

Satellites: none

VENUS

Atmosphere: Composed mainly of carbon dioxide (CO₂), N₂, sulfur dioxide (SO₂), hydrogen sulfide (H₂S),
and sulfur dust. Dense clouds of sulfuric acid droplets and sulfur dust perpetually shroud the surface. There are
traces  of  water vapor.

Surface: Atmospheric pressure is 90 times greater than the sea level pressure on Earth. The surface temperature
is about 800 F.  Very high- speed winds blow continuously. Radar surveys from Earth and orbiting satellites
have revealed several mountain ranges, volcanic cones, and many large, eroded craters. There is evidence of
volcanic activity and crustal movements (plate tectonics).

Density: Less than Mercury but greater than Earth.

Interior: Still molten, thereby providing volcanism and plate tectonics.

Satellites: none

MARS

Atmosphere: Very low density; surface pressure too low to support human life. Composed mostly of CO₂ and N₂.
Occasional thin clouds of ice crystals. There are occasional dust storms that blur the surface features as seen
from the Earth.
 

Surface: Covered with red sand (the regolith) containing iron oxide. Average temperature is -40 F. Seasonal pole
caps consisting of a thin layer of water ice and frozen carbon dioxide. Many shallow craters discovered in 1965 by
Mariner IV fly-by, but these are fewer per unit area than on the Moon. Several extinct volcanoes, including the
largest volcano in the Solar System: Mons Olympia (600 km by 25 km), and the largest canyon: Valles Marineris
(4000 km x 250 km x 6 km). The latter is believed to be a fault in the crust.

Satellites: Phobus and Deimos, two small, irregularly shaped, rocky bodies averaging 22 km and 12 km in
diameter.  Both are heavily cratered.

JUPITER

Atmosphere: Very cold, dense, and deep. It comprises a large fraction of the diameter of the planet.   One
observes only the tops of variegated cloud bands that are parallel to the equator. Numerous vortices,  e
specially in the polar regions. A very large, red colored, hurricane-like structure has existed for the last few
hundred years.

Interior: Unknown, but probably various layers of ice and perhaps a solid core about the size of Earth.  Overall
density is comparable to liquid water on Earth.

Satellites: 62, not counting the more than 250 very small objects revolving beyond the orbit of Callisto.  The
four largest, Ganymede, Callisto, Europa, and Io were discovered by Galileo in 1609 and are called the Galilean
satellites.  Io has the greatest number (7) of active volcanoes in solar system.  These exude sulfur dioxide gas
and molten sulfur.  The other 3 Galilean moons have ice in their crusts.  All moons have craters similar to those
seen on Mars.

Rings: Several, thin, concentric rings, comprised of many dark, kilometer-sized and smaller bodies.

SATURN

Atmosphere: Similar to Jupiter, but colder. The lower temperature results in a helium haze that partly obscures
the cloud bands

Interior: Unknown, but similar to Jupiter.

Satellites: 61. Titan is the largest and has an atmosphere of ammonia and methane. Most have ice incorporated
into their surfaces and have layers of ice in the interior. All have cratered surfaces.

Rings: Comprised of millions of small, ice-covered bodies that move in various, relatively thin, concentric
bands in the plane of Saturn's equator. They are noticeable in a small telescope.

URANUS

Atmosphere: It gives a distinctive greenish appearance to the planet. Comprised mostly of hydrogen, helium,
methane and some ammonia. Faint cloudbursts have been photographed by the Voyager II spacecraft. The
cloud belts are more conspicuous in the polar regions.

Interior: Unknown but low in density.

Satellites: 27, 8 of which were discoverd by Voyger 2 in 1986.  Also 8, well separated, sparse rings of small
bodies. Ice is an important component in the structure of all satellites, which are heavily cratered.

NEPTUNE

Voyager 2 images reveal a dark blue (due to methane) atmosphere with thin bands of white clouds.  According
to a NASA website, the white cloud bands seen in the Voyager 2 images are comprised of methane crystals.
There are also several large, storm-like vortices. The largest is a dark blue vortex that may be similar in nature
to Jupiter's great red spot.  There is also a smaller vortex that resembles a human eye.

There are 13, heavily cratered satellites, 6 of which were discovered by Voyager 2  in 1989. The satellites have
a large proportion of various ices in their composition. The largest is Triton, which has erupting geysers of
various gases. This activity is caused by internal heating as a result if tidal interactions with Neptune.

PLUTO

Pluto once had the distinction of being the smallest major planet, but it is now classified as a dwarf planet.
Recent measurements indicate the planet is composed of 70% rock and 30% ices. Most of the ices comprises
a thick upper layer with embedded rocky particles. Pluto has 5 moons; Charon is its largest satellite and is
1/3 the size of Pluto. It contains even more ice.  Two other very small satellites were discovered in 2006.
No space probes have visited this planet. Most new information has been obtained with the Hubble
Space-telescope.
 

Above montage courtesy of NASA.
 

B.    SUMMARY OF PLANETARY SURFACE FEATURES

 

1.    Meteorite-impact craters are ubiquitous throughout the solar system, wherever there is a solid surface.

3.    The surfaces of the terrestrial planetary bodies, moons, and minor planets (asteroids)  are cratered to
        various degrees  depending on how much erosion has occurred since the body formed.

3.     Mercury and the Moon have have surfaces covered with numerous craters of various sizes and ages.
        There are slso numerous rills, rays, ridges amd mountins.
 

4.    It is believed that Mercury, the Moon, the moons of Mars, and the minor planets have essentially pristine
        surfaces dating back more than 4.5 billion years.

5.    All the other planetary moons have cratering, even those whose surfaces are mostly ice.

 6.   The surface of Mars shows signs of erosion, since the craters are fewer and shallower.  Many extinct volcanic
cones remain intact including the largest known volcanic cone in the solar system, Mons Olympus.

7.    Radar surveys of the Venusian surface reveal large shallow  craters, mountainous regions, and some volcanic
cones,some of which appear to be active.
 

8.    At the opposite end of the cratering spectrum is Io and Earth.  On Io, volcanic activity completely alters the
surface every million years or so. The Earth's surface changes over completely every few hundred million years
as a result of erosion, mountain building, and volcanic activity. The Earth has only a few meteorite impact craters.
The youngest is about 35,000 years old and the oldest remnant of an impact crater is some 210 million years old.

 9.   It has been concluded that when the planets were forming about 4.55 billion years ago,  there was an intense
        meteorite bombardment going on for all the planets. In fact, this bombardment was part of the accretion
        process of planetary growth.

10    Accretion is growth by addition. The great meteorite bombardment essentially ended about 3.5 billion years
        ago, since the lunar maria have only small, young craters.

11.    All planetary bodies less than about 300 km in diameter are irregular in shape rather than spherical or round.
        This is a consequence of (a) the small mass, and hence, weak surface gravities of these objects, and
        (b) the fact that these bodies formed cold  by random collisions and were never completely molten.

12.  The Earth has only a few meteorite impact craters.  The youngest is about 35,000 years old and is located
       in the desert of Arizona.  Yhe oldest remnant of an impact crater is about 250 million years old and is
        located in northeast Canada.
 

C.    THE MINOR PLANETS  (Read appropriate chapter in the Textbook)

        The minor planets (often called asteroids or planetoids) are relatively small planetary bodies less than
1000 km in diameter.

More than 400,000 have been discovered since the first (Ceres) was discovered in 1801.  Ceres has also been

classified as a dwarf planet.  Most of the asteroids orbit the Sun between Mars and Jupiter, averaging a distance
of 2.8 AU from the Sun.  The diagram below shows the asteroid belt and its position relative the planets.  The white
and green dots represent the approximate locations of the 400,000 known asteroids.

 

Objects smaller than 300 km in diameter are irregular in shape, whereas objects that are larger than 300 km in
diameter tend to be more nearly spheroidal in shape, like the Earth.   This is a consequence of  two factors that
result from the mass of the body:  (1) the amount of heating the body underwent after it was formed, and
(2) the surface gravity of the body which could be sufficiently strong for it to overcome rigid body forces and
therefore become spheroidal. in shape.

    With regard to item (1) above: More massive and therefore larger bodies underwent total surface melting as a
result of an enhanced accretion process. This allowed gravity to form a  more spherical shape.  Less massive
bodies were never heated to the degree that they become totally molten.

    The rotation of a body also influences its shape.   The faster it rotates, the more oblate it is.   Even the Earth is
not perfectly spheroidal  in shape but is somewhat oblate.

    Space probes have been able to telemeter images of  the surfaces of some minor planets.   These have
shown that the minor planets also have many impact craters.
 

        The meteoroids are not usually thought of as planets because they are even smaller than the minor planets.
There has been no agreed upon size that separates a minor planet from a meteoroid, but the value is
somewhere around 10 meters.

Meteoroids can be almost of microscopic size and are then referred to as dust grains.

All meteoroids are made of solid rock and metal with little or no ice.

Meteoroids number in the billions and are irregular in shape.

Most meteoroids move in orbits around the Sun near the minor planets, but a meteoroid's orbit can be located
anywhere.

    When a meteoroid collides with a planet's atmosphere, it does so with very high speed.  This generates friction
at the surface of the meteoroid which quickly reaches a temperature of several thousand degrees.  This results in
the surface of the meteoroid to glow brilliantly and quickly evaporate, layer by layer.

The surface of the meteoroid, where evaporation is taking place is called the ablation surface
and the wearing away of the meteoroid is called ablation.

 When this is observed, the meteoroid is now called a meteor.  Since most meteoroids are very small, the entire
body evaporates in a fraction of a second.  This appears as a sudden bright streak in the night sky.  Meteors are
commonly called "shooting stars" or "falling stars" but have nothing to do with stars at all.

    If a meteoroid is larger than 50 or so centimeters in diameter, the core of the meteoroid survives and lands on
the planet's surface.  This surviving piece is called a meteorite.  So a meteorite is the remains of a meteoroid that
has survivedablation and hits the ground.

    Sometimes meteors explode or fragment, providing a shower of meteors.  Sometimes thesepieces may reach
he ground as meteorites.

E.    Meteorites

1.    Types and Parent Body Theory

    The meteorites that have been found on the Earth are classified according to their physical characteristics as follows:

A. Irons, B. Stony-irons,  C. achondrites,  D. chondrites,  and E. carbonaceous chondrites.

    Each of these different types is believed to have originated from a layer (correspondingly labeled in the diagram
below) in a much larger object called a 'PARENT BODY."   If two such bodies collided, they would break apart into
smaller pieces that would be the different kinds of meteorites that are found.

The above diagram depicts the cross-section of a parent body, after it has cooled and solidified throughout.

    The 'Irons' contain mostly iron and some nickel.  When cut, polished, and etched with acid, the irons show
characteristic criss-cross crystalline patterns called the Widmanstatten figures.  These meteorites are believed
to have come from the core of a parent body, which is labeled A in the diagram.

    The stony-irons originate from the layer labeled as B in the above diagram.  The meteorites are a
mixture of iron and silicate material (rock).

    Achondrites come from layer C.  Achondrites are similar to terrestrial igneous rocks.  They are comprised of
mineral crystals that have resulted from slow cooling of molten rocky material under high pressure..

    The chondrites come from layer D.   This material has not been melted but has been subjected to heating and
pressure.Chondrites are characterized by the inclusion of tiny glass globules  and metallic grains within a silicate
matrix.   The glass globules are called chondrules, which are the result of quick cooling of droplets of molten rock
produced by collision or impact.

    Chondrites are composed of 74% silicates, 20% metallic grains, and 6% ferrous sulfide grains.

    The carbonaceous chondrites originated from the outermost layer of a parent body (E).  They are are
characterized by the presence of delicate carbon compounds found within the grains.  These molecules were formed
on dust particles in space before the dust became incorporated into the parent body.  They are similar to chondrites
except the material was never heated or subjected to high pressure, otherwise the carbon compounds would have
been destroyed.

3. Chemical differention occurs as a result of gravity, causing the heavy elements such a iron
    and nickel to sink to the center and the less dense silicate material to form various outer layers.

4.  After the radioactive elments are spent, the body slowly cools and solidifies by emitting infrared
    radiation into space. It then has an onion-like layered structure of different densities. Each of
    the layers has experienced different degrees of heating or melting and compression.   This is
    diagramatically shown below:

E.    PROPERTIES OF COMETS

   1.    Comets are composed mostly of ice, solid grains, and dust.   The grains and dust  particles are embedded in the icy
           matrix.   The dust is essentially tiny silicate,  metallic, and carbon particles.

    2.    They move in very elliptical orbits that carry them far from the Sun for most of the time (Kepler's 2nd Law).

    3    They average about 20 km in diameter, and have relatively small masses.

    4.    When comets come to within 5 AU of the Sun, their ice begins to sublime, releasing any dust particles or grains.
            The particles and gases then form a large envelope around the nucleus called the coma.

    5.    The Solar wind then blows the gases and dust of the coma away from the comet, thereby forming separate tails.
            The tail of a comet always  points away from the Sun.  Study Fig. 49.8 in S&A.

    6.    Structure:    Nucleus, Coma, & Tail.   The nucleus and coma are referred to as the head of the comet.

    7.    It is believed that most comets originate from a toroidal shaped distribution of  cometary nuclei, extending from
            30 AU to about 55 AU from the Sun called the Kuiper Belt. See the top panel of the figure below, which shows the
            Kuiper Belt extending out to only slightly more than 100 AU., which may be to far. In addition, there is evidence
            indicating that the Sun is surrounded by a cocoon of cometary nuclei centered at a distance of 100,000 AU, called
            the Oort Cloud.  See the bottom panel in the figure below.