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PLANETARY GEOLGY

Understanding Earth focuses on the geology of Earth. This is natural, for Earth

is the planet we know most about and the one we are most interested in.Geology includes the study of all the worlds in our solar system: how planets

and their satellites are born and how they evolve. Geologists are interested in

other worlds because the things we learn about other planets give us new

knolegde into Earth's geologic history and the processes that still continue today.

From the perspective we get on Earth, our planet appears to be big, with an

endless ocean of air. From space, astronauts get the impression that the Earth is

small with a thin, fragile layer of atmosphere. For a space traveler, the

distinguishing Earth features are the blue waters, brown and green land massesand white clouds set against a black background.

Many dream of traveling in space and viewing the wonders of the Universe. Inreality all of us are space travelers. Our spaceship is the planet Earth, traveling atthe speed of 108,000 kilometers an hour.

The Solar System


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Earth is the 3rd planet from the Sun at a distance of about 150 million

kilometers, and obits, together with other 8 planets, around the Sun, in a system

called Solar System. The Solar System consists of an average star we call the

Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune,

and Pluto. It includes: the satellites of the planets; numerous comets, asteroids,and meteoroids; and the interplanetary medium.

THE SUN

The Sun is the most prominent feature in our solar system. It is the largest object

and contains approximately 98% of the total Solar System mass. One hundred

and nine Earths would be required to fit across the Sun's disk. Its interior couldhold over 1.3 million Earths. The Sun's outer visible layer has a temperature of

6,000C. Solar energy is created deep within the core of the Sun. It is here that

the temperature (15,000,000 C) and pressure (340 billion times Earth'spressure) is so intense that nuclear reactions take place.

The Sun have been active for 4.6 billion years and has enough fuel to go on for

another five billion years or so. At the end of its life, the Sun will start to fuse

helium into heavier elements and begin to swell up, ultimately growing so largethat it will swallow the Earth. After a billion years as a red giant, it will suddenly

collapse into a white dwarf - the final end product of a star like the Sun.

The Sun's outer visible layer is called the photosphere and has a temperature of

6,000C. A small region known as the chromosphere lies above the

photosphere. The highly rarefied region above the chromosphere, called the
http://www.solarviews.com/eng/sun.htmhttp://www.solarviews.com/eng/mercury.htmhttp://www.solarviews.com/eng/venus.htmhttp://www.solarviews.com/eng/earth.htmhttp://www.solarviews.com/eng/mars.htmhttp://www.solarviews.com/eng/jupiter.htmhttp://www.solarviews.com/eng/saturn.htmhttp://www.solarviews.com/eng/uranus.htmhttp://www.solarviews.com/eng/neptune.htmhttp://www.solarviews.com/eng/pluto.htmhttp://www.solarviews.com/eng/comet.htmhttp://www.solarviews.com/eng/asteroid.htmhttp://www.solarviews.com/eng/asteroid.htmhttp://www.solarviews.com/eng/comet.htmhttp://www.solarviews.com/eng/pluto.htmhttp://www.solarviews.com/eng/neptune.htmhttp://www.solarviews.com/eng/uranus.htmhttp://www.solarviews.com/eng/saturn.htmhttp://www.solarviews.com/eng/jupiter.htmhttp://www.solarviews.com/eng/mars.htmhttp://www.solarviews.com/eng/earth.htmhttp://www.solarviews.com/eng/venus.htmhttp://www.solarviews.com/eng/mercury.htmhttp://www.solarviews.com/eng/sun.htm


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corona, extends millions of kilometers into space but is visible only during a

total solar eclipse.

Solar energy is created deep within the core of the Sun. It is here that the

temperature (15,000,000 C) and pressure (340 billion times Earth's air pressure

at sea level) is so intense that nuclear reactions take place.Sometimes the Moon comes directly between the Earth and the Sun. This is

called a solar eclipse; if the alignment is imperfect the Moon covers only part of

the Sun's disk and the event is called a partial eclipse. When it lines up perfectly

the entire solar disk is blocked and it is called a total eclipse of the Sun. Partialeclipses are visible over a wide area of the Earth but the region from which a

total eclipse is visible, called the path of totality, is very narrow, just a few

kilometers (though it is usually thousands of kilometers long).

The geometry of a solar eclipse

Total eclipse - the path of totality (the blue narrow area in the photo)


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MERCURY

Mercury is the second smallest planet (after Pluto).

Mercury's surface is very hot, sometimes reaching extremes of more than 470C.

Because of the heat and the planet's low gravity, Mercury has not a significant

atmosphere. Trace amounts of hydrogen, helium, and oxygen above the surface

are probably derived from the solar wind.Mercury orbits the Sun once every 88 days.

Mercury's high density (5.4 g/cm3) suggests a large iron or nickel-iron core.

Mercury's core has a radius of 1800 km - 75 percent of the planetary radius.(Earth's core has a radius of only about 55 percent of the planetary radius.) This

large iron core, part of which is probably molten, is responsible for Mercury'smagnetic field.

Mercury's large iron or nickel-iron core


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Surface Features

Mariner 10 photographed about 40 percent of Mercury's surface in detail.

Ancient cratered highlands (estimated to be about 4 billion years old) cover

much of the observed surface. Plains were also seen. The largest plain, CalorisBasin, is 1300 km wide. It was probably created by the impact of a large

meteoroid during the time when Mercury was forming. Radar observations of

the north polar area in 1991- an area not photographed by Mariner 10 - suggest

that some water ice may exist there.

Caloris Basin of Mercury

VENUS


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Venus, the second planet from the Sun, completes one trip around the Sun every

225 days and rotates on its axis once every 243 days. Its rotation is retrograde.

Venus and Earth have been considered sister planets because of their similardiameter, mass, and composition. Venus has no satellites. The planet is covered

by a thick, reflective cloud layer that produces very high surface temperatures

(about 475C) and pressures (about 90 atmospheres). Venus' clouds are made up

of sulfuric acid droplets and solid sulfur particles.

Earth and Venus Earth compared

The atmosphere of Venus is composed mainly of carbon dioxide (about 98

percent).

Venus has a very pronounced greenhouse effect. Sunlight filtering through theclouds heats the surface, which radiates the heat back into the lower atmosphere.

The heat is easily absorbed by the carbon dioxide in the atmosphere and radiated

back to the surface.

Composition

The composition of the surface was measured by some spacecrafts. Venera 8detected materials similar to the granitic rocks on Earth's continents, along with

uranium, thorium, and potassium. Venera 9 and Venera 10 found basaltic rock

compositions. Venera 13 and Venera 14 also measured the electricalconductivity of rocks.


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Greenhouse effect on Venus

Geology

There are three types of surface on Venus: plains (20 percent), hills (70 percent),and highlands (10 percent). All three areas show evidence of volcanism. Venushas several large shield volcanoes, some of which are thought to be currently

active.

Spectacular volcano on Venus


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Although its surface has been very geologically active in the past, Venus

appears to have not plate tectonics. Heat generated in the interior drives the

volcanic activity.

Venus surface

Mons MaatVenus, with lava flows


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MARS

Mars is the fourth planet from the Sun. Its year is 687 days long, and it rotates

on its axis once every 1.026 Earth days. The planet's axis is tilted at about 25

degrees, which produces seasonal changes similar to those on Earth. The

diameter of Mars is about half the size of Earth and nearly twice the size of theMoon.

Earth and Venus Earth compared

Mars is less dense than Earth (3.95 g/cm3

versus 5.52 g/cm3

). No measurablemagnetic field has been detected. The core is solid. The soil of Mars was


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sampled by Viking that found it consists of clays and iron. This was expected

because of its red color.

Martian atmosphere is composed of: carbon dioxide (95 percent), nitrogen (3

percent), and argon (2 percent). The atmospheric surface pressure is less than 1

percent of Earth's. The surface temperature fluctuates greatly between day andnight and between seasons. The average temperature is about -53C.

Geology

Many geologic features on Mars are much larger than the same kinds of featureson Earth. For example, the shield volcano Olympus Mons, the largest volcano in

the solar system, is three times as tall (25 km) as Mount Everest (the tallest

mountain on Earth).

Mons Olimpus - Viking 1st, 1978, Mars

Several factors contribute to the size of geological features on Mars. Withoutplate tectonics, volcanic activity persist longer in the same place.

These more static conditions allow volcanoes to grow for longer periods of time.

Life on Mars?

A small number of meteorites found on Earth are known to have originated on

Mars. In August 1996, a NASA research team announced that they had

identified organic compounds in a Martian meteorite. This evidence suggests

that primitive life may have existed on Mars more than 3.6 billion years ago.

Mars Pathfinder, a mission to explore the atmosphere, weather, and surface of

Mars, landed on the planet's surface on July 4, 1997. Color panoramas of the

Martian landscape show clear evidence that the surface has been altered bywinds and flowing water. Mars in its infancy appears to have been very like


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Earth, with sedimentary processes and flowing water that created a variety of

rock types and a warmer atmosphere that generated clouds, winds, and seasonal

cycles.

The planet has two moons: Phobos and Deimos.

Rocks on Mars surface

Mariner 6 - February 24, 1969


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VikingMars surface

Marsice cap

JUPITER

Jupiter, the fifth planet from the Sun, is by far the largest in the solar system. It

is 318 times more massive than Earth, and its diameter is about 11 times that of


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Earth. It orbits the Sun once every 12 years. Its rotation period is of about 10

hours.

Jupiter and Earth compared

Jupiter has a low density (1.3 g/cm3), being composed primarily of hydrogen

and helium. Jupiter has a rocky core, surrounded by a massive mantle of liquid

metallic hydrogen. (At the temperature and pressure of Jupiter's interior,

hydrogen is a liquid).

Solid core confirmed on Jupiter


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Jupiter has spectacular bands of clouds ranging in color from white to orange to

blue to brown.

Jupiter has high-velocity winds confined in wide bands of latitude. The winds

blow in opposite directions in adjacent bands.

Jupiters bands of clouds

In 1979, Voyager 1 photographed two rings.

Jupiters rings


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Fragments of comet Shoemaker-Levy 9 collided with Jupiter in 1994, with

spectacular results.

The comet Shoemaker-Levy 9 collision

The Galileo mission, launched in 1989, observed Jupiter and its moons.The four large Moon-sized satellites were discovered by Galileo in 1610 and

named the Galilean satellites after him. Jupiter also has 12 smaller satellites.

The Galilean moons of Jupiter

SATURN

Saturn, the sixth planet from the Sun, is second in size only to Jupiter. Saturn

rotates once on its axis about every 11 hours. Its axis of rotation is tilted byabout 27 degrees, which causes seasons. The average density is only 0.7 g/cm

3..


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Saturn, like Jupiter, is composed primarily of hydrogen and helium.

Saturn's spectacular rings were first seen by Galileo in 1610.Saturn has a spectacular ring system. The particles that make up the rings are

composed primarily of water ice and range in size from sand grains to large

blocks.

Saturn's ring system

Saturn has 18 satellites.

Saturns satellites


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URANUS

Uranus, the seventh planet from the Sun, was the first to be discovered since

ancient times. It was first observed through a telescope in 1781. Uranus orbits

the Sun completing one revolution every 84 years. Uranus rotates on its axis

once every 18 hours. The planet has a very unusual rotation, with its axis tilted

on its side. This results in very strange seasons.

Uranus' rotation axis

Despite the exaggerated seasons, Uranus's winds blow parallel to the equator.

The atmosphere has such a great capacity to store heat that the alternating 42-


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year periods of sunlight and dark have practically no effect on Uranus's

temperature.

Uranus has a diameter four times that of Earth. Its density is 1.21 g/cm3. The

planet's blue green color is caused by methane gas in the atmosphere.

Uranus and Earth compared

Hydrogen and helium make up more than 99 percent of Uranus's atmosphere,

which is about 8,000 km thick. Uranus has a rocky and icy core.

It has a surface temperature of -214C.

Uranus has rings. The planet has five major satellites, but Voyager 2 found 10

small satellites as well.

NEPTUNE


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Neptune, the eighth planet from the Sun, completes one trip around the Sun

every 165 years in an orbit even more nearly circular than Earth's. The rotation

period is 16 hours.

Neptune and Earth compared

Neptune has a mass 17 times that of Earth and an average density of 1.67/cm3.

The planet is believed to have a rocky core surrounded by a liquid water mantle,

which is surrounded in turn by liquid hydrogen and helium. The atmosphere

consists mainly of hydrogen and helium. Neptune is deep blue color.Temperatures on Neptune are about -228C.

Neptune has rings.

Before Voyager's visit were discovered only two satellites. Voyager 2 also

discovered six new satellites.

PLUTO

Pluto, the outermost planet, is also the smallest. Because of its small size, it wasnot discovered until 1930. Pluto can be seen only as a point of light even by the

largest Earth-based telescopes. Much of what we know about Pluto is deduced

from the presence of its satellite, Charon, discovered in 1978. Pluto is the onlyplanet that hasn't been visited by a spacecraft.


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Pluto has high orbital eccentricity. This unusual orbit brings Pluto inside the

orbit of Neptune during its close approach to the Sun (between 1979 and 1999).Pluto rotates once on its axis every 6 Earth days.

The eccentricity of Pluto's orbit

Pluto's composition is unknown, but its density (about 2 g/cm3) indicates that it

is probably a mixture of 70 percent rock and 30 percent water ice. Pluto's very

thin atmosphere probably consists of nitrogen with some carbon monoxide andmethane.


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MOON

The Moon is one-quarter Earths size and its it single satellite.The Moon moves around Earth in an elliptical orbit, at a distance of 385,000

km. It completes an orbit around Earth every 27.32 days, which is the real

month. Because Earth orbits the Sun in the same direction as the Moon, thetime needed to return to the same phasethe synodic monthis longer:

29.53 days. The Moon also rotates on its axis over one sidereal month, so italways keeps approximately the same side toward Earth.

Internal StructureThe Moon is less dense than Earth (3.34 g/cm3 versus 5.52 g/cm3). The lunar

surface rocks brought back by the Apollo missions were close to the averagedensity, which means that the Moons interior is probably about the same

density as the surface.Seismometers installed by the Apollo astronauts showed that the Moon ismuch less seismically active than Earth. It does not have a metallic core.

CompositionThe most abundant element in the Moons crust is oxygen (60 percent),

followed by 16 percent silicon, 10 percent aluminum, 4 percent calcium,magnesium, iron, and titanium. The amounts of oxygen, silicon, and

aluminum on the Moon are comparable to their abundance in Earths crust.


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Moons interior

Surface FeaturesThere are two distinct types of terrain on the lunar surface. The highlands arerough, and occupy 83 percent of the Moons surface. The maria (Latin forseas),

which make up the other 17 percent, are darker and smoother. The old,

heavily cratered highlands formed during a heavy bombardment period earlyin the Moons history.

Moons suface


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MineralogyThe dark crystalline materials filling the basins are basalts, similar to Earthslavas but enriched with iron and titanium. The highlands consist of granites.

Many of the materials found in the highlands are breccias made up of grains

of various minerals. Breccias are angular conglomerates. The structure of breccias indicates that they were formed by metamorphism that occurred

during meteoroid impacts.

Geologic HistoryMany scientists think that the Moon was formed by the collision of a Mars-sized body with Earth about 4.5 billion years ago. They hypothesize that the

gigantic impact started a shower of debris into space, which aggregated toform the Moon.

Radiometric dating of the rocks brought back by the Apollo missions hasrevealed that the oldest rocks are about 4.44 billion years old, which is closeto the proposed time of the giant impact event.

Earth from Moon

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