Minor Bodies of the solar System

Ch 28

• satellite - natural or artificial body that revolves around planet.

• moon - body that revolves around a planet and that has less mass than the planet does.

• Seven of the planets in our solar system have natural satellites, or moons. – The Apollo space program sent six spacecraft

to study the moon. Scientists were able to gather data about the moon’s weak gravity and its effect on astronauts, as well as data about the moon’s surface.

mare - a large, dark area of basalt on the moon• Any feature of the moon is referred to as lunar.

• anorthosites - light patches seen on the moon’s surface

• maria -darker areas on the moon’s surface– Maria are plains of dark, solidified lava which formed

more than 3 billion years ago when lava slowly filled basins that were created by massive asteroids.

Moon

Craters, Rilles, and Ridges crater - a bowl-shaped depression that forms on

the surface of an object when a falling body strikes the object’s surface or when an explosion occurs– The surface of the moon is covered with craters,

rilles, and ridges. Most of the craters formed when debris struck the moon about 4 billion years ago.

– Rilles are long, deep channels that run through the maria. Rilles are thought to be leftover lava channels from the formation of the maria.

– The moon’s surface also has several ridges, which are long, narrow elevations of rock that rise out of the surface and criss-cross the maria.

Some of the unique properties of lunar regolith are shown here. Clockwise from the upper left: Agglutinates are glass-bonded aggregates of other regolith components. They contain countless microscopic grains of metallic iron (shown in the second image) formed by reduction of iron oxide present in the glass. The surfaces of mineral grains, even minerals that contain no iron, are coated with a mixture of silicon-rich glass and nanometer-sized grains of metallic iron (bottom image).

• The moon’s surface is very susceptible to meteorite hits because the moon has no atmosphere for protection.

• Over billions of years, these meteorites crushed much of the rock on the moon’s surface into a layer of dust and small fragments called regolith.– The depth of regolith on the moon varies from 1

m to 6 m.• The interior of the moon is less dense than

the interior of earth.

• The side of the moon that faces Earth is called the near side, while the side of the moon that faces away from Earth is called the far side.

• The moon’s crust is thicker on side than the other.

• The crust on the near side is about 60 km thick. The crust on the far side is up to 100 km thick.

• The difference in thickness was caused by the pull of Earth’s gravity during the formation of the moon.

Giant Impact Hypothesis• Most scientists agree that the moon formed in

three stages.• The giant impact hypothesis states that the

formation of the moon began when a large object collided with Earth more than 4 billion years ago.

• The collision ejected chunks of Earth’s mantle into orbit around Earth. The debris eventually clumped together to form the moon.

• This hypothesis explains when moon rocks share many of the chemical characteristics of Earth’s mantle.

Formation of the Moon

• Earth and the moon revolve around each other. Together they form a single system that orbits the sun.

• barycenter - balance point of the Earth-moon system is located within the Earth’s interior, because Earth’s mass is greater than the moon’s mass.– follows a smooth orbit around the sun.

• The Moon’s Elliptical Orbitapogee in the orbit of a satellite, the point at

which the satellite is farthest from Earth

perigee in the orbit of a satellite, the point at which the satellite is closest to Earth

• The orbit of the moon around Earth forms an ellipse, the distance between Earth and the moon varies over a month’s time.

Moonrise and MoonsetThe moon appears to rise and set at Earth’s

horizon because of Earth’s rotation on its axis.

• The moon rises and sets 50 minutes later each night. This happens because of both Earth’s rotation and the moon’s revolution.

• While Earth completes one rotation each day, the moon also moves in its orbit around Earth. It takes 1/29 of Earth’s rotation, or about 50 minutes, for the horizon to catch up to the moon.

Lunar RotationThe moon’s revolution around Earth and its

rotation on its axis take the same amount of time.

• Because the rotation and revolution take the same amount of time, observers on Earth always see the same side of the moon.

• The part of the moon illuminated by sunlight changes as the moon orbits Earth.

eclipse - an event in which the shadow of one celestial body falls on another

• Bodies orbiting the sun, including Earth and its moon, cast long shadows into space. An eclipse occurs when one body passes through the shadow of another.

• Two parts of Earth and the moon shadows: umbra - inner, cone-shaped part

penumbra - outer part of the shadow

solar eclipse the passing of the moon between Earth and the sun; during a solar eclipse, the shadow of the moon falls on Earth.

• During a total solar eclipse, the sun’s light is completely blocked by the moon. The umbra falls on the area of Earth that lies directly in line with the moon and the sun.

• Outside the umbra, but within the penumbra, people see a partial solar eclipse. The penumbra falls on the area that immediately surrounds the umbra.

lunar eclipse the passing of the moon through Earth’s shadow at full moon

• A lunar eclipse occurs when Earth is positioned between the moon and the sun and when Earth’s shadow crosses the lighted half of the moon.

• When only part of the moon passes into Earth’s umbra, a partial lunar eclipse occurs.

• When the entire moon passes through Earth’s penumbra, a penumbral eclipse occurs.

• As many as seven eclipses may occur during a calendar year. Four may be lunar, and three may be solar or vise versa.

• Total eclipses of the sun and the moon occur infrequently.

• Lunar eclipses are visible everywhere on the dark side of Earth. A total solar eclipse, can be seen only by observers in the path of the moon’s shadow as it moves across Earth’s lighted surface. A partial solar eclipse can be seen for thousands of kilometers on either side of the path of the umbra.

phase - in astronomy, the change in the illuminated area of one celestial body as seen from another, celestial body; phases of the moon are caused by the changing positions of Earth, the sun, and the moon

• As the moon revolves around Earth, different amounts of the near side of the moon, which faces Earth, are lighted.

• Therefore, the apparent shape of the visible part of the moon varies. The varying shapes are called phases.

Waxing Phases of the Moon

• When the size of the lighted part of the moon is increasing, the moon is said to be waxing.

• When a sliver of the moon’s near side is illuminated, the moon enters its waxing-crescent phase.

• When a waxing moon becomes a semicircle, the moon enters its first-quarter phase.

• When the lighted part of the moon’s near side is larger than a semicircle, the moon is in its waxing-gibbous phase.

• At full moon, the entire near side of the moon is illuminated by the light of the sun.

Waning Phases of the MoonWhen the lighted part of the near side of the

moon appears to decrease in size, the moon is waning.

• When the moon is waning, but is still larger than a semicircle, the moon is in the waning-gibbous phase.

• When the moon is waning, and it is a semicircle, the moon enters the last-quarter phase.

• When only a sliver of the near side is visible, the moon enter the waning-crescent phase. After this phase, the moon becomes a new moon, in which no lighted area of the moon is visible from Earth.

• The moon revolves around Earth in 27.3 days, however, the period from one new moon to the next one is 29.5 days.

• This difference of 2.2 days is due to the orbiting of the Earth-moon system around the sun.

• In the 27.3 days in which the moon orbits Earth, the two bodies move slightly farther along their orbit around the sun. So, the moon must go a little farther to be directly between Earth and the sun. About 2.2 days are needed for the moon to travel this extra distance.

Tides on Earth• tidal bulges - Bulges in Earth’s oceans, form

because the moon’s gravitational pull on Earth decreases with distance from the moon. – As a result, the ocean on Earth’s near side is pulled

toward the moon with the greatest force. – The solid Earth experiences a lesser force.

• These differences cause Earth’s tidal bulges. Because Earth rotates, tides occur in a regular rhythm at any given point on Earth’s surface each day.

Causes of Tides

Moons of Mars• All of the planets in our solar systems except Mercury

and Venus have moons. In addition, Saturn, Jupiter, Uranus, and Neptune all have rings.

• Mars has two tiny moons named Phobos and Deimos, which revolve around Mars relatively quickly.

• Phobos and Deimos are irregularly shaped chunks of rock and are thought to be captured asteroids.

• The surfaces of Phobos and Deimos are dark like maria on earth’s moon, and both have many craters.

Moons of Jupiter• Galilean moon - any one of the four largest

satellites of Jupiter—Io, Europa, Ganymede, and Callisto - that were discovered by Galileo in 1610

• In addition to the four large moons discovered by Galileo, scientists have observed dozens of smaller moons around Jupiter.

• Of Jupiter’s four Galilean moons, three are larger than Earth’s moon.

Europa• Europa is the second closest Galilean moon to

Jupiter. Europa is about the size of Earth’s moon, but is much less dense.

• Astronomers think that Europa has a rock core that is covered with a crust of ice that is about 100 km thick.

• Scientists have concluded that an ocean of liquid water may exists under this blanket of ice.

• If liquid water exists, simple life forms could also exist there.

Ganymede• Ganymede is the third Galilean moon from

Jupiter and is also the largest moon in the solar system.

• Ganymede has a relatively small mass because it is probably composed mostly of ice mixed with rock.

• The surface of Ganymede has dark, crater-filled areas, and light areas that may be ridges and valleys.

Callisto

• Callisto is the farthest Galilean moon from Jupiter.

• Callisto is similar to Ganymede in size, density, and composition, except Callisto’s surface is much rougher.

• Callisto may be one of the most densely cratered moons in our solar system.

Moons of SaturnTitan

• Saturn has over 30 moons, but only five moons are fairly large.

• Saturn’s largest moon is Titan, which has a diameter of over 5,000 km.

• Titan’s has a thick atmosphere composed mainly of nitrogen.

Saturn’s Other Moons

• Most of Saturn’s moons are small, icy bodies that have many craters.

• Saturn’s icy moons resemble Jupiter’s icy Galilean moons.

• Saturn’s other smaller moons have irregular shapes. Scientists think that many of the smallest moons were captured by Saturn’s gravitational pull.

Moons of Uranus and Neptune

• Uranus’s four largest moons, Oberon, Titania, Umbriel, and Ariel, were known by the mid-1800’s.

• A fifth moon, Miranda, was discovered in 1948. Scientists now know that Uranus has over two dozen moons.

• Neptune has at least eight moons. One of these moons, Triton, revolves around Neptune in a retrograde orbit.

Moon of Pluto• The relationship between Pluto and its moon

Charon is unusual because Charon is almost half the size of Pluto itself.

• Because Pluto and Charon are similar in size, some scientists consider them to be a double-planet system.

• Charon completes one orbit around Pluto in 6.4 days, the same length of time as a day on Pluto. Because of these equal lengths, Charon stays in the same place in Pluto’s sky.

Rings of the Gas Giants• Saturn’s set of rings was discovered over 300

years ago. Each of the rings circling Saturn is divided into hundreds of small ringlets, which are then composed of billions of pieces of rock and ice.

• Jupiter has a single, thin ring made of microscopic particles that may have been given off by Io.

• Uranus also has a dozen thin rings.• Neptune’s relatively small number of rings are

clumpy rather than thin and uniform.

asteroid - a small, rocky object that orbits the sun; most asteroids are located in a band between the orbits of Mars and Jupiter

• In addition to the planets and their moons, our solar system includes millions of smaller bodies, such as asteroids, comets, and meteoroids.

• The largest of these smaller bodies are asteroids, which are fragments of rock that orbit the sun.

• The composition of asteroids is similar to that of the inner planets.

• Classification of asteroids:– asteroids made mostly of silicate material -

most common – iron and nickel - have a shiny, metallic

appearance, especially on fresh surfaces– made mostly of carbon materials – has dark

color –the rarest type

• Near-Earth asteroids - have orbits that sometimes bring them very close to Earth, 1000+

comets - a small body of rock, ice, and cosmic dust that follows an elliptical orbit around the sun and that gives off gas and dust in the form of a tail as it passes close to the sun

• The most famous comet is Halley’s Comet, which passes by Earth every 76 years. It last passed Earth in 1986, and will return in 2061.

• Every 5 to 10 years, the Hale-Bopp Comet is visible from Earth.

Composition of Comets• core, or nucleus - made of rock, metals, and

ice.• coma - spherical cloud of gas and dust,

surrounds the nucleus. The coma can extend as far as 1 million kilometers from the nucleus.

• tail - most spectacular part of a comet, form when sunlight causes the comet’s ice to change to gas. The solar wind pushes the gas away from the comet’s head.

• second tail -made of dust and curves backward along the comet’s orbit.

• The nucleus and the coma form the head of the comet.

Oort cloud a spherical region that surrounds the solar system, that extends from just beyond Pluto’s orbit to almost halfway to the nearest star, and that contains billions of comets

• Scientists think that most comets originate in the Oort cloud.

• The Oort cloud surrounds the solar system and may reach as far as halfway to the nearest star.

The Kuiper Belt• Kuiper belt the flat region beyond Neptune’s orbit

that extends about twice as far as Neptune’s orbit, that contains leftover planetesimals, and that is the source of many short-period comets.

• Advances in technology have allowed scientists to observe many small objects beyond the orbit of Neptune.

• Most of these objects are from the Kuiper belt and are called Kuiper-belt objects. Pluto is located in the Kuiper belt during much of its orbit

meteoroids - a relatively small, rocky body that travels through space

• Most meteoroids have a diameter of less than 1 mm. Scientists think that most meteoroids have a diameter of less than 1 mm.

• Large meteoroids, which are more than 1 cm in diameter, are probably the result of collisions between asteroids.

meteor - a bright streak of light that results when a meteoroid burns up in the Earth’s atmosphere

• When a meteoroid enters Earth’s atmosphere, friction between the object and the air molecules heats the meteoroid’s surface.

• As a result of the friction and heat, most meteoroids burn up in Earth’s atmosphere.

• As the meteoroid burns up, it produces a bright streak of light called a meteor. Meteors are often called shooting stars.

Meteorites

• Meteoroids that do not burn up, but fall to Earth’s surface, are called meteorites.

• Most meteorites are small and don’t cause much damage, but occasionally large meteorites strike Earth’s surface with the force of a large bomb.

• There are three types of meteorites: stony, iron, and stony-iron.

Lunar Landings