Solar System Review:

Our solar system consists of a star of average size and luminosity we call the Sun, the planets (in order of their distance from the Sun) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, the satellites or moons of the planets, numerous comets, asteroids, meteoroids and the interplanetary medium.

The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction (counterclockwise), in nearly circular orbits (ellipses, but close to circles). When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction.

The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets.

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined.

Terrestrial Planets:

The four primary terrestrial worlds are the innermost planets in the solar system, Mercury, Venus, Earth and Mars. There are an additional 8 other terrestrial worlds; the Moon, Io, Europa, Ganymede, Callisto (the four Galilean moons), Titan (a moon of Saturn), Triton (a moon of Neptune) and Pluto.

They are called terrestrial because they have a compact, rocky surface like the Earth's and are spherical in shape. The other moons are not spherical and are more asteroid-like (i.e. irregular). Venus, Earth, Mars and Titan have significant atmospheres, the rest have little to zero.

Planet vs. Moon:

A planet revolves around the Sun. A moon or satellite revolves around a planet. The term planet or moon is not selected by mass or size of the body (for example, Titan is bigger than Mercury).

The period of revolution of a planet is determined by timing and astrometry (the science of measuring stellar and planetary positions). Periods of rotation are determined by either

  1. timing surface features
  2. timing clouds and atmospheric features
  3. reflected sunlight (light curves)
  4. Doppler radar measurements of planet limb
Note that timing atmospheric features usually reveals that Jovian planets have differential rotation (meaning their equators rotate faster than the pole regions, i.e. the planet is not solid).

Information about the planets is obtained by:

Earth:

The planet which we know the most about, due to our ability to explore its interior as well as exterior, is the Earth Most of this course will compare values and processes on other planets to those on the Earth, i.e. it is our yardstick for understanding other worlds. Therefore, knowing about our home world is crucial to appreciate the Universe, besides being necessary for our own survival.

In many ways, Earth is unique in the solar system. Its most obvious feature is the vast amounts of liquid water on its surface, as well as the ability to sustain intelligent life.

Earth rotating

The above color image of the Earth was obtained by the Galileo spacecraft when it was about 1.3 million miles from the planet, displays our world as would be seen by a space probe from another solar system. Galileo was making the first of two Earth flybys on its way to Jupiter. South America is near the center of the picture, and the white, sunlit continent of Antarctica is below. Picturesque weather fronts are visible in the South Atlantic, lower right.

The above is an infrared image of the Earth was taken by the GOES 6 satellite on September 21, 1986. A temperature threshold was used to isolate the clouds. The land and sea were separated and then the clouds, land and sea were separately colored and combined back together to produce this image.

The above image is a map of North and South America using radar altimetry to reflect the underlying topography of the oceans and continents.

The above image is part of the Rocky Mountain Range in the Yukon Territory of Canada is an excellent example of young mountains on Earth. This space shuttle image was taken when the sun was low on the horizon; the sharp shadows on the snow-covered peaks show how rough and uneven the area is.

The above picture is a space shuttle image of the Colorado River in Arizona captures the Grand Canyon. The canyon is 30 km (18 miles) across at its widest point and 1.6 km (1 mile) deep at rock bottom. It is 446 km (277 miles) long and covers an area that is over 5000 square km (about 2000 square miles). The Grand Canyon was created by the erosional action of the Colorado River on the surface as this region has continued to rise high above sea level over the last several million years.

Observations of the Earth's weather and ocean changes require permanent space platforms such as the Mir space station.

Where do people live? i.e. where are the population centers? The fastest way to check this is to observe the USA at night

Moon:

The Moon has fascinated mankind throughout the ages. By simply viewing with the naked eye, one can discern two major types of terrain: relatively bright highlands and darker plains. By the middle of the 17th century, Galileo and other early astronomers made telescopic observations, noting an almost endless overlapping of craters. It has also been known for more than a century that the Moon is less dense than the Earth. Although a certain amount of information was ascertained about the Moon before the space age, this new era has revealed many secrets barely imaginable before that time. Current knowledge of the Moon is greater than for any other solar system object except Earth. This lends to a greater understanding of geologic processes and further appreciation of the complexity of terrestrial planets.

The Moon is 384,403 kilometers (238,857 miles) distant from the Earth. Its diameter is 3,476 kilometers (2,160 miles). Both the rotation of the Moon and its revolution around Earth takes 27 days, 7 hours, and 43 minutes. This synchronous rotation is caused by an unsymmetrical distribution of mass in the Moon, which has allowed Earth's gravity to keep one lunar hemisphere permanently turned toward Earth. The above full disc of the Moon was photographed by the Apollo 17 crew during their trans-Earth coast homeward following a successful lunar landing mission in December 1972. Mare seen on this photo include Serentatis, Tranquillitatis, Nectaris, Foecunditatis and Crisium.

This image shows Earthrise over the Moon's limb.

Above is what the lunar surface was thought to look like in the 1800's.

Now we know it looks like the above image taking from the Apollo missions.

Many people have also seen imaginary faces on the Moon such as the Man on the Moon, the Lady on the Moon and the Bunny on the Moon.

This is a movie of Lunar rotation taken from the Clementine mission in 1994. Notice how the farside of the Moon does not have the large, smooth-looking maria that the nearside has.

The most obvious feature on the Moon is its many craters, due to impacts by space debris onto the surface. An Apollo 16 astronaut stands near the rim of Plum Crater (30m, or over 200 yards, in diameter). Although Earth has experienced many meteorite impacts throughout its history, the action of wind and water quickly erases the resulting craters.

The major surface features on the Moon are craters, highlands and maria.

craters

Highlands: The lighter colored, heavily cratered regions are called the lunar highlands. The bare, chaotic terrain indicates that these regions are primordial and one would expect the oldest rocks in these regions. Mountains on the Moon are not due to tectonic activity, but rather are due to overlapping impact rims.

Maria: Dark-colored regions which turn out to be smooth plains of basaltic lava. The are the remnants of large impact events that cracked the crust and allowed the lava from the mantle to flow upward and erase early cratering. Note that the impacts must have occurred after the initial phase of cratering.

The minor features are:

1) wrinkle ridges
2) scarps
3) domes
4) rilles

Craters:

The typical features to an impact crater are shown below:

Craters range in size from microscopic to large basins of order 1000's km (see Orientale Basin below).

Normal, round craters are due to impacts from objects up to a couple thousand meters. Object larger (asteroids) will typically crack the crust and form impact basins.

Erosion is slow on a world without an atmosphere and is caused by:

1) slumping (gravity)
2) other craters
3) temperature changes
4) moonquakes

The result is that young craters have sharp edges (usually less than 2x108 years old) and old craters are rounded, smoother (with ages of order a billion years old).

Mercury:

Mercury is the smallest of the major planets with a radii of only 2440 km (1590 miles). Since it is the closest planet to the Sun its greatest elongation is only 28 degrees (i.e. its only visible right after sunset or right before sunrise).

From its radius (i.e. volume) and mass we calculate it has a mean density of 5.4 g/cm3 which implies a dense iron (Fe) core. All planets form as molten balls, then cool and solidify. The cooling rate is proportional to the amount of material (the planet's mass) and its surface area (its radius). Since Mercury is low in mass (less heat stored from formation), its core is probably solid rather than liquid.

The rotation period of Mercury is 58.6 days and its orbital period (year) is 87.9 days. Notice that 2/3 times 87.9 is 58.6; thus, Mercury suffers from spin-orbit coupling where the tidal forces from the Sun has locked Mercury's rotation into a resonance number (1/2, 2/3, 4/5, 5/6, etc...).

Daytime temperatures on the surface of Mercury hover around 700 degrees Kelvin (enough to melt lead). Whereas, at night the ground temperature plunge to a mere 100 Kelvins (air turns to liquid at 77 Kelvins). Dawn is ten times more brilliant than on the Earth, since the Sun is ten times larger. The lack of any significant atmosphere means that before dawn you can see the Sun's corona spreading over the horizon.

The above image is a photomosaic of Mercury was constructed from photos taken by Mariner 10 six hours after the spacecraft flew past the planet on March 29, 1974. The north pole is at the top and the equator extends from left to right about two-thirds down from the top. A large circular basin, about 1,300 kilometers (800 miles) in diameter, is emerging from the day-night terminator at left center. Bright rayed craters are prominent in this view of Mercury. One such ray seems to join in both east-west and north-south directions.

The above mosaic shows the Caloris Basin (located half-way in shadow on the morning terminator). Caloris is Latin for heat and the basin is named this because it is near the subsolar point (the point closest to the sun) when Mercury is at aphelion. Caloris Basin is 1,300 kilometers (800 miles) in diameter and is the largest know structure on Mercury. It was formed from an impact of a projectile with asteroid dimensions. The interior floor of the basin contains smooth plains but is highly ridged and fractured. North is towards the top of this image.

The above ``weird terrain'' best describes this hilly region of Mercury. This area is at the antipodal point from the large Caloris basin. The shock wave produced by the Caloris impact was reflected and focused to this antipodal point, thus jumbling the crust and breaking it into a series of complex blocks. The area covered is about 100 kilometers (62 miles) on a side.

The above picture is a Mariner 10 image shows Santa Maria Rupes, the sinuous dark feature running through the crater at the center of this image (note how is passes through the crater rim indicating that it was created after the impact crater). Many such features were discovered in the Mariner images of Mercury and are interpreted to be enormous thrust faults where part of the mercurian crust was pushed slightly over an adjacent part by compressional forces. The abundance and length of the thrust faults indicate that the radius of Mercury decreased by 1-2 kilometers (0.6 - 1.2 miles) after the solidification and impact cratering of the surface. This volume change probably was due to the cooling of the planet, following the formation of a metallic core three-fourths the size of the planet. North is towards the top and is 200 kilometers (120 miles) across.

In general, the surface of Mercury is similar to the Moon (i.e. heavily cratered due to a lack of a heavy atmosphere to erode away primordial impacts). However, there are some key differences:

1) There are few maria on Mercury and they are small. No large impact era like the Moon. Therefore, Mercury must have cooled faster.

2) Cratering is less heavy, more plain region between craters. Due to the higher surface gravity on Mercury (you weight more than on Mercury than the Moon). This means impacts did not throw debris as far, fewer secondary craters and more concentrated around primary crater.

3) Long scarps or wrinkles are found on the crust and the tops of craters (i.e. after cratering epoch). After Mercury cooled, its crust solidified first. Mercury was still rotating quickly back then and had an equatorial bulge. As Mercury slowed in its rotation, due to tidal forces with the Sun, gravity pulled Mercury into a more spherical shape and the crust had to fold producing long scarps.

Venus:

Venus, the jewel of the sky, was once know by ancient astronomers as the morning star and evening star due to its low elongation with respect to the Sun. Venus, which is named after the Roman goddess of love and beauty, is veiled by thick swirling cloud cover.

Venus is often called the twin planet to Earth because 1) it has a similar radius/size, 2) is has a similar mass, 3) it has a similar density and 4) it has an atmosphere. However, the environment of Venus is very different from the Earth, its ground temperature is over 800 degrees F and its atmosphere is carbon dioxide and sulfuric acid.

cloud cover

The above image of Venus is a mosaic of three images acquired by the Mariner 10 spacecraft on February 5, 1974. It shows a heavy atmosphere covered with thick clouds that prevents optical observation of the surface of Venus.

radar mapped terrain

This hemispheric view of Venus is the result of more than a decade of radar investigations culminating in the 1990-1994 Magellan mission. The effective resolution of this image is about 3 kilometers. It was processed to improve contrast and to emphasize small features, and was color-coded to represent elevation.

Radar imaging shows that a Venusian day is 243 Earth days and is longer than its year of 225 days. Oddly, Venus rotates from east to west. To an observer on Venus, the Sun would rise in the west and set in the east.

The atmosphere of Venus is composed of 97% CO2, 2% N2 and less than 1% of O2, H2O and CH4 (methane). There is also a substantial amount of sulfuric acid in the lower atmosphere.

The surface temperature of Venus is around 890 degrees F, the hottest average temperature in the Solar System. This is due to the rich amount of CO2 which leads to a runaway greenhouse effect.

Due to an optically thick atmosphere, the surface features on Venus are known only through radar mapping. Venus' surface is relatively young geologically speaking. It appears to have been completely resurfaced 300 to 500 million years ago. Its not clear how and why this occurred. The Venusian topography consists of vast plains covered by lava flows and mountain or highland regions deformed by geological activity. Some of the prominent features revealed by the Magellan mission are:

1) Craters - impact craters, Venus is scarred by numerous impact craters distrubuted randomly over its surface. Small craters less that 2 kilometers are almost non-existent due to the heavy Venusian atmosphere. The exception occurs when large meteorites shatter just before impact, creating crater clusters. Note the lava flows around the rims.

2) Volcanoes - Volcanic features are common on Venus with at least 85% of the Venusian surface is covered with volcanic rock. Hugh lava flows, extending for hundreds of kilometers, have flooded the lowlands creating vast plains. More than 100,000 small shield volcanoes dot the surface along with hundreds of large volcanos. Flows from volcanos have produced long sinuous channels extending for hundreds of kilometers, with one extending nearly 7,000 kilometers.

3) Fault lines - In images of the Alpha Regio, bright terrain is shown to be a series of troughs, ridges, and faults that are oriented in many directions. The lengths of these features generally range from 10 kilometers to 50 kilometers.

4) Arachnoids - As the name suggests, arachnoids are oval features with concentric rings and a complex network of fractures extending outward. The arachnoids range in size from approximately 50 kilometers to 230 kilometers in diameter. They might have resulted from an upwelling of magma from the interior of the planet which pushed up the surface to form "cracks".

Although there is no direct evidence for tectonic activity (active volcanoes, plumes, vents, etc.), three of the above features suggest weak geological activity in the last 107 years.

Due to the intense heat and pressure, a mission to the surface of Venus was challenging. The Venera missions were a series of unmanned Soviet planetary probes that were sent to Venus. Venera 5 and 6 (1969) made soft landings on Venus, but ceased transmitting data before reaching the surface because of the extreme heat and pressure. Venera 9 and 10 (1975) sent back the first closeup photographs of the planet's surface; these images showed that certain parts of Venus were covered with sizable sharp-edged rocks and others with fine-grain dust.

In general, the Venusian soil is rocky material intermixed with gravel. The rocks appear to be igneous in nature and smooth from erosion.

Mars:

Before photography, the only recorded observations were drawings. Below, Old Mars displayed many features which were later shown not to exist.

Up till the early 1920's, we thought Mars looked like the drawings above and below. Note the ``canals'', which originally described as ``channels'' in Italian, but then was badly translated to ``canals'' which implied they were built by intelligent beings.

Today, Mars looks like this from the Hubble Space Telescope

Even before 1800's, it was known that Mars had some large surface features (grey-green regions between larger red regins). And it was know that Mars had seasons because the size of the polar caps changed.

Since 1976, we have send several probes to Mars. Orbital probes have produced detailed visual and radar maps of the surface. Some of the most notable surface features on Mars, such as:

valleys and canyons

The above image is a mosaic of the Valles Marineris hemisphere of Mars. The center of the scene shows the entire Valles Marineris canyon system, more than 3,000 kilometers long and up to 8 kilometers deep. Although it appears to be a canyon formed by water, in fact, Valles Marineris is a deep crust fracture. Many huge ancient river channels begin from the chaotic terrain and north-central canyons and run north. Many of the channels flowed into a basin called Acidalia Planitia, which is the dark area in the extreme north of this picture. The three Tharsis volcanoes (dark red spots), each about 25 kilometers high, are visible to the west. Very ancient terrain covered by many impact craters lies to the south of Valles Marineris.

landslides

Although Valles Marineris originated as a tectonic structure, it has been modified by other processes. This image shows a close-up view of a landslide on the south wall of Valles Marineris. This landslide partially removed the rim of the crater that is on the plateau adjacent to Valles Marineris. Note the texture of the landslide deposit where it flowed across the floor of Valles Marineris. Several distinct layers can be seen in the walls of the trough.

islands

The water that carved the channels to the north and east of the Valles Marineris canyon system had tremendous erosive power. One consequence of this erosion was the formation of streamlined islands where the water encountered obstacles along its path. This image shows two streamlined islands that formed as the water was diverted by two 8-10 kilometer diameter craters lying near the mouth of Ares Vallis in Chryse Planitia. The water flowed from south to north (bottom to top of the image). The height of the scarp surrounding the upper island is about 400 meters, while the scarp surrounding the southern island is about 600 meters high.

outflows

This image of the head of Ravi Vallis shows a 300-kilometer (186-mile) long portion of a channel. Like many other channels that empty into the northern plains of Mars, Ravi Vallis orginates in a region of collapsed and disrupted ("chaotic") terrain within the planet's older, cratered highlands. Structures in these channels indicate that they were carved by liquid water moving at high flow rates. The abrupt beginning of the channel, with no apparent tributaries, suggests that the water was released under great pressure from beneath a confining layer of frozen ground. As this water was released and flowed away, the overlying surface collapsed, producing the disruption and subsidence shown here. Three such regions of chaotic collapsed material are seen in this image, connected by a channel whose floor was scoured by the flowing water. The flow in this channel was from west to east (left to right). This channel ultimately links up with a system of channels that flowed northward into Chryse Basin.

polar caps

This image shows the south polar cap of Mars as it appears near its minimum size of about 400 kilometers. It consists mainly of frozen carbon dioxide. This carbon dioxide cap never melts completely. The ice appears reddish due to dust that has been incorporated into the cap.

 Phobos       Deimos

Mars also has two small moons, Phobos and Deimos (Fear and Panic). They are not regular moons like our Moon, but rather irregular-shaped objects, which probably means that they are captured asteroids.

  1. craters: impact craters with heavy erosion due to atmosphere

  2. featureless terrain: large regions devoid of faults or craters (not maria since no young craters are found). Observations by Viking Orbiter showed them to be deserts with dunes.

  3. chaotic terrain: highlands and broken hills, probably old tectonic regions.

  4. Polar caps: change in size with seasons, but since the temperature is less than 0 degrees C at all times, the ice is mostly CO2 ice with an H2O ice core. Viking Lander confirmed that the atmospheric pressure goes up in the summer as the CO2 ice melts.

    Olympus Mons

  5. volcanoes: The Tharsis and Elysium regions are rich in old cone volcanoes, averaging over 500 km across and 25 km high. The largest is Olympus Mons, shown above. These are ``hotspot'' volcanoes like the Hawaii Islands. There extreme size due to the fact that there is no tectonic plate motion on Mars.

  6. canyons: long, fractured regions. Most canyons on Earth are caused by either a) wind erosion, b) H2O flow or c) lava flow. But, the atmosphere of Mars is too thin for wind erosion, H2O is all ice, all volcanoes are inactive. Therefore, Martian canyons must be tectonic features leftover from early epochs.

Viking Lander 1   Viking Lander 2

Our first views of the Martian surface came from the Viking Lander 1 and Viking Lander 2. They indicated a surface and soil that is mostly old impact debris with sand-blasted gravel. The soil is rich in Si and Fe, Fe oxide (rust) given Mars its red color.

Viking also showed that Martian winter is think with frost, but that this frost was mostly CO2 ice.

In the summer of 1997, Mars Pathfinder landed on the surface of Mars. The lander contained cameras and metrological instruments, and also carried a robot rover (shown below) whose job was to take soil and rock samples.

Galilean Satellites:

The satellites discovered by Galileo with his small telescope form a small, `mini' solar system around Jupiter. They each have special characteristics related to their formation process, but have the following traits in common:

1) all orbit Jupiter
2) they all are tidally locked to Jupiter
3) they all have radii larger than our Moon
4) the inner moons have densities higher than outer moons (implies that Jupiter was much warmer in the past, such that the moons formed near Jupiter have less of the volatile elements such as CO2 and H2O)

Io:

Io is the innermost world, closest to Jupiter and can be classified as one of the most unusual moons in our solar system. Its unique properties include:

Europa:

Europa is the next world out from Io and is considered a strong candidate to find primitive life. Its characteristics are:

Ganymede:

Ganymede is much less dense than Europa or Io. Its interesting characteristics are:

Callisto:

Callisto is the outermost of the 4 primary satellites:

Titan:

Titan is the largest satellite of Saturn, unique in its methane atmosphere: