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Content Benchmark E.8.B.3
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Earth Science
Atmospheric Process and Water Cycle
Solar System and Universe
Earths Composition and Structure
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Earth Science
Physical Science

Students know characteristics of the planets in our solar system. I/S

I've known things about the planets since I was a kid.

Mercury is tidally locked to the Sun, with one side always facing the Sun and the other side always in perpetual darkness. Thus, Mercury is both the hottest and the coldest planet in our solar system. And, Mercury is home to bulbous vegetable entities which would one day serve as hosts to the transferred minds of the Great Race of Yith.

Venus? What a fascinating planet! It is a tropical, Carboniferous paradise teeming with exotic life in the various jungles, swamps and oceans. Though you have to watch out for dinosaurs and carnivorous plants (and sometimes lizardmen) the sentient inhabitants, whether aquatic or terrestrial, are generally peaceful and amiable in their interaction with humans.

Mars is so intriguing. When the red planet entered an elonged period of drought, the inhabitants built an extensive system of canals, so large they were telescopically visible to observers on Earth (as are also the seasonal changes in the Martian polar ice caps - Figure 1). Martians, themselves, are a diverse group, some looking exactly like humans (if you disregard their vivid coloration), others very tall, yet humanoid, and others still quite alien in appearance, and intent on attacking Earth and killing humans.

Figure 1. Percival Lowell's drawings of the Martian Canals

The floating, city-sized lifeforms found in the thick, Jovian atmosphere immediately draw one's attention. Of course, the vast area of land present on Jupiter's surface is most appealing. Enough room there for Earth's population one hundredfold. Granted, Earthling's have to be genetically altered for locomotion on such a strong-gravity planet, but the atmosphere has such wealth in gaseous resources it is worth it. And you cannot forget the lake from which the waters are able to bring inanimate objects to "life".

When Micromégas first visited Saturn, he found intelligent beings over 6,000 feet tall, possessing 72 senses and living at least 15,000 years on the rocky, desolate surface of the planet. Then of course, are the swordsmen of Saturn, who live around the volcanic regions and fly atop pterodactyl-type creatures.

Uranus is still the only know location in the universe from which the mineral taranium can be mined. Neptune, an inhabited world, contains vast regions of sea as well as of glaciers. And Pluto, so recently ignominiously demoted from planetary status, is an icy world upon which was found evidence revealing our solar system to be a failed alien engineering project.

My early "knowledge" of the planets of our solar system came from extensive research - in the forms of total immersion both in science fiction literature and in science fiction movies and television shows. As invested as I was with the childhood view of my planets, that excitement paled in comparison to the awe I felt when viewing images from the multitude of planetary orbiters and landers humanity sent out to explore the solar system, and in reading the analyses of the collected data.

Mercury, at an average distance of 57,910,000 km, or 0.38 Astronomical Units (AU), is the closest planet to the sun. A year on Mercury (one complete revolution about the sun) requires 88 Earth days. Once believed to be the smallest planet in our solar system, Mercury (diameter of 4880 km) is actually bigger than Pluto (diameter of 2274 km). Still, with Pluto now classified as a "dwarf planet" it would be fair to again list Mercury as the smallest "actual" planet. With a density of 5.43 g/cm3, Mercury is the second-most dense planet in our solar system (Earth is first, with a density of 5.52 g/cm3). Mercury has the widest range of temperature extremes (coldest to hottest) of the planets. Mercury's twilight hemisphere can reach temperatures as low as -184°C, while its daylight hemisphere will range up to 427°C. The temperature extremes are related to the length of Mercury's rotational period. It takes about 59 Earth days for Mercury to complete one rotation about its axis, leaving a portion of Mercury exposed to direct sunlight for a very long time while another portion is exposed to no sunlight for lengthy periods. The fact that there is virtually no atmosphere prevents convection from tempering the extreme heat and cold. What little atmosphere there is appears to consist of small amounts of hydrogen and helium captured from solar wind, and tiny quantities of sodium and potassium vapors (formed as the crust degrades in the heat) which cling near the surface.

Mercury was visited by Mariner 10 from 1974-1975, which revealed a heavily cratered surface, similar to Earth's moon as well as steep cliffs as tall as 3km (Figure 2).

Figure 2. Mariner 10 mosaic of Mercury

Geological analyses of the photos sent back by Mariner 10 suggest that Mercury's crust solidified completely before its core solidified, and as the core solidified from molten iron to solid iron, it shrunk, which caused Mercury's crust to inwardly contract, leading to compressional forces and reverse faulting (Figure 3).

Figure 3. Mariner 10 photo of fault cutting through Mercury surface features

Just in January of 2008, a new probe, Messenger, flew by Mercury and gathered even more data about the planet. Messenger also flew by Venus (2007), and by 2012 will establish an orbit around Mercury. Messenger took a photo from the closest approach ever (200 km above the surface) as seen in Figure 4.

Figure 4. Messenger photo of Mercury from 200 km above the surface

The website Views of the Solar System provides a Quicktime movie (in two parts) describing the formation and early history of Mercury. The links are as follows: and,

The above information about Mercury is a compilation of data from the Nine Planets website ( and the Windows to the Universe website (

Venus is the second planet of our solar system, located at a distance of 108,200,000 km (0.72 AU) from the sun. Its size (diameter of 12, 104 km), mass (4.87 x 1024 kg), density (5.24 g/cm3) and rock composition (mafic and ultramafic igneous rock) is similar to that of Earth, which led us to once believe Venus was Earth's "twin." A year on Venus is equivalent to just over 226 Earth days. Venus has the distinction of having the longest day of any of the planets. Not only does Venus have retrograde rotation (rotates clockwise about its axis instead of counterclockwise), it takes 243 Earth days to make one complete rotation. This means a Venus day lasts longer than a Venus year! Temperatures on Venus average around 457°C, hotter even than Mercury. The extreme temperatures can be attributed to the exceptionally thick Venusian atmosphere, which is composed primarily of carbon dioxide, sulfuric acid and some water vapor. Figure 5 depicts layers of atmosphere around Venus, and indicates the average temperatures of those layers.

Figure 5. Atmospheric Column of Venus

Venus is so hot that its oceans were long ago evaporated and boiled away. With no ocean water in which to dissolve appreciable amounts of carbon dioxide, atmospheric carbon dioxide rose and the greenhouse effect was compounded past the point of no return (a phenomenon referred to as a run-away greenhouse effect). Surface temperatures on Venus are hot enough to melt lead (melting point around 327°C), and there are over 90 Earth atmospheres of pressure at the surface, so it is clear that if Venus is Earth's twin, it would be the "evil twin".

Various space probes have explored Venus. Mariner 2, in 1962, was the first successful probe to fly by Venus. It revealed a very hot, cloud-covered planet with a carbon dioxide atmosphere. Venera 7 (1970), launched by the Soviets, was the first probe to return data from the surface of another planet. Venera 9 (1975) made a soft landing on Venus and sent back remarkable pictures of the surface (Figure 6).

Figure 6. Surface images of Venus taken by Venera 9

Even better were the color images sent back in 1982 by Venera 13, which had a color TV camera onboard (Figure 7).

Figure 7. Surface images of Venus taken by Venera 13
(From and

In 1978, Pioneer Venus (an orbiter with four atmospheric probes) made the first high-quality map of Venus' surface (Figure 8).

Figure 8. Topographic Map of Venus from Pioneer Venus data

Magellan, launched in 1989, mapped 98% of the surface of Venus at better than 300 meter resolution. Gravimetric data was obtained for 95 percent of Venus. The images returned from Magellan revealed a wealth of information about the external and internal geology of the planet. The totally random distribution of craters on Venus' surface suggests a uniform age for the surface (about 800 million years old). This is unusual, and may indicate Venus was completely resurfaced by intense, global volcanic activity. This has fueled a debate over whether Venus has a thin lithosphere or a thick lithosphere. The debate has not been resolved. Magellan revealed craters with unusual, spider-like appearance, including one which resembles a tick (Figure 9).

Figure 9. Arachnoid Craters from Magellan photos
(From and

Other unusual features were exceptionally flat volcanoes, referred to as "pancake" volcanoes (Figure 10).

Figure 10. Pancake Volcanoes from Magellan

Detailed, 3-D images of the surface topography were compiled, revealing steep cliffs and deep canyons (Figure 11).

Figure 11. Mountains and Scarps on Venus from Magellan photos

Venus Express arrived at Venus in 2006. It was launched by the European Space Agency (ESA) with the intent of studying Venus even more intensely. Data gathered from Venus Express suggest that contrary to the popular theory of Venus being "dead" geologically, it might actually be geologically active, having from a few hot spots to many active volcanoes. Venus Express has detected gaseous markers in the lower layers of the atmosphere and variations in its temperature which may be indicative of volcanic activity. Figure 12 is an artist's rendition of how a Venusian volcano might appear.

Figure 12. Artist's Rendition of Volcano on Venus suggested by data from Venus Express

Figure 13 is intended as an overarching image to show the topography on Venus as a whole. It is a mosaic of photos taken from Magellan. The website Views of the Solar System provides a Quicktime movie which nicely follows Magellan's exploration of Venus. The links is as follows:

Figure 13. False color image of Venus

The above information about Venus is a compilation of data from compilation of data from the Nine Planets website ( and the Windows to the Universe website (

Earth, colloquially the "third rock from the Sun", is located 149,600,000 km (1.00 AU) from the sun. As our planet of residence, it is the planet about which we've learned the most, and of which we are taught in Earth Science classes. As such, I'll cover just a few fundamentals. Earth has a diameter of 12,756.3 km and a mass of 5.972 x 1024 kg. Its density, at 5.52 g/cm3, places Earth as the densest planet. Earth's lithosphere is comprised of mafic, ultramafic and felsic rocks. Earth hosts the only known life in our solar system. That fact, while important, also explains the presence of free oxygen in Earth's atmosphere. Oxygen is a very reactive gas; if it were not constantly released into the atmosphere by photosynthetic organisms most of the oxygen would soon be tied up in compounds on Earth's land and seas. Earth is the only known solar body upon which water exists in all phases (solid, liquid and gas). We believe Earth to be the most geologically active planet, replete with active volcanism, seismic activity and plate tectonics. Though Earth is around 4.6 billion years old, the majority of Earth's surface is quite young. Over 500 million years of tectonism, weathering and erosion has erased evidence of Earth's early geologic history in favor of more recent geologic history. To put Earth into perspective with the rest of the solar system, Figure 14 shows an image of Earth taken in 2003 by the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) when it was orbiting Mars approximately 139 million kilometers from Earth.

Figure 14. Earth as viewed from Mars (image on left is the actual photo taken by Mars Orbiter Camera; image on right has computer overlay of lines of longitude and latitude, as well as North and South America, to help viewer understand what
portion of Earth they are seeing on the right)

The above information about Earth is a compilation of data from the Nine Planets website ( ) and the Windows to the Universe website (

Mars is the fourth planet in our solar system, located 227,940,000 km (1.52 AU) from the Sun. It has a diameter of 6,794 km, a mass of 6.4219 x 1023 kg, and a density of 3.911 g/cm3. A year on Mars is equivalent to about 687 Earth days, but a day on Mars, at 24.6 hours, is almost identical to an Earth day. The tilt of Mars' axis (25°) is also similar to that of Earth's (23.5°), thus Mars has the same four seasons found on Earth. We have observed the Martian polar ice caps growing in size during winter and diminishing in size during summer. We have discovered that the polar ice caps of Mars are "bi-layered". The lower, "permanent" layer is composed of water ice. The upper, "temporary" layer is made of frozen carbon dioxide. It is the carbon dioxide layer which exhibits seasonal growth and shrinkage. In Figure 15, the regions of the ice cap colored pink are comprised of frozen carbon dioxide, while the regions in blue and in green represent water ice.

Figure 15. False color image of Mars southern ice cap from the ESA Mars Express

The seasonal changes are associated with violent winds and dust storms as huge volumes of carbon dioxide gas are added to the atmosphere. The dust storms are global in coverage, as photographed in 2001 by the Mars Global Surveyor and depicted in Figure 16.

Figure 16. Global Dust Storm on Mars during June and July, 2001

Temperatures from -133°C to 27°C have been recorded from the Martian surface. Due to the thin atmosphere, that warm reading of 27°C exists on the dayside during the summer, and is found only within a few centimeters of the surface. Regarding the atmosphere, it made up of over 95% carbon dioxide, with small concentrations of nitrogen and argon. Oxygen is found as 0.13% of the atmosphere of Mars. The average surface pressure of Mars is around 7 millibars (less than 1% of Earth's surface pressure).

We have sent many space probes to Mars over the years, starting with the Mariner 4 fly by in 1965 which gave us our first close-up pictures of the surface of Mars (Figure 17). Mariner 9, in 1971, was first to achieve orbit around Mars. Evidence of huge volcanoes, deep, long canyons, and surface water flows were provided by the images Mariner 9 sent back to Earth.

Figure 17. Images of craters taken by Mariner 4

Later that same year, the Soviets orbited Mars 2 about the planet, but crashed the lander as a result of too steep a descent. Just in time for the 1976 United States bicentennial, Viking 1 and Viking 2 successfully landed on the surface of Mars. Both searched for life in the form of Martian microorganisms. The tests revealed unusual chemical activity, but were not conclusive as to the presence or absence of life. The seismometer on Viking 2 recorded a single marsquake. And, both craft sent back detailed color panoramic landscape images. The photos Viking 1(Figure 18) and Viking 2 took revealed Mars to have a crust made of reddish rock and sand, and showed a pink sky from sunlight reflected off red-colored dust in the atmosphere (when dust is largely absent, the sky is blue).

Figure 18. Viking 1 landing site

The Mars Pathfinder (with rover Sojourner, 1997) and Mars Expedition (with rovers Spirit and Opportunity, 2004) landed on and roamed across the Martian surface. These rovers were able explore regions of Mars far from the sites upon which their landers settled (Figures 19 and 20).

Figure 19. Sojourner investigating Martian rock and soil

Figure 20. Stratified layers of rock photographed by Opportunity along a rim in Victoria Crater

The rover Spirit captured video of a large dust devil traversing the Martian surface (Figure 21). Click on the weblink to view the actual video of the dust devil.

Figure 21. Dust devil filmed by Spirit

Three Mars orbiters (NASA's Mars Global Surveyor and Mars Odyssey, as well as ESA's Mars Express) have also provided valuable data and imagery during the early to mid 2000s. The following link, (
) provides access to the Mars Global Surveyor highlight website. Quicktime video created by a compilation of images obtained by Mars Odyssey provides an animated flight through Valles Marineris (

Enormous geological features have been found on Mars. Olympus Mons, though inactive, is the solar system's largest volcano (27 km tall and over 600 km wide, Figure 22). There are no active volcanoes on Mars.

Figure 22. Olympus Mons

Valles Marineris dwarfs Earth's Grand Canyon. This Mars canyon is over 4000 km in length and ranges to 200 km in width and 7 km in depth (Figures 23 and 24).

Figure 23. Global image of Mars showing entirety of Valles Marineri

Figure 24. Close-up view of Candor Chasma, a portion of Valles Marineris

Many images of the Martian surface show evidence of surface erosion. The various fluvial features tell us Mars once had great quantities of flowing water (Figures 25 and 26).

Figure 25. Network of ancient streams on Mars

Figure 26. Gulleys on Mars

In 2005, surprising images were obtained from Mars Express. The photos were taken near the Martian equator, and show what appears to be a dust-covered, frozen sea. Evidence suggests that it was liquid as few as 5 million years ago. Scientists have compared the plates observed in Figure 27 with photos of ice blocks off the coast of Antarctica, and believe they formed by the same process.

Figure 27. Possible plates of dust-covered ice near the equatorial region of Mars

Images of various regions of Mars show the planet has been glaciated in the past, and indeed, may still be undergoing active glaciation. Figure 28 is of Deuteronilus Mensae, and displays typical steep-sided features associated with glaciation. A more recent, close-up image ( Figure 29) shows what is believed to be an active glacier in this same region.

Figure 28. Glaciated features in Deuteronilus Mensae on Mars

Figure 29. Active glacier in Deuteronilus Mensae on Mars

Mars has two tiny, irregularly-shaped moons which likely were captured from the asteroid belt. Phobos is closest to Mars, and is around 25 km wide; it orbits Mars once every 7 hours. Deimos is furthest from Mars, is about 13 km across, and takes around 31 hours to make one orbit about Mars (Figure 30).

Figure 30. Phobos (left) and Deimos (right)
(From and

On an aside, some people believe Viking took images, on Mars, of what appear to be artificial structures created by intelligent life (Figure 31). The face and pyramids of Mars, though intriguing, are artifacts of erosion.

Figure 31. Face and Pyramids on Mars
(From and

The above information about Mars is a compilation of data from the Nine Planets website ( and the Windows to the Universe website (

Jupiter is the fifth planet, at a distance of 778,330,000 km (5.20 AU) from the sun. Jupiter is the first of the gas giants in our solar system. It is largest of all the planets, with a diameter of 142,984 km. One Jupiter year takes about 12 Earth years, but a day on Jupiter is the equivalent of just 9.8 Earth hours. Though Jupiter has a mass of 1.9 x 1027kg, its density is just 1.326 g/cm3. It is interesting to note that Jupiter is the most massive planet; it actually has more mass than all the other planets combined. It is possible Jupiter has a rocky core equivalent to around 10 - 15 Earth masses. The core appears to be as hot as 30,000°C, leading Jupiter to radiate more energy back into space than is received from the sun. Since Jupiter is not massive enough to have initiated nuclear fusion, it is thought that gravitational compression must be producing the heat. Liquid metallic hydrogen surrounds the core (and comprises the bulk of Jupiter's mass). Conditions must be extreme (pressure of over 4 million atmospheres) to permit the existence of liquid metallic hydrogen. This form of hydrogen likely makes possible Jupiter's intense magnetic field. The magnetosphere of Jupiter (Figure 32) is the largest thing in our solar system (wider, even, than the diameter of the sun).

Figure 32. Magnetosphere of Jupiter

What we see of the planet is its extraordinarily thick atmosphere made of striped bands of hydrogen gas, methane, ammonia, carbon dioxide and water. The bands are classified as zones if they are lighter-colored and belts if they are darker-colored. Minor variations in temperature and chemical composition are believed to be responsible for the differences in color of the bands. It has been observed that the winds in the bands blow at speeds of as fast as or greater than 540 km/hr. Winds in adjacent bands blow in opposite directions, as depicted in this Quicktime movie: Data from atmospheric probes indicate the atmospheric turbulence extends thousands of kilometers towards Jupiter's interior, thus suggesting the turbulence and winds are driven by internal heat as opposed to insolation (heating from the sun) of the surface atmosphere. Figure 33 shows the banded atmosphere of Jupiter.

Figure 33. The banded atmosphere of Jupiter

The Great Red Spot of Jupiter (Figure 34) is a cyclonic storm (like a hurricane). It is wide enough that 2½ Earths could fit across. The Great Red Spot is telescopically visible from Earth, and has been observed by astronomers for over 300 years. Scientists are uncertain as to how this same storm could have lasted continuously for such a long time.

Figure 34. The Great Red Spot of Jupiter

Jupiter has thin, dark rings (Figure 35), composed of small particles of rocky material. It appears that, unlike Saturn's rings, they contain no ice.

Figure 35. Jupiter's Main Ring

Jupiter has over 60 known satellites, but the most famous are the Galilean moons (Figure 36). Io is the most volcanically active object in our solar system, spewing forth sulfurous lava from its many active volcanoes. It has a thin atmosphere of oxygen, sulfur and sulfur dioxide. Europa has surface composed of a 100 km thick layer of water ice. There might be a 50 km deep ocean of liquid water beneath the frozen layer. Imagery suggests possible tectonic activity. There appears to be a thin atmosphere with water vapor and oxygen present. Ganymede is the largest moon in the solar system (and is bigger than planet Mercury). It has a thick crust of water ice, and possibly slush beneath this, but no apparent liquid water. A very thin hydrogen atmosphere has been detected. Callisto has a crust made of ice and rock. Possibly, a very saline liquid ocean exists beneath the frozen crust. A thin atmosphere of hydrogen, oxygen and carbon dioxide is present.

Figure 36. Composite of the Galilean Moons of Jupiter

Jupiter has been visited by a few space probes. Pioneer 10 was first to flyby Jupiter in 1973, followed by Pioneer 11 in 1974. The Pioneer probes were designed to test the ability of spacecraft to survive passage thru the asteroid belt (an easy accomplishment) and Jupiter's magnetosphere (the ions of which nearly fried the electronics on the spacecraft). Both Pioneers are heading into interstellar space (the first Earth vessels to do so). As such, each carries a gold-anodized plaque with information about their origin (Figure 37) in the event either craft is someday discovered by extraterrestrial intelligence interested in contacting us.

Figure 37. Drawing of plaque attached to Pioneer 10 and Pioneer 11

Voyager 1 and Voyager 2 were launched in 1977. These two probes extended our knowledge of Jupiter (as well as Saturn, Uranus and Neptune) and the rings and satellites of those planets. They revealed Jupiter to have complicated atmospheric dynamics, with lightning and aurorae. Ulysses, a product of collaboration between the ESA and NASA, was launched in 1990, first visiting Jupiter, then moving into a solar orbit ranging from 1.5 AU to 5.2 AU from the sun (it was designed as a long-range solar studies spacecraft). Ulysses detected streams of dust particles flowing from Jupiter. These dust streams are made of grains no larger than smoke particles, and are believed to originate in volcanoes of Io. The particles are propelled outward by Jupiter's magnetic field. The Galileo Jupiter orbiter and atmospheric probe made extensive surveys of Jupiter's moons and of Jupiter's atmosphere. In 2003, Galileo was deliberately crashed into Jupiter to prevent any possibility that it might eventually crash into Europa. Conditions on Europa might favor development of life, and scientists wanted to avoid contamination of such life that might be there.

The above information about Jupiter is a compilation of data from the Nine Planets website ( and the Windows to the Universe website (

Saturn, at 1,429,400,000 km (9.54 AU), is the sixth planet from our Sun. Saturn's diameter is 120,536 km (making it the second largest planet) and its mass is 5.68 x 1026 kg. With a density of only 0.678 g/cm3 (less than that of water), Saturn has the lowest density of any planet. Like Jupiter, it is a gas giant. A day on Saturn is equal to 10.67 Earth hours, and a Saturn year is equal to 29.5 Earth years. Saturn seems to have a rocky core surrounded by liquid metallic hydrogen. And Saturn, with a core temperature of over 12,000°C, also radiates more energy than it receives from the sun. Saturn has a thick atmosphere with faintly colored bands (within which wind speeds reach 1440 km/hr). The atmosphere is comprised mainly of hydrogen and helium with trace amounts of water, methane and ammonia. One of the most notable features of Saturn is its rings. The rings are quite thin (less than one kilometer thick) but extend 250,000 km in diameter. The particles comprising the rings are primarily water ice, but may also include rocky particles with icy coatings. Figure 38 displays the banding and rings of Saturn.

Figure 38. Bands and rings of Saturn

Pioneer 11, Voyager 1 and Voyager 2 visited Saturn after each completed their fly bys of Jupiter. These spacecraft gave us the first detailed, clear images of Saturn's banded atmosphere and of its rings. They also provided images of Saturn's moons. The most detailed imagery has come from Cassini, a craft launched in 1997 as a joint venture between the ESA and NASA. Cassini, consisting of an orbiter and a probe, arrived at Saturn in 2004, and sent back very sharp images of Saturn and its rings (Figure 39).

Figure 39. Cassini images of Saturn and its rings

High resolution observations of Saturn's radio emissions have recently been released by NASA. These observations were made with the radio and plasma wave instrument aboard the Cassini spacecraft. To listen to the "sounds of Saturn" click on this link:

Of Saturn's many moons (around 60), Titan is of particular interest to scientists because it is one of the few moons with an appreciable atmosphere. Titan is surrounded by a thick, smog-like haze which may be very similar to the atmosphere of primordial Earth more than 3.8 billion years ago. Cassini has revealed oceans of hydrocarbons on Titan, and precipitation of hydrocarbons from the atmosphere. It appears the liquid hydrocarbons on the surface are primarily ethane and methane. Read the recent NASA release (February 13, 2008) to learn more about the organic matter on Titan ( Mimas, resembling the Death Star from Star Wars due to its enormous crater, and Iapetus, with its one-of-a-kind equatorial ridge, are two more of Saturn's interesting moons (Figure 40).

Figure 40. Mimas (left) and Iapetus (right)
(From and

The above information about Saturn is a compilation of data from the Nine Planets website ( and the Windows to the Universe website /

Uranus, third of the gas giants, is the seventh planet of our solar system. It is located 2,870,990,000 km (19.218 AU) away from our Sun. Uranus has a diameter of 51,118 km and a mass of 8.683 x 1025 kg. It has a density of 1.279 g/cm3. A day on Uranus lasts 17.24 Earth hours, while a Uranus year takes 84 Earth years. Uranus is unusual in that its rotational axis lies within its orbital plane, and might have retrograde rotation. Uranus appears to be composed of uniformly distributed rock and various ices. The atmosphere is made of around 83% hydrogen, 15% helium and 2% methane, and exhibits very faint banding. The only spacecraft to visit Uranus is Voyager 2, which found that Uranus' magnetic axis was highly skewed from the already completely skewed rotational axis, leading to a strange magnetosphere with a twisting structure. The photographic images, taken by Voyager 2, show little variation in color within Uranus' atmosphere (Figure 41). Uranus does have rings. They have a dark coloration, and are composed of fairly large particles (up to 10 meters in diameter) in addition to fine dust.

Figure 41. True and False-Color Images of Uranus

Uranus has at least 27 moons. Its moon, Miranda, is so oddly-shaped that we believe it was smashed into pieces after colliding with another moon, after which gravity reassembled the fragments in a hodge-podge fashion (Figure 42).

Figure 42. Miranda

The above information about Uranus is a compilation of data from the Nine Planets website ( and the Windows to the Universe website

Neptune is the last of the gas giants. It is the eighth (and recently reassigned last) planet of our solar system. Neptune is located 4,504,000,000 km (30.06 AU) from the Sun, with a diameter of 49,532 km, a mass of 1.0247 x 1026 kg and a density of 1.638 g/cm3. A day on Neptune lasts 16.11 Earth hours. It takes almost 165 Earth years for Neptune to complete one revolution about the sun. Neptune was discovered in 1864, so we have not yet witnessed Neptune complete one orbit about the sun (that event will occur in the year 2011). The composition of Neptune is probably similar to Uranus (more or less uniform distribution of rock and ice), but there might also be a small, Earth-sized core of rocky material. Its atmosphere is mostly hydrogen and helium, with a small amount of methane. The blue color of Neptune may be due to the absorption of red light by methane in the atmosphere. Wind speeds on Neptune are the fastest in the solar system, reaching 2000 km/hour. Like Jupiter, Neptune has large cyclonic storms. Neptune had a Great Dark Spot in the southern hemisphere which, since its initial observation, has disappeared (Figure 43).

Figure 43. Neptune, exhibiting its Great Dark Spot

Like Jupiter and Saturn, Neptune radiates more heat energy than it receives from the sun, most probably due to gravitational compression. Neptune has dark rings encircling the planet. Earth-based observations suggested the rings were incomplete, just clumps of matter in arcs. Voyager 2, the only spacecraft to visit Neptune, revealed the rings are complete, and are twisted about each other in places (Figure 44).

Figure 44. Twisted rings of Neptune

Neptune has 13 known moons. Its largest, Triton (Figure 45), has a very thin atmosphere composed mostly of nitrogen and a trace of methane. At -235°C, Triton is as cold as, if not colder than, Pluto. At such cold temperatures methane, nitrogen and carbon dioxide are all frozen solid. Voyager 2 captured images on Triton of ice volcanoes (or are they geysers?). On one image, a plume (presumably of liquid nitrogen, dust and methane) is show rising 8 km above the surface. The Quicktime video linked to at this website ( ) shows animation of such a volcano/geyser. Besides Earth, only Triton, Io and Venus have been determined to be volcanically active at the present time. Triton is also different in that it has a retrograde revolution about Neptune.

Figure 45. Triton

The above information about Neptune is a compilation of data from the Nine Planets website ( and the Windows to the Universe website

Pluto has been demoted! In 2006, the International Astronomical Union (IAU) reclassified Pluto as a "dwarf planet". Additionally, Ceres (the largest asteroid in the main asteroid belt) and Eris (for awhile nicknamed Xena) have been classified as dwarf planets. Eris was originally designated 2003 UB313, and is a trans-Neptunian Object with a very eccentric orbit (perihelion of about 38 AU and an aphelion of around 98 AU). Eris most likely escaped from the Kuiper Belt. Figure 46 depicts the orbits of Ceres and Eris.

Figure 46. Ceres and Eris orbital paths


Figure 47 is a graphic which compares the sizes of Pluto, Ceres and Eris to Mars, Earth and the Moon (Luna). Additionally, the larger moons of these dwarf planets are depicted.

Figure 47. The Dwarf Planets as compared to other known solar bodies

Pluto is 5,913,520,000 km (39.5 AU) from the Sun (on average, as its orbit is highly eccentric). Pluto has a diameter of 2274 km, a mass of 1.27 x 1022 kg and a density of 2.063 g/cm3. Pluto is believed to have retrograde rotation, with its day equivalent to 6.387 Earth days. It takes Pluto 248 Earth years to make one complete revolution about the sun. Surface temperatures on Pluto are around -235°C. Pluto was discovered in 1930 by Clyde Tombaugh. Tombaugh was searching for a planet he believed to be perturbing Neptune's orbit. His discovery of Pluto was fortuitous, as Pluto was too small to have actually caused such a disruption to the orbit of Neptune. Pluto has three moons, Charon, Nix and Hydra (Figure 48). Charon is about half the size of Pluto. As of yet, no spacecraft have visited Pluto, but New Horizons, launched in 2006 should reach Pluto in 2015.

Please visit the following websites for additional information about Ceres ( and Eris (

Figure 48. Pluto and its moons

The above information about NeptPluto une is a compilation of data from the Nine Planets website ( and the Windows to the Universe website

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Content Benchmark E.8.B.3

Students know characteristics of the planets in our solar system. I/S

Common misconceptions associated with this benchmark

1. Students incorrectly believe that all planets have an appreciable atmosphere.

Students have been taught about Earth's atmosphere since they were in elementary school. It is a natural extension for them to believe that all planets are capable of developing an encompassing envelope of atmosphere. It is not immediately apparent to students that the strength of the planet's gravitational field is critical in its ability to hold onto an atmosphere.

Misconception reference is located at For further explanation about this topic, please visit the following websites:

2. Students have a mistaken belief that all planets have a rotational axis tilt similar to that of Earth, and that the rotational axes of all planets are perpendicular to the plane of their orbit.

Students understand that Earth rotates about an axis (which causes day and night). Many students believe this axis to be perpendicular to Earth's plane of the ecliptic (orbital plane) and do not realize Earth's axis is actually tilted almost 23.5° from perpendicular. Mercury's axis actually is perpendicular to its orbital plane. At 178° from perpendicular, Venus' axis has been flipped almost completely from top to bottom. Mars, at 25°, has an axis tilt close to Earth's. Jupiter has a tilt of 3.1° and Saturn has a tilt of 26.7°. Uranus, with a tilt of 98°, has an axis which lies virtually within its orbital plane. Neptune's axis it tilted 29.5° from perpendicular, and Pluto, with a tilt of 118° has its axis several degrees below its orbital plane (Figure 49)

Misconception reference is located at Further information about tilts of axes of rotation of the planets in our solar system can be found by visiting the following website (
) and clicking, individually on each planet's name, then from that page, click on the link to Planetary Facts.

Figure 49. Axial tilts of the planets in our solar system

3. Students have the misconception that all planets have similar chemical composition and similar densities.

Students relate the other planets to the one upon which we live, Earth. It makes sense to them that the other planets would also be composed of the same rocky exterior and iron-rich core of which they have learned in Earth Science classes. Since they believe all planets to have similar compositions, it follows that the planets should have similar densities. In reality, the different terrestrial (Earth-like) planets do have rocky exteriors and metal interiors, but the chemical composition of the crusts and cores of those worlds (Mercury, Venus and Mars) are neither identical to those of Earth, nor do they exist in the same proportions. The extraterrestrial planets (also called the gas giants) have far greater concentrations of light elements comprising the bulk of their mass.

Misconception reference is located at To better understand this topic, please visit the following websites:,


4. Students incorrectly believe that all planets undergo prograde rotation (planets rotate counterclockwise about their axes).

While it is true that the majority of the planets do exhibit prograde rotation, two planets operate differently. Venus has retrograde rotation (rotates clockwise about its axis) and the jury is still out on Uranus. The axis of rotation for Uranus is within its plane of the ecliptic. Astronomers disagree as to whether the rotation observed in this unique position is prograde or retrograde. The dwarf planet Pluto also has retrograde rotation.

Misconception reference is located at To learn more regarding the direction of rotation of planets about their axes, visit the following websites:,

5. Students mistakenly believe that all planets have solid surfaces; more specifically, that the diameter of the planet represents the approximate solid surface of that planet.

Students walk on the surface of Earth. Even when they view images of Earth from space in which clouds are visible, it does not appear to them that the atmosphere adds appreciably to the diameter of the planet. So, when they see the images of the various planets, they assume the seemingly solid disk they are viewing is the solid surface of the planet. It is true that the atmospheres of terrestrial planets are thin compared to the diameter of the planets, but for the Jovian (Jupiter-like) worlds (a.k.a. gas giants), there exists thick, layered atmospheres, often underlain by very deep, global oceans of liquid metallic hydrogen. A solid surface, if it exists at all on those worlds, lies at the core of the planet.

Misconception reference is located at More information about the surfaces of the planets in our solar systems can be found at the following websites:,

6. Students have the misconception that for all planets, their year (one revolution) is longer than their day (one rotation).

Our personal experience tells us that a year on a planet (one revolution) lasts far longer than a day (one rotation). It seems almost inconceivable that it could take longer for a planet to complete one rotation than it takes for it to complete one revolution. But, there is one planet, Venus, which does the inconceivable. A day on Venus lasts 243 Earth days while a year on Venus lasts just 226 Earth days.

Misconception reference is located at For information on the periods of rotational and periods of revolution of all the planets, visit the following website: and individually click on the planets to view the planet profiles.

7. Students have a misunderstanding that Mercury is the same temperature worldwide, and that Mercury has the hottest surface temperature of any planet in the solar system.

Students know Mercury is the closest planet to the sun. They surmise that as Mercury rotates, heat from the sun is absorbed and stored in the crust. They suspect Mercury will radiate some of this heat during its nighttime, but imagine overall, the planet has a relatively uniform very hot temperature. Due to Mercury's proximity to the Sun, it seems clear Mercury must be the hottest planet. The fatal flaw in the reasoning is failure to account for Mercury's almost imperceptible atmosphere and lack of realizing Mercury has a slow rotation (59 Earth days for one Mercury day). Without an atmosphere, the night side of Mercury, during its very lengthy night, radiates virtually all the heat energy which was absorbed back into space. Far from a uniform temperature, Mercury is very hot on the day side and very cold on the night side. And, even though Mercury is closer to the sun, Venus has a higher surface temperature due to its thick atmosphere and super-greenhouse effect.

Misconception reference is located at
More information about the surface temperatures of Mercury and Venus can be found at the following websites:,

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Content Benchmark E.8.B.3

Students know characteristics of the planets in our solar system. I/S

Sample Test Questions

1st Item Specification:  Describe general planetary motions.

Depth of Knowledge Level 1

  1. Which of the following is the BEST statement to describe the orbital path of the planets?
    1. All planets revolve in a clockwise direction.
    2. All planets revolve in a counterclockwise direction.
    3. The inner planets revolve clockwise while the outer planets revolve counterclockwise.
    4. The inner planets revolve counterclockwise while the outer planets revolve clockwise.
  1. Which statement regarding the amount of time it takes planets to complete one revolution is correct?
    1. All planets take the same amount of time to make one complete revolution.
    2. Gas giants require less time than rocky planets to make one complete revolution.
    3. The further a planet is from the Sun, the less time required to complete one revolution.
    4. The closer a planet is to the Sun, the shorter the time required to complete one revolution.

Depth of Knowledge Level 2

  1. The table below shows the direction of rotation of the planets and from which direction the sun rises. CCW is counterclockwise rotation and CW is clockwise rotation.


  1. Which planet, when viewed from north, has a clockwise rotation which causes the sun to rise in the west and set in the east?
    1. Venus
    2. Mars
    3. Jupiter
    4. Neptune
  1.   Use the diagram illustrating the axial tilt of the planets of our Solar System to answer the following question.


  1. Which three planets have axial tilts closest to that of Earth?
    1. Mercury, Venus and Jupiter
    2. Jupiter, Mars and Uranus
    3. Uranus, Pluto and Venus
    4. Mars, Saturn and Neptune

2nd Item Specification:  Explain the relationship between moons and planets.

Depth of Knowledge Level 1

  1. Besides Earth, one planet and two moons are believed to be volcanically active.  Which of the following statements regarding such volcanic activity is INCORRECT?
    1. Venus appears to have active volcanoes as evidenced by gases in its atmosphere.
    2. Mars has the largest active volcano in the solar system.
    3. Io is a volcanically active moon of Jupiter, spewing forth sulfurous lava.
    4. Triton is a volcanically active moon of Neptune, erupting plumes of liquid nitrogen, dust and methane.
  1. When comparing moons to the planets they orbit,
    1. the Moon is closer in size to Earth than any other moon is compared to its planet.
    2. the inner planets are found to have far more moons in orbit than the outer planets.
    3. all moons are observed to orbit their planet is a counterclockwise direction.
    4. Mercury has been discovered to be the only inner planet which has no moons

Depth of Knowledge Level 2

  1. The photograph below displays the two moons of Mars.

(From and

  1. Careful observation of these two moons suggests that they have
    1. life which mined rock from the surface.
    2. a dense atmosphere of carbon dioxide.
    3. been impacted by meteorites.
    4. a composition mostly of gases.
  1. Which of the following is a reasonable explanation for the existence of volcanoes on Jupiter’s moon, Io?
    1. Friction between Io and Jupiter’s atmosphere cause portions of Io’s crust to melt.
    2. Tidal forces due to Jupiter’s gravity cause heating and melting of Io’s interior.
    3. Io is the escaped core of the planet Jupiter, so it is still extremely hot.
    4. The strong magnetic field of Jupiter attracts Io’s lava to the surface.

3rd Item Specification:  Understand the characteristics of rocky (terrestrial) and gaseous planets.

Depth of Knowledge Level 1

  1. Venus has the hottest surface temperature of any planet in our solar system because it
    1. is the closest planet to the sun.
    2. radiates more energy than it receives from the sun due to gravitational forces.
    3. has a thick, CO2-rich atmosphere which holds most of the energy received from the sun.
    4. has a very rapid rate of rotation which induces a high degree of friction.
  1. Which statement about the properties of planets is INCORRECT?
    1. All of the gas giants have rings.
    2. The rocky planets are more massive than the gas giants.
    3. Atmospheres have been found both on rocky planets and on gas giants.
    4. The gas giants have far shorter periods of rotation than do the rocky planets.
  1. Each statement about the characteristics of planets is correct EXCEPT
    1. rocky planets have higher densities than the gaseous planets.
    2. gaseous planets have thicker atmospheres than the rocky planets.
    3. all the rocky planets and all the gaseous planets have solid surfaces.
    4. craters are found on all the rocky planets but not on the gaseous planets.
  1. One of the planets has a core which appears to be at least 30,000°C.  This planet radiates more energy back into space than it receives from the Sun. The planet is
    1. Jupiter.
    2. Mercury.
    3. Pluto.
    4. Venus.

Depth of Knowledge Level 2

  1. The diagram below displays the average density of planets in our solar system.


  1.  When compared to the density of Earth,
    1. Mars is the inner planet with the least difference in density.
    2. Mercury is the inner planet with the most difference in density.
    3. Uranus is the outer planet with the least difference in density.
    4. Saturn is the outer planet with the most difference in density.
  1. The photograph below shows a portion of the surface of Mars.


  1. The photo provides evidence suggesting Mars once had
    1. aquatic life.
    2. extensive glaciation.
    3. flowing surface water.
    4. a system of canals.
  1. The table below has values comparing several properties of the planets in our solar system.


  1. From this table, it can be concluded that the
    1. inner planets have more mass than the outer planets.
    2. outer planets have longer days than the inner planets.
    3. inner planets have a longer solar revolution than the outer planets.
    4. outer planets have greater diameters than the inner planets.
  1. Which of the following is an accurate statement when comparing the terrestrial and gaseous planets?
    1. Terrestrial planets are denser even though they are smaller than gaseous planets.
    2. Gaseous planets are less massive because of the lower density of their materials.
    3. Terrestrial planets are smaller than gaseous planets, but both have the same mass.
    4. Gaseous planets are more likely to float out of their orbit than are terrestrial planets.

Constructed Response E.8.B.3

1.  Use the following table and information to answer the questions below.


Distance in AU
(from Sun)

(compared to Earth)

Period of Rotation
(compared to Earth)

Period of Revolution
(compared to Earth)




25.4 days (equator)
36 days (poles)





59 days

0.24 years




243 days

0.62 years




1 day

1.00 years




1.025 days

1.91 years




0.408 days

11.86 years




0.445 days

29.46 years




0.718 days

84.01 years




0.671 days

164.79 years

(Derived from

Using graph paper, construct a scale model depicting the distance of each planet from the Sun.  Next to each planet on this model, indicate its distance, period of rotation and period of revolution.  Include a key indicating the units and scale used for distance.

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Content Benchmark E.8.B.3

Students know characteristics of the planets in our solar system.  I/S

Answers to Sample Test Questions

  1. B, DOK Level 1
  2. D, DOK Level 1
  3. A, DOK Level 2
  4. D, DOK Level 2
  5. B, DOK Level 1
  6. C, DOK Level 1
  7. C, DOK Level 2
  8. B, DOK Level 2
  9. C, DOK Level 1
  10. B, DOK Level 1
  11. C, DOK Level 1
  12. A, DOK Level 1
  13. D, DOK Level 2
  14. C, DOK Level 2
  15. D, DOK Level 2
  16. A, DOK Level 2

Constructed Response E.8.B.3 Score Rubric:

3 points

Response addresses all parts of the question clearly and correctly.

Student graph has been constructed at a suitable scale with a key including the units and scale used.  Each of the eight planets is represented at the correct distance, and each is appropriately labeled with its distance in AU, period of rotation and period of revolution. 

2 points

Response addresses all parts of the question and includes only minor errors.

1 point

Response does not address all parts of the question. 

0 points

The response is totally incorrect or no response provided. 

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Content Benchmark E.8.B.3

Students know characteristics of the planets in our solar system. I/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Nine Planets and Windows to the Universe - Our Solar System

These two websites are packed with information and multimedia regarding objects in our solar system. Clicking on the various links to each planet provides valuable information, as well as links to subcategories about the planets (atmospheres, comparison of exterior and interior, etc.).

Nine Planets is accessed at . Windows to the Universe - Our Solar System is located at

2. Journey Through the Solar System

There are a wide variety of lessons appropriate for middle school students at this website. Students have the opportunity to create various models of the solar system, and to engage in creative writing.

To access these lesson plans visit

3. Gander Academy's Solar System Theme Page

This site is flush with activities appropriate for middle school science students. There are many lessons, complete with black line masters, covering each of the planets. There are also links to several websites for each planet which could be utilized in student research.

To access these activities visit


Very up-to-date information on the planets (as well as other solar bodies) is found on this page. The latest images from the various planetary spacecraft (fly bys, orbiters and landers) are available to be viewed.

To access this information, visit:

5. Astronomy Answerbook - Planets

This is an excellent site to answer questions your students may have about the planets. There are dozens of commonly asked questions, each linked to a detailed answer.

To access this information, visit

6. Exploring Planets in the Classroom

This is another great website providing many hands-on lessons covering many aspects of the planets in our solar system. Near the bottom of the main page are links to several other websites with valuable information on the solar system.

To access these lessons, visit


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Support Pages

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Sample Questions:
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Intervention Strategies & Resources:
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Benchmark Related Vocabulary

Solar system