Introduction to the Planets for NSO Students

On this single page is my introduction to the planets. I hope to give everyone a general "fly-over" of the Solar System, starting at Mercury and traveling out to Pluto and beyond. There are many more details that are really exciting, and you can jump to the pages that I have created at earlier dates for those pieces of information that make this unit equally as exciting to me as stars. A quick look at the basic planetary data can be found at Planet Introduction. This page will compare every planet to the Earth in terms of mass, diameter, composition, and orbital parameters. You will be able to look all of the planets on one chart to see which is biggest, smallest, most dense, and most elliptical. You can also see which has the most moons and which as no moons at all.

If we could build a large rocket with renewable food, water, and oxygen supplies, we could begin a tour of the planets at Mercury. This planet is the only planet that is tipped up without any tilt. It looks like our Moon except there are no large "seas." Mercury has no atmosphere and therefore no means to trap heat from the Sun. The daytime temperature is 800 F and the nighttime temperature drops to -300 F. This means that there is a drop of 1100 F when you go from sunlight into shadow. The largest impact basin in the Solar System is found on Mercury, and is called the Caloris Basin. Mercury is unique since it has a core of iron that makes up 75% of the entire planet. Planetary geologists have assumed that Mercury is geologically dormant with a solidified core, but there is some recent evidence from a 2007 study that indicates the possibility of an at least partially molten core.

Venus is second on our tour, and it is almost the same size as the Earth. Venus is a mysterious planet covered in clouds. No person has ever seen the surface since the clouds are so very thick. As a result, planetary scientists proposed the possibility of a waterworld or even an "Everglades-like" world with lush vegetation and giant insects. When the Soviets finally landed a probe on the surface they were surprised to find a most inhospitible world. The thick clouds of carbon dioxide also contain sulfuric acid raid. The temperature at the surface is 900 F ... every day, every night, all the time, but at least there is little or no wind at the surface. Venus is most strange because it spins in the opposite direction of every other planet, causing the Sun to appear to rise in the west and set in the east. Since astronomers like to make comparisons where everything is the same, they describe Venus as a planet that is rotating counter-clockwise, but in a completely upside-down position, causing it to only appear to be rotating clockwise. But the sad news is that you never see the Sun from Venus' surface anyway due to the clouds. These clouds trap the solar heat and have turned Venus into a runaway greenhouse planet that Al Gore would be proud to campaign there for climate change. To make the planet that much hotter, it takes Venus 240 days to spin just one time, so the Sun can really beat down on the planet. Even more strange is that Venus orbits the Sun in only 221 days, making a Venus "day" longer than its "year." To add the finishing touches to your surface visit, the pressure is 90 times greater than on Earth, and you would feel 1200 points of pressure per square inch. Geologists were astounded by the apparent random distribution of craters and the relatively thin crust. The current theory holds that Venus is currently geologically inactive, but experiences a global upheaval every 500 million years or so. In one massive event, the entire surface of Venus appears to have been sucked into the interior and replaced by brand new crust.

Mars is next on our visit, and is the planet with the most storied history. Since Mars has a similar tilt, spins in less than 25 hours, and is close to the Sun, science fiction writers have composed many stories of Martians. This fascination with Martian life can be traced to the work of Percival Lowell who spent his profssional life looking at Mars and seeing things that no one else saw. Arnold Schwarzenegger made Mars interesting in "Total Recall," many ufologists were excited by the "face on Mars" formation with its accompanying pyramids, and even "Transformers" poked at the reason why the Mars lander Beagle died. In truth, and due to repeated landings on Mars by robot spacecraft and rovers, Mars is a geologically dead and inhospitible world. The core solidified long ago, and Mars lacks a global magnetic field that would otherwise protect Mars from lethal solar radiation. Life cannot exist at the surface, but there are some who think it might exist protected beneath the surface dust and rock. Mars boasts the largest mountain in the Solar System (a giant extinct volcano called Olympus Mons), and the largest canyon (Valles Marineris). The canyon is as long as the USA and over 400 km wide a parts. Mars has an atmosphere, but it is less than 1/1000th of the Earth, and mostly carbon dioxide. The surface pressure is so low that a glass of water would spontaneously boil, even though the temperature is ear freezing. Mars has two little moons (Phobos and Deimos) that are probably asteroids that were captured by Mars' gravity.

Between Mars and Jupiter is the Asteroid Belt. No worrries. You will not crash your spaceship in a dense pile of rocks like what Han Solo weaved through in "Empire Strikes Back." There are thousands of asteroids, but they are so spread out that you need to literally steer and aim your ship very precisely just to find one. Asteroids are made of iron, iron and stone, stone, and carbon-based compounds. Most remain in a stable orbit in the main belt, but there are many that occasionally cross the orbit of Earth (Near Earth Asteroids) and some were the probable causes of major extinction level events. Many paleontologists are convinced that a 10 km asteroid hit the Earth on the tip of the Yucatan Penninsula 65 million years and wiped out the dinosaurs. It is true that rocks from space continue to hit the planet, and we can expect other large rocks to do serious damage in the future.

Past the Asteroid Belt, we arrive at Jupiter, the first of the gas giant planets, and the largest planet in the Solar System. Jupiter is made of hydrogen and helium gases like the Sun, but it does not have enough mass to ignite nuclear fusion at become a star. Jupiter is a thousand times bigger than Earth and yet it spins completely around in less than 10 hours. The rapid spin creates complicated weather bands, and the different chemicals in the gases make the bands stand out in a telescope. In spite of this complex weather organization, a giant hurricane has persisted in one area for almost 400 years. This giant Red Spot would engulf two Earths. The Voyager spacecraft made two spectactular discoveries in 1979: One of Jupiter's big moons, Io, turned out to be the most volcanic object in the entire Solar System; and Jupiter has rings similar to Saturn, but less prominent. Additionally, another of Jupiter's big four moons, Europa, has a frozen ocean of water ice, under which is a liquid ocean that is geologically heated and may harbor primitive life. Jupiter, with is large mass, has captured many stray asteroids, and presently binds 63 moons in orbit around it. One final extremely important feature of Jupiter is it's immense magnetic field that traps energetic particles from the Sun. No human can survive within 3 million km of Jupiter because of this lethal magnetic field.

Saturn is next, and it most definitely the "Lord of the Rings." Saturn's globe is a little smaller than Jupiter, but made of the same hydrogen and helium gases. The same weather bands exist, but there are less colors because the chemicals in the atmosphere are different from Jupiter. Saturns rings are very wide and complicated, but incredibly thin. The ring particles are made of ice and rock pieces ranging in size between a sand pebble and a house. Saturn has 60 moons, and none is more exciting that Titan. Titan is larger than our Moon, and it has a thick atmosphere. In fact, it is the only moon in the Solar System with an atmosphere. When the Cassini spacecraft dropped a probe onto the surface of Titan in the summer of 2004, it discovered rivers and lakes of methane, hydrocarbon rain, and volcanoes of ammonia-water mixture. With a surface temperature of -180 C, life there is certainly not possible, but the chemicals on the surface and in the air mimic what some consider the earliest Earth to be like.

Next is Uranus, a planet where everything is tipped over on its side. The planet spins on its side, the rings spin on its sides, and even the 27 moons orbit on the side. No one know why this is happening. Uranus is made of gas and liquid, and is much bigger than Earth, but much smaller than Jupiter or Saturn. The rings of Uranus are darker in color than Saturn's rings, but just as complicated in structure. While there are many Uranian moons, Miranda is the most curious since it looks as it the entire moon was somehow shattered and consequently reassembled.

Finally (if you hold to the IAU meeting result of August 26, 2006) we arrive at Neptune. Like Uranus, Neptune is made of gas and liquid and owes its striking blue color to a small percent of methane gas in the clouds. Some conjecture that these methane droplets might sink into the high pressure interior or Neptune and be heated and pressurized into diamonds! Neptune is tipped more like the Earth and Mars, and it has only 3 narrow and thin rings. Neptune has 13 moons, of which Triton is the biggest (in fact it is bigger than our Moon). Triton stunned geologists when volcanic activity was observed by Voyager in 1989. Here is a moon whose surface temperature is 34.5 K ... down near where molecules stop moving. Even at this extremely cold temperature (the coldest place in the Solar System), nitrogen gas is erupting and making black stains on the surface.

Pluto is next, but because it has been downgraded by the "professionals," it is no longer called a major planet. It is now just another "dwarf" planet. Still, I like to think of Pluto as planet #9. We have a space satellite flying out there today, but it is still a few years from arriving. Pluto has a major moon named Charon that is more than half the diameter of Pluto. Together, Pluto and Charon act more like a double planet than a planet with a moon. They orbit each other so closely that gases from the atmosphere of one object are exchanged with the other. We know very little about either Pluto or Charon except that Pluto seems to be made of brighter material than Charon.

In the same vicinity as Pluto is a vast belt of comets called the Kuipter Belt, and way beyond this belt is the giant Oort Cloud of comets. Comets are small balls of dry ice, water ice, rock, and some other chemicals that occasionally get pushed toward the Sun. As their orbit is changed, they now become occasional visitors to our night sky. The ices of the comet evaporate as the comet approaches the Sun, and the solar wind blows the evaporated gas and dust away from the Sun to create the comet's tail. The most famous is Halley's Comet which visits the Earth every 76 years, and will not be back here until 2061.

You are now encouraged to enter the Main Introduction Page for the Planets and explore to your heart's content.

Course Syllabus