Mercury is an ancient, cratered hunk of iron and rock with a surface temperature that gets hot enough to melt lead.
HISTORY:
Mercury orbits the Sun so closely that it never appears far from the Sun in Earth’s skies. Mercury is never up in the sky after dark, making it very hard to study in visible light. The planet can be seen with an unaided eye if you know where, and when, to look. Astronomers could make out very little detail on the surface and so had to guess what Mercury was really like. We assumed Mercury always kept the same face toward the Sun, making one side perpetually hot and bright, and the other perpetually cold and dark. Radar studies in 1965 disproved this model, showing that the planet actually takes 59 days to spin once on its axis. This rotational period is exactly 2/3 of a Mercurian year.
Most of what we now know about Mercury was revealed to us by the space probe Mariner 10 in 1974-75, during three flybys. Unfortunately, due to the position of Mariner 10’s orbit, we were only able to image 1/2 of Mercury’s surface. The side we did see looks remarkably similar to Earth’s moon. Mercury’s surface seems to be made up of uninterrupted impact craters.
Mariner 10 remains the only space probe to have visited this tiny world.
OUTSTANDING FEATURES:
•Mercury is very near to the Sun, making the Sun’s radiation is almost 7 times more intense than on Earth.
•Mercury’s ancient cratered surface is much like Earth’s moon and is probably 3.8 to 4.2 billion years old. The period of most intense cratering probably occurred in the first 800 million years of Mercury’s estimated 4.5 billion year existence.
•The period of rotation is exactly 2/3 as long as the period of revolution making Mercury’s “day” twice as long as its year.
•Because Mercury’s orbit lies inside Earth’s orbit Mercury is never far (within 28 degrees) from the Sun in our skies.
•Mercury has almost no atmosphere (except a few stray particles it diverts from the solar wind).
MYSTERIES:
What does the other half of Mercury look like? Mariner 10 only imaged one side of the planet. (For all we know, there could be a giant sign saying “hello Earthlings” on the other side!)
SURVIVING ON MERCURY:
•You’ll need both warm and cool clothing. At night, which lasts 88 days, it gets down to -270 F. During the day it gets up to 660 F.
•Wear a pressurized suit with oxygen to breath. With practically no atmosphere on the planet you would live for about 10 seconds without oxygen. (“Wait a minute. I can hold my breath for over a minute,” you say. Not in a vacuum you can’t. Imagine trying to hold your breath while your lips are wrapped over the intake hose of the world’s most powerful vacuum cleaner. Don’t try this, unless you want a new set of lungs.)
•Better wear a sunscreen visor with that suit, not only is the sunlight 7 times brighter than on Earth but Mercury has no ozone layer to filter out harmful ultra violet radiation. You also need a shield to protect against ionic radiation from the Sun, as Mercury’s magnetic field is too weak to divert particles around the planet.
VENUS
Venus is Earth’s nearest neighbor in space, with a dense atmosphere blanketing the hottest surface in the Solar System.
HISTORY:
Although Venus is close and bright, clouds perpetually obscure the surface of the planet. We have long speculated about the conditions on our mysterious neighbor. In 1600 a contemporary of Galileo named Giordano Bruno was burned at the stake for suggesting that there might be people on Venus. (Speculating about the heavens was more hazardous to your health back then.) In 1610, Galileo discovered that Venus shows phases like the phases of Earth’s moon. Well into the twentieth century astronomers speculated that “Earth’s twin” could have warm oceans and a climate that might even be able to support life. Then, in 1958, radio astronomers on Earth learned that the surface of Venus is extremely hot. In 1962, the first interplanetary spacecraft, Mariner 2 confirmed this finding. Meanwhile, radar studies from Earth penetrated the cloud cover and determined that Venus has an incredibly slow 117 day, retrograde (backward) rotation. The Pioneer Venus 1 spacecraft arrived at Venus in 1978, and provided us with the first global radar maps of the planet. In the same year Pioneer Venus 2 probed the atmosphere and confirmed the existence of a “greenhouse” effect.
The Venus that emerged from the clouds was not a lush greenhouse where life could thrive. There was a greenhouse, but not the kind you would want to grow anything in. This greenhouse was more like a pressure cooker, and any water that might once have been present would have boiled away long ago. The former U.S.S.R. was able to land 7 spacecraft, beginning with Venera 7 on the surface in 1970, recording searing temperatures and crushing pressures. In 1985 the Soviet made Vega 1 and 2 probes released balloons into Venus’ atmosphere. These balloons were tracked from Earth and experienced a much bumpier ride than expected. Pioneer Venus 1 was designed with an 8 month life expectancy, but remained in orbit around Venus for 14 years. Finally in the fall of 1992, all propellant had been used up and the orbiter began falling toward the surface, continuing to return data until it burned up in the atmosphere.
In 1990 the Magellan spacecraft began returning stunning radar images of the planet showing details as small as 120 meters across. (For comparison, the best resolution obtained by Pioneer Venus had been 25 kilometers!) These images reveal Venus to be much more volcanically active than we had previously thought.
OUTSTANDING FEATURES:
•Venus is almost the same size as Earth and is our nearest neighbor in space.
•Its dense atmosphere produces the highest temperatures (870 F) and pressures(90 times Earth’s) found on the surface of any terrestrial planet. (Terrestrial planets include the 4 innermost planets, each of which has a solid surface.) A “greenhouse” effect is responsible for the searing temperatures on Venus. Visible light from the Sun warms the planet’s atmosphere and surface. Warm objects become cooler by emitting infrared (heat) radiation, but carbon dioxide gas absorbs infrared radiation. The atmosphere of Venus is 97% carbon dioxide, and so prevents heat from escaping from the planet.
•There are winds up to 250 mi/hr in the atmosphere.
•100,000+ volcanoes cover most of Venus’ surface. The fact that Venus has almost no impact craters means that the surface is very young. We suspect there are volcanoes which are erupting even now!
•The highest mountain, Maxwell Montes, is taller than Mt. Everest.
•Venus has an unusual retrograde rotation, so that if you were standing on the planet the Sun would rise in the west and set in the east. Of course you couldn’t see the Sun because the atmosphere is too thick! Like Mercury, Venus’ day is longer than its year.
•Like Mercury, Venus also orbits the Sun inside the Earth’s orbit, so Venus never appears more than 48 degrees from the Sun in Earth’s skies. If someone tells you they saw Venus overhead at midnight you would know they were mistaken. Venus appears featureless in ordinary, visible light. Cloud details can only be seen in ultraviolet light.
MYSTERIES:
Could Earth’s atmosphere end up like Venus’ with a run away greenhouse effect? Did Venus have oceans like Earth billions of years ago? Was there primitive life in these oceans?
SURVIVING ON VENUS:
•You will need an air-conditioned pressurized suit of body armor to survive the searing heat and crushing pressure.
•Make sure your suit is made out of something that Venus’ sulfuric acid clouds won’t eat through!
•Watch out for active volcanoes!
•Get off the planet as soon as possible. Armored space probes have yet to last more than 2 hours on the surface!
EARTH
Earth is a beautiful planet with a unique ocean and atmosphere system capable of supporting life.
HISTORY:
Earth is the planet we know best, having lived here for about a million years. Still, it is only relatively recently that we have begun to look at our own world as a planet at all. In 340 BC. the ancient Greek philosopher Aristotle argued that the Earth must be a round ball rather than a flat disc. Within ten years, another Greek, Eratosthenes used shadows cast by sticks to roughly calculate the size of this ball. Around 240 BC. Aristarchus calculated the relative sizes of the Earth, moon and Sun, and the distances between them and concluded that the Earth moved around the Sun! Other Greeks argued “if the Earth is moving so fast why can’t I feel it?” They dismissed this Sun-centered (heliocentric) model in favor of the more popular Earth-centered (geocentric) model. The geocentric model of the universe reached its pinnacle of popularity in about 150 AD. in a work put forward by Ptolemy. Ptolemy’s system describes a central fixed, round Earth surrounded by transparent concentric spheres carrying the other 5 known planets, the Sun, moon and stars around the Earth.
After the fall of Greece and the destruction of the great library at Alexandria in 389 AD., many of the achievements of the Greek astronomers were lost. The flat Earth theory regained favor. By the time of Columbus scholars again agreed that the Earth must be round but argued over its size. Looking for support to open a new route to the East, Columbus argued that the Earth was much smaller than most scholars currently believed. Fortunately for Columbus, a new land mass (the New World) lay along his route, otherwise he and his men would have perished on the long sea journey. Even after discovering the New World, Columbus stubbornly refused to believe the world was as big as it really was, and remained convinced even until his death that he had landed on the shore of India.
The geocentric model put forth by Ptolemy remained unchallenged until 1514, when Copernicus proposed that the Sun, not the Earth, was at the center of the universe. (Aristarchus would have been pleased, if he had not been dead for 1700 years!) Galileo championed the cause of the heliocentric or Sun-centered theory in the early 1600’s, for which he was nearly burned at the stake. People paid more attention to the work of astronomers in those days...
Our first views of Earth from space came during the late 1940’s from brief unmanned rocket flights. John Glenn took the first hand-held camera color snapshots of Earth on his historic orbital flight aboard Friendship 7 in 1962. Since then we have studied Earth with a variety of orbiting satellites, including the early Tyros weather satellites, Skylab, and Landsat satellites. Today we use the TOPEX/Poseidon satellite to study our oceans; we may possibly continue using it into the year 1997.
OUTSTANDING FEATURES:
•Earth’s orbit is the perfect distance from the Sun to sustain our life-giving oceans. If we were any closer to the Sun our oceans would boil away; any farther and they would freeze solid.
•A dynamic interaction between land, ocean and atmosphere keeps the surface of our planet very young. Note the relative absence of impact craters compared with Earth’s moon. Earth must have been bombarded at about the same rate as the Moon, but erosion has removed the evidence. Earth’s oldest rocks are on the continents, and its youngest are on the ocean floor.
•Earth’s oceans smooth out temperature fluctuations on a daily and annual basis. Think about desert climates for example. Without water around, desert sands become very hot during the day and very cold during the night.
•An ozone layer in Earth’s atmosphere shields life from harmful ultraviolet radiation from the Sun.
•A strong magnetic field shields Earth from the Sun’s ionic radiation, and made it possible for life to originate on our planet. Ionic radiation would have destroyed the complex organic molecules necessary for life.
•Earth’s continental drift is unique in the solar system. North America is moving away from Eurasia at the rate of 1 cm/yr—about the same speed as your fingernail is growing!
•Temperature at the center of the Earth is about 5,000.C, the same as on the surface of the Sun. Radioactive decay keeps our planet’s iron core molten.
•Earth has a single, relatively large moon.
The Moon raises the tides on Earth, slowing the Earth’s rotation down. Fossil coral research suggests that the Earth was spinning faster in the past and that 400 million years ago a full day (sunrise to sunrise) took only 21 hours. Originally, an Earth day may have been as short as 6 hours. The Moon was also spinning faster in the past, and similar tidal braking slowed it down until it became locked in synchronous rotation with the Earth, which is why it always shows its same face to Earth. As Earth’s rotation slows down, the moon is required to move farther away from Earth. Currently each day is getting longer by 5 hundred millionths of a second, and the moon is moving away from the Earth at a rate of 4 cm/year. Tidal braking will continue to slow the Earth down until it is locked in synchronous rotation with the moon’s orbit. In other words, the same side of the Earth will always face the moon! Earth and moon will be locked into a slow dance with each other, like Pluto and its moon Charon are!
MYSTERIES:
How did Earth come to have such a large moon? Will our atmosphere go the way of Venus’ and become super-heated because of the green-house effect? What has caused the periodic mass extinctions recorded in the fossil record, one of which killed off the dinosaurs 65 million years ago?
SURVIVING ON EARTH:
•Earth has been generally hospitable to humans in the last million years or so that we have been around, but man made environmental problems may make survival more difficult in the future. As we continue to destroy our ozone layer, harmful ultraviolet radiation from the Sun will cause more skin cancer and cataracts (clouding of the lens of the eye). Global warming caused by the release of carbon dioxide into Earth’s atmosphere may significantly alter life here.
•Some biologists are predicting that one-half of the species currently alive on Earth will go extinct within the next century!
•Looking on the bright side, no matter how bad things get here on Earth they will almost certainly be better than anywhere else in the Solar System.
MARS
Our rusty neighbor Mars is only half the size of Earth, yet Mars is the most Earth-like planet in the Solar System.
HISTORY:
Mars has inspired human imagination more than any other planet. From Earth the best telescopic views reveal about the same level of detail as a person can see looking at the Moon with a naked eye. You can see broad light and dark patches and that is about it.
The great astronomer Kepler predicted that Mars would have two moons, because Earth had none, and Jupiter was known to have four. He reasoned that the number of moons should double with each successive planet, so Mars should have 2. About 150 years later in 1877 these two moons were actually discovered! Also in 1877 an Italian astronomer Schiaparelli claimed to have seen faint streaks on Mars which he called “canali” Italian for “channels”. “Canali” was mistranslated into “canals” in English, leading to widespread speculation about giant artificial irrigation systems. A few astronomers led by Percival Lowell insisted they could see the canals, and argued loudly for the existence of intelligent life on Mars. It wasn’t until the 1920’s that skeptical astronomers (who couldn’t see the canals) were able to reject the intelligent Martian theory. The life on Mars idea got another boost when seasonal variations in light and dark regions were observed. Some regions appeared to darken in the summer as if they were experiencing extensive plant growth. Some observers even claimed to see a greenish hew! What was going on here?
The answers had to wait until we could image the planet close-up by spacecraft. In 1965 Mariner 4 sent back the first close-ups, showing impact craters characteristic of an inactive surface. Mars was labeled a “dead” world and the public lost interest. Two more probes found more craters and missed the most dramatic features on Mars. In 1971 Mariner 9 became the first spacecraft to orbit another planet, and after waiting for a planet-wide dust storm to subside began discovering some of the most spectacular terrain in the Solar System. Mariner 9 found giant volcanoes, huge canyons and yes, huge runoff channels.
Interest was rekindled and two landing probes were designed to explore the Martian surface directly. The Viking landers remain among the most complex spacecraft ever designed. The landers were about the size of a small car. Onboard laboratories were designed to carry out a variety of sophisticated experiments. Three central experiments were to test Martian soil for signs of life. Great pains were taken to sterilize the landers so that they would not contaminate Mars with microbial life from Earth. Viking 1 landed on July 20, 1976, followed by Viking 2 about a month later. At first scientists were excited by positive looking results in the life experiments, but closer study revealed that Viking had not found life after all. An independent test showed Martian soil to be completely devoid of carbon, a required element for life as we know it. Seasonal variations in brightness turned out to be the result of light colored dust deposited and removed by seasonal winds over dark regions. Public interest waned once again. The Viking 2 lander continued to function for over three Earth years, greatly exceeding its planned 90 day life. Viking 1 sent us information from the Martian surface until November 5, 1982 when a programming error redirected Viking’s antenna away from Earth. Viking 1 has been donated by NASA to the Smithsonian, making it the only museum display on another planet.
Despite the incredible success of the Viking program, the U.S. abandoned Mars for ten years. Then in 1992, we launched the Mars Observer. The orbiter should begin to return images and data in December of 1993. Look for more surprises in these results! Renewed interest in Mars may lead to an expedition with a human crew aboard early in the next century. There remains a remote chance of finding life on other parts of Mars, or finding fossilized life from a more hospitable epoch in Martian history.
OUTSTANDING FEATURES:
•Mars is the most Earth-like planet. While only half Earth’s size, Mars has almost the same day length and tilt of its rotational axis. This tilt produces seasons on Mars similar to the ones we have on Earth.
•Giant volcanoes dominate the surface. Olympus Mons is the largest volcano in the Solar System, and is 3 times the height of Earth’s Mt. Everest.
•Spectacular canyons such as Valles Marineris stretch 5,000 km across planet (the distance from Los Angeles to New York). Huge landslides may still be occurring this canyon.
•Dust storms have been observed to cover the entire planet.
•Mars has ice caps at its poles. In the winter time these poles can absorb up to one-third of the planet’s whole atmosphere as gaseous carbon dioxide freezes into “dry ice,” the same stuff ice cream stores use to keep their ice cream cold.
•Low atmospheric pressure, only about 1/100 of that on Earth.
•Low surface temperature never rises above freezing (0.c).
•Some scientists hoped to find life on Mars but probe tests showed (Viking, 1976) that the element carbon is completely missing from Martian soil, making life as we know it impossible.
•Mars has two tiny moons, Phobos (fear) and Deimos (panic). Both moons always keep the same face toward Mars. Phobos’ orbit is decaying and the moon will plow into the surface of the planet in about in about 30 million years.
MYSTERIES:
Where did all the water go? Massive channels suggest that there was liquid water flowing on the surface long ago, but there is no liquid water present now. Did life once exist in primitive oceans and lakes?
SURVIVING ON MARS:
•Dress warmly, it’s only about -10 F during the day and drops to -150 F at night.
•Wear a pressurized suit with oxygen to breathe. With practically no atmosphere on the planet you would live for about 10 seconds without your suit. (See Surviving on Mercury.)
•Better wear a sunscreen and visor with that suit; Mars’ tenuous atmosphere has no ozone layer to filter out the Sun’s harmful ultraviolet radiation. You also need a shield to protect against ionic radiation from the Sun, as Mars has no magnetic field to divert particles around the planet.
•Watch out for dust storms and landslides if you are exploring canyons.
JUPITER
Jupiter is an enormous ball of gas with beautiful swirling clouds and a Great Red Spot.
HISTORY:
The ancients who named Jupiter after their most powerful god did not know how impressive the planet really was. In 1610 Galileo became the first person to turn a telescope on the giant planet and found 4 new worlds never before seen by human eyes. He had discovered Jupiter’s 4 largest moons (now called the Galilean Moons), forming a kind of miniature Solar System.
As telescopes improved, so did the amount of detail that could be seen. Light and dark bands were resolved, along with the Great Red Spot. The first spacecraft to encounter Jupiter were Pioneers 10 and 11 in 1974 and 1975. These spinning probes proved that space craft could survive a passage through the asteroid belt between Mars and Jupiter, and withstand bombardment from charged particles in Jupiter’s magnetosphere. The Pioneers made measurements and sent a few low resolution images of Jupiter back to Earth. Pioneer had paved the way for the spectacular Voyager missions.
The Voyager probes were state-of-the-art spacecraft. Onboard computers enabled the probe to function relatively autonomously, a requirement since it would take over a hour for round-trip communication with Earth from Jupiter. After traveling from Earth through space for nearly 2 years, Voyagers 1 and 2 encountered Jupiter in spring and summer of 1979. The Voyagers made measurements of Jupiter’s intense magnetic field, and studied radiation given off by the planet in radio, visible and infrared spectrums. Earth received tens of thousands of images of Jupiter, its ring, and satellites, which experts are still studying today. Several new moons were discovered.
The Galileo Jupiter orbiter and probe was launched in October, 1989, on a complicated path using one gravitational assist (in which a spacecraft steals a little momentum from a planet as it swings closely by) from Venus and two from Earth to gain speed on its way to rendezvous with Jupiter in 1995. An atmospheric probe will be released from the orbiter and plunge down through Jupiter’s atmosphere. The orbiter will also examine the four Galilean satellites up close, nearly 400 years after their discovery by Galileo himself. Watch for these exciting results.
OUTSTANDING FEATURES:
•Jupiter is the largest planet in Solar System. Like the Sun, Jupiter is made up of mainly hydrogen and helium. Jupiter is about as big as a planet can be. Calculations show that if you added more mass the planet would actually shrink under the increased gravity.
•If it were about 70 times more massive it would be a star! As it is, Jupiter radiates 1.7 more energy than it receives from the Sun. This energy is given off as infrared radiation and comes from the gravitational shrinking of the planet. Today we think Jupiter is shrinking about 1 mm/year. When it was first forming about 4.5 billion years ago, Jupiter was probably shrinking fast enough to glow with a deep red color.
•Complex, beautiful clouds. White clouds are the highest in Jupiter’s atmosphere, with red belts lower down and brown clouds being the lowest. Light bands (zones) are blowing toward the east and dark bands (belts) are blowing in the opposite direction, toward the west.
•Jupiter’s Great Red Spot is a giant swirling eddy big enough to hold 2 Earths inside it. It is at least 300 years old.
•Lightning bolts 10,000 more powerful than those on Earth were detected by Voyager.
•Short day of only 9 hours and 50 minutes. Jupiter is spinning so fast that it bulges noticeably at its equator. (Picture a slightly flattened tennis ball.)
•Strong magnetic field 20,000 times stronger than the Earth traps charged particles in it. A huge magnetosphere (region where magnetic field traps charged particles) exceeds the size of the Sun!
•Faint ring
•At least 16 moons, 4 of which can easily be seen from Earth with binoculars! Jupiter and its moons resemble a miniature solar system. Jupiter’s moon Ganymede is the largest moon in the Solar System, and is even larger than Pluto and Mercury!
•Jupiter’s moon Io has active volcanic geysers caused by tidal forces from Jupiter. Tidal forces occur when gravity pulls harder on one side of an object than the other, stretching it. Io is currently the most active body in the Solar System.
MYSTERIES:
•Why is there only one Great Red Spot on Jupiter? Why is it red? (phosphorus?)
•Why do certain cloud formations seem confined to certain regions?
SURVIVING ON JUPITER:
•Stay in your spaceship, as there is no firm surface to stand on.
•Watch out for lightning bolts and high winds.
•Ballooning in the troposphere might be fun, with balmy temperatures and moderate pressures of only a couple of atmospheres.
SATURN
Saturn is another giant gas ball like Jupiter, but it is only about half as dense. Saturn has the largest, brightest rings in the Solar System.
HISTORY:
Saturn was the most distant planet known to the ancients. Almost twice as far from the Sun as Jupiter, it is still bright enough to be seen easily with the unaided eye. In 1610, Galileo saw bulges on either side of Saturn with his new telescope. Better telescopes later revealed these bulges to be rings. Even small telescopes today show Saturn’s beautiful rings.
Pioneer 11 served as a trail blazer in 1979, paving the way for the Voyager 1 and 2 flybys in 1980 and 1981. Fresh from their encounters with Jupiter, the Voyagers studied Saturn’s magnetic field, and the radiation given off by the planet in radio, visible and infrared spectrums. Earth received tens of thousands of images of Saturn, its rings, and satellites, which experts continue to study today. Voyager discovered at least 6 moons (more moons are suspected but unconfirmed). Another mission to Saturn is scheduled for launch in 1997. The Cassini spacecraft will use gravity assists around Venus, Earth, and Jupiter arriving at Saturn in the year 2004. An orbiter will continue studying the Saturn system, and a probe will be released to descend through the atmosphere of the mysterious moon Titan.
OUTSTANDING FEATURES:
•Huge ball of gas second in size only to Jupiter. Made up of hydrogen and helium gas like Jupiter, but because of its smaller mass, gravity doesn’t compress Saturn as much, leading to a lower density. Saturn’s density is even lower than water so Saturn could float in a bathtub if you could find one big enough (but imagine the ring it would leave!)
•Saturn radiates 3 times more energy than it receives from the Sun. You can think of Saturn as a giant two hundred million billion watt heat lamp. This energy probably comes from helium droplets falling through liquid hydrogen on their way toward the core of the planet, like a kind of helium rain.
•Delicate cloud formations are much more subtle on Saturn than on Jupiter. With lower gravity than Jupiter, Saturn’s atmosphere is much more extended with a thicker cloud layer that makes the planet appear much more uniform.
•Equatorial jet stream of 1300 km/h (very fast!)
•Huge magnetosphere (region where planet’s strong magnetic field traps charged particles) exceeds the size of the Sun.
•Bright beautiful grooved rings made up of countless particles. Light from the rings softly illuminate the night side of Saturn, keeping the night sky from ever really being dark. These rings are incredibly thin, being only about 10 meters thick. Saturn’s main ring is almost 30 million times wider than it is thick. When we see Saturn’s rings edge on from Earth, they disappear!
•21 satellites orbit Saturn, including mysterious Titan, a huge moon that is bigger than Mercury and has an atmosphere that is even thicker than Earth’s!
MYSTERIES:
•How did Saturn’s magnificent rings form? Did a moon get too close to the planet and get torn apart by gravitational tidal forces, or was this debris there from the beginning and never able to form into a moon?
•What lies beneath the giant moon Titan’s clouds? Could there be oceans here?
•Why is Saturn’s magnetic field aligned with its axis of rotation, when all the other magnetic fields on other planets are skewed?
SURVIVING ON SATURN:
Similar to conditions on Jupiter:
•Stay in your spaceship, as there is no firm surface to stand on.
•Ballooning in the troposphere might be fun, with balmy temperatures and moderate pressures of only a couple of atmospheres.
URANUS
Uranus is a nearly featureless blue green gas giant.
HISTORY:
On March 13, 1781 William Herschel was looking through his homemade six-inch reflecting telescope when he discovered a fuzzy spot in the sky, which turned out to be a new planet, Uranus. Nobody expected there to be another planet past Saturn, and this discovery excited the world. Because Uranus orbits the Sun at twice the distance of Saturn, the size of the known Solar System had just doubled!
In 1977 astronomers on Earth watched Uranus pass in front of a star. They watched the star blink on and off several times while moving near the planet and concluded that Uranus must have a set of thin dark rings. The rings were confirmed by the space probe Voyager 2 in 1986. Voyager 2 was also able to help astronomers with another problem. Uranus’ surface is so bland that we couldn’t see enough detail to mark its rotation with. In other words we couldn’t tell when it had spun once completely around, so we couldn’t measure the length of Uranus’ day. Voyager 2 was able to use the planets rotating magnetic field to time a complete spin, thereby measuring the length of a Uranian day. Voyager 2 also discovered 10 new Uranian moons in addition to the five moons that can be seen with telescopes from Earth. Uranus can just barely be seen under favorable conditions with the unaided eye.
OUTSTANDING FEATURES:
•Unique tilt of rotational axis is almost in the plane of orbit. Moons orbit the planet in a kind of bulls-eye. Equatorial regions experience two summers and two winters in a year! If you were in the polar regions you would not be able to see the Sun for about half of the Uranian year, or 42 Earth years.
•Despite long “days” Uranus’ temperature is nearly the same on light and dark sides.
•Uranus’ density is less than the density of rock but more than the density of water, suggesting that the planet is actually composed of a mixture of ice and rock.
•There is even some evidence to suggest that Uranus’ dense clouds may gradually merge with deep oceans of warm water (tinged with ammonia and methane) as you move farther into the planet.
•Uranus radiates the same amount of energy it receives from the Sun, unlike the other gas giants which all radiate more energy than they receive.
•Methane (the gas you cook with) in the atmosphere gives Uranus its characteristic blue green color.
•Uranus has thin dark rings.
MYSTERIES:
•Why does Uranus rotate on its side? The next most tilted planet, Saturn, is tipped 70 degrees less than Uranus.
•Why do winds travel east/west instead of north/south as you would expect from the planet’s tilt? How is heat distributed between the northern and southern regions, keeping them nearly the same temperature?
•Why doesn’t Uranus have a thermal inversion layer like all other planetary bodies with an atmosphere?
•How did the moon Miranda come to have such incredibly varied topography?
SURVIVING ON URANUS:
•As with the other gas giants, stay in your spaceship, since there is no firm surface to stand on.
•Keep your heater on. At 1 atmosphere the temperature is about -200 C. Uranus does not seem to have an internal source of heat, unlike the other gas giants, to warm it up.
NEPTUNE
Neptune is a giant blue gas ball, of similar size and mass as Uranus.
HISTORY:
Disturbances (known as perturbations) in the orbit of the newly discovered Uranus led astronomers to search for another new planet. Two mathematicians, Adams of England and LeVerrier of France, independently predicted the position of the planet but they couldn’t get other astronomers with telescopes interested enough to look for the planet. Finally a German astronomer Johann Galle, undertook the hunt and found Neptune on September 23, 1846 very near its predicted position. It turned out that Galileo saw and sketched Neptune positioned near Jupiter, way back in 1612. Galileo even noted in his log that the “star” appeared to have changed its position from one night to the next. Despite this near miss, we would have to wait 233 years for Neptune to be discovered.
Neptune is too faint to be seen with the unaided eye. Telescopes on Earth revealed that Neptune had two moons but the planet is so far away that astronomers had to wait until Voyager 2’s 1989 flyby to learn much more about it. Voyager discovered 6 new satellites and confirmed the existence of Neptune’s faint rings. As with Uranus, Voyager 2 was able to determine the length of a Uranian day using magnetic field studies. Neptune was Voyager 2’s final planetary encounter before heading out of the Solar System.
Because Neptune was discovered so recently, it won’t be until the year 2011 that we will have witnessed one complete revolution of Neptune around the Sun. In other words, we have not yet seen a complete Neptunian year.
OUTSTANDING FEATURES:
•Neptune’s center is a hot 7,000 C. Heat escaping from the center warms up the planet’s outer regions, making its atmosphere about as “warm” (-214 C) as Uranus, despite being one and one half times farther from the Sun.
•Neptune’s atmospheric features are much more distinct than those found on Uranus. Neptune’s Great Dark Spot is as big as the entire Earth.
•The highest winds in the Solar System are found here, with speeds up to 2,000 kilometers per hour, almost breaking the sound barrier on Neptune.
•Neptune, like Uranus, may have deep warm oceans.
•Neptune has thin dark rings, with brighter patches forming arcs.
•Neptune’s largest moon, Triton, has a tenuous atmosphere. This moon may be the coldest surface in the Solar System, except for Pluto whose temperature is uncertain. Despite its cold, Triton has active nitrogen geysers!
MYSTERIES:
•What is the source of Neptune’s interior heat?
•Why are Neptune’s winds so high?
•Why are Triton’s geysers still active?
•Why does Triton revolve around Neptune in the opposite direction from the other moons and opposite to the direction Neptune spins?
SURVIVING ON NEPTUNE:
•As with the other gas giants, stay in your spaceship, as there is no firm surface to stand on.
•Keep your heater on here too because it’s less than -200 C in the outer atmosphere.
PLUTO
Mysterious, tiny Pluto is actually a double planet, with its moon Charon to keep it company on the outskirts of the Solar System.
HISTORY:
Astronomers began looking for a ninth planet when they noticed disturbances in the orbits of Uranus and Neptune. Painstaking photographic searches finally led to the discovery of Pluto by Clyde Tombaugh at Lowell Observatory on February 18, 1930. Ironically, Pluto’s mass was later found to be too small to disturb the orbits of its neighbors. Is there another more massive planet out there waiting to be discovered? Pluto is a tiny, distant world 10,000 times too faint to be seen with the naked eye. Pluto has only moved along a tiny part of its orbit since its discovery. It won’t be until the year 2178 that humans will have witnessed one complete revolution.
In 1978 Pluto’s moon Charon was discovered. Pluto’s mass could finally be determined from Charon’s orbit. Because Charon has fully 1/10 the mass of Pluto, the Pluto-Charon pair are considered a double planet. (Earth’s moon is only 1/80 the mass of Earth.) By chance, Charon’s orbit allowed it to pass right in front of Pluto as seen from Earth recently. Ingenious telescopic studies of these eclipses from 1985 to 1991 have allowed us to learn a little more about this double planet, including the fact that Pluto has polar caps.
Pluto is the only planet that remains unvisited by a space probe. Voyager 2 could not be redirected from Neptune to encounter Pluto because of Pluto’s position. We may never see a close up image of Pluto in our lifetimes!
OUTSTANDING FEATURES;
•Pluto is the runt of the Solar System at only 1/27 of the mass of the next smallest planet Mercury.
•Pluto’s orbit is highly inclined and eccentric compared to the other planets. Pluto’s orbit is so flattened (eccentric) that sometimes it can be closer to the Sun than Neptune. In fact Pluto is inside Neptune’s orbit now and will be until the year 2000.
•Pluto’s companion moon Charon is 1/2 Pluto’s size. Both objects revolve around a common center outside of the body of Pluto.
•Pluto and Charon always show the same face to each other. If you were standing on Pluto looking at Charon, it would not appear to move in the sky, and it would never set!
•Has polar caps. When Pluto is closer to the Sun its polar caps boil to give the planet a tenuous nitrogen, methane atmosphere. As the planet moves farther away its atmosphere freezes completely and is deposited back on the polar caps.
MYSTERIES:
•Pluto is very similar in size and composition to Neptune’s moon Triton. Do they have a common ancestry?
•Will Pluto someday give us clues about the early solar system, preserved like a frozen time capsule far from the Sun?
SURVIVING ON PLUTO:
•There is little or no atmosphere so wear your space suit.
•Make sure your heater is on; this is the coldest planet in the Solar System.
•Bring a flash light, the sky on Pluto is black even in daytime. The distant Sun appears 1600 times fainter than when it is seen from Earth and looks more like a bright star.
