SOLAR SYSTEM

Travel Guide to the Solar System

by David Wright

Earth is a very special place. Of course, we humans have a biased opinion about this planet, because it is our home. The temperature, pressure, and chemical composition on Earth are ideal for supporting a wide variety of life forms. However, the conditions at other planetary locations in our solar system are quite different. A quick trip to each of our planetary neighbors reveals quite a bit of useful information.

Earth is just one of nine planets that orbit Sol, the star we call the Sun. The four inner planets are Mercury, Venus, Earth, and Mars. Each is a relatively small, dense, and rocky planet.

The outer planets include Jupiter, Saturn, Uranus, and Neptune. Each is primarily a large ball of dense gas. Pluto is the outermost of the nine planets. It is a very small, rocky ball of ice and frozen methane.

Conditions on other planets are hard to imagine. But scientists have learned a lot about our solar neighborhood during the past few decades.

Mercury is the planet closest to the sun. It receives tremendous amounts of solar radiation. There is almost no atmosphere to help distribute this energy evenly.

The temperature on the sunlit side of Mercury is 800 degrees Fahrenheit, hot enough to melt lead into a gooey soup. But a different world exists on the dark side. Temperatures there can plunge to a frigid 280 degrees below zero. No other planet is so different from one side to the other.

Mercury also lacks an atmosphere to protect the surface from meteorite impacts. As a result, Mercury is heavily cratered, just like Earth's moon.

Venus is Earth's closest planetary neighbor. The second planet does not receive as much solar radiation as Mercury. However, Venus has an atmosphere heavy in carbon dioxide, a powerful greenhouse gas. The thick atmosphere traps heat from the sun causing the temperature on Venus to soar above 900 degrees Fahrenheit. That's hotter than the sunny side of Mercury!

On Venus, thick clouds of sulfuric acid are blown around by hurricane winds of up to 220 miles per hour. The air pressure there is about 90 times that found at sea level on Earth, which would not make for a pleasant visit. You would be cooked, crushed, and dissolved in minutes.

Here on Earth, a wide range of conditions exist that produce many different environments for living things. Planetary geologists study the extreme conditions on Earth and other planets in an effort to understand what those bodies are like and how they formed.

In winter, the areas at Earth's poles are frigid. In summer, the equator is sweltering. On top of the highest mountains, the air is too thin for us to breath. At the bottom of deep ocean trenches, the pressure of seawater is so great it can crush a submarine. Near a volcano, acidic gases burn skin, lungs, and eyes. Near the Dead Sea and Great Salt Lake, soil is severely alkaline.

Despite these hostile conditions, most of Earth's surface supports life that is well suited to its environment. Astrobiologists study life under Earth's most hostile conditions in an effort to predict where life might exist on other planets.

The surface is just one part of our planet. The Earth also has an atmosphere, crust, mantle, outer core and inner core. The surface is the thin layer between the atmosphere and crust. Too date, the only place life has been found is on the surface. Conditions elsewhere appear intolerable to life.

Ten miles above Earth's surface, the temperature is 110 degrees below zero and the pressure is only one tenth of that found at the surface. The upper atmosphere is bombarded with high-energy solar radiation. The gases there are chemically reactive.

Earth's crust is made of pieces called tectonic plates. Earthquakes often occur when tectonic plates slide next to each other. Volcanoes are found in the areas where these plates crash into one another.

Below the crust lies the mantle. It is made of rocky materials. Below the mantle lie the outer and inner cores, made of molten iron and nickel.

Gravity squeezes all of Earth's layers toward the center. The temperature at the inner core is 13,000 degrees Fahrenheit. That's as hot as the surface of the sun!

The pressure at Earth's center is 4 million atmospheres. That's 50 times greater than the pressure needed to make diamonds! Compared to Earth as a whole, the surface has only a very narrow range of conditions.

Mars is known as the Red Planet because it is covered with red iron oxide dust. The atmosphere contains mostly carbon dioxide, some nitrogen, and a little water vapor. The pressure on Mars is only 1/100th of an atmosphere, yet there are strong winds that kick up the red dust into storms than can cover the entire planet and last for a year or more.

Standing on Mars, the sky would appear pink from tiny particles of iron oxide in the atmosphere. There is water on Mars, but the average temperature is a frigid 40 degrees below zero. Most of that water is frozen as tiny ice crystals found in clouds or as part of the large ice caps that cover the planet's poles.

During the Martian winter, it gets so cold that carbon dioxide from the atmosphere condenses, covering the polar ice caps with a frosting of dry ice.

Jupiter is the largest gas giant. A fish bowl the size of Jupiter could hold 1,300 Earths, yet Jupiter is only 318 times heavier than Earth. Why? Because Earth is four times denser than Jupiter.

The atmosphere of Jupiter contains mostly hydrogen, some helium, and trace amounts of water, ammonia, and other gases. It is very cold there, about 180 degrees below zero.

One of the best-known features on Jupiter is the Great Red Spot, first observed by astronomers on Earth more than 300 years ago. In reality, the spot is actually a giant storm large enough to swallow two Earths. Wind speeds within the storm can reach hundreds of miles per hour.

Below Jupiter's atmosphere lies an outer mantle, which is made of liquid molecular hydrogen and helium. Below that, the inner mantle is made of liquid metallic hydrogen, a form of hydrogen unknown on Earth. Jupiter's rocky core is much larger than Earth. It also is extremely hot - more than 54,000 degrees Fahrenheit!

Jupiter also has an unsolved mystery. Because it is so far from the sun, the giant planet receives very little solar radiation. Yet Jupiter radiates more energy than it receives from the sun. No one knows why.

Saturn, Uranus, and Neptune are gas giants similar to Jupiter. They are made of hydrogen and helium, mostly in liquid form. Each of these planets is very cold on the surface and extremely hot at the core. They also are very far away from the sun.

Pluto is the smallest planet. Astronomers at the Lowell Observatory in Flagstaff, Ariz., discovered Pluto in 1930. It is the only planet known to have solid methane ice. Some scientists believe Pluto is about the same today as it was when the Solar System was formed 4.6 billion years ago.

In addition to the nine planets, there are 61 moons within our Solar System. Some of these moons have dramatically different conditions.

Triton is a moon of Neptune. At 391degrees below zero, Triton is the coldest object in the Solar System. Jupiter's moon Io has many active volcanoes that shoot plumes of lava hundreds of miles above the surface. On Miranda, a moon of Uranus, cliffs stretch more than 12 miles high.

Compared to these other places, the surface of Earth seems rather mild, but that's what makes it home.

David Wright is a research professional at ASU's Center for Solid State Science. He is also a mentor for the "Science is Fun" internship in the ASU Service Learning Program. For more information, see the article on page 32.