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Science
by Pat Murphy & Paul Doherty

A Visit to Mars

FOR MORE than a century, science fiction writers have been transporting people to Mars.

Back in 1912, Edgar Rice Burroughs sent John Carter to Mars, where this former officer of the Confederate Army had many adventures and fell in love with a princess. (Burrough's books are among the many in which men travel to Mars and fall in love. Pat's favorite discovery, in our brief survey of early sf about Mars, is the 1893 novel Unveiling a Parallel: A Romance by Alice Ilgenfritz Jones and Ella Marchant, in which a young man visits Mars and is shocked by the emancipated women there. But among them, he finds the woman of his dreams. Go figure.)

Recently NASA sent a couple of six-wheeled Rovers named Spirit and Opportunity to Mars. To date, neither vehicle has turned up any women (princess, emancipated, or otherwise). But these robotic explorers have returned a great deal of information on the Martian environment.

As part of his job at the Exploratorium, San Francisco's museum of science, art, and human perception, Paul used information from the Mars Rovers to give people a chance to experience aspects of the Martian environment. We thought those of you who missed the opportunity at the Exploratorium might like some instructions on how to experience aspects of the Martian environment in the comfort of your own home. No Martian princess is required for this experience—and emancipated women are welcome.

SEEING MARS

A good way to experience the look of Mars while keeping your feet on terra firma is to don a pair of "Blue Blocker" sunglasses. The lenses of these sunglasses are sandy-butterscotch in color. Put them on, and the world around you will look a lot more like Mars—particularly if you are standing in a desert area without any vegetation.

Now when John Carter arrived on Mars, he didn't comment on the color of the sky. He was busy fighting aliens mere moments after his arrival, so perhaps he can be forgiven for that oversight. But you, as an armchair traveler to Mars, can take your time in examining the Martian sky in the photos of Mars available on the Exploratorium's Mars Website www.exploratorium.edu/mars. You'll immediately notice that the sky is not a lovely blue, but rather a dusty butterscotch brown.

The Martian atmosphere is full of dust and the dust of Mars is butterscotch tan. This dust is primarily "weathered limonite," a brown iron oxide. Mars's surface dust may also include red hematite, Fe2O3 (one of the main ingredients of rust on Earth) and magnetite, Fe3O4, a magnetic mineral. The dust is blown into the sky on Mars by winds and settles out slowly. The finest dust remains suspended for a very long time since it is never washed out of the air by rain.

Without this dust, the sky of Mars would be black, as is the sky on Earth at 115,000 feet above sea level. That's the elevation at which the Earth's atmosphere has the same density as the atmosphere at the surface of Mars. To get an idea of what the clear Mars sky might look like, glance out the window of your airliner the next time you are on an intercontinental flight cruising at 45,000 feet. If you look up at the sky at that altitude, you'll see a sky that's a blue so dark as to be almost black. If you could fly higher the sky would be black as it would be on Mars if the atmosphere were free of dust.

The dust in the Martian atmosphere makes sunsets particularly interesting. On Earth we have red sunsets in a blue sky. The sky is blue because blue light scatters when it encounters molecules in the atmosphere. Red light goes straight, without scattering. When you're watching the sun set, the light traveling straight from the sun to your eyes moves at an angle through the Earth's atmosphere. As the light passes through the atmosphere, the blue light scatters away, leaving reddish light to reach your eyes.

The color of light scattered by a particle depends partly on the particle's size. Molecules in Earth's atmosphere are the right size to scatter blue light. Martian dust particles, on the other hand, are the right size to scatter red light. When light from the setting sun passes through the Martian atmosphere, red light scatters and blue light continues in the forward direction. As a result, the setting sun on Mars is blue-green, surrounded by the butterscotch-colored sky.

If you visit the Exploratorium's Mars Website to check out the sky, spend a bit of time perusing the other photos as well. Each Mars Exploration Rover carries nine black-and-white digital cameras. One camera gives geologists a close-up view of the Martian rocks, acting as a magnifying lens. The other eight are arranged in pairs. Views from these paired cameras are used to create 3-D images of the Martian landscape. (You need two cameras to make a 3-D view for the same reason you need two eyes to see in depth. Each camera provides a slightly different view of the landscape. Combining these views creates a 3-D image.)

The Rovers' panoramic cameras have filter wheels that rotate in front of each lens, so that photos can be taken through different filters. Using red, green, and blue filters, the cameras can take photos that can be combined to make an image that resembles what a human eye would see on Mars.

In addition, there are several infrared filters that let the cameras create images using infrared radiation. (Infrared radiation is a form of light that's just outside the range of human vision.) Snakes with infrared vision and geologists both appreciate the value of infrared images. Rattlesnakes and other pit vipers use their infrared-sensitive pits to make a 3-D, infrared image of the world. In this view, warm-blooded mice are bright against the cool night background of desert rocks—a handy thing if you're a snake looking for dinner.

Geologists, who generally aren't looking for mice to eat, use infrared imaging for other purposes. Silicate minerals that look uniformly reddish brown in visible light look entirely different in the infrared. That's why so many images from the Mars Rovers are taken using infrared filters. This means that many of the images coming back from Mars aren't in true color—geologists use infrared filters instead of red filters so that they can learn more about the rocks surrounding the Rovers.

This use of infrared filters is a great example of how scientists choose to capture images using different forms of light (including light that's beyond the range of human vision). In false-color infrared images of Earth, trees are red, clean water is black, but rocks have interesting and subtle shades—obviously a palette of colors chosen by a geologist.

You can look at lots and lots of rocks in the photos that were sent back from Mars—great fun for geologists, but Pat thinks it gets a bit dull for those of us raised on Burroughs. But there are other things to check out as well. Each Mars Rover carries a sundial, and images of those sundials are among those on the Exploratorium Website. Scientists use these sundials to adjust the Rovers' panoramic cameras, and students participating in NASA's "Red Rover Goes to Mars" program can monitor the sundials to track time on Mars.

On Mars, only half the light hitting the sundial's surface comes from the Sun; the other half comes from the sky. When you check out the images of the sundial on the Exploratorium's Website, take a close look at the shadow of the gnomon, the upright in the center of the sundial. The shadow is not completely black because light from the sky shines into the shadow.

SMELLING MARS

Back in Burroughs's day, a stalwart hero (like John Carter) could arrive on Mars (or Barsoom, as the Martians called it) stark naked—and still survive. (The mysterious force that brought John Carter to Mars mysteriously left his clothes behind.) But Carter (and many of the other visitors to Mars in early science fiction) had no trouble breathing the air.

It was fortunate for John Carter that he was protected by an author who was not noted for scientific verisimilitude. In truth, if you were to step naked onto the surface of Mars, all the gas inside you would come out through every orifice in your body. You would remain conscious for ten to fifteen seconds. Your ears would "pop" as they adjusted to Martian pressure. After fifteen seconds your blood would boil.

On top of all that, the atmosphere doesn't have enough oxygen to sustain you. The Martian atmosphere is ninety-five percent carbon dioxide. In the Martian atmosphere, only one out of a thousand molecules is oxygen. (On Earth, it's more like two hundred and ten out of a thousand.) On both Earth and Mars, most of the remainder of the atmosphere is nitrogen and argon.

If you want to find out what Mars smells like, open a bottle of unflavored soda water and take a sniff from the neck of the bottle. That aroma of carbon dioxide is the smell of Mars.

At the Exploratorium we put a block of dry ice (that's solid carbon dioxide) into an empty fish tank. After a while, the carbon dioxide sublimes (making the transition from solid ice to gas without ever becoming liquid). Carbon dioxide gas fills the fish tank. Since carbon dioxide is heavier than Earth air, the gas will stay in the tank until you sweep your hand through it.

Sweep some of the carbon dioxide gas into your nose. Stick out your tongue and plunge it into the top of the tank and you will experience the tangy taste of Martian air.

LIGHTING FIRES ON MARS

If you were to lower a burning candle into the fish tank, it would go out. An ordinary fire, where hydrocarbons combine with oxygen to produce carbon dioxide and water vapor, will not burn in the Martian atmosphere.

But that doesn't mean you can't have fire on Mars. Our friend Eric Muller, a teacher at the Exploratorium, can make a fire that burns quite well in a carbon dioxide atmosphere. Eric takes two slabs of dry ice and carves a hemispherical hollow in the center of one block. He fills the hollow with powdered magnesium, lights the magnesium, and puts the second flat block over the first. A bright glow shines through the dry ice blocks for a long time as the magnesium cheerfully burns in the carbon dioxide atmosphere.

When the fire goes out, a quick look into the center of the dry ice blocks reveals a collection of black carbon and white magnesium oxide. The burning magnesium rips oxygen from the carbon dioxide in order to burn, leaving carbon behind.

There is an important life-lesson here for Earthlings: never try to put out a metal fire with a carbon dioxide fire extinguisher. If you do, you'll be spraying oxidizer onto the burning metal.

The lesson for anyone camping on Mars is simple: if you want to make a campfire, don't forget to bring along some magnesium logs.

LISTENING TO MARS

We have a vacuum chamber at the Exploratorium, which allows us to reduce the pressure inside the chamber to Martian atmospheric pressure. We put a tape player and a tape recorder into the chamber at Earth pressure and then pump the chamber down to Martian surface pressure, which is only about one percent of the pressure on Earth. (Meteorologists call this pressure ten millibars.)

Under Martian atmospheric conditions, the sound picked up by the recorder gets much quieter. When we let Earth air pressure back into the chamber, the sound grows loud again. Sound does travel through the thin air of Mars, but not as well as sound travels on Earth. It is harder to make loud sounds and harder to hear them.

The Mars Polar Lander carried a microphone but it failed to land safely so we don't have direct measurements yet of the sounds of Mars. If the recorder had worked, we might have been able to hear the wind whispering around the rocks.

TOUCHING MARS

The Rovers can't transmit the tactile sensation of Mars. Their instruments dust off rocks, take photos, drill into rocks, and analyze the dust. Yet here you sit ten or more light-minutes away from the Martian surface, longing to touch Mars rocks.

You can fulfill that desire by visiting the Exploratorium (which is a lot closer than Mars). We have a small piece of a rock from Mars.

Back in October of 1962, a meteorite landed about ten feet away from a farmer in a Nigerian corn field. This forty-pound chunk of rock, known as the Zagami meteorite, was sliced up, distributed to various museums, and studied extensively. In 1995, scientists analyzed gas contained in bubbles in the meteorite. They discovered that the composition of the gas matched that of the Martian atmosphere. Scientists now think that a comet or asteroid slammed into Mars about 2.5 million years ago, flinging the rock that became the Zagami meteorite into space.

An enterprising meteorite dealer managed to trade other specimens from his collection for chunks of the Zagami meteorite—and offered slivers of this Martian rock for sale. The Exploratorium has only a small piece because Martian meteorites are among the rarest rocks on Earth. Like diamonds, they're priced by the carat. Slivers of the Zagami meteorite sold for hundreds of times the price of gold.

The rock of the Zagami meteorite is greenish-gray basalt with crystals big enough to see with a geologist's hand lens. It feels rough to the touch. It is quite a wonderful sensation to actually run your finger along a piece of Mars.

WORTH A VISIT?

The science fiction writers who wrote about Mars when it was just a blurry image in a telescope let their imaginations run wild, giving Mars a population, a breathable atmosphere, and a network of canals. Their work inspired kids who grew up to be scientists interested in finding out what Mars was really like. These scientists and engineers (emancipated women and men) sent robot representatives to Mars. Now that the robots have sent back a more up-to-date portrayal of Mars, it's up to the science fiction writers to weave that new vision of the Red Planet into their fiction.

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To learn more about Pat Murphy's science fiction writing, visit her web site at www.brazenhussies.net/murphy. For more on Paul Doherty's work and his latest adventures, visit www.exo.net/~pauld.

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