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Mars by Andrew Fraknoi

Mars is the 4th planet from the Sun and the one that has most intrigued humanity over the years. It was named after the god of war because its reddish soil gives the planet a scarlet hue in the night sky, like the color of blood. The red planet is also an easy object for stargazers and amateur astronomers to observe, and it's been a favorite target for telescopes since they were invented.

Mars is roughly half the size of Earth and contains just a little over 10% of our planet's mass. These differences combine to give Mars only 38% of Earth's gravity. In other words, if you weighed 100 pounds on Earth, you would weigh only 38 pounds on Mars. Early telescopes showed Mars had an atmosphere, polar caps (layers of frozen ice) at both its poles, and a 24-hour day, which led some observers to leap to the conclusion that the little planet was similar to the Earth in many ways.

But today, with robot explorers that have wandered over the Martian surface and orbiting spacecraft mapping the red planet with unprecedented detail, we know a lot more about our neighbor in space than those early observers.

What Happened to Ancient Mars

Here's the story that our spacecraft exploration has uncovered about the history of Mars. There is ample evidence in the Martian rocks and minerals that billions of years ago, when Mars was young, it had a thick atmosphere. With significant air pressure (and heat from the Sun), water on Mars was a liquid and could make rivers, lakes, and even perhaps small oceans. Planetary scientists now believe Mars had all of these, and thus could even have experienced the first stirrings of life.

A dried-up river bed on Mars
A dried-up river bed on Mars, as seen from the Mars Global Surveyor spacecraft, in orbit around Mars. The valley is about 1.6 miles (2.5 km) across. (NASA)

But when we see Mars today, it is dry as dust. All the old riverbeds are empty, sand has filled in many lakebeds, and the temperatures are much colder than in those early days. What has made such a dramatic change?

The answer is the gravity. Unlike its big brother Earth, little Mars could simply not hold on to its thicker atmosphere. The air slowly escaped into space, and reduced the pressure in the atmosphere over billions of years until water could no longer be a liquid. Liquids require air pressure, which is why things boil more easily at high altitudes. With less air pushing down, there is less reason for a liquid to stick together and it becomes easier to turn it into gas. This is just what happened on Mars. With the falling air pressure, more and more water became vapor.

The absence of air also cooled Mars, because a blanket of air allows a planet to store up the heat of the Sun. As Mars cooled, water froze into the soil and the ice caps, leaving the Martian surface (and any possible life forms) high and dry.

Today, the air pressure on Mars is about seven tenths of one percent of Earth's pressure, and the average temperature is in the minus 60 degrees Fahrenheit range. On the surface, your first deep breath would be your last, since the thin air is made mostly of carbon dioxide. The absence of an ozone layer would mean that nothing was filtering out the Sun's ultraviolet rays. Overall, it's not a place where humans could survive even briefly without significant protection.

Still, scientists can imagine domed cities where visitors from Earth might live comfortably in the future, particularly in the warmer equatorial regions. And Mars has great tourist attractions, including the largest volcano in the solar system—so large at its base that it would cover the entire state of Missouri. And the possibility exists that fossils or frozen examples of early Mars life may yet survive deep in the soil, where future missions may discover it. It's no surprise then that the little red point of light in the sky still intrigues dreamers and scientists, as it did thousands of years ago.

Observing Mars for Yourself

Even though Mars is being closely monitored by spacecraft in orbit around it, amateur astronomers continue to make useful observations from their vantage point on Earth. Mars has its own kind of weather, including giant dust storms, and amateur astronomers can follow the larger weather patterns with their backyard telescopes. In summer 2003, for example, a dust storm grew to be over a thousand miles wide, covering about 1/4 the diameter of the entire planet. Occasionally, these storms can grow until they encircle the entire planet. (By coincidence, this happened in 1971, just as the first of our spacecraft to orbit the red planet reached its target.) Amateur astronomers constitute a kind of "early-warning" system for storms on Mars and other planets.

Mars, Earth orbit diagram
Mars at a close opposition (left) and a more distant opposition (right).

Mars is best observed when it is at or near "opposition," meaning that it is opposite in our skies to the Sun. (See diagram.) When Mars is at opposition, it rises when the Sun sets, and is in the sky all night long. It is at these times that Mars looks largest through a telescope.

The reason has to do with the orbits of Mars and Earth. Mars takes about 1.9 Earth years to go once around the Sun. Think of Earth and Mars as cars speeding around a big round race track: the Earth goes around faster on an inner track, while Mars moves more slowly on an outer track. Sometimes the two cars will be next to each other, while at other times they may be on different sides of the race course. In the solar system, too, Mars and Earth are sometimes "next" to each other; this is an "opposition."

All oppositions are not of equal quality. This is because Mars' orbit is not circular, but a bit oval (the technical term is elliptical.) The best oppositions occur when Mars' orbit brings it nearest the Earth. The two factors—Mars and the Earth lined up in the race track and Mars' orbit bulging toward the Earth—can bring Mars as close as 33 million miles (54 million km).

A Mars opposition happens roughly every 26 months (2 years and 2 months). Close oppositions are separated by 15 to 17 years. We had one of the closest oppositions in history in August 2003; the next close one will not be until July 2018. Still, there will be regular opposition in late December 2007. Weather permitting, Mars will then be a target for astronomy enthusiasts around the world.

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