© 1987, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112
You skip southward lightly in reduced gravity; red dust puffs upward where the boots of your pressurized body suit touch the ground. A plastic cup of icy cola, grasped firmly in your glove, boils violently in the thin poisonous air. The early-morning sky is pink all around, mutedly illuminated by a small-looking Sun, rising to your left. You stop at what seems to be the edge of the world: a stomach-dropping cliff falls away to an arid floor miles below you. Far off to the south, dimly visible through a morning haze of ice crystals, is another mammoth wall. The chasm extends out of sight toward the rising Sun. To your right, a distant mountain top rises impossibly high; a wisp of white carbon dioxide ice condenses downwind from its summit. Another morning dawns on Mars.
To make matters just a bit more complicated, Mars's orbit is slightly eccentric, which means that it is not at the same distance from us at every opposition — it can be as far away as 101 million kilometers (63 million miles) and as close as 56 million kilometers (35 million miles) — quite a range!
The closest close approaches — called favorable oppositions — come every 15 to 17 years on average. The last favorable opposition was in 1971 (how many of your students had even been born then?), but the 1988 one will be one of the best in the 20th century. On September 22, Mars will be only 58.6 million kilometers (36 4 million miles) from Earth, only about 2.9 million kilometers farther than the record closest approach. Mars should be very easily visible in the night sky — a brilliant crimson point of light, beckoning to us as it has for centuries.
Olympus Mons on Mars. (NASA photo)
To begin with, the planet's atmosphere and weather are very hostile — and in quite different ways from the wet, stormy hazards Earth's climate poses. The most dangerous thing about the atmosphere of Mars (to humans) is that it's very thin. As measured by the weather stations on the U.S. Viking probe landers, Mars's pressure varied between .006 and .015 of Earth's. Such a near-vacuum would have a peculiar effect on liquid water: if the temperature is even just a little above freezing, a pool of water — or the water in a human's bloodstream — would quickly come to a boil.
The boiling point of water (or any other liquid) depends in large part on the weight of atmosphere pushing down on it. The higher the pressure, the harder water has to "work'' (the higher its temperature has to be) in order for it to boil. That's how pressure-cookers work: by sealing a pot of water under high pressure, the water reaches a higher temperature before it boils, and potatoes can cook faster. Mars can be thought of as a pressure cooker in runaway reverse; the boiling point of water on Mars is very close to the freezing point, and a glass of water at room temperature would bubble away vigorously and quickly.
Even if protected by a pressure suit, a tourist on Mars would have to contend with other strange inconveniences. Imagine our tourist trying to boil water over a campfire. First, she'd find no liquid water — it's all either frozen solid or a very thin vapor in the air. Second, she'd find no fuel for her campfire: no trees, no twigs, no organic material of any kind. Most frustratingly, though — even if she brought her own water and her own newspapers to light with matches — nothing would burn! Mars's air contains virtually no oxygen at all, and oxygen is necessary for fires. (Of course, this means that our tourist would need to have oxygen tanks in order even to breathe!) The atmosphere of Mars is about 95% carbon dioxide; oxygen makes up only about 1/10th of 1%. (Earth's air is roughly 80% nitrogen and 20% oxygen.)
Even a suitably protected and supplied tourist would find Mars's weather to be a little tough. Martian temperatures are brutally cold, as a rule. In their six years of operation on the surface from 1976 to 1982, the two Viking landers seldom recorded any temperature higher than about minus 30 degrees Celsius (20 degrees below zero Fahrenheit). While it never rains on Mars, there are storms: but they are sand storms. Periodically (about every Martian year or so), the entire planet is choked by global, sandblasting dust storms that shroud its entire surface. Even when no sandstorm is in progress, there is enough red Martian dust in the air to turn the sky pink in the daytime.
The most impressive mountain in the Tharsis region is called Olympus Mons ("Mount Olympus''). This mammoth cone is about 500 kilometers (300 miles) across at its base — on Earth, it would cover Arizona or New York State — and the 80-kilometer-wide crater (or "caldera,'' as the depressions at volcano tops are called) at its summit is 25 kilometers (15 miles) above the surrounding plain. To put its height in perspective, note that the top of Mt. Everest is only about 9 kilometers above sea level.
The Valles Marineris canyons were probably formed by a stretching of Mars's crust (akin to the geologic forces that pulled Earth's continents apart from one another) long ago, not by running water which carves the canyons we're used to on Earth.
As we've seen, liquid water cannot exist in large amounts on Mars now — the atmospheric pressure is just too low—but the now-arid scars of ancient rivers and floods show clearly that water could (and did) flow on the red planet's surface sometime in the past. No one is sure Just how long ago that was—in fact, it may have happened more than just once—but it is clear that Mars used to be a very different place from what it is now. It appears that in the past the air was thick enough, and the temperatures warm enough to sustain pools, rivers, rain, and floods.
(We should emphasize that the dry river channels on Mars are not the "canals'' of science fiction. Such canals — artificial waterways — apparently never existed on Mars.)
Most of the evidence concerning the question of life on Mars comes from a battery of ingenious experiments that were carried out by the Viking automated lander craft on Mars in 1976 (and for a few years thereafter). These experiments included cameras (which continually monitored the surface near them for large scale organisms), three micro-biology experiments (which looked for changes that microbes would cause in soil samples brought aboard), and one that looked for organic molecules that might have been left in the soil by life long ago, even if Mars is lifeless now (Earth's soil is permeated by such chemical "droppings'' of life). None of the experiments produced results that conclusively showed life to be on Mars — or ever to have been there.
Of course, the Viking landers could only explore their immediate environment — Lander 1 in Chryse Planitia ("the plains of gold'') at latitude 22 degrees North and Lander 2 on the plains called Utopia at latitude 48 degrees North. Scientists would like to explore many other parts of the red planet before coming to any final conclusions about the presence or history of life on Mars.
"Expedition Mars'' and "The Exploration of Mars'' in Odyssey magazine, May 1984.
Moche, D.: Mars. 1978, Watts. A nice picture book for primary grades.
Beatty, J. "The Amazing Olympus Mons'' in Sky & Telescope, Nov 1982, p. 420
Carroll, M. "The Changing Face of Mars'' in Astronomy, Mar 1987, p. 6
Cooper, H. The Search for Life on Mars. 1980, Holt, Rinehart & Winston
Gore, R. "Sifting for Life in the Sands of Mars'' in National Geographic, Jan 1977
Sagan, C. Cosmos. 1980, Random House. The chapter called "Blues for a Red Planet'' is an eloquent introduction to Mars.
Washburn, M. Mars At Last. 1977, Putnam's
The Mars Kit. 1987, Astronomical Society of the Pacific. Six color slides and background information.
Following up on Dennis Schatz's very popular "Invent an Alien'' activity (published in the first issue of this newsletter), you might assign students the task of putting together an alien life form that could survive conditions on Mars as we know them today. Students could submit written reports, drawings, or models, and even use costumes, props, and make-up to transform themselves into plausible Martians.
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