Mars : The Lure of the Red Planet

by Sheehan, William

• ISBN13: 9781573929004
• ISBN10: 157392900X
• Format: Hardcover
• Copyright: 2001-04-01
• Publisher: Prometheus Books

Summary

No other planet has so fired the human imagination as Mars. The possibility of life on the planet was an obsession in the 19th century. It is not surprising to find that it has become so once again, because of all the planets in the solar system (apart from Earth), Mars has always been regarded as the most likely to serve as the abode of life. Now that there are hints that the planet may have held "liquid water" (a prerequisite for life, in contrast to the frozen water of the ice caps) much more recently than anyone had supposed, excitement over Mars exploration has never been higher.

In this engaging, eloquent account of our love affair with the Red Planet, William Sheehan and Stephen James O'Meara review the history of human fascination with our neighbor in the solar system, and look at the prospects for manned spaceflight to Mars in this new century. The authors portray the history of Mars investigations through the eyes of the dreamers and achievers that have made the planet such an integral part of the human psyche. They reveal the discoverers' hardships

Table of Contents

Preface	9	(4)
Acknowledgments	13	(4)
Part I. Pale Red Dot
Lilac Dawn	17	(10)
Mars in the Solar System	27	(10)
Wanderers and Wonderers	37	(14)
The Untrackable Star	51	(14)
The War with Mars	65	(18)
Part II. A Swelling Orb
The Sails of Imagination	83	(12)
Mappers of Strange Seas	95	(8)
``A Less Difficult and Bloody Conquest''	103	(22)
Sidebar: A Color-Blind Astronomer	116	(8)
``Mars,'' excerpt of a poem by Percival Lowell	124	(1)
Lowell's Mars	125	(24)
La Grande Lunette	149	(36)
Sidebar: The Canals of Mars	176	(9)
Part III. The Mars of Romance
Invaders from Mars	185	(12)
War of the Worlds	197	(20)
Part IV. Marsfall
Water, Water---Anywhere?	217	(18)
Woodstock Mars	235	(14)
The Brave New World of Mariner 9	249	(22)
Part V. The Abode of Life?
Paradise Lost?	271	(30)
If Stones Could Talk	301	(18)
Epilogue: The Mars of Global Surveyor	319	(7)
Appendix A. A Square Old Yellow Book of Mars	326	(13)
Appendix B. The Colors of Mars	339	(6)
Appendix C. Exploring Mars with Your Telescope	345	(22)
Appendix D. Table of Data for the Planet Mars	367	(2)
Appendix E. The Satellites of Mars	369	(2)
Appendix F. Oppositions of Mars, 2001--2035	371	(2)
Appendix G. Most Favorable Oppositions of Mars, 3000 B.C.E. to 3000 C.E.	373	(2)
Notes	375	(18)
List of Illustrations	393	(4)
Index	397

Excerpts

Chapter One

Lilac Dawn

The rosy-fingered dawn appeared, the early-born.

--Homer, The Iliad

July 4, 1997.We were back. Twenty-one years after we established our first human outpost on Mars with the Viking 1 lander, fifteen years after we received our last transmission from that mechanical pioneer, a hopeful emissary from our planet hurtled out of the bleak Martian sky at more than 26,000 kilometers per hour (16,000 miles per hour) and headed straight for the ground 130 kilometers (81 miles) below. It was 2:55 A.M. Local Mars Time, and the end of a seven-month-long journey was just minutes away.

    Back on Earth, mission specialists at Cal Tech's Jet Propulsion Laboratory (JPL) in Pasadena, California, braced themselves for impact; in five minutes Mars Pathfinder would smash into the red and rocky soil at 80 km/hr (50 mi/hr) ... they hoped. The final minutes passed in nervous anticipation. It was like watching a skydiver jump out of a plane and waiting for the parachute to open. Although no one likes to say it, interplanetary travel has its risks; failure is not an option, though it's a possibility.

    The last mission to Mars was a bust. In August 1993, Mars Observer , a billion-dollar-baby with a $22 million camera and a host of other expensive instruments designed to help us understand the history, geology, and space environment of Mars, died of a ruptured aneurysm in a fuel line just before the spacecraft entered orbit. The death of Mars Observer ended twelve years of hard work and countless scientific dreams. No one wanted to see Pathfinder repeat that dismal performance.

    In the wake of the Mars Observer disaster, NASA'S chief, Dan Goldin, adopted a new slogan: "smaller, faster, better, cheaper." Mars Pathfinder was advertised as the first of a new wave of inexpensive, quick-turnaround missions that, it was hoped, would revivify the American Mars Program. In all it cost only $280 million to fly. A masterpiece of miniaturization, it was built largely with off-the-shelf components. Three triangular petals covered with solar cells powered the landers; one of them housed a weather station equipped with windsocks and a thermometer. The base between the petals had a mast which could hoist a camera 1.5 meters (5 feet) above the ground and thereby furnish a human's-eye view of the Martian terrain.

    The lander also carried a rover, called Sojourner , after Sojourner Truth, an ex-slave who became a leader of black emancipation and women's suffrage in nineteenth-century America. (Valerie Ambroise, a twelve-year-old from Connecticut, suggested that name in a winning essay, which was submitted to a contest sponsored by JPL and The Planetary Society; students were asked to name the rover after a heroine who had blazed a trail for humanity. Thus, Sojourner became the first American spacecraft named for a woman.) The toy-sized buggy measured 0.6 meters (2 feet) long and 0.3 meters (1 foot) high. It had six steel wheels that could roll independently over the Martian surface. A tiny microprocessor "brain" gave it roughly the intellectual capacity of an insect. If it arrived safely, Sojourner would become the first automated vehicle to roam another planet (not, however, the first to roam another world; in the early 1970s the Soviet Russians had successfully deployed similar landers, the Lunokhods, on the Moon, and used them to carry out sample-return missions.)

    Unlike other Mars landers, Pathfinder would bypass the complicated and costly orbit-and-descent choreography usually employed and instead make a beeline for the planet's surface--parachutes and retrorockets would slow the craft until a system of gas bags inflated and allowed the lander to bounce to a stop. On paper, Pathfinder 's purpose was to get something--anything--to land on Mars. "After [that]," project scientist Matthew Golombek said, "whatever we did was pretty much considered gravy."

    But in the hearts of the millions of earthlings who were also watching, waiting, and hoping (by the end of the first day, Americans would log 40 million hits on NASA's Pathfinder Web pages, 500 million by the end of the month), Pathfinder 's journey to Mars symbolized the latest American Dream, In truth, scientists and lay persons alike all wanted the gravy. More than any other planet beyond Earth, Mars alone has had the power to pique the human imagination, to cause us to ponder the possibility that we may not be alone in the universe, that planets with life--no matter how small or faceless--may be as abundant as the number of stars overhead.

    Mars has offered us that hope for more than two centuries. But not until Mariner 9 orbited the planet in 1971 did the real Mars come into focus. The spacecraft's unprecedented images showed us a world with features much like those on our own planet, features that displayed legible marks of historical change. Seasons of ice ages had scarred the landscapes around the planet's icy polar caps. Sand dunes attested to the incessant action of wind erosion and deposition. There were signs of great geological upheavals, like shield volcanoes--towering mountains built by episodic outpourings of molten lava; some of these volcanoes soared 22,860 meters (75,000 feet) into the thin Martian air, more than twice the height of the loftiest mountains on Earth. There were networks of canyons, with one so vast it rips across one-sixth of the planet's circumference, thereby dwarfing even the Grand Canyon of Arizona. But above all we saw for the first time evidence of ancient floodplains and dry river valleys. One of them, Ares Vallis, an ancient flood channel near the planet's equator, was Mars Pathfinder 's final destination. The area had been identified as one of the most intriguing places on the planet, where possibly massive floods had once raged across the surface. If true, evidence would be forthcoming. As Ambroise concluded her essay, " Sojourner will travel around Mars bringing back the truth."

    But first the craft had to arrive safely. And not everyone was so sure it would. In the months leading up to impact, JPL's Tony Spear had been haunted by dreams of Pathfinder smashing into a thousand pieces and ruining the mission's chance for success.

* * *

When Pathfinder penetrated the upper atmosphere of Mars, it sped like a missile toward the ground with a descent angle of only 13.9°, just shallow enough to keep it from burning up as a meteor (a protective shell and heat shield, whose efficiency proved worthy on previous Mars missions, also helped). Two minutes into its fiery descent, Pathfinder dropped from 130 kilometers (81 miles) to 9 kilometers (6 miles) above the planet's surface. A parachute billowed open, slowing the craft's speed to 1,336 km/hr (830 mi/hr). One hundred seconds from impact, the heat shield was jettisoned. Eighty seconds from impact, a tether lowered the lander 20 meters (65 feet) from its protective shell; this maneuver would safeguard the lander from the braking rockets when they fired. Thirty-two seconds from impact, the lander, now traveling at 241 km/hr (150 mi/hr), turned on its ground-detection radar system. Ten seconds from impact, a bulbous panoply of air bags made of Vectran (a bullet-proof vest material) inflated to surround the craft. Six seconds: the braking rockets fired 304 meters (1,000 feet) above the surface. Four seconds: the tether was cut, and Pathfinder , nestled within its airbags, smashed into the surface at 3 A.M. Mars Time. But the journey was not over. The force of the impact sent the craft bouncing 15 meters (50 feet) back into the air. Pathfinder bounced more than 15 times in the first minute, before it rolled end over end for another minute and a half until--not quite a mile from where it had first touched down on Mars--it settled in a safe and secure resting place. By sheer chance it happened to roll over into an upright position, thus eliminating the need for a planned righting maneuver.

    From the surface of Mars, Pathfinder beamed a faint but hopeful "Hello." The message was borne on radio waves across 191 million kilometers (119 million miles) of space to the morning-star Earth. Pathfinder had reached its destination, well within the planned 97 x 193 kilometer (60 x 120 mile) landing ellipse. After deflating its airbags and unfolding its three solar panels, Pathfinder triumphantly raised its weather mast and windsocks. Shortly after 2:00 P.M. Pacific Daylight Time, the Sun crested over the horizon of Mars and activated Pathfinder 's solar cells, enabling its stereoscopic camera to click open and take its first picture of its new environment. The mission specialists back at JPL were not disappointed: displayed before them in wall-sized wonder were the black solar panels atop Pathfinder 's open petals, deflated air bags, and a waiting Sojourner ready to roll down its ramp onto the lone and level sands of Mars, beneath the butterscotch skies of a clear Martian morning.

    What captured the scientists' eyes lay beyond, and presumably around, the lander, for Pathfinder 's camera was looking out upon a rocky expanse unlike any that had ever been seen on Mars before. The images showed that the craft had landed among a field of gently sloping dunes sculpted by winds and littered with rocks ranging in size from a few millimeters to boulders several meters across. The most common size was from a few centimeters to about 20 centimeters (8 inches). "We wanted rocks," Golombek said, "and we got rocks." Ares Vallis was supposed to be a geological wonderland, and it was.

    Donna Shirley, the thirty-five-year-old aerospace engineer in charge of managing the Mars Exploration Program at JPL that had built Sojourner , was especially intrigued by an area southwest of the lander, where "rocks were crammed together over 30 percent of the surface." The rounded rocks in this plot of Martian real estate, dubbed the Rock Garden, lay on the far wall of a trough and were stacked and slanted to the northwest, like books on a shelf. The consensus swelled unanimously: "The area," Golombek said, "shows the effect of catastrophic flooding." Torrents of water, perhaps having a volume equal to that of all the Great Lakes combined, had washed down the valley from the southeast, carved the trough, and deposited the boulders. "In a typical flood like this on Earth," Golombek explained, "we would expect to see big rocks deposited during the first rush of water. Then, as the water volume and speed lessen, we see dust and smaller particles deposited around the rocks."

    Indeed, much of the landing area was powdered with the silt-fine red drift materials so ubiquitous on the Martian surface. Sandbars tailed leeward of some of the rocks, which displayed all manner of shapes, sizes, colors, and textures. Some rocks appeared to have flattened forms and sharp edges, suggesting they might have been tossed there from an impact crater--perhaps from the one whose rim was visible as a low knob in the distance, only 2.25 kilometers (1.4 miles) away--or from an unseen volcano. Other rocks displayed a layered structure suggestive of sedimentary origin--meaning it was created by debris laid down horizontally by the action of rivers, lakes, glaciers, or wind. Many of the rocks exhibited interesting textures; about half had long flutes on their surfaces that were probably carved by windborne particles. Other rocks were pitted, suggesting they were formed from the exsolution of fluids from volcanic rock, the weathering away of soft minerals within the harder rock, or erosion by the wind. Then again, perhaps the sockets had contained pebbles within conglomerate rocks, like puddingstone on Earth.

    To make identifying the rocks easier, the scientists quickly began naming them. "Within hours," Shirley recalls, "scientists were jostling for position in front of wall-sized prints of the lander's pictures and demanding to name the rocks. The crush was so great that one young scientist was given the policing job. He would put a sticky note with the selected name onto the picture. Squash. Half Dome. Wedge, Cradle, and Flat Top got stickies. Stimpy and Ren were named after cartoon characters, and there was even a Scooby Doo, a flat white area which later proved to be more likely a dried up mud puddle than a rock." A pair of mountains, the Twin Peaks (named after the popular David Lynch television series) loomed over the horizon, along with gullies, drift deposits, and terraces fashioned, presumably, by repeated episodes of flooding. Even the lander was renamed: NASA christened it the Sagan Memorial Station, in honor of Carl Sagan, the late great doyen of planetary studies and science popularizer, who had died in December 1996.

    Sojourner 's deployment was delayed for two reasons. First, the lander and Sojourner had trouble communicating. Apparently, the problem was caused by a temperature difference between the modem in the rover and the modem in the lander. "If the temperatures were different the [modem] frequencies were different," Shirley explains, "and they couldn't communicate." The other, though not unforeseen, problem was that one of the landing airbags crumpled against the petal the rover was to slide down. This forced mission controllers to carry out an extra maneuver. Just before sunset on Sol 1, they sent a command to the Pathfinder to lift one of its petals 45°. With the petal tilted, the lander was reoriented, the airbag further retracted, and the petal reextended. Although it took longer than expected to deploy the rover, this wasn't really a problem, because there wasn't any real deadline for getting the rover off the lander. Just before the fast-sinking Earth set below the horizon of Ares Vallis, moments ahead of the Sun, and severed the communications link with ground control, Pathfinder captured an image showing that the maneuver had succeeded. Though part of the airbag still drooped over the petal, there was room enough for the rover to roll down the ramp.

    As the Sun dipped toward the west-northwest horizon, the surface of Mars began to cool; the atmospheric pressure dropped, and the atmospheric temperature plummeted as heat rushed off the surface and radiated into space, Thin waifs of icy clouds reflected the dying embers of the Sun. A pink twilight gave way to the cold black expanse of night with Venus burning in the west and the summer Milky Way raining down a multitudinous starlight.

    The temperature differences between day and night on Mars are both wild and extreme. Because the atmosphere of Mars is about 140 times thinner than the Earth's, a separate temperature regime exists between ground level and approximately 1.5 meters (5 feet) from the surface of the planet. At the hottest point in the daytime, a person standing on the surface would feel the ground beneath his feet a comfortable 47° C (65° F) but the top of his head would be chilled at -9° C (15° F). This is because the thin Martian atmosphere lets most of the sunlight pass through unimpeded to the surface where it is absorbed. Daytime high temperatures are usually much higher for the ground than for the air. "An extreme example," explains JPL's Mike Mellon, "might be walking on the sidewalk or a beach, barefoot on a hot summer day. The air is hot, but the ground is much hotter and you could even burn your feet." At its coldest point during the night, the Martian surface dips to a frigid -90 C (-130° F) while the atmosphere 1.5 meters (5 feet) above is only -19° C (-105° F). Come morning, the Sun warms the surface and heat rises in eddies as clouds blow in from the northeast.

July 5, 1997. The radio link was restored on Sol 2, and Sojourner glided down the ramp to begin its epic trek across the Ares Vallis flood plain. (Once the rover moved off the petal, the communication's link functioned as expected.) Its maximum speed was only 46 meters per hour (150 feet per hour)--about the speed of a tortoise. Built-in gyroscopes equipped it with a primitive vestibular system to prevent it from toppling over as it explored nearby boulders. Five laser beams helped it "feel" its way, in blindman's-bluff fashion, across the surface. Sojourner 's progress across the flood plain, traced in the thin tracks it left in the rust-red Martian soil, was recorded by the cameras of the fabric-draped Sagan Memorial Station.

    As Sojourner explored, the skies overhead presented an intense, ever changing panorama. Despite the stratospheric thinness of the atmosphere, it had been known, ever since the Viking 1 landing in 1976, that the Martian skies were remarkably bright ("like the skies over Los Angeles on a smoggy day," one Viking scientist had quipped). Early in the Viking mission, their color had at first been rendered in evocatively Earth-like blue. In the end it was decided it was no true-blue, only the product of our having too long drunk to intoxication the idea of an Earth-like Mars. Later images were recalibrated with greater sophistication to a more Martian appearing salmon-pink (brownish-orange might have been still more accurate). Because the cold Martian atmosphere is so thin, common sense requires the sky to appear dark. But it isn't. It's bright. That's because the atmosphere of Mars--95 percent carbon dioxide, 3 percent nitrogen, and traces of oxygen, argon, carbon monoxide, and water vapor--is seeded with micron-sized dust particles, which extend to altitudes of 30 kilometers (19 miles). By Sol 16, predawn images from Ares Vallis showed thin wisps of water-ice clouds in violet skies. The clouds, 16 kilometers (10 miles) high, floated across the video screen from the northeast, wafted by winds blowing at 24 km/hr (15 mi/hr). Another intriguing picture showed the tiny Martian moon Deimos suspended in the sky--the first time a Martian moon had ever been captured from the surface of the planet.

    The Martian sunrises were grand Homeric stirrings: glorious bursts of color and light in a white sky tinged with the faintest hint of blue. They were majestic lilac- rather than rosy-fingered affairs. The Sun, as seen from Mars, was noticeably smaller than as seen from the Earth--which was hardly surprising, since the planet is one and a half times the distance of the Earth from the Sun. As the Sun climbed higher above the horizon the wispy evanescent water-ice cirrus clouds burned off. By noon they had vanished without a trace, and the dusty skies had turned to murky brownish-orange--the strange skies of a strange rust-red world.

    Sunsets were events of breathtaking grandeur as darkness and repose settled once more upon the lonely plains of Ares Vallis. The feeble glow of twilight faded to dark and muddy brown; the brown gave way to violet, the violet to eerie blue (the blue color was produced by the same process of forward-scattering of blue light by fine dust particles that on rare occasions causes blue moons and blue suns on Earth). The fading blue of the sunset gave way at last to the vast and unfathomable solitude of the night, as the landscape became a realm dark and mysterious, haunted, perhaps, by howling Martian ghosts. A realm just as mysterious to us now as the red dot of the planet wandering in the inky skies of night was to humanity's first skywatchers.

Copyright © 2001 William Sheehan and Stephen James O'Meara. All rights reserved.