Category Dreams of Other Worlds

The comet cloud defines the outer edge of the Solar System, ex­tending a thousand times further than the planets from the Sun, and a significant fraction of the distance to the nearest stars. It had always been presumed that these small bodies were frozen rel­ics from the formation epoch, essentially unchanged in 4.5 billion years. The expectation was that comets would be made of dust from a previous generation of stars, or pre-solar grains. That’s why the mission was named Stardust.

Instead, something quite unexpected was found: fire and ice.24 Comets contain ice that formed in the frigid zone beyond Nep­tune, but the bulk of a comet’s mass is rock, and that rocky ma­terial seems to have formed under conditions hot enough to va­porize bricks. The comet particles embedded in Stardust’s aerogel included two ingredients that are found in meteorites, debris from asteroids that formed between the orbits of Mars and Jupiter. One is chondrules, rounded droplets of rock found in many primitive meteorites that melted and quickly cooled as they orbited the Sun. The other is a much rarer mineral called a Calcium Aluminum Inclusion, irregular white particles of very unusual chemical com­position that can only form at very high temperature. The Stardust team puzzled over this discovery, but the implication seemed clear. Matter that formed readily in the inner Solar System was some­how transported to the edge of the young Solar System where the comets formed. Wild 2 does contain grains of pre-solar stardust material, but they’re very rare. Comets are not made of material left over from other stars; they’re made mostly from material that formed close to the Sun. As such, they provide insights into how planets and moons were built 4.5 billion years ago.

Stardust’s primary contribution to planetary science is verifi­cation of the idea that there was extensive radial mixing of ma­terial in the solar nebula just as the Solar System was forming. Also, comets are more physically diverse and variegated than any­one imagined. Each of the comets where we’ve got up close and personal—Halley, Borrelly, Tempel 1, and Wild 2—have different shapes, surface textures and features, and levels of activity. Com­ets may be modest in size compared to planets and moons, but their role in planetary systems is anything but modest. They deliver water and organic building blocks of life to terrestrial planets, and as we know from studies of our geological record, when they hit a planet they can decisively alter the history of ecosystems and even the entire biosphere. It’s appropriate that all human cultures have been fascinated by comets, and viewed them as harbingers of life and death.

To learn more requires an increase in our modest amount of di­rect information. So it was very exciting when Stardust was given a new lease on life after its sample return. In 2007, NASA approved the New Exploration of Tempel 1 (NExT) mission, a return to the site of the Deep Impact mission in 2005.25 Stardust had con­served just enough hydrazine fuel to make a second journey to a comet, and the goal was to observe the impact made by the ear­lier spacecraft and see any changes on Tempel 1 caused by its last close approach to the Sun (Deep Impact’s cameras were blinded by dust released from the impact). In early 2010, a controlled burn ensured Stardust would approach at the optimal speed. The flyby was challenging because the camera had to catch the correct side of the tumbling comet nucleus while coming up on it at 7 miles per second. Appropriately, our first second date with a comet took place on Valentine’s Day, 2011. Stardust showed that the 500-foot wide crater created in 2005 was indistinct and had a mound in the center, indicating that much of the ejected material had fallen straight back down in the weak comet gravity. The results indi­cated that the comet nucleus was fragile and only weakly held together.

All good things must come to an end. For Stardust the end came on March 24, 2011, when mission controllers deliberately ordered a “burn to completion.” They told the spacecraft, which was al­ready running on fumes, to fire its main engines until there was no fuel left. NASA used the final burn to refine fuel consumption models for these kinds of engines, ensuring that Stardust gave use­ful data right up to its last gasp. The next day, Project Manager Tim Larson put the spacecraft into a state called safe mode, turned off the transmitter, and walked away from the console. Mission accomplished.

Michael Perryman has dubbed the Hipparcos mission astronomy’s equivalent of the Human Genome Project.22 Perryman explains that as astronomers more accurately map the location, velocity, and vector of stars in our galaxy we can understand the age and morphology of the Milky Way, how our galaxy has evolved in the past, and what the future holds for our Solar System and the gal­axy. For instance, the Hipparcos mission has contributed to our better understanding of the galaxy’s current structure. We know our galaxy is not a perfect spiral, but is instead a barred spiral that’s warped so that the limbs at one end curve up and at the other bend down (figure 8.3). Another major contribution of Hip – parcos, for astronomers and popular audiences, is that the mission improved the estimates of distances to stars harboring exoplanets. In this way, it has crystallized our sense of the growing number of distant worlds in space. We’ve seen in the earlier chapters on the Solar System that planets and moons are potential abodes for life. As the Human Genome is a project to map the underlying structure of terrestrial life, so Hipparcos is a tool to help astrono­mers map plausible sites for extraterrestrial life. The search for life beyond the Earth is a foundational scientific pursuit, and it has attracted attention from some unlikely quarters.

The Vatican has maintained an observatory over the centuries in order to officially determine dates of the calendar year; the Gre­gorian calendar has been used in the Western world since 1582. However, astronomers of the Vatican Observatory more recently

have been focusing on other concerns. In November 2009, Pope Benedict XVI called leading astronomers, astrobiologists, and cos – mologists to Vatican City to spend a week presenting recent find­ings regarding exoplanets orbiting nearby stars and to discuss the possibilities of intelligent life in those star systems.

Of the Vatican’s interest in exobiology, science reporter Marc Kaufman noted: “Just as the Copernican revolution forced us to understand that Earth is not the center of the universe, the logic of astrobiologists points in a similarly unsettling direction: to the likelihood that we are not alone, and perhaps that we are not even the most advanced creatures in the universe. This. . . may conflict with the stories we tell about who and what we are.”23 During the five-day meeting scientists addressed subjects such as the origins of life, extremophiles and their habitats, the likelihood of such life thriving on moons in the outer solar system, and whether life’s bio­signatures could be detected on exoplanets.

As yet, exoplanets are mostly gas giants with little chance of life on them, but as the detection limit has reached Earth mass with NASA’s Kepler satellite, research spurs scientists, philosophers, and theologians alike to contemplate the implications for our place in the universe. “The questions of life’s origins and of whether life exists elsewhere in the universe. . . deserve serious consideration,” explained Jose Gabriel Funes, a Jesuit priest who is also the di­rector of the Vatican Observatory. Co-author Chris Impey, who presented a paper at the meeting and co-edited the written pro – ceedings,24 comments: “Both science and religion posit life as a special outcome of a vast and mostly inhospitable universe. There is a rich middle ground for dialog between the practitioners of as – trobiology and those who seek to understand the meaning of our existence in a biological universe.”25 Reporter David Ariel, who also covered the meeting, aptly noted, “The Church of Rome’s views have shifted radically since Italian philosopher Giordano Bruno was burned at the stake as a heretic in 1600 for speculating, among other ideas, that other worlds could be inhabited.”26

For the moment, most of the vast inventory of stars remains out of reach. But several hundred relatively nearby stars are known to have planets, and Hipparcos has been an essential tool in measur­ing their distances. These new and potentially habitable worlds range from a dozen to a few hundred light-years away. Spanning the entire galaxy, one estimate is of 8 billion terrestrial habitable worlds around Sun-l ike stars, each of which has the potential to host life.27 This number is the same order of magnitude of the number of base pairs derived from the Human Genome Project, making literal the analogy of a vast mapping project to parse life in the Milky Way.

The grip that Mars holds on the popular imagination is grounded in the question of whether Mars ever was, or is, alive. The Viking landers were explicitly designed to search for evidence of life in the Martian regolith or perhaps in the planet’s geological past.41

The short section of the mission science summary on life detection is worth quoting in full because of the surprising ambiguity of the wording: “Three experiments were conducted to test directly for life on Mars. The tests revealed a surprisingly chemically ac­tive surface—very likely oxidizing. All experiments yielded results, but these are subject to wide interpretation. No conclusions were reached concerning the existence of life on Mars.”42

The Viking landers packed a substantial scientific punch in their 200-pound payloads. Power came from a plutonium-238 radioiso­tope thermal generator, eking out 30 Watts of continuous power. Using late 1970s technology, Viking’s data capabilities were even more feeble; the data recorder on each lander could only store 8 Mbytes at a time, thousands of times less than the average mem­ory stick. The landers had cameras that could take 360-degree panoramic images. They had seismometers and instruments to test magnetic fields. They had meteorology booms that measured temperature, pressure, and wind speed and direction. Most impor­tantly, they had robotic arms that could scoop up soil samples and deposit them into temperature-controlled and sealed containers on each spacecraft (figure 2.3).

The biological package contained four instruments. A gas chro­matograph and mass spectrometer heated soil samples and mea­sured the molecular weight of each component of the vapor re­leased, down to a concentration of a few parts per billion. The instrument found no significant levels of organic, or carbon-based, molecules. Mars soil had even less carbon than the lifeless soils tested on the Moon by the Apollo missions. This seemed to be prima facie evidence against life. The gas exchange instrument added nutrients, and then water, to a soil sample, and then looked for changes in the concentration of gases such as oxygen and methane in the sealed chamber. The hypothesis was that a living organism would process one of the gases. The result was negative. The pyrolytic release experiment created an “atmosphere” in the chamber using radioactive carbon, in the hope that a photosyn­thetic organism would incorporate the carbon the way plants do on Earth. After several days of incubation under an artificial Sun (in this case, a xenon arc lamp), the sample was baked at a high temperature to see if any of the radioactive carbon had been con­verted into biomass. The results were also negative.43

The only wild card was the labeled release experiment. A sample of soil had nutrients dissolved in water added to it, and the nutri­ents were “tagged” with radioactive carbon, which was once again used as a tracer. To the surprise of the instrument team, radioactive carbon dioxide was detected in the air above the samples, suggest­ing that microbes had metabolized one or more of the ingredients. The same result was seen in both Viking landers. However, when the experiment was repeated a week later, the air was free of radio­active carbon. The data were declared inconclusive.44

Overall, the results were disheartening for those who hoped that Mars might be a living world. Terrestrial life is based on com­plex molecules with a carbon backbone—organic ingredients like carbohydrates, proteins, nucleic acids, and lipids. While organic does not mean biological, all life on Earth is carbon-based and so is made of organic ingredients. The Viking experiments detected virtually no organic compounds in the Martian regolith. This was somewhat surprising, since they are fairly common on small Solar System bodies like comets, asteroids, and meteorites. With no organic material, the biological experiments would have been doomed to failure, since their aim was to detect a metabolism that could incorporate carbon from the atmosphere, as microbes on Earth do. The lander could only gather samples from the top few centimeters of the regolith, and that layer is blasted with ultravio­let radiation and cosmic rays from space (Mars has no protective layer of ozone). The surface is strongly oxidizing, as the “rusty” red color of iron oxide indicates.45 So the conventional interpreta­tion is that activity seen in the experiments was caused by chemical reactions involving oxidizing molecules in the soil, with no bio­logical explanation required.

Former NASA historian Steven Dick recounts the claims, first, by American physicist Nikola Tesla and a few decades later by Ital­ian innovator of wireless telegraphy Guglielmo Marconi that they had received wireless signals from Mars. Collier’s Weekly in 1901 reported that Tesla was convinced he was “the first to hear the greeting of one planet to another” and that the supposed radio signals were most likely from Mars.40 By the early 1920s, Marconi apparently repeatedly attempted to receive short-wave radio mes­sages from Mars. Dick writes: “Marconi’s interest in interplanetary communication peaked during a trip from Southampton, England, to New York City aboard [his yacht] the Electra from May 23 to June 16, 1922. The New York Times noted that Marconi ‘spent the time crossing the Atlantic performing many electrical experiments, principally by listening for signals from Mars.’”41 William Sheehan and Steven O’Meara report that two years later, when Mars was at its closest since 1804, “radio stations around the world were urged to simultaneously cease transmissions at specified intervals, so as not to interfere with any attempts by Mars to radio the Earth.”42 Even the U. S. Navy, notes Dick, monitored radio transmissions for potential Martian messages.

News reportage of such events may be the reason so many as­sumed a Martian invasion in October 1938 when a radio broad­cast, seemingly interrupting The Mercury Theatre on the Air pro­gramming, indicated Martians had landed at the Wilmarth farm in Grover’s Mill, New Jersey. Orson Welles’s radio presentation, discussed in chapter 1, is one of the most singular events in radio history. As Bruce Lenthall points out, broadcast radio was a major news source in the early twentieth century. For audiences in the United States, Lenthall claims: “Radio ownership more than dou­bled in the 1930’s, from about 40% of families at the decade’s start to nearly 90% ten years later. By 1940 more families had radios than had cars, telephones, electricity, or plumbing.”43 Len – thall estimates that 6 million people actually heard the radio play that Howard Koch adapted from H. G. Wells’s The War of the Worlds (1898) and of that number, approximately 1 million listen­ers literally thought Martians had landed. Subsequent to Tesla and Marconi’s independent claims of having detected radio messages from Mars, some contemplated whether Mars was inhabited by a sophisticated civilization whose radio signals the authorities had been listening for. T. S. Eliot noted in the poem “The Dry Salvages” (1941) that popular commentary on means to “communicate with Mars” was one of the “usual [p]astimes.”44

Perhaps interplanetary travel from Mars didn’t seem entirely fantastic given that the twentieth century emerged as an unprec­edented era of global travel. In the first few decades of the cen­tury, train and ocean liner travel expanded exponentially, while motoring, and flight, first achieved in 1903, quickly developed as everyday experiences. Large numbers of people became mobile in ways that just a few years prior had been extremely arduous, or even unimaginable. With the serious development of rocketry in the 1920s and the genesis of the American Interplanetary Society in 1930, followed by the British Interplanetary Society in 1933, travel between Mars and Earth may have finally seemed possible and well within the realm of the imagination.

With large numbers for the first time traversing multiple time zones in a day, British author Virginia Woolf suggested that people began to internalize in finer detail Earth’s global topography. She claimed that her own travel experiences afforded her a better sense of Earth’s surface so that she could easily imagine and rehearse its large topographical contours. When she and her husband Leonard bought a used automobile from sales of her novel To the Light­house (1927), Woolf observed that motoring had been “a great opening up in our lives” that allowed her to “expand that curious thing, the map of the world in ones [sic] mind.”45 As early as 1909, while traveling through Italy, Woolf recorded in her diary: “It is strange how one begins to hold a globe in ones [sic] head; I can travel from Florence to Fitzroy Square [in London] on solid land all the time.”46 She meant that she could easily envision the con­tours of Earth’s globe, as if she could turn the Earth around in her fingers and trace its continents, mountains, islands, and shorelines.

Coincident with the increase in global travel in the first few decades of the twentieth century was the construction of a new generation of large telescopes like the 100-inch at Mount Wilson Observatory. News reports covering astronomical discoveries fre­quently appeared in newspapers and weeklies, so much so that references to lunar and planetary landscapes were taken up in advertising and in the common parlance. In January 1926, while motoring through Persia, Woolf’s close friend and celebrated au­thor Vita Sackville-West wrote to Virginia describing the hills near Thebes in Egypt as a “mountains-of-the-moon landscape.” A year later, in March 1927, Sackville-West was again touring in Persia and wrote to Woolf from Tehran, “[T]here is one little asteroid, called Ceres I think, only four miles across, the same size as the principality of Monaco, on which I have often thought I should like to live, revolving in lonely state round the Sun.”47 Presum­ably Sackville-West had read a news report regarding our larg­est asteroid, currently estimated at 975 kilometers or 606 miles in diameter. Prompted perhaps by Vita’s evocation of other plan­etary landscapes, Woolf in September that year noted in her diary: “What I like. . . about motoring is the sense it gives one of light­ing accidentally, like a voyager who touches another planet with the tip of his toe, upon scenes which would have gone on, have always gone on, will go on, unrecorded, save for this chance glimpse.”48 Decades later, Spirit and Opportunity offered our first extensive, close-up glimpse of Martian geological processes that have gone unrecorded for eons, and what we have found allows us to more fully internalize the red planet’s landscapes and its geomorphology.

Saturn’s ring system became etched in the human imagination some time after the mid-1600s when Dutch mathematician and astronomer Christiaan Huygens first illustrated the rings.4 He also was first to identify its moon Titan.5 However, in recent memory, it was Chesley Bonestell’s renderings of Saturn that brought the planet into the public purview. By training Bonestell was an ar­chitect, but he’s probably best known for the stunning paintings of Saturn published in May 1944 in LIFE magazine. The most famous and striking of these is titled “Saturn as Seen from Titan.” Readers were amazed by the suite of paintings that appeared in the May 29 issue, perhaps in part because the issue was largely dedicated to news and advertisements related to the war effort. Among 130 pages of news reports on American soldiers in Europe, war-themed ads, and largely black and white photos of troops, Bonestell’s realistic and full color renderings of Saturn lumbering in the sky of Titan transported readers into exotic and delightful planetary vistas.

Bonestell recounted how in 1905 he was inspired to paint the spectacularly ringed planet: “When I was seventeen, an important event occurred in determining my future career, although I little suspected it then.” Having once hiked with a friend to Lick Ob­servatory at the summit of Mount Hamilton, Bonestell recalled, “That night I saw for the first time the Moon through the 36-inch refractor, but most impressive and beautiful was Saturn through the 12-inch refractor. As soon as I got home I painted a picture of Saturn.”6 Although that painting was lost in the fires caused by the Great Earthquake of 1906 in San Francisco, Saturn had made a lasting impression on the young artist.

When nearly forty years later Bonestell submitted the series of paintings that included “Saturn as Seen from Titan” to the edi­tors of LIFE, they quickly agreed to publish them. Ron Miller and Frederick Durant explain: “No one had ever before seen such paintings—they looked exactly like snapshots taken by a National Geographic photographer (figure 5.2). For the first time, renderings of the planets made them look like real places and not mere ‘art­ists’ impressions.”7 Miller and Durant write that Carl Sagan main­tained “he didn’t know what other worlds looked like until he saw Bonestell’s paintings,” while science fiction writer Arthur C. Clarke suggested that in a sense Bonestell had walked on the Moon long before Neil Armstrong and reportedly quipped, “Tranquility Base was established over Bonestell’s tracks and discarded squeezed-out paint tubes.”8

Wyn Wachhorst has explored why Bonestell’s famous painting “Saturn as Seen from Titan” is so compelling: “Since Titan is the only satellite in the Solar System with an atmosphere, the giant Saturn looms low in a dark blue sky like an alien ship, a thin,

gleaming crescent bisected by the glowing edge of its rings, afloat between jagged cliffs that jut from a frozen sea. . . . A hint of dawn lights the far horizon; and beyond a lofty pinnacle, out under the glow of the great crescent, lies a distant patch of noonday plain.” Among the other Bonestell paintings in the LIFE layout were imagined scenes of Saturn from its moons Phoebe, Iapetus, Mimas, and Dione. One depicted Saturn’s rings passing overhead from the perspective of the planet’s cloud tops. Wachhorst explains that the suite of paintings was intended to offer varying views of Saturn on approach from its outer moons.9

Though he painted numerous panoramas of planetary land­scapes ranging from Mercury to Pluto, Bonestell was aesthetically captivated by Saturn, a subject he repeatedly returned to through­out his life. He painted numerous iterations of Saturn from Titan and its other moons. In 1949, for instance, he completed paintings of Saturn from Dione, in which the full body of Saturn is glimpsed from the mouth of a cave. His panorama for the Griffith Observa­tory, completed in 1959, featured a prescient vision of the frozen landscape of Titan with Saturn low on the horizon. Throughout the 1960s, Bonestell reworked different views of Saturn from Titan, changing the lighting or subtly altering Titan’s landscape. In 1972, he completed two separate paintings of Saturn from Iapetus, as well as a painting of Saturn from Enceladus for Arthur C. Clarke’s book Beyond Jupiter. Bonestell returned to the subject of Saturn again and again, in various configurations, settings, and lighting.

All this from an architect whose work included contributions to the design of San Francisco’s Golden Gate Bridge, the layout of the well-known Seventeen Mile Drive at Pebble Beach in Monterey, the Eagle gargoyles and art deco facade of the iconic Chrysler building in New York City, and the design of buildings for the Cal­ifornia Institute of Technology in Pasadena.10 He became the high­est paid special effects artist in Hollywood, working on films like The Hunchback of Notre Dame (1939) and Citizen Kane (1941). Bonestell’s turn to space art played out in popular magazines such as LIFE and Collier’s, and in films like Destination Moon (1950), When Worlds Collide (1951), and Conquest of Space (1955). His work inspired generations to imagine the stark and beautiful plan­etary landscapes in our Solar System and in far-flung star systems of the galaxy. “Bonestell brought the edge of infinity out of the abstract and into the realm of direct experience,” comments Wach – horst.11 His paintings suggested planetary vistas human eyes had not yet seen, and sometimes included figures of astronauts dwarfed by a vast surrounding terrain. This was true of his painting of Sat­urn from Mimas in the LIFE layout, and of an iteration of Saturn from Titan completed in 1969, which situates three tiny astronauts on a cliff, looking out at a fully lit Saturn as one astronaut points to the rings.

Bonestell apparently learned the technique for rendering his realistic paintings from science illustrator Scriven Bolton while working in the 1920s at the Illustrated London News. Bolton, a Fellow of the Royal Astronomical Society, constructed plaster cast models of planetary landscapes, photographed them, and then painted in planets and stars.12 Working from this technique, Bon – estell would project light onto his plaster landscapes to get a sense of how sunlight and shadows might fall across terrain, and then painted based on photos of these lit scenes. This resulted in land­scapes that seemed reachable and tangible. His widely celebrated renderings “invited viewers into the possible planetary landscapes that exist on moons of the outer solar system. In Bonestell’s depic­tion, Titan’s landscape resembles that of the American Southwest or perhaps the craggy cliffs of the Rocky Mountains in winter. The deep blue of the sky recalls that of Earth.”13 Though such spectacular views from Titan may be unlikely given the moon’s hazy, methane-rich atmosphere, Bonestell was prescient in suggest­ing the sublime experience of standing on the shore of one world to view another in close proximity.

Wyn Wachhorst contends that Bonestell’s art purposely evokes “a kind of cosmic shoreline, a composite of stark and eerie beaches on the near edge of the starry deep,” and that the seashore is the “root metaphor” of Bonestell’s art, meant to evoke Earth’s hori­zon as the shoreline between Earth and outer space. Bonestell’s art reminds us that from Earth we stand “on the shore of the cosmic ocean, riding our wisp of blue and white like mites on a floating leaf, in the whorls and eddies of a great galactic reef.”14 Carl Sagan wrote in Pale Blue Dot that humans have from time immemorial been innately drawn to the horizon. Ancient Egyptians identified their god Horus with the Sun on the horizon and with the planet Saturn, thought to represent Horus the Bull.15 The Great Sphinx in Giza apparently was associated with Horus and specifically is ori­ented toward, and draws the eye to, the Eastern horizon.16 With­out question, Bonestell’s work inspired Sagan, whose first episode of the PBS series Cosmos was titled “The Shores of the Cosmic Ocean.”

In August 1929, the New York Times science section ran an article titled “The Star Stuff that is Man.” Astronomer Harlow Shapley had been popularizing the point that humans are the mere by­products of stars. In a radio talk series a few years earlier produced by the Harvard College Observatory, Shapley pointed out that “we are made out of the same materials that constitute the stars.”26 In the Times article, Shapley similarly observed: “We are made of the same stuff as the stars, so when we study astronomy we are in a way only investigating our remote ancestry and our place in the universe of star stuff. Our very bodies consist of the same chemical

elements found in the most distant nebulae.”27 At the time, some were disconcerted to think that humans might be little more than the product of fission and fusion occurring in stars.28

Cosmologists have good evidence that all the hydrogen and most of the helium in the universe have existed since close to the time of the big bang. Hydrogen, the simplest element, is truly primor­dial, while most of the helium was formed soon after in a process known as primordial nucleosynthesis or big bang nucleosynthe­sis. “The primordial nuclei of the matter constituting the universe were formed in the first three minutes,” explains CERN theoretical physicist Luis Alvarez Gaume. “The cosmic oven produced a num­ber of nuclei, made up of about 75% hydrogen and 24% helium. Small amounts of deuterium, tritium, lithium and beryllium were also produced, but hardly any of the other atoms that make up our bodies and the matter around us: carbon, nitrogen, oxygen, sili­con, phosphorus, calcium, magnesium, iron, etc. All of these were formed in the cosmic ovens of subsequent generations of stars. As Carl Sagan put it, we are just stardust, remains of dead stars.”29

The Oxford English Dictionary lists a definition for the term stardust as “fine particles supposed to fall upon the Earth from space; ‘cosmic dust.’” The OED likewise cites one published com­mentary from 1879 that claimed “the very star-dust which falls from outer space forms an appreciable part” of the mud accumu­lated on the ocean floor.30 In fact, the estimate for interplanetary dust particles swept up each year by the surface of the Earth as it churns through space is a substantial 10,000 tons, but that’s a tiny fraction of the material added to the seafloor. The most pristine reservoirs of stardust or interplanetary dust are comets. Having formed early in the life of our Solar System approximately 10 mil­lion years after the Sun’s protoplanetary disc stabilized, comets spend most of their time in the cold extreme outskirts of the Solar System. As a result, comet nuclei are largely preserved from heat­ing, melting, and collisions with other planetary bodies. The ice composition and dust grains of comets reveal elements present in the Solar System’s primordial past.

Nuclear fusion reactions in stars create heavier elements by pro­cessing or fusing together lighter atomic nuclei. The base of the fusion “pyramid” is the fusion of hydrogen into helium, which oc­curs in the Sun and low-mass stars. Stars approximately the mass of our Sun can also synthesize helium, carbon, and oxygen, but thereafter, they never reach a dense enough or hot enough state to fuse yet heavier elements. Massive stars have the ability to fuse their hydrogen to produce helium, carbon, and oxygen, but then progress in successive stages of evolution to neon, magnesium, sili­con, sulfur, nickel, and iron. Iron is the most stable element, so typ­ically the fusion chain stops there. Still heavier elements are forged two ways: by the slow capture of neutrons in the atmospheres of massive stars and more rapidly by stellar explosions known as novae and supernovae.31 Since rarer, larger mass stars are required to generate the heaviest elements, these elements are cosmically scarce compared to the light elements hydrogen and helium.

The first generation of stars probably formed approximately 200-300 million years after the big bang.32 According to cur­rent theoretical ideas, none of those stars still exist and certainly none have yet been found. They have since exploded in novae or massive supernovae, collapsed into black holes, or have oth­erwise expended their fuel and burned out. The oldest stars cur­rently are generations removed from the first stars that shone in the universe.33 Astronomers estimate that approximately twenty – five novae occur each year in an average galaxy generating inter­stellar dust enriched with heavier metals. From the clouds of gas, dust, carbon, silicon, and metals produced by repeated generations of novae and supernovae, new stars and their attendant planets are born.

A nova is a sudden brightening of a star believed to occur when the outer layers of a star are pulled by gravity onto the surface of a companion white dwarf. Pressure due to the mass that accumu­lates on the white dwarf causes nuclear fusion of hydrogen into he­lium at its surface that in turn blows material off the surface of the white dwarf. One type of supernova is an extreme form of a nova, where mass acquired from a young companion star causes a white dwarf to begin carbon fusion and explosively detonate. Another type, a core collapse supernova, occurs when a single massive star exhausts its nuclear fuel and suffers a core collapse followed by explosive detonation. In comparison to a nova, a supernova will produce a million times the energy and can for weeks shine as brightly as all the stars of an entire galaxy. In the case of the Milky Way, that’s equivalent to about 400 billion Suns. A supernova is a prodigious alchemical event; one that detonated in a nearby gal­axy in 1987 generated enough stardust to build 200,000 Earths.34

The Milky Way formed approximately 10 billion years ago, but our Sun is only 4.5 billion years old. The Sun and its neighbor­ing stars were likely born in a nearby region of dense, coalescing interstellar gas and dust. Isotopic studies of meteorite samples in­dicate that our Sun formed from the detritus of a massive super­nova explosion in nearby space roughly 1 million years prior to the formation of the protoplanetary disc that became our Solar System.35 We know this simply by the presence of metals that our Sun could not have produced, such as the iron that makes up the Earth’s core. Other than hydrogen, which is primordial and dates back to cosmic genesis, almost all the other atoms in the universe have been recycled, silent witnesses to amazing trips through fiery cauldrons and into frigid space, some experiencing multiple adven­tures. Unfortunately, no atom bears the imprint of its particular passages through the core of a star; astronomers can only describe the origins statistically.

To judge the scientific contributions of Hipparcos, we start by rec­ognizing that measuring the positions of stars is both fundamental and unglamorous. It’s fundamental because it’s the key to measur­ing the physical properties of celestial objects. Positions are the keys to the trigonometric determination of distance, and distance is needed to calculate the size, mass, and intrinsic brightness of any planet, star, or galaxy.28 Without distances we’re stuck with the appearance of stars in the sky, and a star that’s far away and lumi­nous can appear to be the same brightness as a star that’s nearby and dim. That ambiguity is fatal to any reliable understanding of the denizens of the night sky. It’s unglamorous because measuring a position is the simplest and most obvious way to characterize a star: there’s no image, just two angles to identify a unique spot on the sky, with no units. Needless to say, the people who do such prosaic work don’t always get their due.

It was not always that way. On the spinning Earth, the measure­ment of star positions is critical for keeping time and navigating. Early cultures noticed and tracked star positions as if their lives depended on it, which they did! In the third century BC, Timo – charis and Aristillus produced the first star catalog in the Western world while working for the Great Library at Alexandria. About a century later, Hipparchus extended their work, generating a cata­log with 850 star positions. He also divided the stars into intervals of logarithmic brightness that form the basis for a system that as­tronomers still use. This was a natural way to classify brightness since the eye has a nonlinear or logarithmic response to light. Ptol­emy increased the catalog to 1,022 stars.29 These star catalogs are among the most impressive intellectual achievements of antiquity; later generations of admiring astronomers called Ptolemy’s stellar compendium Almagest, which means “greatest” in Arabic.30

As in many other aspects of astronomy, the torch for mapping stars was then taken up by Arabs for a millennium. Around AD 964, the Persian Abd al-Rahman al-Sufi wrote his Book of Fixed Stars, which depicted the constellations in glorious, natural color. Al-Sufi was the first to catalog the Large Magellanic Cloud and the Andromeda Nebula, two distant star systems whose true na­ture would not be fully understood until the 1930s. The pinnacle of pre-telescopic observations was reached by Tycho Brahe in the sixteenth century. Through relentless attention to detail and the control of systematic errors, he improved on the positional er­rors of earlier catalogs by a factor of fifty. His reputation didn’t suffer from doing these mundane measurements; Brahe was cel­ebrated in his lifetime and is considered the greatest observer before Galileo.

The big prize in astrometry was its use to measure the distance to a star. Stars are so far away that the apparent seasonal shift of a nearby star with respect to more distant stars—the effect called parallax—was not observable for the first two centuries of use of the telescope. Friedrich Bessel won the race to detect parallax by showing that 61 Cygni, one of the closest stars, was nearly 10 light – years away, or a staggering 60 trillion miles.31 Bessel didn’t have a university education, but his meticulous calculations elevated him to fame as one of the most noted scientists and mathematicians of the nineteenth century. The parallax shift is extremely subtle, and far more difficult to detect than the large-scale migration of constellations through the night sky as the Earth spins on its axis and orbits the Sun (figure 8.4). Almost all stars have parallax shifts over the course of a year of about one second of arc or less, and most stars visible to the naked eye have parallax shifts smaller than 0.1 second of arc. For comparison, each of the letters on an eye chart that defines 20/20 vision spans an angle of five minutes of arc, a 3,000 times larger angle.

Thereafter astrometry lapsed into the status of a worthy but dull aspect of astronomy. In part, this was because it was so chal­lenging to measure parallax; in the half century after Bessel’s mea­surement, new star distances were only added at the rate of about one per year. Through the twentieth century, photographic plates made it easier to capture and measure star positions, and Her – schel’s project to map the Milky Way galaxy was carried out by researchers in Europe and the United States. But the air blurs out the light of all stars to about 1 arc second in diameter (1/3600 of a degree), larger than the size of the angle that had to be measured to detect parallax. Refraction and telescope flexure also complicate a parallax measurement. Astronomers were bumping up against the limitations of the atmosphere and the only solution was to go into the pristine environment of space.

Gil Levin has never wavered. The principal investigator of Viking’s labeled release experiment is now approaching ninety, but he’s very active and stays current with research on Mars. Levin had an unusual career path, starting as a sanitation engineer before join­ing NASA. In addition to authoring 120 scientific papers, he owns fifty patents for items ranging from artificial sweeteners to thera­peutic drugs.46 Levin insists that at two locations on Mars, and in seven out of nine tests, his experiment detected biological activity. No purely chemical reactions have been identified that would fully reproduce the labeled release results, which explains the cautious and equivocal wording of the science mission summary.

Levin is in the minority, but he’s not alone in believing the Vi­king results bear revisiting. Rafael Navarro-Gonzalez and his team went to the highest and driest parts of the world, places like the Atacama Desert and Antarctic dry valleys, and duplicated the tests that Viking did more than thirty-five years ago.47 In northern Chile, they found arid soils with levels of organic material that would have been undetectable by the Viking instruments, yet there were bacteria in the soil. In other words, Viking was not sensitive enough to detect either organics or life in terrestrial locations that are the closest analogs to Mars.48 The team speculated that the organic material on Mars might have been too stable to turn into a gas, even at the blistering temperature of 500°C (932°F) reached inside the oven of the Viking experiment. They also noted that iron in the soil might oxidize the organic material and prevent its detec­tion by the mass spectrometer. This would account for the carbon dioxide that was detected in the experiments.

Mars is an alien chemical and physical environment, so it makes sense to think “outside the box” when considering how biology might operate there. If we only look for life as we know it, that’s all we’ll be able to detect and we may miss life with a different biochemical basis. Dirk Schulze-Makuch and Joup Houtkooper have speculated that Mars might be home to microbes that use water and hydrogen peroxide as the basis for their metabolism.49 Hydrogen peroxide is a toxic household disinfectant and seems implausible as a basis for life, but the researchers note that it’s more life-friendly under the extreme conditions typical of Mars. It attracts water and when mixed with water freezes at -57°C, yet it doesn’t form cell-destroying ice crystals at even lower tem­peratures. Hydrogen peroxide is tolerated by many terrestrial microbes and is the basis of the metabolism for Acetobacter per – oxidans. It’s used as a defensive spray by the bombardier beetle and even performs useful functions in the cells of some mammals. As Schulze-Makuch pointed out in 2007, “We can be absolutely wrong, and there might not be organisms like that at all. But it’s a consistent explanation that would explain the Viking results. . . . If the hypothesis is true, it would mean we killed the Martian mi­crobes during our first extraterrestrial contact, by drowning—due to ignorance.”50

Another speculation was spurred by the discovery of perchlo­rate in a Martian polar region by the Phoenix lander in 2008.51 Phoenix carried out a wet chemical analysis of Martian soil, find­ing it to be alkaline and low in the type of salts found on Earth, but it had enough perchlorate to act as antifreeze and allow the soil to hold liquids for short periods during the summer. Perchlorate is strongly oxidizing and so generally considered to be toxic for life, but some microbes metabolize it.52 In 2010, the Viking results were reinterpreted in the light of the Phoenix discovery, giving support to Gil Levin’s lonely position.53 When perchlorate was added to Atacama Desert soils and analyzed in the manner of the Viking samples, it released chlorine compounds. When these were seen in the Viking experiments back in the 1970s, they were presumed to be cleaning fluid contaminants from Earth. But if perchlorate is present in the mid-latitude Martian soil, it would explain the data. Since perchlorate becomes a strong oxidant when heated, it would have destroyed any organics and so explain why Viking didn’t detect any. It’s remarkable that the Viking legacy continues to provide so many surprises and unanswered questions.

A leitmotiv in the reinterpretation of the Viking biological ex­periments is the amazing range of life on Earth—ihere are mi­crobes that can tolerate or thrive in conditions that would be fatal to plants and animals. Collectively, forms of life found in physi­cally extreme conditions are called extremophiles. The envelope of habitability is much larger than we thought in the 1970s. There are microbes than can live below the freezing point of water and with toxic and alkaline conditions as seen on Mars. Even high doses of ultraviolet radiation are not an impediment to life, since mi­crobes can survive conditions in the upper stratosphere that match the radiation environment of Mars.54 In science, proof is the gold standard, but it’s a very high bar to clear, usually requiring copious amounts of evidence. While Viking didn’t find life on Mars, it was unable to prove the converse hypothesis, that biology is absent. It’s premature to declare Mars dead. Surprisingly, this harsh planet inspired a new way of thinking about life on Earth.

By the late 1960s and early 1970s, the Moon landings became the grand travel narratives of human history. Mars has subsequently emerged as the next great travel destination. There are multiple reasons why the planet so intrigues us. Through MER and other missions, Mars has become familiar and tangible. While our oceans obscure 70 percent of Earth’s topography, on Mars we can visually survey the entire surface, so that planetary scientists to some extent have a better understanding of its surface than of our home planet. Global Surveyor’s multi-colored elevation maps, for instance, re­veal that Mars’s northern hemisphere lies low in elevation com­pared to the southern highlands. Far fewer impact craters in the northern hemisphere may indicate that at one time this region was covered by a primordial sea. Prominent on the Tharsis bulge is the tallest volcano in the Solar System, Olympus Mons (figure 3.5), accompanied by three nearby shield volcanoes. Looking toward Mars’s north pole, Arsia Mons is the most southern, then Pavonis Mons, and finally Ascraeus Mons. These adjacent volcanoes occur in a chain-l ike sequence reminiscent of the tectonic plate move­ment that produced the Hawaiian Islands, which raises the ques­tion of whether geothermal processes are still at work on Mars.

Moreover, extremophiles may thrive in possible underground water reserves, deep in the crust, or at the Martian polar caps. Re-

searchers have detected methane in the atmosphere, which could suggest active volcanism or a more exciting, possible microbial source.49 Despite Mars’s tenuous atmosphere, images by the Mars Reconnaissance Orbiter indicate groundwater sporadically breaks through the surface to briefly flow down steeply sloped terrain, and data from the 2008 Phoenix Lander indicate its soil is simi­lar in acidity to Terran soil. Added to this are recent findings that ancient fossilized life on Earth, dating to more than 3.4 billion years ago, metabolized and lived on sulfur instead of oxygen, as Earth’s atmosphere was not then oxygen-rich.50 All of this suggests that Mars could harbor life deep within its ruddy dirt. If the Mars

Science Laboratory’s rover Curiosity finds evidence of ancient or extant extremophiles, we might discover how life emerged on the early Earth.

The other tremendous appeal of Mars is that it is the only planet astronauts could travel to, and explore, in the near future. Allud­ing to the passion with which we dream of stepping foot on worlds beyond our Moon, Ray Bradbury has observed, “We know not why we thread an architecture of travel in a fiery path across a winter space and warm far worlds with our breath.”51 Perhaps we do so simply because we survive on our own relatively cold planet only as a result of technology. Anthropologist Ben Finney points out that we inhabit the globe, from pole to pole, solely with the aid of our technological ingenuity. Finney contends that the Moon or Mars represent a logical next step for human migration. He re­searched the ocean routes Polynesian peoples navigated via canoe over as much as 1,000 miles of open ocean, from South China and Southeast Asia to Hawaii and other islands, to demonstrate how humans might hopscotch to nearby planets. Finney has shown that Polynesian peoples sailed with all the supplies they needed to es­tablish themselves on islands that they could have only speculated existed but had never charted or seen. This was an enormous feat without even the aid of a simple compass or sea charts. And yet, they accomplished it using horizon markers and the rising and set­ting of the Sun and stars.

The Polynesians succeeded, asserts Finney, precisely because “[t]hey did not regard the ocean as an alien environment, but one which was utterly natural—and essential to the spread of human life.”52 Even as ancient seafarers ventured far beyond known shores to settle uninhabited islands in the wastes of the Pacific, Finney ar­gues that we could and should strike out across the Solar System. The extremely challenging act of humans venturing to Mars or an asteroid could mark the first step. Noted physicist Stephen Hawk­ing would agree. Hawking contends that the future of our species depends on our becoming a truly space-faring species as a means of surviving natural or human-produced global catastrophes.

Archaeologist Peter Capelotti observes that extreme explora­tion, “begun more than two million years ago with a few rough stone tools, in what would become Africa’s Olduvai Gorge, would culminate in Apollo 11.” Our desire for travel and exploration will take us far beyond Earth, suggests Capelotti:

By the time a human finally claimed to have stood at the North Pole, that icy coordinate had come to signify—and continues to represent—the human search for something larger in our culture and ourselves: the sense and the knowledge that our journey is a continu­ous one, and that the geographic “firsts” we credit to ourselves are merely waypoints on a continuum begun two million years ago. If our culture. . . (and, increasingly, our robots) allow it, this path may con­tinue for millions of years more. We have set human feet upon only one North Pole; in space, there are millions of North Poles waiting to be discovered.53

As of this writing, Opportunity continues sampling the Martian terrain, and with each turn of its wheels, we take in more of the landscape, not just in images downloaded to NASA’s archives or as attached links to Google Mars, but into the human imagina­tion and into our dreams. As Capelotti indicates, there are many mountaintop vistas in our Solar System yet to be glimpsed, through the stereoscopic eyes of our robotic co-explorers and, eventually, through our own eyes. The sublime heights of Martian volcanoes entice the explorer in us. Mountain climbers and space enthusiasts dream of summiting Olympus Mons, rising 69,480 feet above the Martian surface, and imagine the sweeping vistas they would take in from the rim of its calderas, or from the top of nearby Ascraeus Mons reaching 59,699 feet, or Arsia Mons’s summit of 57,085 feet.54 Our tallest peaks above sea level pale in comparison, with Mount Everest reaching a mere 29,035 feet. Rock climbers like­wise anticipate the grandeur of venturing into Valles Marineris, ex­tending more than 2,500 miles in length and whose canyon walls soar from three to six miles high. But we don’t have to wait for a brave explorer to rappel over the six-mile vertical scarp of Olym­pus Mons. Robotic eyes are even now taking us there.

The journey to, and across, Mars’s cracked and cratered ter­rain is one of the great travel narratives of the twenty-first cen­tury. With our rovers, we have alighted upon and are motoring through its fascinating desertscapes. In July 1997, the Pathfinder Mission successfully landed the Sojourner Rover at Ares Val – lis. Project scientist Matthew Golombek commented that, at the time, “an entire generation had never witnessed a landing on an­other planet” and reported that within the first month of opera­tion, roughly 566 million people accessed the mission website.55 The twin MER rovers captivated even larger audiences. As sci­ence studies scholar Robert Markley points out, “Sojourner had traveled a few yards to take readings of the mineral composition of several rocks; the 2004 rovers provided a photographic record that seemingly mimicked the experience of humans walking across a Martian landscape reminiscent of terrestrial deserts.” Markley characterizes Spirit and Opportunity’s images as “the first vacation photographs from another world” that in turn have become “part of the semiotics of human visualization.”56 Through these rovers, we’ve sampled the planet’s sedimentary and volcanic rocks, as well as its ubiquitous dust. We’ve sniffed the air, measured barometric pressure, tasted the soil to determine its alkaline or saline qualities, and bored into rocks searching for fossilized evidence of water. On balmy afternoons, by Martian standards, we glimpsed the si­multaneous dance of multiple dust devils. We’ve witnessed silent sunrises, seen morning fogs cling to the valleys and dissipate, and watched from Husband Hill through Spirit’s cameras as the Sun sank below the horizon. In winter, the rovers weathered the cold, dim days and we too waited them out. Rather than our rovers exploring with us, it seems that we’ve tagged along with them as they traverse, photograph, and engage with this captivating and stark landscape.

Humans have a presence on Mars, even if for now it is limited to remote-controlled rovers ambling over its arid plains. Little could Steve Squyres know that his sturdy MER rovers would continue doing science for years beyond their ninety-day mission. Squyres wonders about the astronauts who might someday find the derelict rovers: “Above all, I simply hope that someone sees them again,” writes Squyres. “Spirit and Opportunity have become more than just machines to me. The rovers are our surrogates, our robotic precursors” until astronauts someday leave their “boot prints in our wheel tracks at Eagle Crater.”57 Those astronauts are just as likely to find remnant tracks of Mars Science Laboratory’s rover Curiosity. If W. H. Auden was right in his poem “Moon Landing” about the eventuality, from the first flaked flint, of humans walking on the Moon, then boot prints on Mars seem equally probable— even inevitable.58

The seashore marks a liminal space and appeals to us in part due to the sheer tenacity of life to survive there despite its harsh envi­ronment. Marine zoologist Rachel Carson characterized the sea­shore as a harsh but vibrant biome. Known primarily for her book

Silent Spring, Carson’s area of expertise and real passion was the ocean. She published three best-selling books on the topic, but The Edge of the Sea specifically focused on the rigor necessary for spe­cies to survive in the hostile terrain of the shoreline.17 Describing the seashore as “the place of our dim ancestral beginnings,” Car­son writes:

Only the most hardy and adaptable can survive in a region so mu­table, yet the area between the tide lines is crowded with plants and animals. In this difficult world of the shore, life displays its enormous toughness and vitality by occupying almost every conceivable niche. Visibly, it carpets the intertidal rocks; or half hidden, it descends into fissures and crevices, or hides under boulders, or lurks in the wet gloom of sea caves. Invisibly, where the casual observer would say there is no life, it lies deep in the sand, in burrows and tubes and pas­sageways. It tunnels into solid rock and bores into peat and clay. It encrusts weeds or drifting spars or the hard, chitinous shell of a lob­ster. It exists minutely, as the film of bacteria that spreads over a rock surface or a wharf piling; as spheres of protozoa small as pinpricks, sparkling at the surface of the sea; and as Lilliputian beings swimming through dark pools that lie between the grains of sand.18

Celebrated anthropologist and naturalist Loren Eiseley, in his essay “The Star Thrower,” similarly contemplated the formidable environment of the shoreline in Sanibel, Florida, which he de­scribes as “littered with the debris of life”: “Shells are cast up in windrows; a hermit crab, fumbling for a new home in the depths, is tossed naked ashore, where the waiting gulls cut him to pieces. Along the strip of wet sand that marks the ebbing and flowing of the tide, death walks hugely and in many forms. Even the torn fragments of a green sponge yield bits of scrambling life striving to return to the great mother that has nourished and protected them.”19

Eiseley recounts walking along the beach and encountering a man picking up starfish washed ashore by the rugged waves. Eise – ley pauses and together the men notice a starfish that “thrust its arms up stiffly and was holding its body away from the stifling mud.” The man picks up the star and tosses it beyond the breakers back into the sea. At some point, Eiseley joins in the effort to save at least a few of the beached starfish. He describes lifting up one star, “whose tube feet ventured timidly among my fingers while. . . it cried soundlessly for life.”20 In such prose Eiseley offers a snap­shot of life’s tenacity to survive in a threshold landscape. Evolved approximately 500 million years ago, starfish are ancestors to more complex organisms, including us. In that momentary em­brace between Eiseley’s fingers and the tube feet of the starfish, life primordial touches the present as two beings communicate a sim­ple and mutually understood message, each wishing only to cling to life. Such vivid depictions of species surviving by a bare foot­hold, or tubed foothold, inspire us to imagine the distant shores on Titan, with its carved river channels and methane lakes, and won­der what microbes or variants of terrestrial mollusks might subsist on its frozen shorelines. While Bonestell painted his spacescapes at the dawn of the Space Age, we have since sent our spacecraft bil­lions of miles from home, and in time we’ll seek out the answers to what must have been Bonestell’s questions regarding life-forms that might cling to the rocks and crevasses on Saturn’s icy moons.