The Geologist's Guide to the Galaxy

NOVEMBER 19, 2019

The Geologist’s Guide to the Galaxy

Fifty years after the first person walked on the moon, the relatively new field of planetary geology provides fresh insights into the origin of Earth, our solar system and the essence of life.

By Paul A. Griffin

WorldQuant, LLC 1700 East Putnam Ave. Third Floor Old Greenwich, CT 06870 www.weareworldquant.com 11.19.19 THE GEOLOGIST’S GUIDE TO THE GALAXY PERSPECTIVES

PROLOGUE picture of what special conditions were present in our solar system IN THE FIELD OF PLANETARY GEOLOGY, 1969 WAS A TRANS- that generated the basis for life on Earth. This information may formational year. In early February, a meteorite fell near the not answer the probability-of-extraterrestrials question, but it village of Pueblito de Allende in northern Mexico, scattering does help us appreciate what factors make us unique and hence more than two tons of fusion-crusted stones over an area of better estimate the likelihood of similar life existing elsewhere in 75 square miles. Five months later, Apollo 11 made the first the universe. moon landing and brought 22 kilograms (48.5 pounds) of lunar material back to Earth. Then, at the end of September, To understand planetary geology, readers need to be familiar with a meteorite blazed over the southern Australian town of some astronomical objects, as these are the actors in our story. Our Murchison, showering more than 100 kilograms of rocks solar system consists of the four inner terrestrial planets (Mercury, containing amino acids essential for life. Venus, Earth and Mars), the main asteroid belt, the four ice giants (Jupiter, Saturn, Uranus and Neptune) and the Kuiper belt. The At the time, these three events may not have seemed to have asteroid belt is the ring of rocks, rubble and other material orbiting anything in common, but over the past 50 years the data collected the sun between Mars and Jupiter that, because of the velocities from them and similar occurrences have helped scientists piece generated by the gravitational resonances with Jupiter, cannot together answers to a cosmic puzzle. The questions they’ve pon- coalesce into larger bodies. Collisions in the asteroid belt can lead dered include: What processes led from the origin of the cosmos to material falling toward the terrestrial bodies, causing meteorite to the creation of our solar system? What caused the formation impacts on Earth. Most meteorites are mainly bits of asteroids, but of the moon? And what makes Earth so special that we have life, some come from Mars and our moon. which as far as we can tell seems to be unique in the universe? The Kuiper belt is a second outer ring, just beyond Neptune, and The potential existence of extraterrestrial life has long fascinated is much larger than the asteroid belt. While most asteroids are scientists. During a lunch with colleagues at the Los Alamos composed of rock and metal, the objects in the Kuiper belt are National Laboratory in 1950, physicist Enrico Fermi famously made up of ices, including methane, ammonia and water. Pluto, asked, “Where are they?” Fermi, who had won the Nobel Prize in which astronomers demoted to dwarf planet status in 2006, is a Physics in 1938, reasoned that some of the billions of stars in the member of the Kuiper belt. Comets (basically, cosmic snowballs) Milky Way similar to our sun should have had planets capable of are former members of the Kuiper belt originating from the farthest sustaining intelligent life. Furthermore, he posited, some of these region of our solar system, the Oort cloud. life forms should have developed advanced technologies capable of interstellar travel. Given that many of these stars are billions The sun is truly the center of our world, containing 99.86 percent of years old, there has been ample time for aliens to have visited of our solar system’s mass. (Jupiter and Saturn comprise more our planet, yet there are no signs of extraterrestrial life — the than 90 percent of the rest.) Jupiter, the largest planet, is about five Fermi paradox. times farther from the sun than Earth is, and most of the planets are locked into orbital period resonances with it. (Saturn, for example, In 1961, astrophysicist Frank Drake tried to quantify the answer is in a 2:5 resonance with Jupiter, meaning it orbits the sun twice to the Fermi paradox as a series of conditional probabilities even- for every five of Jupiter’s orbits.) tually leading to the detection of alien life. The Drake equation, which attempts to calculate the number of alien civilizations in the But our solar system represents a tiny corner of the universe. The Milky Way capable of communicating with Earth, has been a core Milky Way, our galaxy, has roughly 100 billion stars, some larger piece of the debate over the search for extraterrestrial intelligence and some smaller than our sun. The universe has about 10,000 (SETI). “A search of hundreds of thousands of stars in the hope of galaxies in each of its three dimensions, or 1 trillion galaxies in detecting one message would require remarkable dedication and total (10,0003).2 would probably take several decades,” Drake wrote in a January 1997 Scientific American article co-authored with renowned astron- PART 1: THE MOON omer Carl Sagan.1 On July 20, 1969, at 10:56 p.m. EDT, astronaut Neil Armstrong began climbing down the ladder of the Apollo 11 lunar module. More than two decades later, searching for a definitive answer Moments later, he proclaimed, “That’s one small step for a man, to Fermi’s question, thousands of people around the world are one giant leap for mankind.” Buzz Aldrin joined Armstrong on the using computers to monitor radio signals for signs of extrater- moon’s surface 19 minutes later. I watched these historic footsteps restrial intelligence through projects like SETI@home. Advances on a black-and-white television with my brother and parents at in planetary geology over the same period have provided a clearer home in Monterey, California. I was not alone: The first of six moon

Geology may be the most that after Armstrong said his famous words and inspected the lunar module exterior for damage, he gathered contingency lunar important science when it comes to samples and placed them in his collection bag in case an emergency understanding the formation of the ascent was required.3 If just this material had been recovered, the entire space program would have been a scientific success. moon and solar system. Fortunately, the Apollo moon missions brought back significantly more, a total of 220 kilograms of rocks,4 providing samples from landings was estimated to have been watched live by 600 million six sites chosen for their distinctive geological features. people, nearly one-fifth of the world’s population at the time. Late Heavy Bombardment Fifty years later, the Apollo 11 lunar mission is still one of mankind’s The moon’s surface is actually fairly homogeneous. When we look greatest scientific and technical accomplishments. For me, like at it with our naked eyes, the first thing that stands out are the many others, the event inspired an interest in the sciences most lunar maria (Latin for “seas”), multiple large dark impact craters clearly associated with the space program: physics and astronomy. so deep that they once generated volcanic outflows. The maria are Certainly, the space program deserves recognition for advancing surrounded by the highlands, or terrae, bright areas created by mete- those fields. But geology, the one science I did not appreciate at orite impacts that did not penetrate the moon’s crust. Rocks called the time of the lunar landings, may be the most important science regolith breccias5 make up some 80 percent of the lunar highlands, when it comes to understanding the formation of the moon and which almost completely cover the far side of the moon.6 (Earth solar system. In fact, the only Ph.D.-level scientist in the astronaut has regolith breccias as well, but they are not commonly seen due program who walked on the moon, Apollo 17’s Harrison (Jack) to the effects of weather and recycling of the surface.) Using fairly Schmitt, was a geologist. precise isotopic dating of the Apollo lunar samples, scientists were surprised to discover that the recovered breccias were created over a relatively narrow time period 3.9 billion to 4.0 billion years ago, MOONFALL during a period now called the Late Heavy Bombardment. Nearly all This lunar meteorite found in Northwest Africa is similar of the visible craters on the moon were created during this period.7 to the breccias collected by the Apollo missions. The Late Heavy Bombardment was a time of great turmoil and activity in the formation of our solar system. In fact, the asteroid bombardment affected Earth even more than the moon, but because of the recycling of Earth’s crust, it is not as obvious. Geologists have noticed for years that the rocks in the crust have an unusually large amount of iron-loving elements such as gold, palladium and platinum, which should have ended up in the planet’s core during the initial stages of its formation. These metals are believed to have come from a continuous process of asteroid and comet bombardment — in particular, the accretion of a late veneer on Earth’s surface roughly 3.5 billion to 4.5 billion years ago. The peak accumulations of the late veneer8 took place during the Late Heavy Bombardment.9

The moon’s gravity is very weak — during the Apollo 11 mission, Aldrin and Armstrong bounced along the lunar surface as they set up experiments and collected rocks. As a result of this weak gravity, any major asteroid impact on the moon’s surface would generate projectiles with speeds greater than the moon’s escape velocity. Computer simulations indicate that most of these projectiles would be directly captured by Earth.

The first meteorite recognized as originating from the moon was In 1969, the main scientific reason to visit the moon was to bring identified during the Japan–U.S. Antarctic expedition of 1982 and back lunar samples. If you read the literature or watch the many confirmed by isotopic and chemical matching with Apollo lunar documentaries and recent movies about Apollo 11, you will notice samples.10 (The surface of Antarctica’s ancient ice floes turns out

to be an excellent place to find meteorites.) Cosmic ray exposure Cosmic ray exposure data indicate data, which measure the time meteorites traveled in space outside the protected atmosphere of Earth, indicate most of these lunar most of these lunar meteorites were meteorites were ejected from the moon less than 20 million years ejected from the moon less than 20 ago. In total, there are now about 300 known lunar meteorite finds, representing approximately 30 distinct lunar impact events and million years ago. totaling 190 kilograms of material.11 This is slightly less in total mass than the amount brought back by the Apollo astronauts, but 50 million years after the birth of the solar system, early Earth was it samples more distinct sites. The lunar meteorites found on Earth struck by a Mars-sized planet, now called Theia.14 are randomly distributed samples of the lunar surface, including from the far side of the moon.12 They are entirely consistent with Theia may have initially formed at one of Earth’s two stable the lunar samples from the Apollo missions and have the same Lagrangian points. These are positions in space where an object formation dates as the Apollo lunar samples — namely, the period can remain in orbit equidistant between two larger bodies — in of the Late Heavy Bombardment. this case, the sun and Earth. It is unclear what may have dislodged Theia from its orbit, as there are currently no known coincident Lunar Origin Story events recorded in geological or meteorite history. Whatever the So let’s consider the origin of the moon. Was it captured by Earth cause and wherever Theia initially orbited, the impact of Theia on or generated by some other process? Why does it exist at all? The Earth was violent enough to eject large parts of the original Earth’s answers to these questions can be found in the bulk properties of mantle into space and vaporize the entire planet into what is called the lunar samples. When scientists compare the ratios of various a synestia, a spinning torus of vaporized rock.15 The moon cooled elements in them with those of ordinary rocks on Earth, they find off faster and condensed first, followed by the new Earth. many exact matches, indicating that the moon and Earth share a common geological origin.13 It took until the mid-1980s for scien- PART 2: THE SOLAR SYSTEM tists to come up with enough additional data and analysis to put On February 24, 1969, in preparation for their lunar landing, Buzz forth a narrative to solve this puzzle. They hypothesized that about Aldrin and Neil Armstrong went on a geology field trip near Sierra Blanca in West Texas.16 There they practiced collecting rock and soil samples among the ruins of an old U.S. Army garrison. PRELUDE TO SPACE The Allende meteorite provides clues to the origin Two weeks earlier, the Allende meteorite had fallen about 400 miles of the solar system. south of Sierra Blanca, in the Mexican state of Chihuahua.17 The meteorite, estimated to be roughly the size of a car, had exploded as it entered Earth’s troposphere, raining down more than two tons of material for geologists to study. The timing was propitious, as many scientists were in the process of preparing their laboratories to analyze lunar rocks.

There are two types of meteorites: falls and finds. Falls represent observed events. They are very dramatic, as meteorites like Allende enter Earth’s atmosphere at speeds typically greater than 11 kilometers per second (25,000 mph), causing bright streaks, flashes and glass-breaking sonic booms as they deaccelerate due to atmospheric friction. Such events occur about 500 times a year, but only about six of them are recovered by meteorite hunters and made available for scientists to analyze, as well as for museums and private collectors.

Finds are discoveries of previous falls — some ancient — and are typically discovered in deserts or on top of ice fields due to the ease of detection. Finds provide us with a record of meteorite falls over eons and can often be dated based on the strata in which they are discovered. As meteorite science is only a few generations old,

there are presently more finds than recovered falls. And ultimately, (1,340 degrees Fahrenheit) during early interactions of the cloud the accumulated mass of finds blends into the continuum of what material as it was swept into and processed by the new sun and we call the planet’s soil and crust; meteorites represent at least subsequently ejected into the planetary disk along with the CAI. 0.5 percent of Earth’s total mass. This is known as the X wind theory, proposed by University of California, Berkeley, astronomer Frank Shu.21 Allende is one of the most primitive types of meteorites, known as carbonaceous chondrites because they’re rich in carbon. Like Chondrules occur in most meteorites. As millimeter-sized objects, most meteorites, the Allende meteorite has a dark fusion crust they clumped together and formed at least 100 planetary seeds. generated during its flight through Earth’s atmosphere; the inside These seeds eventually became the major elements of our present is a dark gray and white grainy material. The meteorite has three solar system. From start to finish, the processing of a giant molec- distinctive features: small pebbles known as chondrules, a dark ular cloud by the early sun to create the solar system’s planets took background called the matrix and an extended white material about 50 million years. known a refractory inclusions. The inclusions are bright white and consist of calcium and aluminum, so they are also known as The Kilonova Event calcium-aluminum-rich inclusions (CAIs). On August 17, 2017, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the first kilon- Solar System Origin Story ova as two neutron stars in the Hydra constellation, 130 million What caused our solar system to form in the first place? The light-years from Earth, collided.22 The explosion set off fireworks Allende meteorite provides key insights that help answer that across the universe; spectral analysis revealed heavy element question. In 1977, a group of scientists from the California Institute signatures in the trailing cloud of dust. Scientists estimate that of Technology reported that Allende showed signs of a magnesium the kilonova could have produced a quantity of gold more than 40 crystal compound that formed when the magnesium was originally to 100 times Earth’s mass. aluminum-26.18 This element has a relatively short 700,000-year half-life and is produced by what are known as Type II supernovas In May 2019, scientists observed that Allende carries excess heavy — dramatic explosions of large stars with masses ranging from elements in relationship to the data gathered by LIGO, consistent eight to 50 times that of our sun. That same year, Harvard University with a similar neutron star event about 80 million years before the astrophysicist Alastair G.W. Cameron published a paper with long- supernova.23 A colliding neutron star event generates a thousand time University of Chicago professor James Truran making the case times more gold than a Type II supernova; this may explain Earth’s that a Type II supernova triggered the collapse of a giant molecular excess of so-called r-process metallic elements, such as gold, gas cloud to begin the formation of our solar system.19 silver, platinum, plutonium and uranium.

The refractory inclusions in the Allende meteorite are enriched Giant Planet Migration with products of that supernova, and the decay process of those Although the sun, Earth, moon and planets formed over a period supernova-based elements in Allende allows scientists to date of about 50 million years, the stability of the system was not its formation, and hence the supernova, with surprising accuracy. assured, because of remaining nebular material that continued to Refractory inclusions were some of the first matter to form around accrete into the planets as they cleared their orbits over a much the early sun. Allende marks it as 4.567 billion years ago, plus or longer timescale. Over time, the planets increased in size, causing minus 1 million years (accurate to four significant figures).20 This a realignment of the “music of the spheres,” defined as the multi- is about 30 million years before Earth’s formation. body synchronization of planetary resonances that stabilizes the solar system by preventing planets from approaching one another Chondrules in the Allende meteorite better represent the other too closely. One of these realignments, called the giant planet main source of material — a giant molecular cloud. The chondules migration, was a major event, second in importance only to the were produced by flash heating to about 1,000 degrees Kelvin initial formation of the solar system, and occurred about 560 million years after the initial system formation, or about 4 billion years ago. Meteorites like Allende enter Earth’s Giant planet migration is supported by the Nice computational atmosphere at speeds typically greater hydrodynamic model, now widely accepted as the standard sce- than 11 kilometers per second, causing nario for the evolution of the solar system. In the Nice model — initially developed in 2005 by scientists Rodney Gomes, Harold bright streaks, flashes and glass- Levison, Alessandro Morbidelli and Kleomenis Tsiganis while breaking sonic booms. working together at the Côte d’Azur Observatory in Nice, France

Using the Spitzer infrared space FANTASTIC VOYAGE The Aguas Zarcas meteorite contains prebiotic telescope, scientists have discovered compounds critical to the creation of life. that giant molecular clouds are found throughout most galaxies.

Meteorite hunters, such as Michael Farmer of Tucson, Arizona, specialize in the sometimes dangerous work of rapidly recover- ing such falls. They are modern-day Indiana Joneses, combining adventure, detective work and scientific techniques to preserve the valuable samples for future generations.24 Because the Aguas Zarcas meteorite25 is a rare CM-type chondrite composed of clay, it was important to recover samples before it could be broken down and contaminated by rain. The Aguas Zarcas meteorite has a distinctive musty odor from extraterrestrial organic compounds, validating its origin in a carbonaceous asteroid.

Using astronomical observations from as far away as Brazil, astronomers were able to reconstruct the orbit of the Aguas Zarcas meteorite; as expected, it originated in the outer asteroid belt. From the spectral analysis of flyby missions, the parent body of CM meteorites is now known to be the 19 Fortuna asteroid (the 19th asteroid discovered), which has a diameter of 200 kilometers, — Neptune’s orbital drift caused a flux of the icy Kuiper objects making it one of the largest bodies in the asteroid belt.26 The CM inward, toward the center of the solar system. Jupiter, whose meteorites are relatively fragile in space; hence, it is believed that increasing mass was a major source of Neptune’s instability, scat- Aguas Zarcas was dislodged from Fortuna’s surface less than 1 tered many of the incoming Kuiper objects, and as it lost angular million years ago. momentum it moved inward. At some point, its orbital resonance with Saturn passed through the 1:2 ratio, temporarily increas- Aguas Zarcas has drawn comparisons to another CM meteorite, ing Saturn’s orbital eccentricity and pushing Neptune out farther. which fell near the small riverside village of Murchison in Victoria, Via this feedback loop, most of the original Kuiper belt was scat- Australia, on September 28, 1969 — like Allende, it is another tered, either inward and potentially into unstable planet-crossing well-studied meteorite. Whole stones of the Murchison fall were orbits or outward to what is called the scattered disk, where the recovered largely intact, with the inner portions of material essen- comets now reside. These shifts in Jupiter’s orbit also perturbed tially free of any contamination from terrestrial sources. Yet CM the asteroid belt, causing many asteroids to drift into planet-cross- meteorites like Murchison and Aguas Zarcas contain significant ing orbits. The Nice simulations indicate that this process was a amino acids, phosphoric acids, sugars and alcohols, as well as fairly rapid phase transition compared with the lifetime of the solar meaningful quantities of aromatic hydrocarbons.27 These com- system. This is how the giant planet migration caused the Late pounds are clearly organic in nature but not yet alive or generated Heavy Bombardment. by life, so we call them prebiotic organic compounds.

PART 3: LIFE ON EARTH Amino acids are chiral, meaning they come in two configurations On April 23, 2019, at 9:07 p.m., residents of the town of Aguas — left-handed and right-handed — that are mirror images of each Zarcas in Costa Rica saw a fireball streaking across the sky. The other. The amino acids found in the CM meteorites are largely meteorite, about the size of a washing machine, broke apart during left-handed. Life on Earth, of course, is based almost exclusively its descent, with some of the larger fragments crashing into occu- on left-handed amino acids.28 All this is consistent with the notion pied buildings. One piece smashed through the roof of a doghouse. that the organic compounds on Earth that produced the advent of Fortunately, no people or animals were hurt. life and the prebiotic compounds in the meteorites have a common origin. In particular, if the sun formed close to a cluster of mas- Over the next few days, meteorite hunters descended on Costa sive stars, polarized radiation from those stars may have more Rica in a race to find or purchase the fragile rocks that had fallen. preferentially destroyed the right-handed versions of the organic

Life on Earth began about 3.9 billion best representation of this original cloud material in our current years ago, at the end of the solar system are the comets and carbonaceous asteroids. Late Heavy Bombardment, with a The second part of the cosmic carbon pathway has taken place continuously since the Earth cooled after the Theia impact, accom- single-cell organism known as the plished by the delivery of material to Earth in the form of meteors last universal common ancestor. and meteorites.32 Via this mechanism, the asteroids and comets have provided over time much of the water33 that has formed the molecules. Observations indicate that polarization is a near-uni- oceans on Earth.34 Asteroids have a similar ratio of deuterium versal feature of star-forming regions.29 (heavy water) to regular water as that found on Earth, while comets have a much higher ratio, as determined by the European Space The Cosmic Carbon Pathway Agency’s Rosetta mission.35 Hence, the water provided to Earth Where did these prebiotic organic compounds arise? It turns out came mainly from asteroids, not comets.36 Carbonaceous asteroids they originated in the universe’s first generation of stars — in are from the outer asteroid belt closer to Jupiter and beyond the particular, the carbon stars, red giant stars that have more carbon frost line, where frozen water can exist without being vaporized than oxygen in their atmospheres. In these stars at the end of by the solar wind. These asteroids contain significant amounts of their stellar evolution, the elements oxygen and carbon combine ice, typically exceeding 10 percent of their mass. to form carbon monoxide until the oxygen is consumed. At this point, the carbon is free to form other compounds, giving the star The prebiotic organic molecules on Earth were contributed by a sooty atmosphere. As the star shreds itself in the final stages atmospheric synthesis, comets, asteroids and interplanetary dust. of its life, this matter accumulates to form dense clouds in the Carbonaceous meteorites also have similar relative abundances interstellar medium (ISM). The outflow from these late-stage stars of amino acids created in the famous Miller–Urey experiment, includes such complex molecules as polycyclic aromatic hydro- first conducted in 1953 by American chemists Stanley Miller and carbons (PAHs), as found in the CM meteorites, and small carbon Harold Urey to simulate the synthesis of organic compounds and silicate dust particles. These compounds are modified in the from methane, ammonia, hydrogen and water vapor in Earth’s ISM through various physical and chemical processes.30 In the ISM, early atmosphere.37 nearly all free oxygen combines with hydrogen to form water; with the temperature ranging between 10 and 50 degrees Kelvin (–442 Carl Sagan and Christopher Chyba, an astrobiologist at Princeton to –370 degrees Fahrenheit), most of this water is in ice form University, initially estimated the flow of extraterrestrial organics to and accretes on carbon and silicate grains.31 This forms the giant Earth in 1992, based on lunar cratering data.38 Scientists currently molecular clouds. believe that meteorites contributed at least 1 billion kilograms of organic carbon per year during the Late Heavy Bombardment, five Using the Spitzer infrared space telescope, scientists have dis- times greater than what was being produced in Earth’s atmosphere covered that giant molecular clouds are found throughout most from the Miller–Urey mechanism. galaxies. In our nearby sister galaxy, Andromeda, they form a density distribution coincident with the galaxy’s spiral arms when The Genesis of Life viewed in infrared. So we now understand that the cosmic carbon Life on Earth began about 3.9 billion years ago, at the end of the pathway to Earth begins with the formation and evolution of giant Late Heavy Bombardment, with a single-cell organism known as molecular clouds. the last universal common ancestor (LUCA). The first organism relied on synthesized organic molecules from the prebiotic soup. These clouds became the nurseries of the second and third gen- In 2016, American evolutionary biologist William Martin led a team erations of stars. In our galaxy, which has a total mass of 2 billion of researchers that found conclusive genetic evidence that LUCA solar masses, the rate of star formation from clouds is approxi- originated near anaerobic thermal vents in geochemically active mately one per year. This is currently in equilibrium with the gas environments in the prebiotic oceans,39 challenging the then-pre- ejected by the red giants, making the amount of giant molecular vailing theory that life on Earth began in land-based pools. The clouds in the Milky Way relatively stable. It is this material that genetic evidence specifically indicates that LUCA used a set of blocks our view of the central part of our galaxy. Looking up at biochemical reactions called the Wood–Ljungdahl pathway in a the sky, you can perceive the dark clouds, backlit by the glow of hydrothermal vent. LUCA fixes carbon from the environment by the Milky Way’s stars. converting carbon dioxide into carbon monoxide using metals available in the water with a temperature between 86 and 212 Our own solar nebula originated from a dark molecular cloud. The degrees Fahrenheit and without the need for proteins.

The genesis of life benefited significantly from the fact that Earth’s During the space race in the 1960s outer shell is divided into plates, which move over a rocky mantle. Earth’s plate tectonics, which coincidently began 3.9 billion years and ’70s, the Soviet Union and the U.S. ago, provided the consistent, hydrothermal environment. Stable launched multiple spacecraft intended convection zones formed in the mantle, and ocean water filtered down through the deep crust and upper mantle to be reinjected to fly by Mars. into suboceanic ridges at high temperatures. Plate tectonics turned Earth into a giant incubator, with thousands of miles of thermal between the northern lowlands and southern highlands on its vents bubbling away, pumping methane, carbon dioxide, metals surface, known as the Martian dichotomy. This change in the thick- and hydrogen into an ocean of cooler prebiotic organic com- ness of Mars’s crust runs along an apparently irregular boundary pounds. At some point during the incubation process, there was between its northern and southern hemispheres. Scientists believe a LUCA metabolic event40 in a hydrothermal vent. According to that the elliptical nature of this boundary was caused by the impact Martin, the porous iron sulfide 3D structures that formed in the of a giant body (again during the Late Heavy Bombardment), creat- vents served as primitive cell walls and a stable environment for ing the largest such impact structure thus far identified in the solar the eventual creation of the RNA molecule ribozyme from the system and initiating a period of plate tectonics.42 existing prebiotic compounds — providing the genesis of life’s replicating mechanism. Supplementary evidence indicates that Mars consists only of these two plates, north and south.43 The northern depression was actually How did plate tectonics start? It’s possible that during the Late filled with water for a significant period, just like Earth’s oceans.44 Heavy Bombardment a single massive asteroid impact, or a fortu- The conditions on Mars may have been remarkably similar to nate sequence of impacts, pushed one large section of Earth’s crust Earth’s for some time period; this also suggests that water-based below another, setting up the physics required for the continual plate tectonics may not be a rare event. Unfortunately, during the subduction of plates. Though this is currently only one scenario end of the Late Heavy Bombardment, one collision disrupted Mars’s for what sparked life on Earth, it may be the most likely one, given magnetic field, significantly reducing protection from the solar wind the timing of all the overlapping events. If this scenario is true, the and causing the ocean to evaporate and most of the ancient atmo- bombardment fractured the previously solid crust into the parts sphere to dissipate into space.45 At present, the location of water we now regard as continents and oceans, setting in motion the across the planet now seems to be mainly below the surface in the conditions required to create the incubator of life on Earth. All of form of ice. Mars is still, however, occasionally active geologically, the timing for this narrative lines up nicely as a causal relationship. and with significant remaining surface ice at the polar regions, likely pockets of subsurface water and an atmosphere that is 96 percent Other Life in the Universe carbon dioxide, the planet is worthy of further examination for life. Looking beyond Earth, the conditional probabilities set forth in the Drake equation provide a framework for analyzing the amount of EPILOGUE life in the universe.41 However, the estimates of the combined prob- As with the Copernican revolution, in which the sun replaced Earth abilities are overwhelmed by the uncertainty associated with the as the center of the universe nearly 500 years ago, we now know derived values, making it impossible to come to firm conclusions. that Earth itself is not at the center of the creation of materials Still, we can now endeavor to improve the accuracy by examining essential for life. The organic compounds that seeded the early the local planetary systems for candidates that might have had oceans have a mainly extraterrestrial origin via the cosmic carbon thermally activated prebiotic compounds in an aqueous setting. pathway, and they are observable as giant molecular clouds in galaxies throughout the universe. Circular radiation from larger In our solar system, we have at least four such candidates: Jupiter’s nearby stars, or the supernova that collapsed the cloud, may have moons Europa and Ganymede, Saturn’s moon Enceladus and the preferentially destroyed the right-handed polarized amino acids in planet Mars. Europa has plate tectonics of surface ice crust and our solar system, leading to chiral organics. is geologically active, and Enceladus and Ganymede both indicate the existence of volcanic activity in their subsurface oceans. There The delivery pathway of this material from the heavens to Earth are active scientific programs to study all three: the NASA Europa is still robust and continues to this day in the form of meteorites Clipper, the German Aerospace Center Enceladus Explorer and and cosmic dust. Many of the ancient planetary seeds that formed the Russian Space Research Institute Ganymede Lander mission. the early solar system exist on Earth in the form of iron meteorite finds. So Earth’s geological record contains other worlds In the case of Mars, we have much more information. Geologically, to study. Meteorites have proved invaluable to unraveling this the planet’s most prominent feature is the striking difference cosmic puzzle.

During the space race in the 1960s and ’70s, the Soviet Union Mars has been a popular subject for science fiction writers. In and the U.S. launched multiple spacecraft intended to fly by Mars. 1898, British author H.G. Wells published The War of the Worlds, a NASA’s Mariner 4 was the first to reach the Red Planet, on July fictionalized first-person account of a Martian invasion of southern 14, 1965, sending 21 close-up photos back to Earth.46 NASA’s England. Growing up, I enjoyed reading stories about colonizing Viking mission,47 which landed two unmanned spacecraft on Mars Mars by authors like Ray Bradbury and Arthur C. Clarke. In 2011, in 1976, measured the chemical composition of the planet’s soil Andy Weir published The Martian, a novel about an American astro- and atmosphere.48 Based on this data and other factors, scientists naut who is stranded alone on Mars and how he survives while concluded in 1983 that Shergottite meteorites, an igneous rock that waiting to be rescued. had been unclassified for almost 100 years, were of Martian origin. One of the multiple factors pointing to this conclusion is that the gases trapped in a glassy portion of Shergottite meteorites match MARTIAN CHRONICLES the ratio of elements in the Martian atmosphere measured by the The Tissint meteorite supports the theory that Viking landers. there is water on Mars.

There are now more than 100 known finds of Martian meteorites; the total amount of Mars on Earth is more than 100 kilograms.49 However, there are only five observed falls of Martian meteorites, which, interestingly, occur roughly once every 50 years. The last Martian meteorite was a Shergottite called Tissint that fell on July 18, 2011, close to the southern Moroccan town of the same name.50 Tissint meteorites contain trapped carbon-based organic compounds. These compounds were originally thought to be possible markers of life, but today scientists believe they originated from volcanic activity combined with aqueous alteration of the rock material; this is consistent with the theory that there is water on Mars. There is more evidence of water on Mars, 2.5 billion years ago, from the Northwest Africa 7034 meteorite — dubbed “Black Beauty” — found in the Sahara desert in 2011. The oldest known Martian meteorite, Allan Hills 84001, discovered in Antarctica in 1984, shows that liquid water existed on the surface of Mars as far back as 4 billion years ago. All of this is consistent with the delivery of water to Mars by the same pathway that delivered water to Earth. Today a modern-day race to colonize space is playing out among Mankind has long been fascinated with Mars, both because of its private companies like Elon Musk’s SpaceX, Jeff Bezos’s Blue Origin proximity to Earth and because of the possibility that it could sup- and Richard Branson’s Virgin Galactic, and public entities including port life. British astronomer Sir William Herschel believed that the NASA, the Russian Federal Space Agency and the China National dark areas on Mars were oceans; in 1784, he wrote that Martian Space Administration. Musk, who founded SpaceX in 2002 to rev- inhabitants “probably enjoy a situation similar to our own.”51 In olutionize space travel, may have the most ambitious goals: His 1877, Italian astronomer Giovanni Schiaparelli began mapping company plans to transport humans to Mars by 2024, two years Mars; he called the channels he saw canali, which was later mis- after sending its first unmanned cargo ship to the Red Planet “to translated into English as “canals.”52 Two decades later, Percival put in place initial power, mining and life support infrastructure.”53 Lowell, a wealthy American businessman and astronomer, pub- In a March 2019 tweet, Musk wrote that it will be possible to build lished his first of several books on Mars, popularizing the idea “a self-sustaining city” on Mars by 2050. that Martians built the canals to transport water from their polar ice caps. We now know that because of its thin atmosphere Mars Bezos, for his part, believes that the fastest way to get to Mars is supports surface water only in the form of ice, mainly close to its by first establishing an outpost on the moon. “If you’re gonna need poles. However, empty canals formed by surface water in ancient a lot of supplies and fuel and bulk materials to go to Mars, you’re times exist on Mars, and the situation that brought water to Mars much better off lifting them off the moon than you are lifting them was similar to Earth’s, so, ironically, there is truth to Schiaparelli’s off the Earth,” Bezos said in a June 2019 interview, at an event to and Lowell’s early conjectures. celebrate the 50th anniversary of the lunar landing.54

Today a modern-day race to colonize soup delivered by a continuous bombardment of water and organic material that was molecularly chiral onto a relatively cool planet space is playing out among private surface. Although there are specific “Goldilocks” parameters for companies like Elon Musk’s SpaceX, Earth’s special case, this scenario itself may not be extremely Jeff Bezos’s Blue Origin and Richard unusual for solar systems. Branson’s Virgin Galactic. The initial organic materials are available throughout the universe in the form of giant molecular clouds produced by first-generation Hopefully, these human space adventures will include some geol- stars, so planets only need to be capable of the incubation pro- ogists. As billionaires like Musk, Bezos and Branson continue to cess; they do not have to create organic chemistry from scratch. push human exploration beyond the bounds of Earth, and as public Therefore, it seems that for now the Fermi paradox is as con- agencies and their scientists continue to make new discoveries via founding as ever — possibly even more so. In this context, there robotic missions, we may someday accumulate enough data and is significant value in understanding the other candidates in our insights to solve the Fermi paradox. own solar system capable of life to refine the criteria and better appreciate what makes our lovely planet so special.■ So what have we learned over the past 50 years when it comes to the probability of extraterrestrial life? Paul Griffinis a Managing Director, Portfolio Management, at WorldQuant, Well, we now understand that the development of water-based LLC, and has a Ph.D. from Stanford University and BA from the University plate tectonics provided the incubation mechanism for a primordial of California, Berkeley, both in physics.

ENDNOTES

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