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. Milky Way capable of communicating with Earth, has been a core The Milky Way, our galaxy, has roughly 100 billion stars, some piece of the debate over the search for extraterrestrial intelligence larger and some smaller than our sun. The visible universe has (SETI). “A search of hundreds of thousands of stars in the hope of about 10,000 galaxies in each of its three dimensions, or 1 trillion detecting one message would require remarkable dedication and galaxies in 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 Copyright © 2019 WorldQuant, LLC November 2019 2 11.19.19 THE GEOLOGIST’S GUIDE TO THE GALAXY PERSPECTIVES 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 they called the Late Heavy Bombardment. Until 2018 This lunar meteorite found in Northwest Africa is similar it was accepted that nearly all of the visible craters on the moon were to the breccias collected by the Apollo missions. created during this period.7 However, recent analysis indicates that the formation of the moon’s craters may have occurred from 3.9 bil- lion to 4.4 billion years ago; the narrow range indicated by the Apollo samples may be due to the relative closeness of the landing sites. 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.