A Brief Introduction to the Search for Extra-Terrestrial Life

Cesare Barbieri

• Publisher: CRC Press
• Year: 2019
• eBook ISBN: 9780429574696
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

This book provides an introduction, from the astronomical point of view of the author, to the exciting search for extra-terrestrial life, and an overview of the current status of research into 'alien' life in the Solar System and beyond. It also explores the potential future human exploration of the Moon and Mars.

Up-to-date with the latest developments in the field and accompanied by key references for further study, it is a fantastic introduction to the field of astrobiology for non-science majors taking an elective module, in addition to undergraduates studying physics with an interest in this area.

Features:

• 
  
  

• Contains the latest groundbreaking research in the hunt for life outside of Earth
• Discusses the identification of biosignatures in exo-planets
• Reviews future options for human outposts on the Moon and Mars

chapter 1

Introduction

We live in a period of extraordinary discoveries in our Solar System, in the planetary systems of other stars (exoplanets) and in the complex molecules in interstellar and intergalactic clouds. These advancements promote new investigations of the meaning of “life.”

Our knowledge about life developed over the centuries thanks to the many philosophers, physicists, chemists and biologists, who examined such complex matters according to their different points of view. Out of this long history, I wish to quote here only one date, the year 1953. In that year, Miller and Urey carried out their famous experiment about the primordial universal soup, whose foundations had already been expounded by the Russian chemist Alexandre Oparin in 1924. From a mixture of five gases, methane, ammonia, carbon dioxide, hydrogen and water vapor, and an electric discharge as the source of energy, complex molecules were produced, including amino acids. In the same year, James D. Watson, Francis Crick and collaborators discovered the double helix of DNA (deoxyribonucleic acid), a nucleic acid containing the genetic information needed for the biosynthesis of RNA (ribonucleic acid) and proteins. DNA, RNA, proteins and carbohydrates are the main macromolecules essential for all known living beings.

As a source of inspiration for their researches, both Watson and Crick acknowledged the book by Erwin Schrödinger, What Is Life? (1944). In that book, Schrödinger recognized that the question of life requires a multidisciplinary approach. The fundamental importance of quantum mechanics was also pointed out, and far-reaching questions were formulated such as the apparent violation of the second principle of thermodynamics. Life seems to be characterized by a sort of negative entropy, a “negentropy” in his word. See the review of Schrödinger’s book by Ball (2018). Today, we might add another question: How crucial is information for life? In an often-quoted sentence, the famous physicist Archibald Wheeler said, “It from bit,” be the bit a classic or a quantum one, namely from information to matter. Is information a free-floating entity such as electromagnetic or gravitational fields? Does entropy have an entirely different meaning as suspected by Schrödinger? Is life analogic or digital, or both at the same time, as in peptides, digital in the sequence of their amino acids and analogic in the complexity of their macrostructures? These and other questions of a profoundly philosophical nature exceed the purpose of the present astronomically oriented review. See, for instance, the many papers devoted to the role of information on life contained in the book by Walker et al. (2017).

For the past 50 years, the space age has allowed us to study life not only on Earth but also on other planets of the Solar System and their moons, in a variety of extra-terrestrial environments and on exoplanets. A major leap was made more or less at the same time as the primordial soup experiment and the identification of DNA, when Earth was considered and studied as any other planet; thus, comparative planetology flourished. Today, the same step is being made by comparing our Solar System to the planetary systems of other stars.

To study the broad subject of life on Earth and elsewhere, a new discipline has emerged, astrobiology, which involves not only scholars of astronomy, physics, chemistry, biology and engineering, but also sociologists, philosophers and theologians, and arouses the interest of the whole society. The NASA Astrobiology Institute (NAI, https://nai.nasa.gov/) was established in 1998. The NAI is a virtual, distributed organization of teams that integrate astrobiology research and training programs in concert with the national and international science communities. In Europe, a European Astrobiology Institute is proposed (http://europeanastrobiology.eu/), to be finalized in 2019. See the book by Capova et al. (2018) on this topic.

The present review will discuss several topics and questions raised by the fascinating subject of terrestrial and extra-terrestrial life, with the aim of providing at least some information and answers from the astronomical point of view of the author. The review is organized across several chapters.

Chapters 2 and 3 illustrate how the Universe’s chemical composition evolved, from the initial conditions after the Big Bang, to the present abundances of the different elements thanks to nuclear reactions inside the earliest stars. The evolution of large aggregations of matter, namely galaxies, their clusters and the intergalactic clouds of dust and gases, led to the current structure of the Milky Way and the Solar System within.

Chapters 4 and 5 treat questions such as how did life originate? When and where on Earth? Did cosmic bodies such as comets and asteroids have any influence on the appearance of the first building blocks of life? What are the main characteristics of life that will allow us to recognize it in extra-terrestrial environments? The basic characteristics of living organisms and the paramount importance for life of the three phases of water (solid, liquid, gaseous) will be examined. The presence of those phases in many bodies of the Solar System, in interstellar matter and most likely on exoplanets will be discussed.

Chapter 6 discusses the perspectives and difficulties of establishing human bases outside Earth. Instinctively, we tend to associate the adjective “extra-terrestrial” with the life of other living and perhaps intelligent beings somewhere in the universe. Actually, the concept of extra-terrestrial life has another implication, namely the possibility to export humanity to other worlds for permanent outposts, in particular to the Moon and Mars, in the not too distant future.

Chapter 7 reviews the current knowledge on several extra-terrestrial environments that host the building blocks of life or perhaps are capable of supporting life, based on data provided by ground and space telescopes such as the Atacama Large Millimeter/Submillimeter Array (ALMA), the Hubble Space Telescope (HST), Gaia, Kepler and the Transiting Exoplanet Survey Satellite (TESS), and in situ missions such as Rosetta (discussed in Chapter 8). In addition to our instruments, nature itself provides precious information about cosmic matter, thanks to the continuous infall of meteorites, some of them coming from the Moon or Mars or the asteroid Vesta.

Chapter 8 is devoted to the European cometary mission Rosetta. For the first time in space exploration, a spacecraft spent almost two years inside the coma of a comet, from before to after the perihelion. The high number of instruments aboard (imaging devices, spectrographs, thermal mappers, magnetometer, mass spectrometers, etc.) provided a unique amount of information about the morphological and geological surface, the magnetic field and the chemistry of the atmosphere of a comet. Various points in this review quote some of the results obtained by Rosetta, while Chapter 8 will detail only some of the results directly relevant to life.

Chapter 9 illustrates the current evidence about exoplanets, their number and location and possible habitability according to the characteristics of the host star. Furthermore, the main discovery techniques by ground and space telescopes will be exposed.

Chapter 10 examines the possibility of finding intelligent species around us by radio or optical means. The Fermi paradox and the Drake equation will be discussed in such context. The symmetric event, of our Earth being discovered by aliens, will be briefly examined.

The concluding chapter, Chapter 11, will draw some conclusions, with the proviso that the field is in tumultuous expansion, with new discoveries and new theories frequently published. Hopefully, a number of notions in the current review will survive the passage of time.

To conclude this introduction, we underline that the developments in knowledge about our Earth, the Moon, Mars, the moons of Jupiter and Saturn, the many objects populating the outer Solar System and the discovery of planets orbiting other stars, have prompted the publication of a large number of excellent review papers and books. In a very personal and incomplete selection, the following books can be quoted: Goldsmith and Owen (2002), Di Mauro and Saladino (2016, in Italian) and Cockell (2015, 2017). An excellent textbook is Life in the Universe by Bennett and Shostak (2007).

Regarding reviews, Cottin et al. (2017a) summarize the results of a European Space Agency (ESA) topical team of interdisciplinary scientists focused on the utilization of near-Earth space for astrobiology. This paper presents an overview of past and current research in astrobiology conducted in Earth’s orbit and beyond, with a special focus on ESA missions such as Biopan, STONE (on Russian FOTON capsules) and EXPOSE facilities (outside the International Space Station). In an accompanying paper, Martins et al. (2017) review how Earth is used by astrobiologists to investigate life in extreme environments, its metabolisms, adaptation strategies and biosignatures; extinct life from the oldest rocks on our planet and its biosignatures; and changes in minerals, biosignatures and microorganisms under peculiar space conditions.

Several reliable websites are quoted throughout the book because information is quickly updated thanks to the policy adopted by many agencies to distribute their results as soon as they are verified. Some examples follow and other quotations can be found in the relevant chapters.

The International Astronomical Union (IAU) has established a working group on Education and Training in Astrobiology that has prepared a platform of online courses in astrobiology designed for upper-level graduate students and postdocs, as well as the interested and curious public (seehttp://astrobiovideo.com/en/). A very useful site for information on the Solar System, including astrobiological matters, is the Solar System Exploration Resource Virtual Institute (SSERVI, https://sservi.nasa.gov/). See https://www3.nd.edu/~cneal/Lunar-L/ for a repository of lunar-related documents and links. For the earlier Soviet Union missions to the Moon, see https://www3.nd.edu/~cneal/Lunar-L/4_Lunas_Lunokhods_Macau_sm.pdf. The survey of the scientific literature reported in the references was carried out to the end of 2018.

chapter 2

Cosmological Process Leading to Life on Earth

It all started with the so-called “Big Bang,” namely with the beginning of the expansion of the Universe, approximately 13.8 billion years ago.¹ The following notations will be used: Gy and My to indicate billions and millions of years, respectively; cosmological distances will be indicated mostly in light-years, l-y, so Gl-y, Ml-y. Notice that many papers use the notation Ga for gigayears, not Gy as here.

As first proposed by Alpher and Gamow around 1948, when the expanding Universe cooled down and reached temperatures equivalent to the binding energy per nucleon, namely billions of degrees Kelvin (in physical units energies of about 10 million electron volts, MeV), nucleosynthesis occurred. The rapid cooling allowed the formation of the first elements through fusion reactions using protons p (nuclei of hydrogen H, indicated by H⁺) and neutrons n present in the environment. The main processes were the p–p reaction producing deuterium (D), and the D–D reaction producing helium (H³ and H⁴). In addition, minor channels such as D + He³, p + He³ and n + He³ were also present. The light elements lithium (Li⁶ and Li⁷) and beryllium (Be⁷) were produced in minute quantities by subsequent processes involving H, He³ and He⁴. The primordial nucleosynthesis essentially stopped at Li⁷, because there are no stable elements with atomic mass A = 5 or A = 8 (see, for instance, the book by De Angelis and Pimenta, 2018).

In essence, around 75% of the mass of primordial ordinary matter (in physical terms, baryonic matter) was H, 25% was He⁴, with tiny fractions of D, He³, Li⁷ and Be⁷. A first generation of stars produced heavier elements through nuclear fusion processes in their interiors, where temperatures ranged from 108 K to 106 K, with heavier stars being hotter. After no more than 1 Gy, the formation of galaxies, their clusters, stars and perhaps planets, was already underway, although the star-formation rate had its peak at around 3 Gy after the beginning (at cosmological redshifts of z around 2),² as was recently confirmed by the Fermi–LAT collaboration using gamma-ray data (Ajello et al., 2018).

Such an evolution of structures was accompanied by an equally fast chemical evolution. The explosions of first-generation stars as supernovae (SN) enriched the interstellar medium of heavy elements, so that successive generations of stars had those elements and more complex nuclear reactions at their disposal. To be more precise, there are two basic types of SN.

Type I SN occur in binary systems with at least one star of approximately one solar mass, a surface temperature of around 104 K and a radius comparable to that of Earth, namely a so-called white dwarf, the prototype being the faint companion of Sirius. A distinctive feature of Type I SN is the absence of hydrogen lines in their spectra. Type I SN belonging to the subclass “Ia” are the best standard candles with which to measure the expansion of...

Table of contents

• Cover
• Half-Title
• Title
• Copyright
• Dedication
• Contents
• Acknowledgments
• List of Abbreviations
• Chapter 1 Introduction
• Chapter 2 Cosmological Process Leading to Life on Earth
• Chapter 3 Events in the Milky Way and Solar System
• Chapter 4 Main Characteristics of Living Organisms
• Chapter 5 Water and Life
• Chapter 6 Human Outposts on the Moon and on Mars
• Chapter 7 Comets, Asteroids, Meteorites, Death and Life
• Chapter 8 The Cometary Mission Rosetta
• Chapter 9 Planets of Nearby Stars
• Chapter 10 Search for Extra-Terrestrial Intelligence, SETI
• Chapter 11 Conclusions
• References
• Index