Planets and life

Casoli Fabienne, Lequeux James, Thérèse Encrenaz

Description

The Earth is the only planet in the Solar System where liquid water is present on the surface, a condition that seems necessary for the development of life. Its sisters Venus and Mars are extremely different. Why did these three planets, born under fairly comparable conditions, evolve to the conditions we observe today? Understanding the physical or chemical factors that are at the origin of such divergent evolutions is a first step in an approach to the problem of the origin of life on Earth.

Sujets

• Sciences expérimentales
• Astronomie et astrophysique
• Astronomie
• Astrophysique
• Environnement
• Mars
• Science
• Venus
• Earth
• Astronomy
• Solar System
• Environment
• Sciences et techniques
• Space Science

Informations

• Publié par: EDP Sciences
• Date de parution: 20 mai 2021
• Nombre de lectures: 0
• EAN13: 9782759825707
• Langue: English
• Poids de l'ouvrage: 18 Mo

Extrait

Current Natural Sciences

Thérèse ENCRENAZ, James LEQUEUX and Fabienne CASOLI

Planets and Life

A S T R O N O M Y

A S T R O N O M Y

ISBN : 978-2-7598-2563-9

9 782759 825639

Planets and Life

Current Natural Sciences

Thérèse ENCRENAZ, James LEQUEUX and Fabienne CASOLI

The Earth is the only planet in the Solar System where liquid water is present on the surface, a condition that seems necessary for the development of life. Its sisters Venus and Mars are extremely different. Why did these three planets, born under fairly comparable conditions, evolve to the conditions we observe today? Understanding the physical or chemical factors that are at the origin of such divergent evolutions is a first step in an approach to the problem of the origin of life on Earth.

This question takes on a new dimension with the discovery of thousands of planets around the stars of our Galaxy, some of which could resemble the Earth. Could they be home to life? With their discovery, the question “Are we alone in the Universe?” is no longer limited to our Solar System, and the field of possibilities opens up to infinity. It is now possible to approach the problem from a scientific perspective and not only from a philosophical one, as was the case in the past.

The enthusiasm of the public for the subject sometimes results in sensational and premature announcements. This book reminds us that there is still a long way to go before we can detect life outside the Earth.

Thérèse ENCRENAZ is an expert in planetary atmospheres. She has directed the Department of Space Research of the Paris Observatory and was the vice-president of the Scientific Council of this obser- vatory. She has written several textbooks and popular books.

James LEQUEUX has directed the Nançay radioastronomy station of the Paris Observatory and the Marseilles observatory. He has been for fifteen years editor-in-chief of the European journal Astronomy & Astrophysics. He has published several textbooks and popular books, in particular on the history of science.

Fabienne CASOLI is President of the Paris Observatory. Her present scientific interests concern the large national and international projects in radioastronomy: LOFAR, NenuFAR and SKA.

www.edpsciences.org

Current Natural Sciences

Thérèse ENCRENAZ, James LEQUEUX and Fabienne CASOLI

Planets and Life

Cover illustration:The upper image of the cover is a view of the dry, desolated surface of planet Mars (© ESA). The lower image depicts as a contrast water and luxuriant life on planet Earth (Wikimedia Commons).

Printed in France

EDP Sciences – ISBN(print): 978-2-7598-2563-9 – ISBN(ebook): 978-2-7598-2570-7 DOI: 10.1051/978-2-7598-2563-9

All rights relative to translation, adaptation and reproduction by any means whatsoever are reserved, worldwide. In accordance with the terms of paragraphs 2 and 3 of Article 41 of the French Act dated March 11, 1957, “copies or reproductions reserved strictly for private use and not intended for collective use” and, on the other hand, analyses and short quotations for example or illustrative purposes, are allowed. Otherwise, “any representation or reproduction – whether in full or in part – without the consent of the author or of his successors or assigns, is unlawful” (Article 40, paragraph 1). Any representation or reproduction, by any means whatsoever, will therefore be deemed an infringement of copyright punishable under Articles 425 and following of the French Penal Code.

 Science Press, EDP Sciences, 2021 ©

Foreword

With a surface pressure of 1bar and an average temperature of 15°C, the Earth is the only planet in the Solar System with liquid water on its surface. The surface conditions of Venus and Mars are extremely different, with a pressure close to one hundred times the Earth’s value on Venus and less than one hundredth on Mars, and a temperature ranging from more than 460°C on Venus to about –50°C on Mars. How could these three planets, starting from relatively comparable initial conditions, have evolved to the extreme diversity we observe today? Understanding the origin and evolution of the atmospheres of the three terrestrial planets – Venus, Earth and Mars – is a major challenge for planetology. Highlighting the physical or chemical factors that were and are still at play appears as a first step to better understand the context in which life appeared and developed on the Earth. This question takes a new dimension with the discovery, since a quarter of century, of thousands of extrasolar planets,among which are many “rocky” ones, i.e., with a surface like the terrestrial planets of the Solar System. They are called, according to their mass, “exo-Earths” or “super-Earths”. Could some of these exoplanets harbor life? With their discovery, the question “Are we alone in the Universe?”, which is as old as humanity itself, is no longer confined to our Solar System, and the field of possibilities opens up to infinity. In this new context, it is more than ever necessary to understand the evolution of the rocky planets and to identify the factors that determine their habitability,i.e., their capacity to allow the emergence and development of life. These factors can be multiple. Some are of physico-chemical nature (pressure and temperature, atmospheric composition); others are related to the planet’s environment (nature of the star, presence of a magnetosphere) or some of its parameters (ellipticity of the orbit, obliquity of the planet axis, rotation period). For more than two millennia, the quest for extraterrestrial life, present from the earliest ages of mankind, has been based on philosophical considerations. It is only since the end of the 19th century that astronomers were able to begin to approach the question in a scientific manner, first with the observation of the planets that surround us, and then, half a century later, with the search for exoplanets around other stars. The end of the 20th century witnessed an avalanche of discoveries that continued and amplified until now. Increasingly complex planetary space missions explore the soil and subsoil of Mars for possible traces of fossil life; others, in the coming decades, will explore the satellites of the giant planets of the outer Solar

DOI: 10.1051/9782759825639.c901 © Science Press, EDP Sciences, 2021

IV

Foreword

System, some of which may harbor an ocean of liquid water beneath their icy surfaces. In parallel, we now have the possibility to determine the nature of the exoplanets and, in some cases, their atmospheric composition. Among the rocky exoplanets known today, several tens could have a temperature compatible with the presence of liquid water. In one or two decades, this research will be refined to allow, perhaps, to discover on one or more of them oxygen or its derivative,ozone, a possible signature of the presence of life. In this context of abundant and constantly evolving research, it seemed useful to try to better define the criteria for habitability of rocky exoplanets, those that could shelter life. This book is to some extent the continuation of the book “The Exoplanet Revolution”, by J. Lequeux, T. Encrenaz and F. Casoli, published in the same collection in 2020. Like it, it is addressed to all audiences interested in astronomy, planetology and the search for extraterrestrial life. Here, we start from the planets we know well, the three terrestrial planets of the Solar System possessing an atmosphere, to analyze the various physico-chemical mechanisms that could have been responsible for their divergent evolution. Then we will try to extrapolate these results to the rocky extrasolar planets, in order to understand the possible mechanisms of their evolution and to apprehend what could be their conditions of habitability. Finally, we conclude this work by an analysis of the means that could allow us to discover possible traces of life, or even to communicate with eventual distant civilizations.

Contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 2 The Formation of Terrestrial Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 From Antiquity, the Myth of the Plurality of Worlds . . . . . . . . . . . . . . 2.2 The Primordial Nebula Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 The Formation of Stars and Discs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 The Formation of Terrestrial and Giant Planets . . . . . . . . . . . . . . . . . 2.5 The Migration of Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 The Late Heavy Bombardment and Its Consequences . . . . . . . . . . . . . 2.7 The Formation of Planets in Exoplanetary Systems . . . . . . . . . . . . . . . 2.8 The Primary Atmospheres of the Terrestrial Planets . . . . . . . . . . . . . . 2.9 What Atmospheres for Rocky Exoplanets? . . . . . . . . . . . . . . . . . . . . .

CHAPTER 3 The Exploration of Terrestrial Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 The First Modern Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 The Myth of the Martian Canals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 The Physical Nature of Planets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 The Beginning of the Space Era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 The Viking Mission: Hopes and Disillusions . . . . . . . . . . . . . . . . . . . . 3.6 From Mars to Venus… . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 The Renewal of Martian Exploration . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Return to Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Mars and Venus Today . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Between Venus and Mars, the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11 Towards a Comparative Study of the Terrestrial Planets . . . . . . . . . . .

III

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9 9 11 13 16 19 21 23 24 25

27 27 29 30 30 32 33 35 39 41 42 46

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Contents

CHAPTER 4 Venus, Earth and Mars: A Diverging Evolution . . . . . . . . . . . . . . . . . . . . . . 4.1 The Astonishing Variety of Terrestrial Planets . . . . . . . . . . . . . . . . . . 4.2 And Yet…Common Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 The Thermal Structure of the Terrestrial Planets . . . . . . . . . . . 4.2.2 Atmospheric Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Internal Structure and Volcanism . . . . . . . . . . . . . . . . . . . . . . . 4.3 Terrestrial Planets at the Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Secondary Atmospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Primitive Atmospheres Rich in Water . . . . . . . . . . . . . . . . . . . . 4.3.3 The Paradox of the“Young Sun” . . . . . . . . . . . . . . . . . . . . . . . 4.4 History of the Terrestrial Planets: A Divergent Evolution . . . . . . . . . . 4.4.1 Venus: The Ravages of a Runaway Greenhouse Effect . . . . . . . . 4.4.2 Mars: A Planet on the Verge of Geological Extinction . . . . . . . 4.4.3 The Earth, Ideally Located in Relation to the Sun . . . . . . . . . .

CHAPTER 5 The Appearance of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 What is Life? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 From Spontaneous Generation to Primordial Soup . . . . . . . . . . . . . . . 5.3 The First Experiments in Prebiotic Chemistry . . . . . . . . . . . . . . . . . . 5.4 The Building Blocks of Terrestrial Life . . . . . . . . . . . . . . . . . . . . . . . . 5.5 The Origin of Prebiotic Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 The Rise of Complexity from Prebiotic Molecules . . . . . . . . . . . . . . . . 5.7 The Formation of Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Metabolism and the Question of Energy . . . . . . . . . . . . . . . . . . . . . . . 5.9 The Genetic Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 The Ancestor of All Living Beings? . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.11 Life on Earth as a Model for Life on Other Planets? . . . . . . . . . . . . . . 5.12 The Beginnings of Life on Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.13 Life on Exoplanets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 6 The Development of Life on Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 The Paradox of the“Young Sun” . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 The Major Stages in the Evolution of the Earth’s Climate . . . . . . . . . 6.2.1 From Hadean to Archean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 From Archean to Proterozoic: The Great Oxidation Event . . . . 6.2.3 The Phanerozoic: Life on the Continents . . . . . . . . . . . . . . . . . 6.3 What Future for the Earth’s Atmosphere? . . . . . . . . . . . . . . . . . . . . . 6.4 What Lessons for Exobiology? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47 49 52 53 54 55 56 56 57 59 61 61 62 63

65 65 66 68 70 73 75 76 78 79 79 81 82 83

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Contents

CHAPTER 7 Life in the Solar System? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 The Habitability Zone in the Solar System . . . . . . . . . . . . . . . . . . . . . 7.2 A Past Ocean on Venus? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Searching for Traces of Life on Mars . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Other Niches in the Solar System . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 8 How to Search for Life on Rocky Exoplanets? . . . . . . . . . . . . . . . . . . . . . . . . 8.1 The Discovery of Exoplanets: Where Do We Stand? . . . . . . . . . . . . . . 8.2 The Exoplanet Concept: An Old Idea . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Early Discoveries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 The Successes of Velocimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 A New Step: The Transit Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 How to Search for Life on an Exoplanet? . . . . . . . . . . . . . . . . . . . . . . 8.7 Satellites Around Giant Exoplanets? . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8 How to Determine the Atmospheric Composition of an Exoplanet? . . . 8.9 How to Search for Life from the Spectrum of an Exoplanet? . . . . . . . .

CHAPTER 9 Conclusions: Some Future Directions in Exobiology . . . . . . . . . . . . . . . . . . . 9.1 The Future of Mars Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 How to Detect Traces of LifeIn Situ? . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Towards an Inhabited Exploration of Mars? . . . . . . . . . . . . . . . . . . . . 9.4 Towards External Satellites, Other Possible Niches for Life . . . . . . . . . 9.5 Exploring Exoplanets: The Prospects . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 What If We Were Not Alone? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VII

99 99 102 104 106

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135 135 139 139 141 142 146

149 155