Searching for Water in the Universe (Springer Praxis Books Popular Astronomy)

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Searching for Water in the Universe

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Praxis Publishing Chichester, UK

Dr Therese Encrenaz Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA) Paris Observatory Meudon France Original French edition: A la recherche de Veau dans Vunivers Published © Editions Belin 2004 Ouvrage publie avec le concours du Ministere francais charge de la culture - Centre national du livre This work has been published with the help of the French Ministere de la Culture Centre National du Livre Translator: Bob Mizon, 38 The Vineries, Colehill, Wimborne, Dorset, UK SPRINGER-PRAXIS BOOKS IN POPULAR ASTRONOMY SUBJECT ADVISORY EDITOR: John Mason B.Sc, M.Sc, Ph.D. ISBN 10: 0-387-34174-9 Springer Berlin Heidelberg New York ISBN 13: 978-0-387-34174-3 Springer is a part of Springer Science + Business Media (springeronline.com)

Library of Congress Control Number: 2006926438

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. © Copyright, 2007 Praxis Publishing Ltd. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: Jim Wilkie Copy editing: R. A. Marriott Typesetting: BookEns Ltd, Royston, Herts., UK Printed in Germany on acid-free paper

To Pierre

I would like to thank Belin Publishing and Praxis Publishing for their valued and efficient support in the realisation of this book, Fabienne Casoli for reading the original text in the French edition, and Bob Mizon for his excellent translation into English. I am also grateful to Pierre Cox, who, with his specialist understanding of the question of water in the interstellar medium, has helped guide me beyond the bounds of the solar system. Finally, I wish to thank all those colleagues who gave me access to their documents, so useful to the fabric of this text.

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INTRODUCTION W H Y WATER?

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Life on Earth... . . . and elsewhere in the Universe

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A VERY SIMPLE MOLECULE

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The H z O molecule The various states of water Great cosmic abundance The spectrum of the water molecule The ortho and para states of water Heavy water How do we search for water in the Universe?

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THE QUEST FOR COSMIC WATER

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1877: canals on Mars 1950-1970: Mars, Saturn and interstellar water 1970-1990: Mars and the comets 1994-1995: water vapour and the galaxies 1995-1998: the Infrared Space Observatory The post-ISO era Future projects: Herschel and SPICA

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THE ICE LINE AND THE BIRTH OF THE PLANETS

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The solar system today The collapse of the protosolar cloud From protoplanetary disk to planetesimals Terrestrial planets and giant planets A brief chronology of events Where do we look for water in the solar system?

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COMETS AND WATER

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The nucleus: a 'dirty snowball' Halley's comet, 1986: the first detection of water vapour

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Contents

An elusive kind of ice Water ice... and others Cometary matter and interstellar matter Water: historian of the comets Space exploration of comets: recent results and future projects

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WATER IN THE SOLAR SYSTEM The atmospheres of the giant planets Water and the giant planets Satellites of the outer solar system The Galilean satellites Saturn's satellites The companions of Uranus Triton: an example of cryovolcanism Rings and minor satellites of the giant planets Pluto and the trans-Neptunian objects

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AT THE ICE LINE: THE ASTEROIDS Minor planets Asteroid or comet? Meteorites: the possibility of in situ measurement

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WATER AND THE TERRESTRIAL PLANETS Mercury and the Moon: no atmosphere, but traces of water? Phobos and Deimos: Mars' tiny moons Venus, Earth and Mars: three very different worlds Traces of water vapour on Mars and Venus The history of water on Mars and Venus Divergent destinies The history of water on Mars Searching for life on Mars

141 143 144 145 147 152 153 157 159

THE SEARCH FOR OTHER EARTHS How do we define life? How does life begin? Early discoveries of exoplanets Giant exoplanets near stars Are there other Earth-like planets? Possibilities of life on Earth-like planets How do we find extraterrestrial life? The search for extraterrestrial civilisations

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GLOSSARY BIBLIOGRAPHY INDEX

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Introduction

Why water?

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W h y water?

nterestingly, water is present on our planet in three different states: vapour, liquid and solid. In its liquid form, it has played an essential part in the appearance, development and maintenance of terrestrial life. What is its role elsewhere in the Universe? In its gaseous and solid forms, water is omnipresent: in the most distant galaxies, among the stars, in the Sun, in its planets and their satellites and ring systems, and in comets. Is there extraterrestrial life? We still await the answer, and the search for liquid water is an indispensable aspect of that answer. If the Earth's oceans did not exist, we would not be here to ask why. Nowadays, everyone understands the essential and undeniable role that liquid water has played in the emergence, development and maintenance of life on Earth. Liquid water is by far the major constituent of the mass of living organisms, be they animals or plants. And, looking to the future, the availability of our planet's reserves of fresh water is an ever more serious issue for us all, as population numbers soar. Access to that water will present a great challenge during the century which has just begun. We know that the abyssal oceans gave rise to the first signs of life on Earth, but what is less well known is that those oceans have ensured the stability of our habitable planet, with its temperate climates. Since the earliest times, the Earth's temperature has remained relatively stable: we have not had to suffer the kind of runaway greenhouse effect which characterises Venus, leading to scorching surface temperatures of 730 K (about 450° C); neither does the Earth's surface resemble the freezing deserts of Mars, with their average temperature of 230 K (-43° C).

The greenhouse effect is a mechanism causing warming of the surface and lower atmosphere of the Earth or other planet. The surface, heated by solar radiation (especially in the visible part of the spectrum at wavelengths of 0.40.8 jam), reaches a stable temperature regulated by the fraction of solar radiation reflected, known as the albedo (of the order of 0.3 for the Earth), the rest being absorbed and converted into thermal energy. For Earth, this equilibrium temperature is 255 K (-18 C). At this temperature, a black body (absorbing all incident electromagnetic energy) emits mainly in the infrared (A. > 1 jam). In the case of the Earth, the infrared radiation emitted from the surface is absorbed by two atmospheric gases: water vapour and carbon dioxide. The absorption of surface radiation by the lower atmosphere in turn contributes to the warming of the surface, and the process is amplified. This is known as the greenhouse effect, so called because it is analogous to the mechanism

Clouds above the Pacific. An image taken from the International Space Station on 21 July 2003.

Life on E a r t h . . .

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whereby a greenhouse is heated, its glass playing the role of the lower atmosphere and letting through the visible radiation but blocking the infrared. On Earth, this involves a heating effect of 33 C - a modest value kept constant by a self-regulatory mechanism involving the oceans. The phenomenon is less marked on Mars (4 C), but it was undoubtedly much more important in the past. On Venus, the effect is dramatic. The surface heated to a temperature of 730 K (more than 450 C), showing how the mechanism can run wild if no regulation is present. This illustrates the threat to the Earth's climate posed by increased quantities of carbon dioxide, if humans continue to produce it at current rates.

Figure 1. The mechanism of the greenhouse effect Some of the Sun's radiation reaches the surface and warms it. The surface then emits infrared radiation which is absorbed by infrared-active gases in the lower atmosphere (C0 2 , H20). The lower atmosphere, thus warmed, re-emits radiation towards the surface and further warms it, amplifying the phenomenon. The numbers indicate the radiation budget in W/m 2 . (From S. Jousseaume, Climat d'Hier a Demain, CNRS Editions-CEA, 1993.)

The first thing to note is that the water molecule, a very minor constituent (

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