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_'Searching for Water in the Universe', © Springer, Published in association with Praxis Publishing, Chichester, UK. Dr Therese Encrenaz Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique (LESIA) Paris Observatory Meudon France Original French edition: A la recherche de l'eau dans l'univers. Published © Editions Belin 2004. Ouvrage publié avec le concours du Ministère français chargé 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. INTRODUCTION WHY WATER? 1 Life on Earth and elsewhere in the Universe 10 A VERY SIMPLE MOLECULE The H2O 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? 2 THE QUEST FOR COSMIC WATER 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 3 THE ICE LINE AND THE BIRTH OF THE PLANETS 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? 4 COMETS AND WATER The nucleus: a 'dirty snowball' Halley's comet, 1986: the first detection of water vapour 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 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 AT THE ICE LINE: THE ASTEROIDS Minor planets Asteroid or comet? Meteorites: the possibility of in situ measurement 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 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? Figure 8.9 These infrared planetary spectra (between 6 and 17 µm) show the signatures of methane, carbon dioxide and ammonia. This project is to be various gases present in their atmospheres: H2O and CO2 for terrestrial planets, and O3 for Earth. The (vertical) luminance scale refers to the flux emitted by the planet. Sure, a long-term one involving enormous technological challenges, and is unlikely to be realised within the next ten years. The search for other Earths 178 Figure 8.10 An artist's impression of the Darwin mission, consisting of a flotilla of six telescopes - each with an aperture of more than 1 metre - in an orbit beyond 5 AU from the Sun. Working as an interferometer, they will be able to image exoplanets in the vicinity of nearby stars. Albedo The fraction of incident light reflected by a body. Aphelion In an orbit, the furthest point from the Sun. Arcminute A unit of angular measurement: 1° = 60 arcmin (minutes of arc), and 1 arcmin = 60 arcsec (seconds of arc). The angular diameter of the Moon is about 30 arcmin. Asteroid belt The main belt of asteroids, between the orbits of Mars and Jupiter. Asteroids (or minor planets) are solar system objects orbiting the Sun, of sizes between a fraction of a kilometre and nearly 1,000 km. Most of them are in the asteroid belt. Astronomical Unit (AU) The mean distance between the Earth and the Sun - about 150 million km. This unit is used to measure distances within the solar system. Black body A body which absorbs all incident radiation (albedo = 0). Centaur An asteroid travelling between the paths of Jupiter and Neptune, often with a very eccentric orbit. One of them, Chiron, has shown comet-like activity. Circumstellar region The immediate environment of a star within which its radiation and stellar wind interact with the rest of its protoplanetary disc and the nearby interstellar medium. Clathrates Solid matrices of water molecules able to entrap another element, for example a molecule of methane or carbon dioxide, or an atom of argon. Clathrates of methane and carbon dioxide exist on Earth. In the case of methane, a molecule of CH4 is trapped within a matrix composed of about six water molecules. Coma A cometary envelope containing gas, ices and dust ejected from the comet's nucleus as a result of solar radiation. Comet Comets are objects formed from ice and dust, and are a few kilometres in diameter. When they approach the Sun, outgassing of volatile elements causes a coma and a tail to develop, sometimes offering a spectacular sight in the sky. 180 Glossary Ключевые слова: disc, micrometeoroid heated, material, chemical compound, big bang, great variety, satellite, academic press, surface, vapour liquid, radio domain, roche limit, post-iso era, water vapour, mass, mission, outer, weighs tonne, probe, isotopic dating, mars, michael mumma, carbonaceous chondrite, living organism, oxygen, tidal force, time, internal structure, telescope, image, giant planet, energetic radiation, dust, abundance, water, parent body, minor constituent, arizona press, resolving power, initial explosion, wa, orbit, kuiper belt, life, paris observatory, discovered, worth mentioning, mineralogical composition, earth, cosmic, mauna kea, ha, ariane rocket, terrestrial, element, high, semi-major, titan, heavenly body, permanent cap, distance, formed, branching valley, signature, atmospheric circulation, main reservoir, process, outflow channel, model, haute-provence observatory, measurement, electromagnetic radiation, great step, science, form, presses universitaires, rev, phenomenon, high resolution, detected, temperate climate, planet, cumulus cumulonimbus, prebiotic chemistry, radial velocity, velocimetric method, celestial mechanic, atmospheric window, protosolar material, wavelength, molecule, iks-vega spectrometer, photochemical model, millimetric domain, ice line, spectrum, origin, presence, jim, fact box, object, energy liberated, artists impression, observed, solar, water ice, meteorite, moon, altitude, atmosphere, heterodyne spectroscopy, wave number, asteroid, le, energetic particle, neptune, belt, long, temperature, nucleus, universe, atmospheric composition, atreya atmospheres, exoplanets, astronomer, ratio, tour dhorizon, year, vapour, formation, interstellar matter, point, luminous trail, cometary, carbon dioxide, comet, space, chapter, venus, iso, solid, result, chemical, thermal energy, method, au, southern hemisphere, chemical reaction, cotardiere, main belt, reaction leading, planetary, electrically neutral, minor component, spectral, liquid, absorb radiation, cometary physic, cloud, ocean, carbon, uranus, quantity, primordial nucleosynthesis, interstellar medium, celestial body, thermochemical model, infrared, polar cap, carbon monoxide, rosetta probe, chemical characteristic, voyager, interstellar, differential tidal, case, halley, radiation, pressure, gas, ice, tauri phase, protoplanetary disc, van dishoek, molecular nitrogen, total mass, photochemical reaction, energy structure, supernova explosion, type, spatial resolution, question, carbonaceous material, comets tail, ring, nature, atmospheric, density, search, internal energy, oka science, saturn, observation, star, sun, giant, jupiter, atom, composition, terrestrial planet, amino acid, fabienne casoli, body, hydrogen, edgeworth-kuiper belt