Solar System Update Philippe Blondel and John W. Mason (Editors) Solar System Update Published in association with PPraxisraxis PPublishiublishingng Chichester, UK Editors Dr Philippe Blondel, CGeol FGS PhD MSc Dr John W. Mason Senior Scientist Olympus Mons Department of Physics 51 Orchard Way University of Bath Barnham Bath West Sussex PO22 0HX UK UK SPRINGER–PRAXIS BOOKS IN ASTRONOMY AND PLANETARY SCIENCES SUBJECT ADVISORY EDITORS: Dr. Philippe Blondel, C.Geol., F.G.S., Ph.D., M.Sc., Senior Scientist, Department of Physics, University of Bath, Bath, UK; Dr. John Mason B.Sc., M.Sc., Ph.D. ISBN 3-540-26056-0 Springer-Verlag Berlin Heidelberg New York Springer is part of Springer-Science + Business Media (springeronline.com) Bibliographic information published by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available from the Internet at http://dnb.ddb.de Library of Congress Control Number: 2005936392 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. # Praxis Publishing Ltd, Chichester, UK, 2006 Printed in Germany 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 Completed in LaTex: EDV-Beratung, Germany Printed on acid-free paper Latest Advances in Solar System Research Solar System science is advancing faster than ever before, and even professional re- searchers in the field find it hard to keep up with developments. As this volume goes to press, the MESSENGER spacecraft is en route to Mercury, the first spacecraft to visit the planet in over 30 years; Venus Express and Mars Reconnaissance Orbiter have re- cently entered orbit around Venus and Mars, respectively; the Cassini-Huygens mission continues to produce stunning scientific results at Saturn; New Horizons has begun its nine-and-a-half year journey to Pluto; and the Rosetta spacecraft is on its way to a ren- dezvous with Comet 67P/Churyumov-Gerasimenko in 2014. In addition, ground-based telescopes and the Hubble Space Telescope continue to make important discoveries, observing a wide range of Solar System objects, including those such as Uranus and Neptune where no spacecraft is currently active or planned. This first volume of Solar System Update comprises a collection of twelve topical reviews, each presented as a separate chapter, covering important areas of Solar System science. The contributions have been written by scientists at the forefront of research in the selected areas, in a style which, we hope, will be accessible not only to advanced undergraduate students and beginning graduate students, but also to professional as- tronomers, planetary scientists and physicists, helping them to keep abreast of the latest developments in related fields. We have attempted to highlight a diverse range of topics, from recent studies of the Sun and ’space weather’, to research on the major planets and planetary satellites, including the very latest results from the Cassini mission at Sat- urn, and extending to the recent exciting discoveries in the outer Solar System beyond Neptune. As the hub of our planetary system, it is appropriate that Chapter 1 of this volume should focus on the Sun. Volker Bothmer examines early ideas about the solar wind, influences of solar activity on the Earth’s magnetic field and geomagnetic storms. He shows how space-borne instruments, observing particularly in the EUV and soft X-ray regions of the EM-spectrum, have given us new insights into solar physics and revealed details of changing solar activity as never before. Observations of the dynamic solar atmosphere, acquired over many years, by the Yohkoh, SoHO and TRACE spacecraft are comprehensively presented. Taken in combination with observations from near-Earth satellites, these studies have revealed in detail how solar eruptions affect interplanetary and near-Earth space. Discoveries about the origin and evolution of the most intense solar eruptions, the coronal mass ejections, and of the slow and fast solar wind streams, have provided a firm basis for space weather forecasts in near-Earth space. The author also looks ahead to probable future developments in this field. VI Latest Advances in Solar System Research In many respects, Mercury has been the forgotten planet of the past three decades, having been visited by just one spacecraft, Mariner 10, in 1974/75. But as Robert Strom and Ann Sprague show in Chapter 2, Mercury may provide answers to many important questions regarding the formation and evolution of our Solar System. They carefully examine what has been learned about Mercury from the Mariner 10 flybys and from 30 years of ground-based telescopic observations. The authors describe the planet’s exosphere, the exciting discovery of possible water ice deposits near Mercury’s poles, the unique internal structure of the planet and models to explain its significant magnetic field. Only 45% percent of Mercury’s surface were imaged by Mariner 10, but Earth- based images show some features on the side not imaged by Mariner. Important features of Mercury’s surface geology, ground-based spectroscopic observations which have provided clues to Mercury’s surface composition, and features relating to the tectonic, thermal and geologic history of the planet are all reviewed. Some interesting aspects of Mercury’s origin and orbital history are described and the author’s complete their chapter with a discussion of the main objectives of the MESSENGER mission due to make its first flybys of Mercury in 2008 and to go into orbit around the planet in 2011. Although the surface of Venus was mapped in detail by the Magellan spacecraft in the early 1990s, the planet has been relatively neglected for the past 15 years, in spite of the fact that a great many questions remain about its atmosphere, surface and interior. Answering such questions is important in the context of the evolution and sta- bility of terrestrial planet environments and has a relevance to global change problems on the Earth. Now, some of these questions are set to be addressed by two new space missions, Europe’s Venus Express and Japan’s Planet-C. In Chapter 3, Dmitri Titov, Hakan Svedhem and Fred Taylor examine our current knowledge of the atmosphere of Venus, and highlight areas where our knowledge is incomplete or where there is con- tradictory evidence. They discuss the temperature structure, composition and chemistry, cloud layers, radiative effects and atmospheric dynamics, as well as interactions of the atmosphere with the solar wind flow and with the planet’s surface. The probable evo- lution of the planet’s atmosphere and climate are also examined. The authors conclude by describing important aspects of the Venus Express and Planet-C missions and make some predictions for the results which can be expected. A lively debate over the origin and evolution of the Moon continues more than 30 years after the departure of the last human visitors. It is therefore fitting that Chapter 4 has been provided by Apollo 17 astronaut-geologist Harrison Schmitt, the only author in this volume to have actually visited the body they are writing about! A sizeable consensus exists that a few tens of millions of years after the formation of the Earth, the Moon belatedly came into existence as a consequence of a “giant impact” between the very young Earth and a planetoid having about 11–14% of Earth’s mass. Harrison Schmitt reviews the main tenets of this giant impact hypothesis and its consequences. He then goes on to explain why, in his opinion, such ideas deserve intense questioning, since the giant impact scenario is based on computer modelling and simulations rather than also being objectively tested against all the geochemical data available from the collected lunar samples. The author subsequently describes the major problems with the giant impact hypothesis, which lie with geochemical and geophysical information we have about the interior of the Moon below about 500 km. The author subsequently provides Latest Advances in Solar System Research VII some alternative ideas about the origin of the Moon and its subsequent evolution, and he reviews the various episodes of large basin formation early in the Moon’s history. Mars has been the subject of intense scrutiny in recent years and it will come as no surprise that two chapters in this volume have been devoted to the Red Planet. In Chapter 5, Stephen Lewis and Peter Read discuss evidence for climate change on Mars. Although the atmosphere and surface of Mars are extremely dry at present, spacecraft imagery has revealed surface features apparently caused by flowing water on the surface in the past, leading to speculation that the planet was once relatively warm and wet. Recent space missions have provided a detailed picture of the present day climate of Mars, including the discovery of water ice in the permanent polar ice caps as well as in the upper layers of the surface across much of the planet. The Mars Exploration Rovers have also found in situ evidence for ancient water in the nearby rocks and landscape including, at the Opportunity site, rocks which appear to be a mixture of sediments formed during episodic events of shallow surface water.