given the acronym TTAPS （for Richard P。 Turco； （even B。 Toon； Thomas Ackerman； James Pollack； and myself）。 Of the five TTAPS scientists； two were planetary scientists； and the other three had published many papers in planetary science； The earliest intimation of nuclear winter came during that same Mariner 9 mission to Mars； when there was a global dust storm and we were unable to see the surface of the planet； the infrared spectrometer on the spacecraft found the high atmosphere to be warmer and the surface colder than they ought to have been。 Jim Pollack and I sat down and tried to calculate how that could e about。 Over the subsequent twelve years； this line of inquiry led from dust storms on Mars to volcanic aerosols on Earth to the possible extinction of the dinosaurs by impact dust to nuclear winter。 You never know where science will take you。
PLANETARY SCIENCE fosters a broad interdisciplinary point of view that proves enormously helpful in discovering and attempting to defuse these looming environmental catastrophes。 When you cut your teeth on other worlds； you gain a perspective about the fragility of planetary environments and about what other； quite different； environments are possible。 There may well be potential global catastrophes still to be uncovered。 If there are； I bet planetary scientists will play a central role in understanding them。
Of all the fields of mathematics； technology； and science； the one with the greatest international cooperation （as determined by how often the co…authors of research papers hail from two or more countries） is the field called 〃Earth and space sciences。〃 Studying this world and others； by its very nature； tends to be non…local； non…nationalist； non…chauvinist。 Very rarely do people go into these fields because they are internationalists。 Almost always； they enter for other reasons； and then discover that splendid work； work that plements their own； is being done by researchers in other nations； or that to solve a problem； you need data or a perspective （access to the southern sky； for example） that is unavailable in your country。 And once you experience such cooperation—humans from different parts of the planet working in a mutually intelligible scientific language as partners on matters of mon concern—it's hard not to imagine it happening on other； nonscientific matters。 I myself consider this aspect of Earth and space sciences as a healing and unifying force in world politics； but； beneficial or not； it is inescapable。
When I look at the evidence； it seems to me that planetary exploration is of the most practical and urgent utility for us here on Earth。 Even if we were not roused by the prospect of exploring other worlds； even if we didn't have a nanogram of adventuresome spirit in us； even if we were only concerned for ourselves and in the narrowest sense； planetary exploration would still constitute a superb investment。
CHAPTER 15 THE GATES OF THE WONDER WORLD OPEN
The great floodgates of the wonder…world swung open。
—HERMAN MELVILLE； MOBY DICK； CHAPTER 1 （1851）
Sometime ing up； perhaps just around the corner； there will be a nation—more likely； a consortium of nations—that will work the next major step in the human venture into space。 Perhaps it will be brought about by circumventing bureaucracies and making efficient use of present technologies。 Perhaps it will require new technologies； transcending the great blunderbuss chemical rockets。 The crews of these ships will set foot on new worlds。 The first baby will be born somewhere up there。 Early steps toward living off the land will be made。 We will be on our way。 And the future will remember。
TANTALIZING AND MAJESTIC； Mars is the world next door； the nearest planet on which an astronaut or cosmonaut could safely land。 Although it is sometimes as warm as a New England October； Mars is a chilly place； so cold that some of its thin carbon dioxide atmosphere freezes out as dry ice at the winter pole。
It is the nearest planet whose surface we can see with a small telescope。 In all the Solar System； it is the planet most like Earth。 Apart from flybys； there have been only two fully successful missions to Mars： Mariner 9 in 1971； and Vikings 1 and 2 in 1976。 They revealed a deep rift valley that would stretch from New York to San Francisco； immense volcanic mountains； the largest of which towers 80；000 feet above the average altitude of the Martian surface； almost three times the height of Mount Everest； an intricate layered structure in and among the polar ices； resembling a pile of discarded poker chips； and probably a record of past climatic change； bright and dark streaks painted down on the surface by windblown dust； providing high…speed wind maps of Mars over the past decades and centuries； vast globe…girdling dust storms； and enigmatic surface features。
Hundreds of sinuous channels and valley networks dating back several billion years can be found； mainly in the cratered southern highlands。 They suggest a previous epoch of more benign and Earthlike conditions—very different from what we find beneath the tenuous and frigid atmosphere of our time。 Some ancient channels seem to have been carved by rainfall； some by underground sapping and collapse； and some by great floods that gushed up out of the ground。 Rivers were pouring into and filling great thousand…kilometer…diameter impact basins that today are dry as dust。 Waterfalls dwarfing any on Earth today cascaded into the lakes of ancient Mars。 Vast oceans； hundreds of meters； perhaps even a kilometer； deep may have gently lapped shorelines barely discernible today。 That would have been a world to explore。 We are four billion years late。*
* Although in a few places； such as the slopes of the elevation called Alba Patera； there are multibranched valley networks that by parison are very young。 Somehow； even in the most recent billion years； liquid water seems to have flowed here and there； from time to time； through the deserts of Mars。
On Earth in just the same period； the first microorganisms arose and evolved。 Life on Earth is intimately connected； for the most basic chemical reasons； with liquid water。 We humans are ourselves made of some three…quarters water。 The same sorts of organic molecules that fell out of the sky and were generated in the air and seas of ancient Earth； should also have accumulated on ancient Mars。 Is it plausible that life quickly came to be in the waters of early Earth； but was somehow restrained and inhibited in the waters of early Mars？ Or might the Martian seas have been filled with life—floating； spawning； evolving？ What strange beasts once swum there？
Whatever the dramas of those distant times； it all started to go wrong around 3。8 billion years ago。 We can see that the erosion of ancient craters dramatically began to slow about then。 As the atmosphere thinned； as the rivers flowed no more； as the oceans began to dry； as the temperatures plummeted； life would have retreated to the few remaining congenial habitats； perhaps huddling at the bottom of ice…covered lakes； until it too vanished and the dead bodies and fossil remains of exotic organisms—built； it might be； on principles very different from life on Earth—were deep…frozen； awaiting the explorers who might in some distant future arrive on Mars。
METEORITES ARE FRAGMENTS OF OTHER WORLDS recovered on Earth。 Most originate in collisions among the numerous asteroids that orbit the Sun between the orbits of Mars and Jupiter。 But a few are generated when a large meteorite impacts a planet or asteroid at high speed； gouges out a crater； and propels the excavated surface material into space。 A very small fraction of the ejected rocks； millions of years later； may intercept another world。
In the wastelands of Antarctica； the ice is here and there dotted with meteorites； preserved by the low temperatures and until recently undisturbed by humans。 A few of them； called SNC （pronounced 〃snick〃） meteorites* have an aspect about them that at first seemed almost unbelievable： Deep inside their mineral and glassy structures； locked away from the contaminating influence of the Earth's atmosphere； a little gas is trapped。 When the gas is analyzed； it turns out to have exactly the same chemical position and isotopic ratios as the air on Mars。 We know about Martian air not just from spectroscopic inference but from direct measurement on the Martian surface by the Viking landers。 To the surprise of nearly everyone； the SNC meteorites e from Mars。
* Short for Shergotty…Nakhla…Chassigny。 You can see why the acronym is used。
Originally； they were rocks that had melted and refrozen。 Radioactive dating of all the SNC meteorites shows their parent rocks condensed out of lava between 180 million and 1。3 billion years ago。 Then they were driven off the planet by collisions from space。 From how long they've been exposed to cosmic rays on their interplanetary journeys between Mars and Earth； we can tell how old they are—how long ago they were ejected from Mars。 In this sense； they are between 10 million and 700；000 years old。 They sample the most recent 0。l percent of Martian history。
Some of the minerals they contain show clear evidenc