Science Goes to the Moon and Planets: Celebrating 50 years since the IGY
Wesley T. Huntress, Jr.
Carnegie Institution of Washington
This year marks the fiftieth anniversary of the International Geophysical Year (IGY). The IGY was organized by an international council of scientists in 1955 and set to begin on July 1, 1957. It was the largest international scientific endeavor ever undertaken, and it actually went on for about 5 years. The significance of the IGY to the space age is that both the United States and the Union of Soviet Socialist Republics (USSR) proposed to orbit satellites of Earth as part of the IGY. Both succeeded and opened the door to space. The scientific exploration of space began as an element of the IGY.
After the launch of Sputnik on October 4, 1957, and the first U.S. satellite, Explorer, almost 4 months later, the United States established its civilian space agency, NASA, on October 1, 1958. The U.S. National Academy of Sciences had already established its Space Science Board, now named the Space Studies Board, in June 1958 to advise federal agencies on research in space. In commemoration of the IGY, the opening of a new age of space science, and the establishment of NASA and the Space Studies Board, it seems very appropriate now to reflect back on these past 50 years, how far we have come, and where we want to go.
A LITTLE HISTORY
A little more than 50 years ago we had no space program at all. But we did have a vision. Americans had been treated to a dream of space travel authored by Werner Von Braun in magazine articles and on television programs. In 1952, Collier’s magazine began a series of articles about Von Braun and his vision for putting a space station in orbit around Earth and using it as an assembly point to send spaceships first to the Moon and then to Mars. The articles were filled with fabulous paintings by Chesley Bonestell illustrating how all of this would be done (Figure 3.1). It was science fiction brought to reality. The articles were thrilling. And shortly afterwards Walt Disney, who had an immensely popular weekly show on television, made animated movies based on the Collier’s articles that brought it all to life for American audiences.
The first Collier’s article outlined in technical detail and in brilliant illustrations how man would conquer space with new rockets and space stations—written by experts with considerable respect. The second issue showed how we would get to the Moon. It all seemed fantastic but at the same time credible, and a fair amount of it actually came true. We even dreamed in the mid-1950s of going to Mars—the planet foremost and most mysterious in the mind of man—and the third article in Collier’s in 1954 showed how we could do it.
It was not just a U.S. dream either. The Russians also had dreams to go to the Moon and Mars—visions contemplated by Sergei Korolev, the hidden rival in the Soviet Union to Von Braun in the United States. The USSR built its own version of the Saturn V, the N1, but could not make it succeed. After realizing in 1969 that they had lost the race to the Moon, the Russians countered with robotic rovers and sample return mis-
sions to the lunar surface—perhaps not as dramatic as astronautics walking on the surface, but certainly just as scientifically valuable, demonstrating the utility and excitement of robots traveling beyond Earth and exploring the surface of new worlds under control of humans on Earth.
We did set foot on the Moon almost exactly as Von Braun had originally envisioned, but not on Mars.
After Apollo, the political will in the United States evaporated. In 1972 the United States abandoned the Apollo program and the future promise of lunar bases and human flights to Mars. The human space exploration enterprise retreated to Earth and was resigned to remain in Earth orbit.
While human space exploration languished after 1972, robotic exploration flourished (see Figures 3.3, 3.4, 3.5, and 3.6), and that has kept our dreams alive. Humans may not have exploded out into the solar system, but our robots certainly have. We have leapt off the surface of our home planet and sent robotic extensions of our eyes, ears, noses, arms, and legs to the far reaches of the solar system. Our robotic explorers go where we cannot go because of the limitations of our bodies, and they go where we cannot yet go because of the limitations of our own vision and will.
Since the abandonment of human exploration of space beyond Earth, robotic spacecraft have surveyed the solar system from Mercury to beyond Pluto; orbited Venus, Mars, Jupiter and Saturn; and landed on the Moon, Venus, Mars, and Titan to show us the bizarre surfaces of exotic new worlds. In 1957 these places could only be imagined, and traveling to them was in the realm of science fiction. Today, the solar system has become our backyard.
WHY DO WE EXPLORE SPACE?
Why do we find spaceflight so compelling? Because exploration is part of what we are as human beings. We have cultural, scientific, political, and economic incentives to explore space.
Humans have an inborn imperative to explore and to understand. Exploration and the drive to make new discoveries and to learn about what we do not understand are qualities that have allowed humans to survive on Earth. Human beings strive to know and to understand what surrounds them. By exploring the unknown, humans gain security and dispel fear of the unknown, of what is beyond. This survival mechanism is encoded in our genes.
We use the challenge of exploration to advance and to learn, to improve our scientific and technological skills for survival, to sustain our human experience, and to progress. We also exploit the adventure of exploration to provide hope for the future. This is particularly important for our youth, who need to be given a positive vision for their future and inspiration toward achievement.
The development of powered flight and global air transportation in the 20th century created new economic opportunities and ultimately connected societies all over the planet. So too will the exploration of space create new economic opportunities in the 21st cen-
tury in ways that we cannot anticipate today. Spurred by the advent of the space age in the late 1950s, the investment in science and technology by the United States, a relatively young country, drove a half-century of unprecedented wealth and prosperity. Science and technology are the greatest engines of economic growth in America, and this has become obvious to the rest of world as new nations open up their own roads to the Moon and beyond.
In our complex world of national interests and barriers, the exploration of space can and should be a global enterprise. Space exploration is an adventure of and for humankind, not for any nation in particular. It is the perfect place for nations to cooperate, because space is new, unbounded, and open. Achievements in space can be accomplished by nations working together, and thereby each gains security by cooperating in a challenging enterprise where there are no risks to national sovereignty. Space is a place to be utilized by Earth, by the humans of Earth, not by any one nation.
SCIENTIFIC EXPLORATION: WE ARE COMPELLED
The exploration of space is a noble human enterprise with roots in the exploration of our own planet in the 20th century. At the beginning of the 20th century we were exploring the unknown polar regions of our own planet with ships and men. At the end of the 20th century we were exploring the Moon and beyond with spacecraft, robots, and men. Science has been a partner in all of this. We have now stepped upon the Moon and sent robotic spacecraft on flights past every planet in the solar system. We have conducted orbital reconnaissance of four planets and two asteroids (soon to be five planets and four asteroids); landed on two planets, an asteroid, and a moon of Saturn; and we have conducted roving expeditions on both the Moon and Mars.
We are compelled to explore the Moon and the solar system because it is there that we will find answers to fundamental questions humans have been asking themselves as long as we can remember—questions about our own origins and destiny. Questions such as How did we come to be? and What will happen to us in the future? and Are we alone in the universe? The progress we have made in space science over these past 50 years has brought us to the point where we dare voice these big questions, because we believe now that they can be answered through the scientific exploration of space. These human questions can be recast as scientific challenges—goals to be achieved in the course of exploring space. And from these scientific goals, plans can be formulated for both robotic and human explorers, including the destinations and the exploration objectives for each.
How did we come to be? This is a question that approaches the contemplation of existence. Even so, astronomers can address the question by determining how the universe began and evolved; learning how galaxies, stars, and planets formed; and searching for Earth-like planets around other stars. And when we find Earth-like planets, are they habitable or even
inhabited? The answers require large and complex space telescope systems made possible by human construction and servicing in space. We need to find out how Earth developed its biosphere and whether these same processes ever occurred on other planets in our solar system. This requires research on life here on Earth and extensive exploration of other planets in our solar system where there may have been another chance for life, such as Mars and Europa.
What will happen to us in the future? Every human wonders about the future. One form of this question asks if there is any threat to us from space, especially from Earth-crossing asteroids. The answer will come from surveys of the Earth-crossing asteroid population in space and space missions that determine their composition and structure. Another form of this question asks what future humans have in traveling to and living on other planets. Is our species destined to populate space? Ultimately I believe the answer is yes, and it will happen through exploring space and utilizing the resources we find in the most promising places out there.
Are we alone in the universe? Every human being wants to know the answer to this question. We are compelled to find its answer. Some find comfort in the notion that we should be alone; others are fearful of the potential for other life “out there.” Most scientists see the possibilities and are overwhelmed by the notion that the universe might be teeming with life, at least microbial life and perhaps even intelligent forms. We will find the answer by searching for life in the most
promising places in the solar system, such as Mars and Europa, and by looking for signs of life on planets outside the solar system with space telescopes.
So how do we go about all this? Where do we go in the solar system, and what do we do there to try and answer these questions? Let’s start with the Moon. It’s not the only place we need to go, but it’s close, it’s right there in our sky and a convenient trip of only a few days.
WHAT GOOD IS THE MOON?
We probably would not have a space program if it was not for the Moon.
If God wanted man to be become a space-faring species, He would have given [us] a Moon.
—Krafft Ehricke, Saturn V rocket engineer
A bit facetious perhaps, but well crafted and on the mark. The Moon has been dominant in our sky since before the dawn of man. It is another planet large in our sky, demanding attention, drawing our eye and curiosity. And without it, man may never have looked so questioningly at the sky and generated the interest in going to the Moon and beyond.
The Moon is a destination for scientific exploration. There is much we still do not know about the Moon, even after Apollo. It is an archeological site for understanding solar system history, especially the earliest phases where the evidence has been wiped out on Earth. It is also a close-by platform for conducting science, a natural scientific outpost from which to observe the rest of the solar system, the Sun, Earth, and the universe.
The Moon is the first step into deep space for any nation’s space enterprise. It is a nearby planetary destination for flexing the technological muscles of any country’s young space enterprise. It is also a way station to exploring deep space. It can become the new Antarctica, a place for nations to cooperate in peaceful exploration and to develop the trust needed to proceed together in international deep space exploration.
The Moon is potentially a commercial destination. The possibilities have been raised for developing resources, including materials and energy to use locally on the Moon, to support further space exploration, or perhaps even for export to Earth. The Moon will at some point in the next 50 years become a travel destination, first a virtual one through robotic missions and the internet, and ultimately for humans a permanent exploration outpost and then a tourist destination.
Science On and About the Moon
The Moon is a solar system history book, a “Rosetta stone,” providing a template for deciphering and understanding the history and evolutionary processes of the terrestrial planets. Due to its lack of atmospheric weathering and geological activity, the surface of the Moon is a repository of information from the earliest epochs of solar system history. Impact-generated samples of the early Earth, Venus, Mars, and asteroids may lie on the surface, and samples of lunar mantle material may also be exposed as a result of large impacts. The ancient rocks on the Moon may represent our best hope of directly sampling the material from which the Earth–Moon system formed. We can also determine the impact flux of asteroids and comets on Earth over time using the cratering record preserved on the Moon. Material from the solar wind trapped and buried in the lunar surface can also elucidate the history of the Sun.
Comet impacts over the eons may have resulted in an accumulation of water ice in permanently shaded regions at the poles. Some studies have suggested that there may be as much as 10 billion tons of water in the polar regions, potentially a valuable source of oxygen and rocket propellant for future human outposts on the Moon. Finally, the Moon may represent a potential resource for commercial exploitation. There have been proposals to export lunar resources for use on Earth, as well as proposals to use lunar-generated energy and to use the Moon for education, entertainment, or space tourism.
The Moon has also been proposed as a platform for astronomical telescopes. The most compelling of these is a low-frequency radio telescope on the far side where interference from the overwhelming background of commercial radio broadcast traffic is eliminated.
Origin of the Earth–Moon System
The Moon contains a 4.5 billion year old record of the origin of the Earth–Moon system. Apollo and other lunar missions have only scratched the surface of what the Moon can tell us about the history of the inner solar
system. There remain some key elements and isotopes that have not been measured and that are necessary for fully understanding the Moon’s thermal and volcanic history and for making an accurate assessment of the resource potential of the Moon. We need to explore and sample more of the diverse regions of the Moon we have not yet visited. We need samples from the lunar mantle that may await us in the deep basins on the Moon. We need to determine the interior structure and composition of the Moon in greater detail than we know today. This can be accomplished with an in situ network of seismic stations and heat flow measurements distributed around the surface. And the determination of absolute ages of lunar minerals is a requirement for understanding the history of the Moon and its relationship to Earth. These measurements now require analysis by ultra-sensitive, highly complex, and massive instruments in Earth laboratories with extensive sample preparation by human laboratory technicians. While sample return can be done robotically, sample selection and characterization on the lunar surface is a critical function, and there remains a trade-off on the capabilities of lunar robots with human operators on Earth versus human lunar field geologists.
Impact History of the Earth–Moon System
The Moon has recorded the history of impact bombardment since its solidification shortly after the formation of the solar system (Figure 3.9). It is a “witness plate” that can provide the statistics on impacts that must have occurred on Earth, but whose evidence has been erased by Earth’s turbulent tectonic activity. This lunar impact record extends to time periods earlier than the origin of life on Earth, so that the chaotic disruptions caused by impacts on Earth can be used to understand the life forming process on early Earth.
There are meteorites from the Moon found on Earth, and there is every reason to suspect that the inverse is also true. It is possible that material blasted from Earth in its early years rests now on the lunar surface—stones containing secrets to the first billion years of Earth’s history just waiting to be picked up. We have evidence from the oldest rocks available on Earth that life had already arisen more than 3.5 billion years ago at the end of the Hadean eon and the beginning of the Archean eon. Perhaps the clues we need to this early age are waiting to be identified and retrieved on
the Moon. Samples of Earth ranging back into the late Hadean could tell us a lot about the early atmosphere, ocean, surface, and climate when life was first evolving on the planet.
In addition to assessing the effects of bombardment on Earth’s environment in the Hadean, the post-mare cratering record on the Moon can yield information on other critical phases of the evolution of life on Earth. There is evidence that Earth periodically receives large impacts, and these have been linked to mass extinctions. This hypothesis cannot be tested on Earth, but it can be tested on the Moon by a careful examination of its cratering record.
Finally, the Moon preserves a record of the most recent impact history of the Earth–Moon system. There has been an increased awareness of the potential for future large impacts by Earth-approaching asteroids and comets. The time scale for such impacts is a strong function of size, and current statistics are not as accurate as the potential threat dictates they should be. There is a growing program for the identification, orbit determination, and monitoring of Earth-approaching objects in order to provide advance warning of any threat to the planet, but more accurate statistics are required to complement the observational techniques. These statistics could be determined by deciphering the late cratering history of the Moon from samples of a large number of post-mare craters.
A Record of the Ancient Sun
The Sun propels enormous amounts of material into space in the form of hot tenuous plasma known as the solar wind. The solar wind is a sample of the composition of the surface of the Sun. As the Sun burns hydrogen in its interior over time, it produces new elements and isotopes that migrate to the surface and are expelled in the solar wind. The solar wind impacts the Moon and is trapped in regolith material, which is well preserved on the Moon. Age dating of lunar stratigraphy with atomic and isotopic analysis of the implanted solar wind in these layers can be used to determine the past history of solar luminosity as well as to predict its future evolution. This information will help us understand the past climate of Earth over the entire time that life has existed on our planet.
A Platform for Observatories
The far side of the Moon, in permanent shadow from Earth, is the perfect location for a radio telescope. It would be possible to emplace very simple and extremely long, narrow antennas on the far side of the Moon that would have unparalleled spatial resolution on the sky with extreme sensitivity. Antennas that are kilometers in length and deployed from very small roll-up packages are easy to envision. They could be operated remotely and serviced by humans. They could be used to examine solar radio emissions and emissions from the planets, map emissions from Milky Way objects, and look back into time just after the Big Bang when stars had yet to form.
There are also notions to place optical telescopes on the Moon. The advantages are that there is no atmosphere to distort images and filter out large portions of the electromagnetic spectrum, and there is a cold, dark sky for 14 days (except at the poles where permanent night can exist). However, there are significant challenges to emplacing large telescopes on the Moon, including mitigation of lunar dust, the local atmosphere near a human-occupied base, large thermal excursions between lunar day and night, and the large propulsion requirement for the repeated trips into and out of the lunar gravity well that will be needed for construction and servicing.
Resources: Materials and Energy for Space and Earth
The Moon’s regolith contains resources that might be useful for processing into materials and consumables for supporting human explorers on the Moon or for sustaining exploration of space beyond the Moon, or perhaps even for export to Earth. These prospects have been buoyed recently by the discovery that there may be water ice in permanently shadowed regions at the lunar poles. The distribution, form, and amount of any such ice in the polar regolith must be understood before the potential for supporting human exploration can be fully evaluated. This assessment can be accomplished first from lunar orbit followed by in situ measurements on the surface and at depth to characterize these potential deposits in detail. In addition to assessing the value of these deposits for oxygen and fuel production, they
have scientific value in their potential to unravel the history of volatiles in the inner solar system.
In addition to the possibility of usable quantities of water ice at the poles, there may be other useful volatiles implanted in lunar dust grains, such as hydrogen from the solar wind. It may be possible to extract oxygen and metals from lunar rocks and regolith to use for life support, propulsion, and construction.
Solar energy is another resource that could be harvested on the Moon. While storage batteries would be required to survive the long lunar night, solar power plants could be placed in polar locations where there is permanent sunlight. The problem then becomes transmission of that power to other regions where it is needed. This energy could also potentially be exported to cis-lunar space or even back to Earth.
Exploration: Becoming a Space-faring Species
In addition to its intrinsic science value and its potential importance as an observational platform and a resource node, the Moon is a stepping stone into the solar system. The Moon is a natural space station, already in Earth orbit, providing a benign environment with one-sixth gravity for human utilization and exploration. The proximity of the Moon suggests its potential as a training ground for human exploration of space. The Moon is a place to learn the skills we need to live off-planet, to explore planetary surfaces, to learn the respective roles of robots and humans, to develop the means to live as much as possible with local resources, and to confront the societal and psychological impacts of confined living in a hostile, alien environment far from Earth and home.
While the Moon may seem to be a “been there, done that” destination for the American public, the rest of the world has a “go there, do that” attitude, and many nations with emerging space programs have the Moon in their sights. There will be a renaissance in lunar scientific exploration in the next several decades that the United States will not want to miss. The pull of the Moon to emerging space programs around the world can be a catalyst for a new era in space exploration—one of international cooperation instead of the rocket-rattling days of the Cold War and national breast-beating in the days since.
Beyond the Moon
In the long run, the Moon is but one of many exciting places to visit in the solar system. Our robotic spacecraft have free reign of the entire solar system, while humans will be limited to Earth’s vicinity, including the Moon, in the immediate future. The ultimate goal for human spaceflight should be to visit Mars in the next 50 years, but there are two other places for humans and their accompanying robots to visit between the Moon and Mars: one in near-Earth space just beyond the Moon, the Sun–Earth Libration Point L2 (SEL2), and the other just beyond Earth space, a near-Earth asteroid.
SUN-EARTH LIBRATION POINT L2: A PLACE THAT IS NOT A PLACE
In 1772, the French mathematician Joseph L. Lagrange showed that there are five positions of gravitational equilibrium in a rotating two-body gravity field. Three of these Lagrange points—also called “libration points”—are situated on a line joining the two attracting bodies, and the other two form equilateral triangles with these bodies. Figure 3.11 shows a total of seven libration points located in Earth’s neighborhood, five
of which derive from the Earth–Moon gravitational system and two which derive from the Sun–Earth system. Although the collinear points are unstable, very little propulsion is needed to keep a spacecraft at or near one of these points for an extended period of time. This unique gravitational balance and consistent geometry makes the libration points very important locations in space-exploration architecture. In particular, the Sun–Earth L2 point is the ideal location for space telescopes, and it is an excellent stepping-stone to more distant destinations. Sun–Earth L1 is an excellent vantage point for solar telescopes and for viewing the entire sunlit hemisphere of Earth.
The Sun–Earth Libration Points L1 and L2 (SEL1 and SEL2) are located at the very edge of Earth’s gravitational influence in the solar system. From there, you would literally fall off Earth’s gravitational field into the solar system beyond. For this reason it would make an excellent staging point for astronauts traveling beyond Earth–Moon space into the rest of the solar system. The disadvantage relative to Earth–Moon libration points is that the Sun–Earth libration points are farther away and would take more travel time to reach. That extra time may be worth it, however, since SEL1 and SEL2 are energetically inexpensive points to reach in space, taking much less energy than getting into lunar orbit or reaching the Earth–Moon libration points where the Moon’s gravitational field must be countered.
SEL2 is an ideal vantage point for space telescopes (Figure 3.12). It is a thermally benign environment. There are no temperature changes caused by day–night cycles like those on Earth or the Moon. Viewing constraints are minimized because the Sun, Earth, and Moon all lie in the same general direction and are far away. There is no obscuration from a local platform. The entire sky is accessible all the time. There is no dust to contaminate mirrors and clog mechanisms. There is a continuous source of solar power for uninterrupted observations. Because of these advantages, the next generation of space telescopes is targeted for SEL2 beginning with the James Webb Space Telescope.
It is conceivable to construct extremely-large-aperture telescopes at SEL2 because there is no gravity to distort mirrors or impede pointing operations. It is also possible to fly multiple telescopes in formation, their mirrors optically linked by laser metrology, to provide unfilled dispersed apertures with sizes of kilometers or more. Such large sizes can provide spectacular, unprecedented resolution on the sky. With such multiple telescope systems it will be possible to survey the universe across the entire spectrum and look back at the universe to the beginning of time. It will be possible to observe the process of planetary system formation around young stars and to search for terrestrial planets around more mature stars. We can search for evidence of life in the spectra of the extra-solar planets we find, and multiple aperture systems may ultimately be capable of imaging Earth-like planets around other stars. Imagine a time when humanity will be treated to the first image of an Earth-like planet around another star. That image will have an even larger effect on the human consciousness than did the first global image of Earth taken from space by Apollo 8 in 1968.
Such large, complex systems would probably be most cost effective if constructed and serviced by astronauts at SEL2 or at a servicing waypoint in one of the Earth–Moon libration points closer to Earth. SEL2 is
a relatively benign and low-risk destination for human spaceflight. Its unique location at the edge of Earth’s gravitational influence makes it an energy-efficient starting point for missions to deep space. Having developed the capability to travel to SEL2 for telescope construction and servicing, astronauts could use SEL2 to construct and service spacecraft to be staged from here for journeys to more distant destinations.
Near-Earth objects (NEOs), also known as near-Earth asteroids, are nearby remnants of planetary formation and represent valuable storehouses of information on the origin of the solar system. Their structure and composition may hold clues to important scientific
questions about the history of the solar system. In addition, since they pose by far the most significant impact threat to Earth, an understanding of their diversity and their physical characteristics could someday be vital to averting a potential global disaster. These objects impact Earth regularly, with mean times between collisions dependent on size—larger objects fall much less often simply because there are fewer of them. There are recent and dramatic impact scars on Earth, including the 50,000-year-old crater near Winslow, Arizona, and the massive blow down scar of the 1908 Tunguska event (probably a comet impact) in Siberia.
The primary properties of composition and bulk density must be determined in order to understand NEO structure, the nature and severity of possible impact threats, and the efficacy of various mitigation strategies. NEOs also represent substantial mineral resources in space relatively near Earth. Because NEOs have very low gravity, transportation of these resources to other locations can be done relatively inexpensively. These resources could be used for in-space operations or may have commercial potential for export to Earth.
While most of the scientific exploration of NEOs can be done robotically, NEOs are ideally situated to provide an important stepping-stone for human missions to Mars. A mission to an NEO provides an opportunity to exercise many of the required human transportation elements for Mars in a relatively low-risk manner. NEOs are relatively easy to reach, with lower energy requirements than achieving lunar orbit in many cases, but require longer flight times on the order of 6 months to a year. Their locations and physical characteristics will stretch the capabilities of human exploration just enough to greatly reduce the risk of the Mars missions to come. In addition, development of the capability for human operation on and near NEOs, in advance of the discovery of any specific impact threat, could turn out to be a wise investment.
MARS IS FOR ROBOTS AND HUMANS
Virtually ever since it was discovered, the planet Mars has been special to humankind. For centuries it has been a centerpiece for much of our scientific speculation and imagination, and it has been explored in the space age more intensively than any other body in the solar system except Earth.
Mars is the most Earth-like planet in the solar system and almost certainly had a warmer and wetter environment early in its history, with flowing and standing water on its surface. Mars may have developed life, and while its surface appears lifeless today, an early biosphere may have survived at depth where liquid water might still exist. Mars is the most accessible place in the solar system where we can search for evidence of an independent origin of life. From Mars we can learn about the origin and history of an Earth-like planet that has taken a different path in planetary evolution. By comparing the geological and climatological histories of Earth and Mars, we will gain clues to what it takes to construct a habitable planet and how that habitability may be sustained or lost over time. All of these objectives share a common thread—water. When in the planet’s history did liquid water exist? Where was it and where is it now? In what form (rain, rivers, lakes, and oceans)? How much?
There is evidence that Mars had more habitable climates in its past and has undergone climatological cycles, linked to orbital and obliquity changes, that episodically may have produced a warmer and wetter surface environment. There is every reason to believe that 4 billion years ago the surface environments of the two young planets, Earth and Mars, were very similar when life first arose on Earth. So by extension, there is a fair possibility that life may have arisen on Mars at that time. If there was life on the young Mars, there may be fossil indications of that life to be found. If life has survived and still exists today, then it is hidden below the surface of Mars where liquid water and sources of chemical energy may still be found.
By exploring the geological and climatological history of Mars, we are examining the evolution of another Earth-like planet. The knowledge to be gained will help us to understand how terrestrial planets are built and how they evolve, how a habitable environment can be established and maintained, how that environment can evolve to become biological, and what the prospects may be for other habitable planets in planetary systems around other stars.
Where Is the Water?
The key to exploring Mars is to “follow the water.” Scientific results from robotic missions show that the history of martian water and the distribution of its repositories are strongly coupled to the geological and climatological histories of the planet. Water has been a major erosional force on the planet in the past; its features are written all over Mars. Water has also been a major climatological force during the history of the planet. There is enough water locked up in the seasonal polar caps today to cover the surface of Mars to a depth of 20 to 30 meters. Water is a major element in present day martian weather. While present in only trace amounts, the atmosphere is quite often fully saturated with water. There are hidden reservoirs of ice, and perhaps water, in the subsurface, evidenced by seepage channels in cliffs and crater walls and the strong near-surface signals of ice seen by orbiting neutron detectors and radar sounders. The layered polar deposits probably contain as much as 20 to 50 percent water ice.
Liquid water in extensive aquifers could exist below the surface, and there is evidence of catastrophic outbreaks of such aquifers in the past. Global remote sensing and sounding can identify potential aquifers where there might be subsurface ice or water that can be reached with in situ exploration using additional sounding and drilling. A source of liquid water would be a key resource for establishing a human outpost on Mars.
Was There or Is There Life on Mars?
On Earth, wherever there is liquid water there is life, and the same may be true of Mars. Mars may have developed an independent origin of life early in its history when it was warmer and wetter, and that life might have survived at depth where liquid water may remain. Mars is therefore a most compelling place to answer the question, Did life ever arise elsewhere in the solar system?
Life requires a source of liquid water (only a dab will do), a source of energy, and a source of chemical materials. Life is not particular about what it “eats” and seems able to adapt to whatever chemical energy source is available. Life on Earth can exist on hydrogen, carbon dioxide, hydrogen sulfide, iron, manganese, and host of other energy sources, so that martian life could do the same on mineral deposits and gases escaping from the interior. But liquid water is an absolute requirement. The search for extant life should therefore focus on areas where surface mineralogy and subsurface sounding indicates a concentration of ice, and perhaps water, at accessible depths below the surface. Sites where volatiles are escaping, particularly with some reduced components such as hydrogen and methane, would be particularly exciting.
Likewise, the search for past rather than extant martian life is the search for locations of past liquid water on the planet. Tectonic activity on the planet will likely have left samples on the surface with some fossil evidence of past biological processes. This fossil evidence could be in the form of characteristic elemental distributions, isotope ratios, organic residue and perhaps even microfossil morphologies in carefully selected rock samples. Finding such evidence can be accomplished with an orderly procedure starting with a search from orbit for a set of promising sites, followed by an in situ examination of the identified sites to certify which is most suitable, and finally concluding
with in-depth study at one more particularly promising site to include very detailed local geochemical examination, drill samples, and perhaps even sample return to Earth, depending on in situ measurement capability at that time.
For its scientific value as well as for its enduring place in human consciousness, Mars is the ultimate destination for humans in the next 50 years. Until that time, robotic missions will play an important role in defining the activities of human explorers, characterizing the martian environment, searching for potential resources, and emplacing assets on the surface.
The ongoing U.S. Mars Exploration program has maintained a continuing presence at Mars since 1997. U.S. and European spacecraft operating in orbit and the Mars rovers Spirit and Opportunity operating on the surface are still making discoveries, one after another, that add to the impression of a once-wet Mars and a place ever more interesting for further exploration.
Mars beckons. There will be a day when humans will join our robots on the surface of the fourth planet from the Sun. The first stop may be the Moon, with a layover at an asteroid, and perhaps a stop at one of the moons Phobos or Deimos. But on the 100th anniversary of the space age, 50 years from now, humans will have walked on Mars.
Beyond Mars lies the outer solar system, land of the giant planets Jupiter, Saturn, Uranus, and Neptune. And fencing off the outer solar system from the inner lays the asteroid belt, with Mars at its inner boundary and Jupiter beyond its outer boundary. These regions beyond Mars are out of reach for human exploration in the next 50 years, but they have been and continue to be regions explored by our robotic spacecraft. Fifty years ago at the onset of the IGY, and even after the launch of Sputnik, it might have seemed that the Moon and the nearest planets, Mars and Venus, might be attainable, but we have explored well beyond them. We have ventured throughout the entire solar system with our spacecraft, and the adventure continues. At this writing, a U.S. spacecraft is on its way to the innermost planet, Mercury. Europe has spacecraft in orbit about Venus and Mars, Japan and China have spacecraft orbiting the Moon, with India and the U.S. to follow. Japan has a spacecraft returning from a near-Earth asteroid, and
the United States has a spacecraft on the way to two main belt asteroids. The United States has two orbiters and two rovers at Mars, with another lander on the way, the U.S./European Cassini spacecraft continues to orbit Saturn, and the U.S. New Horizons spacecraft is on its way to Pluto-Charon and the Kuiper Belt.
Japan’s Hayabusa spacecraft approached the surface of Itokawa to within a few feet and activated its sampling system. It is returning from its trip and hopefully will land a sample of the asteroid on Earth in 2010. The U.S. Dawn spacecraft is on its way to two of the largest main belt asteroids, Vesta and Ceres, where it will orbit each in turn to carry out the first ever comprehensive investigation of such bodies.
Jupiter and the Galilean Moons
Beyond the main asteroid belt lies massive Jupiter with its large Galilean moons. There seems little doubt after the Galileo orbiter mission that Callisto, Ganymede, and Europa harbor subsurface oceans, but their fluidity, depth, and extent are unknown. The most intriguing is ice-covered Europa with surface manifestations of a more mobile fluid underneath (Figure 3.19). If there is an ocean beneath this ice, then it is sustained by heat flow from the interior of Europa, quite likely by
tectonic processes that might also bring mineralogical nutrients into the ocean. On Earth, this is a recipe for life. There are plans in the United States and Europe for sending a robotic spacecraft to investigate the Galilean satellites in more detail and in particular to orbit Europa and investigate the depth and extent of its subsurface ocean.
Saturn and Titan
Beyond Jupiter lies perhaps the most magnificent and spectacular planet in the solar system, ringed Saturn. The Cassini spacecraft now orbits Saturn—investigating the planet, its atmosphere, magnetosphere, rings, and retinue of moons. While Jupiter has four large Galilean moons, Saturn has seven somewhat
smaller icy moons and one giant moon, Titan, which is larger than the planet Mercury. Titan is the only moon in the solar system with a thick atmosphere. Shortly after it arrived at Saturn, the Cassini spacecraft dispatched the European Huygens spacecraft to Titan, where it entered the atmosphere, floated down on a parachute, and landed on Titan’s surface. Cassini continues to make close passes at Titan, examining it with its cameras, spectrometers, and imaging radar.
Titan’s surface and atmosphere show strong analogies to Earth, although it is much colder—near 93 K at the surface where methane plays a similar role on Titan that water does on Earth. Photolysis of methane leads to an atmosphere loaded with organic aerosols and hydrocarbon clouds. There are clear signs of hydrocarbon rain and mist near the surface with coastal deltas very similar to Earth’s that open onto dark flat plains resembling oceans in appearance. These plains appear episodically to become covered with liquid hydrocarbon. Huygens landed on such a plain, and its instruments indicated that the surface seems to have the consistency of a mud wet with methane and other hydrocarbons. The Cassini radar has returned images of large lakes of hydrocarbon fluid in the polar regions of Titan (see Figure 3.21). Titan has indeed turned out to be a fascinating place and may hold clues to the earliest chemistry on Earth that enabled the origin of biology on our planet.
Pluto-Charon, the Kuiper Belt, and Comets
Launched in January 2006, the New Horizons spacecraft will arrive in the vicinity of the dwarf double planet Pluto-Charon in July 2015, returning data and high-resolution images of one of the largest members of the Kuiper Belt of icy dwarf objects at the fringe of our solar system. It will proceed onward to visit at least one other yet unidentified member of the Kuiper Belt.
The Kuiper Belt is the storehouse of comets that occasionally get perturbed through close encounters and are thrown inward towards the Sun where they make their spectacular appearance with large halos of gas, dust, and ionized curved tails. We have even sent our spacecraft to intercept these objects, returning data and images on flybys, one returning samples after flying through the coma of Wild 2, and another impacting Tempel I to examine the resulting plume for clues to its composition and structure.
Beyond Our Own Solar System
Beyond our solar system, one day we will find another Earth-like planet in the sights of our space telescopes, a blue dot circling another star, with oceans, continents, and signs of life. This will be one of the most enduring products from space exploration in the 21st century, just as that first image of our own planet from Apollo 8 is one of the most enduring products of the 20th century. It is the Moon that first intrigued us and drew us into space, next is Mars, and finally it is the notion that we are not alone, that somewhere out there is a planet like our own, a planet with life on it, and perhaps a civilization—galactic neighbors with whom we can share the glory of the universe.
WE’VE COME A LONG WAY IN THE LAST 50 YEARS
It is amazing how far we have come since we began this adventure 50 years ago, from confinement on Earth to spacecraft on their way to the very extreme boundaries of the solar system and beyond. We have walked on the Moon and will again. We will eventually walk on Mars. And in the meantime we will continue to explore and make new discoveries with our robotic spacecraft, extensions of our human senses, that can go where no human can ever go and where we have not yet seen fit to send them.