Forty years ago this month the lunar surface reverberated with life for the first time. Forty years from now will Mars, too, come alive? President Barack Obama has affirmed the broad goals for human spaceflight that his predecessor put forward in 2004: retire the shuttle in 2010, develop a replacement line of rockets (named Ares), return to the moon by 2020, and go to Mars, perhaps in the mid-2030s [see “To the Moon and Beyond,” by Charles Dingell, William A. Johns and Julie Kramer White; Scientific American, October 2007]. The program is known as Constellation.

For now, policy makers are worried less about Mars than about the downtime between the last shuttle launch and first Ares flight, during which the U.S. will depend on Russia or private companies to launch its astronauts into orbit. What was originally supposed to be a two-year gap has widened to six, and in May the Obama administration announced that former aerospace executive Norman Augustine will lead a review of the program to see how it might get back on track.

Although Mars is still far off, at least NASA is designing spacecraft with an eye toward an eventual interplanetary flight. Planners are guided by the experiences that Harrison H. Schmitt relates in the following article. —The Editors

Mountains higher than the walls of the Grand Canyon of the Colorado towered above the long, narrow valley of Taurus-Littrow. A brilliant sun, brighter than any sun experienced on Earth, illuminated the cratered valley floor and steep mountain slopes, starkly contrasted against a blacker-than-black sky. My crewmate Gene Cernan and I explored this nearly four-billion-year-old valley, as well as the slightly younger volcanic lava rocks and ash partially filling it, for three days in 1972—concluding the Apollo program. It was the first and, so far, only time a geologist has ever done hands-on study of another world. Now the U.S., the European Union, Russia and other international partners are contemplating sending astronauts to Mars to do fieldwork there, probably beginning within the first third of this century. What will be new and what will be familiar to the first geologist to step before a red Martian sunrise?

Most accounts of the Apollo missions focus on their historic firsts and their high-tech achievements, but those of us who participated also remember the low-tech, human side: hiking over the terrain, chipping away with a geology hammer, hauling rocks and getting our bearings under the alien conditions. Any geologist would recognize the principles and techniques of field exploration that we applied. The fundamentals did not change. The goal was still to document and graphically represent the structure, relative age, and alteration of natural features so as to infer their origins and the resources they might provide to civilization one day. Nor did leaving Earth change the principles related to expedition planning and execution, such as how to collect and document samples; if anything, those principles become more important as revisits to the same place become less likely. Particularly unchanged was the need for human touch, experience and imagination in fully realizing the scientific and humanistic value of exploration.

For each new body that people explore, we must build on our experience exploring the last place we have been—as geologists have done on Earth for more than two centuries. We must continually ask what may be similar and what may be different. How will Martian geology, accessibility, exploration strategy and optimal crew composition compare with the experience of Apollo?

In the Lunar Field
Extremely complex influences affect geologic features on Earth. The crust, magma, water and atmosphere interact; oceanic plates and continents break and collide; objects from space impact; and the biosphere, including humans, alters the landscape. On the moon, the influences in the past four billion years largely have been external, confined to the effects of impacts and of energetic particles that constitute the solar wind [see “The New Moon,” by Paul D. Spudis; Scientific American, December 2003, and “The Carbon Chemistry of the Moon,” by Geoffrey Eglinton, James R. Maxwell and Colin T. Pillinger; Scientific American, October 1972].

The absence of an atmosphere exposes surface materials to the extraordinarily hard vacuum of space. Meteors and comets, some as small as dust grains and traveling at tens of kilometers per second, strike and modify the rocks, rock fragments, glass and dust. This process has produced what passes for “soil” on the moon: a covering of fragmented and partially glassy debris, called the lunar regolith, that blankets most older volcanic flows and older impact-generated formations to a depth of several meters. Therefore, field exploration on the moon requires that a geologist have x-ray vision of a sort. To identify the interfaces, or contacts, between major rock units, I had to visualize how the gradual formation and spreading of regolith by impacts had broadened and subdued the original contrasts in the rocks’ minerals, color and texture.

For example, in the valley of Taurus-Littrow, I explored the contact between dark, fine-grained basalt flows and older, gray, fragmented rocks, known as impact breccias. When this contact formed, it must have been sharp—an abrupt juncture between the rock types. But 3.8 billon years of exposure to space had smeared it out over a few hundred meters. Elsewhere, a contact between a dust avalanche deposit and the dark regolith had spread only a few tens of meters in the 100 million years since the avalanche occurred. By understanding the processes actively modifying these contacts, I could determine their original positions. Similarly, a geologist on Earth must determine how terrestrial erosion obscures or covers underlying rock contacts and structures.

Field identification of different rock types in exposed boulders on the lunar surface required understanding the effects of continuous micrometeorite bombardment. When extremely high velocity particles hit the surface, they create a localized, high-temperature plasma and melt rock at the point of impact. The ejected plasma and molten rock redeposit on nearby surfaces, producing a thin, brownish, glassy patina—containing extremely small iron particles—over the entire boulder. Just as a geologist on Earth must look through the desert varnish on exposed rocks in Earth’s dry regions, I had to quickly scan and interpret what lay underneath this patina until I could chip or break the boulder with a hammer.

Small impact pits that interrupt the lunar patina contain glass of varying colors, reflecting variations in the chemical composition of impacted minerals. Where the pit formed on a white mineral (such as plagioclase feldspar, a major component of volcanic rocks), the results are a light-gray glass and a distinctive white spot, caused by very fine spider cracks in the mineral grain. Where an iron- or magnesium-rich mineral has been hit, the result is a green glass. Knowledge of this process allowed me to determine a rock’s composition just by looking.

What Will Explorers Find on Mars?
On Mars, scientists expect influences that combine those affecting Earth and the moon, because the Red Planet is intermediate in size. Indeed, our growing geologic knowledge about Mars already confirms this blend of processes. Since the first photographs provided by orbital cameras and the Viking landers, we have known that geologic features on Mars resulted from combinations of internal and external influences.

Unlike the moon, Mars has a thin atmo­sphere, with a ground-level pressure of about 1 percent the pressure at sea level on Earth. The existence of this atmosphere changes the geologic overprint that explorers will have to evaluate and look through to identify, analyze and understand the underlying rock units. The atmosphere filters out small meteors and comets—those capable of forming craters smaller than about 30 meters in diameter. Consequently, the surface is not covered in impact-generated spray, as on the moon. Instead the dominant migrating material is windblown dust. The dust comes from a variety of sources, such as rocks eroded by wind, landslides, impacts and chemical reactions. It forms soft dunes that explorers may need to avoid, much like deep, wind-formed snowdrifts in the plains and mountain passes of Earth. Indeed, the Spirit and Opportunity rovers have gotten stuck on occasion.

In spite of the filtering effect of the atmosphere, impact-related geology still dominates the surface and near-surface fabric of most exposed Martian formations. The first geologists must decipher the ejecta, fractures and shock modification of rocks. Not all the rocks are impact-related, however. In many rift valleys as well as throughout other regions, layered rocks resembling sedimentary or volcanic strata dominate. The impact-generated regolith is not continuous, and many outcrops of underlying Martian bedrock formations are accessible for normal geologic examination and sampling.

Whereas the moon is dry, liquid water sculpted landforms and created new minerals on Mars. Laboratory inspection of lunar samples identified no water-bearing minerals in them, but orbital sensors and robotic analyses of Martian minerals have detected a variety of water-
containing clays as well as sulfate salts that probably precipitated from water. Further, unlike the moon, whose rocks contain nonoxidized iron metal, Mars has extensive deposits of oxidized iron (hematite, Fe2O3), another sign of processing by liquid water [see “The Red Planet’s Watery Past,” by Jim Bell; Scientific American, December 2006, and “The Many Faces of Mars,” by Philip R. Christensen; Scientific American, July 2005]. The Martian geologist must be prepared to interpret a much larger spectrum of minerals than we encountered on the moon. Water also transports material. It carved valleys, and some impacts appear to have melted subsurface ice to generate mudflows.

In sum, the Martian regolith generally consists of impact ejecta and debris from mudflows or floods, interstratified with windblown dust. In polar regions, it also contains water and carbon dioxide ices and frosts, as the Phoenix lander recently confirmed. The lunar regolith was not nearly as complex.

As a consequence of these differences from the moon, new challenges will face the Martian field geologist. The explorer will still need x-ray vision; however, it will be more like that required on Earth, where one must take into account the effect of wind, gravity or water-transported materials. In other ways, exploration may be easier than on the moon. Images from Mars show that although fine, windblown dust forms a very thin, patinalike coating on many rocks, the wind frequently cleans surfaces, so that dust coatings will not be a significant impediment to visual rock and mineral identification.

One similarity to lunar exploration may be visual distortion. In a vacuum or a thin atmosphere, our brain tends to underestimate distances. People experience the same problem in the clear air of Earth’s deserts and mountains; the absence of familiar objects such as houses, trees, bushes, power poles and the like worsens matters. Neil Armstrong first noticed this problem after landing Apollo 11. I learned to compensate by comparing the known length of my shadow to what it seemed to be and then scaling up my distance estimates by about 50 percent.

Surface dust also plays tricks on the eyes. On the moon, it caused an intense backscatter of light whenever we looked directly away from the sun. This so-called opposition effect—which looks like a bright, diffuse spot—is the same phenomenon that one sees looking toward one’s shadow on snow or a plane’s shadow when flying over a leafy forest or cropland. Mars astronauts will see it, too. Backscattering provides some light into shadows, whereas shadows seen looking in the direction of the sun are lit only by the small amount of light scattered from other surface features. We had to adjust the f-stop of our cameras relative to the sun line for every photograph we took. Future exploration cameras and video systems should be able to adjust to lighting conditions automatically.

The Difficulty of Access
I personally felt very at ease while on the moon. I attribute this comfort level to being highly motivated and highly trained, as well as having great confidence in the support team on Earth. But the moon was only three and a half days away. Mars, using conventional chemical rockets, is eight to nine months away at best. Even with fusion or electric propulsion, which speeds the journey by continuously accelerating and decelerating the ship, the trip will take months [see “How to Go to Mars,” by George Musser and Mark Alpert; Scientific American, March 2000]. Because of its isolation, a Mars crew will have to be much more self-reliant than the lunar crews were.

That said, I suspect that psychological issues will not be much of a problem. A minimum of several months to return home versus a few days might affect some individuals in adverse ways, but explorers of Earth have surmounted this and worse challenges. Historically, adventurers have been subjected to separations from home comparable to those of early Mars crews—and without any means of telecommunication. Mars astronauts’ motivation, training, team confidence and survival instincts will be much the same as they were for Apollo. Everyone will be extraordinarily busy with spacecraft operation and maintenance, scientific tasks, physical exercise, simulation training for future tasks, updating of the plans for exploration, and many other duties. In fact, if the history of spaceflight to date is any indication, finding personal time to relax may be the main psychological challenge facing the crew. Planners on the ground will need to bear that in mind.

The primary constraint on exploration efficiency on Mars, as it was on the moon, will be the need to wear a pressurized space suit. The Apollo 7LB suit we used during the exploration of Taurus-Littrow allowed us to do a remarkable amount of fieldwork in a very hostile environment. The suit was pressurized to 3.7 pounds per square inch, about a quarter of atmospheric pressure at sea level on Earth. I could have run in it at about six miles per hour at a steady pace for several miles if need be, using a cross-country skiing gait. With the tools we had and working as a team, we could take samples, document them photographically and bag them at a reasonable rate. In about 18 hours of exploration we collected 250 pounds of rocks and regolith. I would have liked much better leg, waist and arm mobility, but what we had with the A7LB worked.

What almost did not work, or at least created significant fatigue and hand trauma, were the suit gloves. Something must be done about the technology of the gloves when we return to the moon and go on to Mars. Finger flexibility was limited, and my forearms became tired after about 30 minutes. It was like squeezing a tennis ball continuously. After an eight-hour rest period, I felt no residual muscle soreness—one advantage of more efficient cardiovascular circulation in one-sixth gravity. But after three eight- to nine-hour pressurized excursions, I am not sure how many more would have been possible with the hand abrasions and fingernail damage that the gloves caused.

Space-suit technology may evolve so that the suit glove or its equivalent will approach the dexterity of the human hand and that the suit itself will become as mobile as cross-country ski clothing [see “A Spaceship for One,” by Glenn Zorpette; Scientific American, June 2000]. Conceivably, robotic field assistants may help with preplanning traverses. In addition, based on the experience of astronauts constructing the International Space Station, we now know of physical training techniques that provide superior conditioning of arm muscles for continuous hand exertion. Other new procedures and equipment could further enhance exploration efficiency.

Forming a Crew
The political urgency and test-flight nature of early Apollo planning and development left few options for selecting experienced field geologists as regular members of lunar mission crews. NASA chose mostly professional test and military pilots with only one pilot-trained field geologist (me). All crewmembers needed to be accomplished, experienced and confident in the use of the machines and methods necessary for flight. There was no room for field geologist “passengers.”

That should change beginning with the return to the moon by the Constellation program a decade or so from now. Professional field explorers should be part of every crew sent to the moon, establishing the precedent for Mars exploration. As with the last several Apollo missions, all crewmembers and their operational support teams should receive as much terrestrial field training on real geologic problems as possible. The optimum crew size for early exploration appears to be four: two professional pilots cross-trained as field explorers and systems engineers, as was done for Apollo lunar crews; one professional field geologist cross-trained as a pilot, systems engineer and field biologist; and one professional field biologist cross-trained as a physician and field geologist.

With this cross training, mission success depends not on any one individual but on teamwork. In addition to being fully prepared to contribute specialized skills to an integrated team, each member of a Martian crew must be completely and unhesitatingly comfortable and compatible with a hierarchical command structure. Historically, small, isolated teams of explorers have achieved greatest success when they worked under a clear, experienced leader.

In many ways, Martian exploration will differ from lunar exploration. First, because the trip will be measured in months rather than days, the crew will need to continue practicing landing and other flight procedures throughout their trip. For the Apollo missions, we rehearsed landing in a simulator on the ground, and our last dry run was a few days before launch, when we had less than a week before we would begin our powered descent to the moon. The gap between launch and landing would be on the order of nine months for Mars trips—clearly too long without regular training activity onboard.

Second, ground control on Earth will not be able to perform the traditional mission-control functions because of the long delays in communications (as long as 22 minutes, one-way). Earth will instead handle activities that do not require live interaction with the crew, such as data analysis and synthesis, weekly planning, systems and consumables monitoring and analysis, maintenance planning, and scenario development. The actual live mission-control functions will need to be performed by the astronauts themselves. For instance, the mission might consist of two crews, one of which lands while the other remains in orbit to act as an orbiting mission-control center. When the first returns to orbit, the second descends and explores a different site.

This degree of autonomy is not unprecedented. Even during Apollo, although we planned the lunar exploration activities before launch using available photographs, NASA left significant latitude to the crews to pursue unanticipated targets of opportunity. For instance, late in the second exploration period of Apollo 17, I discovered orange volcanic glass in the rim of Shorty Crater with only 30 minutes left to work at that site. Without waiting for suggestions from mission control, Gene and I began to describe, photograph and sample the deposit. We did not have the time to discuss this plan with controllers, but we knew immediately what needed to be done. Exactly this approach will be required of the crews on Mars, all the time, with mission control on Earth finding out everything tens of minutes after the fact.

A third difference from Apollo is that, in light of the expense and historical importance of each Mars exploration mission, the mission philosophy must be totally success-oriented. If something goes wrong, the astronauts should still be able to continue their mission and achieve all its major goals. For example, the ship should ideally carry two landers in case one cannot be used. Further, if systems or software anomalies occur during the entry, descent and landing sequence, the astronauts should be able to abort to land rather than abort to orbit, as was the plan during Apollo. The problems can be resolved over time, in consultation with Earth, once the crew lands safely on Mars.

Young people now alive will have the privilege and adventure of exploring Mars, if their parents and grandparents provide them the opportunity. It will not be easy. As with anything worthwhile, risks exist. Not only are the rewards from new knowledge great, but the costs of ceasing our exploration also would be great. Postponing the exploration of Mars beyond what is already planned would leave Americans to follow in the footsteps of other explorers and nations. Moreover, without a gradual effort to learn how to explore and eventually settle on other worlds, the very existence of humankind will remain vulnerable to the impact of asteroidal and cometary travelers of the solar system. Curiosity, the lessons of history and our self-preservation instinct all demand that we continue to move outward.

Note: This article was originally printed with the title, "From the Moon to Mars."