 |
| Sedimentary rock on Mars as viewed and analyzed by the Curiosity rover [NASA]. |
Paul D. Spudis
The Once & Future Moon
Smithsonian Air & Space
The news of the day is abuzz with the new and astounding discoveries from the Curiosity rover that Mars once had an environment conducive to life. Once it was warmer, wetter, more hospitable. Water flowed over its surface. The chemicals necessary for life’s emergence and development are present on Mars, suggesting that life may have arisen there in the distant past. So why do I have this sense of déjà vu? Perhaps because this new “result” gets trumpeted anew every few years.
The fixation on the possibility of martian life has been a constant throughout the history of the space program, starting before the first planetary mission to Mars in 1965 (Mariner 4) and then waxing and waning in likelihood every few years. Mariner 4 showed us a moon-like Mars, with a rough, cratered surface and thin cold atmosphere. The stock for martian life fell accordingly.
The fixation on the possibility of martian life has been a constant throughout the history of the space program, starting before the first planetary mission to Mars in 1965 (Mariner 4) and then waxing and waning in likelihood every few years. Mariner 4 showed us a moon-like Mars, with a rough, cratered surface and thin cold atmosphere. The stock for martian life fell accordingly.
A few years later, the twin probes Mariners 6 and 7 flew by Mars, again returning pictures of a cratered surface, but with hints of the presence of unusual terrain, possibly the result of subsurface ice. The stock of the life story rose slightly, but the barren cold desert of the martian surface was hardly a Garden of Eden.
A big breakthrough came with the flight of Mariner 9 in 1971. To the astonishment of most planetary scientists, it revealed a world of giant volcanoes, canyons much larger than the Grand Canyon on Earth, and amazingly, channels that looked as though they were carved by running water. The idea of life on Mars – at least in the distant past – gained credence and served as a springboard for the Viking missions of 1976, America’s bicentennial year. These two missions consisted of both a lander and an orbiter and were specifically designed to test the surface of Mars for the possibility of life. Both landers returned results that were immediately interpreted as negative (although there was some dissent); the surface materials of Mars had a very reactive chemistry, but no organic material was found in the soil, even at concentration levels measured in parts-per-billion. Thus, we had the conundrum of abundant landform evidence for an early, warm and wet climate yet chemical evidence for an almost sterilizing environment at present. If Mars had life, it must have been present only in the distant past. The results from Viking were considered so definitive that no mission was sent to Mars for over 20 years.
What precipitated the new flurry of interest in Mars about twenty years ago was the finding that, astonishingly enough, we have samples in our possession from Mars in the form of meteorites, the so-called “SNC meteorites” (the initials of Shergotty, Nakhla, and Chassigny, the first three meteorites recognized to be of martian origin). It had been thought that the preservation of rocks intact during ejection from the planet at escape velocities and greater was not possible, but in this case, observations trumped theory. Even more amazing, it was claimed that in one of these putative martian rocks, small features within it were actually fossils of ancient bacteria. Although highly controversial then (and now), this finding was given widespread publicity (including even a Rose Garden Presidential statement) and the agency used this discovery to sell a program to send a series of probes to Mars at every two-year opportunity for the next decade.
This fleet of orbiters and landers returned an abundance of new, high-quality data on the martian surface, its composition, the locations of water and its environment. Each mission confirmed that water had once been present on Mars. Each mission confirmed that at present, the surface was not conducive to life. Each lander went to a site that was thought to have been more promising for the development of life than the ones that preceded it. As the years rolled on, each “new discovery” of the former presence of water and favorable environmental conditions on Mars became something of a joke among my colleagues in the planetary science business – how many times can you claim the discovery of something already known?
Lest you think that I am simply expressing my lunar parochialism, I note that this same media phenomenon occurs in regard to the existence of water ice at the poles of the Moon. The theoretical possibility of ice on the Moon had been known for many years. We first found direct evidence for it in 1996 with an improvised radio experiment on the Clementine mission. Subsequent studies from Earth and a variety of other space missions caused the stock for lunar polar water to rise and fall, depending on who issued the latest press release for their published work. Finally, the collision of the LCROSS impactor in 2010 removed all doubt – there was and is ice there, at least at the south pole and in quantities greater than could be reasonably expected to have resulted simply from solar wind deposition. Yet each new finding was announced as a new “discovery” in the press. So this media frenzy is not simply related to Mars mania or even to the over-preoccupation with finding life elsewhere.
The basic fact is that most in the news business do not understand (or at least, do not fully appreciate) the incremental, cumulative nature of modern science. It is seldom indeed when a single experiment or observation causes a scientific revolution. Moreover, it is equally seldom that a breakthough comes from one person or even one research team. Science is a complex, interdisciplinary effort. It makes progress, but slowly and in a manner that includes both leaps forward and (sometimes) backward. Only over long periods of time (decades and greater) is it apparent what the key observation or measurement is and how it fits into a pattern of understanding. Each new mission result adds knowledge, sometimes in great leaps and sometimes in increments so tiny that one can question whether anything new is being learned at all. But even a repeated observation has value in science – in fact, if an observation is not repeatable, it is not a valid scientific observation.
The new inferences from Curiosity suggest a more benign and hospitable environment for life, but few working Mars investigators doubted that such existed in the past. Even if it did not, we have found in the past few decades that even extreme environments on the Earth can support certain types of microbial life. So the new results broaden and deepen our understanding of martian surface properties and processes, they do not revolutionize them. That’s just how science normally works. If some scientists tend to oversell their results, well, they’re only human.
A big breakthrough came with the flight of Mariner 9 in 1971. To the astonishment of most planetary scientists, it revealed a world of giant volcanoes, canyons much larger than the Grand Canyon on Earth, and amazingly, channels that looked as though they were carved by running water. The idea of life on Mars – at least in the distant past – gained credence and served as a springboard for the Viking missions of 1976, America’s bicentennial year. These two missions consisted of both a lander and an orbiter and were specifically designed to test the surface of Mars for the possibility of life. Both landers returned results that were immediately interpreted as negative (although there was some dissent); the surface materials of Mars had a very reactive chemistry, but no organic material was found in the soil, even at concentration levels measured in parts-per-billion. Thus, we had the conundrum of abundant landform evidence for an early, warm and wet climate yet chemical evidence for an almost sterilizing environment at present. If Mars had life, it must have been present only in the distant past. The results from Viking were considered so definitive that no mission was sent to Mars for over 20 years.
What precipitated the new flurry of interest in Mars about twenty years ago was the finding that, astonishingly enough, we have samples in our possession from Mars in the form of meteorites, the so-called “SNC meteorites” (the initials of Shergotty, Nakhla, and Chassigny, the first three meteorites recognized to be of martian origin). It had been thought that the preservation of rocks intact during ejection from the planet at escape velocities and greater was not possible, but in this case, observations trumped theory. Even more amazing, it was claimed that in one of these putative martian rocks, small features within it were actually fossils of ancient bacteria. Although highly controversial then (and now), this finding was given widespread publicity (including even a Rose Garden Presidential statement) and the agency used this discovery to sell a program to send a series of probes to Mars at every two-year opportunity for the next decade.
This fleet of orbiters and landers returned an abundance of new, high-quality data on the martian surface, its composition, the locations of water and its environment. Each mission confirmed that water had once been present on Mars. Each mission confirmed that at present, the surface was not conducive to life. Each lander went to a site that was thought to have been more promising for the development of life than the ones that preceded it. As the years rolled on, each “new discovery” of the former presence of water and favorable environmental conditions on Mars became something of a joke among my colleagues in the planetary science business – how many times can you claim the discovery of something already known?
Lest you think that I am simply expressing my lunar parochialism, I note that this same media phenomenon occurs in regard to the existence of water ice at the poles of the Moon. The theoretical possibility of ice on the Moon had been known for many years. We first found direct evidence for it in 1996 with an improvised radio experiment on the Clementine mission. Subsequent studies from Earth and a variety of other space missions caused the stock for lunar polar water to rise and fall, depending on who issued the latest press release for their published work. Finally, the collision of the LCROSS impactor in 2010 removed all doubt – there was and is ice there, at least at the south pole and in quantities greater than could be reasonably expected to have resulted simply from solar wind deposition. Yet each new finding was announced as a new “discovery” in the press. So this media frenzy is not simply related to Mars mania or even to the over-preoccupation with finding life elsewhere.
The basic fact is that most in the news business do not understand (or at least, do not fully appreciate) the incremental, cumulative nature of modern science. It is seldom indeed when a single experiment or observation causes a scientific revolution. Moreover, it is equally seldom that a breakthough comes from one person or even one research team. Science is a complex, interdisciplinary effort. It makes progress, but slowly and in a manner that includes both leaps forward and (sometimes) backward. Only over long periods of time (decades and greater) is it apparent what the key observation or measurement is and how it fits into a pattern of understanding. Each new mission result adds knowledge, sometimes in great leaps and sometimes in increments so tiny that one can question whether anything new is being learned at all. But even a repeated observation has value in science – in fact, if an observation is not repeatable, it is not a valid scientific observation.
The new inferences from Curiosity suggest a more benign and hospitable environment for life, but few working Mars investigators doubted that such existed in the past. Even if it did not, we have found in the past few decades that even extreme environments on the Earth can support certain types of microbial life. So the new results broaden and deepen our understanding of martian surface properties and processes, they do not revolutionize them. That’s just how science normally works. If some scientists tend to oversell their results, well, they’re only human.
Originally published at his Smithsonian Air & Space Magazine blog "The Once and Future Moon," Dr. Spudis is a senior staff scientist at the Lunar and Planetary Institute. The opinions expressed are those of the author and are better informed than average.
No comments:
Post a Comment