Mars caught the fancy of the general public in the late 1870s, when Giovanni Schiaparelli, an Italian astronomer, reported that he observed ‘canali’ through his telescope. “canali,” or channels, on Mars. Schiaparelli was a prolific Italian astronomer whose research ranged widely.
He joined the staff of Milan’s Brera Observatory in 1860 and became its director two years later. The small instruments at Brera led Schiaparelli to focus his research initially on meteors and comets. Scientifically speaking his most important contribution to astronomy was not Mars, but his discovery that swarms of meteors, which give rise to annual showers on Earth, and comets follow similar paths through space.
As reward for this breakthrough a more powerful (8.6-in.) refractor was installed at Brera which allowed him to engage in serious planetary work. He first wanted to test the powers of the new instrument, and the opposition of Mars in September 1877 seemed the ideal opportunity to do so. Partly due to a possible mistranslation of canali into ‘canals’ in English, partly because Schiaparelli chose romantic and evocative marine names for the features he observed on Mars, the world soon believed Mars to have seas and canals.
That word “canals” may have fired the imagination of Percival Lowell, an American businessman with an interest in astronomy. Lowell founded an observatory in Arizona, where his observations of Mars convinced him that the canals were dug by intelligent beings — a view that he energetically promoted for many years.
Added to this were the fictional visions of Mars. H.G. Wells’ 1898 novel “The War of the Worlds” portrayed an invasion of Earth by technologically superior Martians desperate for water.
In the early 1900s novelist Edgar Rice Burroughs, known for the “Tarzan” series, also entertained young readers with tales of adventures among the exotic inhabitants of Mars, which he called Barsoom.
As the world in 1965 eagerly awaited results of the first spacecraft flyby of Mars, everything we knew about the Red Planet was based on what sparse details could be gleaned by peering at it from telescopes on Earth. Since the early 1900s, popular culture had been enlivened by the notion of a habitable neighbouring world crisscrossed by canals and, possibly, inhabited by advanced life forms that might have built them — whether friendly or not.
Astronomers were highly sceptical about the canals, which looked more dubious the closer they looked. About the only hard information they had on Mars was that they could see it had seasons with ice caps that waxed and waned, along with seasonally changing surface markings. By breaking down the light from Mars into colors, they learned that its atmosphere was thin and dominated by an unbreathable gas: carbon dioxide.
Fact began to turn against such imaginings, when the first robotic spacecraft were sent to Mars in the 1960s. Pictures from the 1965 flyby of Mariner 4 and the 1969 flybys of Mariner 6 and 7 showed a desolate world, pocked with impact craters similar to those seen on Earth’s moon. Mariner 9 arrived in 1971 to orbit Mars for the first time, but showed up just after an enormous dust storm had engulfed the entire planet. When the storm died down, Mariner 9 revealed a world that, while partly crater-pocked like Earth’s moon, was much more geologically complex, complete with gigantic canyons, volcanoes, dune fields and polar ice caps.
Those holding out for Martians were further discouraged when NASA’s two Viking landers and two Viking satellites in 1976. The landers carried a suite of experiments that conducted chemical tests to detect life. The experiments turned up no conclusive sign of biological activity. And most scientists interpreted the results of these tests as negative, deflating hopes of identifying another world where life might be or have been widespread. However, Viking left a huge legacy of information about Mars that fed a hungry science community for two decades.
Mars as we came to know it was cold, nearly airless and bombarded by hostile radiation from both the sun and from deep space. However, the science community had many other reasons for being interested in Mars, apart from the direct search for life. They wanted to know how the geology of Mars had developed to compare it to Earth’s geology. What was Mars’ climate and why? Over the next 20 years new findings in laboratories and in extreme environments on Earth came to change the way that scientists thought about life and Mars.
Since then, however, new possibilities of a more hospitable Martian past have emerged. Mars is a much more complex body than Earth’s moon. Scientists scrutinizing pictures from the orbiters of the 1970s detected surface features potentially shaped by liquid water, perhaps even the shoreline of an ancient ocean.
Another development shaping ideas about extraterrestrial life was a string of spectacular findings on how and where life thrives on Earth. The fundamental requirements for life as we know it today are liquid water, organic compounds and an energy source for synthesizing complex organic molecules.
In recent years, it has become increasingly clear that life can thrive in settings much harsher than what we as humans can experience. In the 1980s and 1990s, biologists found that microbial life has an amazing flexibility for surviving in extreme environments; environments that are extraordinarily hot, or cold, or dry, or under immense pressures; environments that are completely inhospitable to humans or complex animals. Some scientists even think, that life on Earth may have begun at hot vents far under the ocean’s surface.
This, in turn, had its effect on how scientists thought about Mars. Martian life might not be so widespread that it would be readily found at the foot of a lander spacecraft, but it may have thrived billions of years ago in an underground thermal spring or other hospitable environment. It even might still exist in some form in niches below the currently frigid, dry, windswept surface, perhaps shielded in ice or in liquid water aquifers. Each successful Mars mission uncovers more pages of the planet’s story. After years of studying pictures from the Mariner 9 and Viking orbiters, scientists gradually came to conclude that many features they saw suggested that Mars may have been warm and wet in an earlier era.
Mars Global Surveyor’s camera detected possible evidence for recent liquid water in many settings, including hundreds of hillside gullies. That orbiter’s longevity even allowed before-and-after imaging that showed fresh gully-flow deposits in 2005 that had not been present earlier in the mission. Observations by Global Surveyor and Odyssey have also been interpreted as evidence that Mars is still adjusting from a recent ice age as part of a repeating cycle of global climate change. The cycle results from changes in the tilt of the rotational axis of Mars on time scales of hundreds of thousands to a few million years. Mars’ tilt varies much more than Earth’s.
Although it appears unlikely that complex organisms similar to advanced life on Earth could have existed on Mars’ comparatively hostile surface, it is not unlikely that life in some form (e.g. simple microbes) may have existed on Mars when it was wetter and warmer.
Even more, it is not unthinkable that life in some form could persist today in underground springs warmed by heat vents around smoldering volcanoes, or even beneath the thick ice caps. To investigate those possibilities, NASA’s productive strategy has been to learn more about the history of water on Mars: How much was there? How long did it last? Where are formerly wet environments that make the best destinations for seeking evidence of past life? Where might there be wet environments capable of sustaining life today?
The strategy for answering those questions balances the examination of selected sites in great detail on the one hand, while also conducting planet-wide surveys to provide context for interpreting the selected sites. This enables researchers to extrapolate from the intensively investigated sites to regional and global patterns, and to identify which specific sites make the best candidates for targeted examination.
Orbital and surface missions work hand in hand to get us answers.Mineral mapping by NASA’s orbiting Mars Global Surveyor identified a hematite deposit that made Meridiani Planum the selected landing site for NASA’s Mars Exploration Rover Opportunity. The hematite suggested a possible water history.
Opportunity’s examination of the composition and fine structure of rocks where it landed confirmed that the site had had surface and underground water. Opportunity added details about the acidity of the water and the alternation of wet and dry periods at the site.
Meanwhile, halfway around the planet at Gusev Crater, the Spirit rover found evidence of materials altered in an ancient hydrothermal system. The carbonate deposit offered evidence for a wet environment that was less acidic and found a nearly pure silica deposit formed by a hot spring or steam vent.
Either of those long-ago environments deduced from Spirit’s observations could have been even more favourable for microbial life than the conditions that left the clues found by Opportunity during Opportunity’s first seven years on Mars. In 2011, the long-lived Opportunity also found veins of gypsum deposited by water that might not have been acidic. This “ground truthing” by the rover improves the interpretation of the observations by Mars satellites of the surrounding region
The European Space Agency’s Mars Express has identified exposures of clay minerals that probably formed in long-lasting, less-acidic wet conditions even earlier in Mars history than the conditions that produced the minerals examined by Opportunity through 2010. Mars Express and telescopic studies from Earth have found traces of atmospheric methane at Mars that might come from volcanic or biological sources. A radar instrument co-sponsored by NASA and the Italian Space Agency on the European orbiter has assessed the thickness and water content of icy layers covering Mars’ south polar region, yielding an estimate that the quantity of water frozen into those icy layers is equivalent to a 11 meter thick (36-foot-thick) coating of the whole planet.
The Phoenix Mars Lander investigated a site with intriguing characteristics discovered from orbit. Spectrometers on NASA’s Mars Odyssey orbiter found evidence of copious water ice within the top 3 feet (1 meter) of the surface in high-latitude and some mid latitude regions. Phoenix landed at a far-northern site and confirmed the presence of plentiful water ice just beneath the surface. In the soil above the ice, Phoenix found a chemical called perchlorate, which could serve as an energy source for microbes and as a potent antifreeze enabling water to be liquid at low temperature.
The Mars Reconnaissance Orbiter had found Phoenix a less rocky, safer landing site. The orbiter also examined more than 30 potential landing sites for the Mars Science Laboratory. Four finalist sites were examined in thorough detail to identify specific mineral deposits of interest and potential landing hazards down to the scale of individual rocks before Gale Crater was chosen as the landing site.
Since 2006, NASA’s Mars Reconnaissance Orbiter has radically expanded our knowledge of the Red Planet. Observing the planet at the highest spatial resolutions yet achieved from orbit, this mission has provided an enormous amount of information about ancient environments, ice- agescale climate cycles and present-day changes on Mars. It has mapped water-related mineral deposits, including multiple types of clay minerals, in hundreds of locations, and carbonates in several locales, confirming a story of alteration by water in a diversity of environments early in Martian history and a dramatic change to very acidic water in many areas after an era of more neutral conditions.
With regard to recent Mars history, the Mars Reconnaissance Orbiter has added evidence of ongoing climate-change cycles linked to how changes in the tilt of the planet’s rotation axis alter intensity of sunlight near the poles. This evidence includes radar observations of episodic layering within the polar ice caps and of debris-covered ice deposits.
Other observations have revealed water ice just below the surface in middle latitudes. If found water-ice exposed in small craters formed by fresh impacts identified in before-and-after observations. These findings and their implications for ongoing cycles of climate change put water ice closer to the equator on modern Mars, including possibly beneath the Viking 2 Lander, than most researchers imagined a few years ago.
The surface pressure of the Mars atmosphere varies over the year as the seasonal carbon-dioxide frost forms and then sublimes at each pole in turn. A major finding by the radar on Mars Reconnaissance Orbiter is a thick, hidden layer of carbon-dioxide ice deep in the water ice that forms the bulk of the south polar ice cap. This deposit may contain nearly as much carbon dioxide as today’s Martian atmosphere. This implies that the total mass of the atmosphere on Mars can change several fold on time scales of hundreds of thousands of years or less. Computer modelling indicates that during larger tilt, when summertime solar heating at the poles is more intense, most of the frozen carbon dioxide rejoins the atmosphere, and that during smaller tilt, most of the atmosphere freezes out.
Orbital studies in recent years have also revealed some processes on Mars very unlike those on Earth related to temperatures low enough to freeze some of the carbon dioxide gas out of the planet’s thin atmosphere. Thisnew carbon-dioxide ice blankets the ground around whichever pole is in winter. In the spring, sunlight penetrates the translucent ice, heating it from below. As the carbon dioxide thaws back into gas, it triggers geyser-like eruptions in some areas and fresh flows of sand on slopes in other areas.
Eight successful Mars missions since the mid-1990s have advanced the story. Their accumulated evidence shows that the surface of Mars was indeed shaped by flowing water in hundreds of places. Also we know now for certain that some Mars rocks formed in water. Orbiters proved that significant amounts of water as ice and in hydrated minerals still make up a fraction of the top surface layer of Mars in many areas. Even today water may occasionally emerge from the ground to flow briefly before freezing or evaporating.
Modern Mars is a more dynamic planet today than we realized before the advent of frequent, high-resolution imaging: Hundreds of sets of before-and-after images from Mars Global Surveyor and Mars Reconnaissance Orbiter document soil flowing down gullies, rocks bouncing down hills, dunes migrating, craters forming, icy layers receding and dust blowing.
Cameras in orbit have even caught avalanches and tall whirlwinds as they happen. Image sequences from Spirit show the motions of dust devils dancing across the landscape and clouds scooting across the sky. Thus, the Martian landscape, which has changed dramatically in the past, continues to change today.