CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)
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Mars Exploration History

Mars has fascinated scientists and the general public since the invention of the telescope. In the late 1800s, interest in Mars really began to ramp up. The wealthy astronomer Percival Lowell had an observatory built in Flagstaff, Arizona, for the sole purpose of observing and gathering as much information as possible about the Red Planet. After his own observations and much reading about other astronomers' findings, Lowell announced his startling conclusion that Mars was inhabited by intelligent beings. He based his rather incredible theory on what he interpreted to be lines crisscrossing the surface of Mars. The lines were originally discovered by the Italian astronomer Giovanni Schiaparelli. Astronomers of the time lacked a modern perspective that the surfaces of other planets could be shaped by processes much different than on Earth's surface. So, Schiaparelli interpreted Mars' dark areas as seas (mare) and bright reddish areas as dry, desert-covered continents (terra). The lines he thought he saw appeared to connect the dark areas, so he called them channels (canali). This word was mis-translated into English as canals.

These canals, Lowell believed, were the work of intelligent beings desperately trying to bring water from melting polar ice caps to the warmer regions at the equator. Lowell wrote a book about his findings, Mars as the Abode of Life. Lowell had unknowingly set the world on edge with his remarkable ideas. But they were only ideas, not lasting scientific results. Over the next decades, scientists came to the conclusion that the canali were only optical illusions, whereby astronomers' eyes unconsciously connected unrelated dots (craters, sand dune fields, etc.). Lowell's having reached wrong conclusions about intelligent Martian beings shows how "science" can go wrong when it isn't really science at all, and doesn't use the scientific method of formulating and testing hypotheses (and taking the results of those tests seriously). Reality of the canali was at least thrown into doubt, if not disproven outright, by observations by other scientists. But Lowell was dismissive of the other scientists' work. Even if the canali were real, other astronomers pointed out, sort-of-straight lines could form by much simpler means than Martian engineers working on a giant public works project - for example, as faults, or crater rays. But Lowell ignored them, too.

One of Percival Lowell's drawings of linear features he thought he saw on Mars. Image courtesy of Wikipedia Commons.

Regardless of the truth of canals, artists shared fictional visions of what Lowell's Mars might look like if it were visited by an Earthly being. In H.G. Wells' famous science fiction story The War of the Worlds, Martians leave their dying planet to capture Earth's vast resources. Ruthless and uncaring, they destroy everything in their path. The alien invaders eventually die as a result of a common microbe to which they have no immunity.

After advances in telescope making, and better observations, it was generally agreed that Mars was not populated with intelligent life. The new observations also showed that Mars' atmosphere was thinner and colder than thought previously, with little or no oxygen, challenging any life. Still, speculation remained that vegetation growing in the Martian soil might be responsible for creating dark areas on the planet.

In late 1964, the United States launched the first successful space probe to Mars, beginning the modern era of scientific exploration of the Red Planet. When it flew by Mars in July, 1965, Mariner 4 sent back 22 close-up images. These new data showed a heavily cratered surface lacking Earth-like mountains, with no sign of water. And measurement of Mariner 4's radio signal as it passed behind Mars showed that the atmospheric pressure was only less than 1% of that on Earth, not enough for liquid water to persist. Then in 1969, just after the Apollo 11 landing on the desolate surface of the Moon, Mariner 6 and Mariner 7 each flew by Mars, imaging more of the surface. The images looked similar to those from Mariner 4, leading scientists to be nearly certain that Mars was devoid of any kind of life. In fact, they compared Mars to the Moon – and declared it geologically dead. Now, the pendulum of prevailing opinion has swung to the opposite extremely from where it had been to the followers of Percival Lowell. But prevailing opinion still wasn't right. Unknowingly, the Mariners had seen only the oldest, most battered parts of Mars, and the low resolution of their images missed evidence for dry riverbeds, dry lakebeds, lava flows and tectonic faults preserved from the distant past.

mariner images
Early Mariner images only covered the oldest, most cratered parts of Mars. Mariner 4 image (left) and Mariner 6 image (center) courtesy of National Air and Space Museum. Mariner 7 picture (right) courtesy of the Jet Propulsion Laboratory. The Mariner 4 image actually covers lava flows and tectonic faults that were not resolved in the image.

It wasn't until the first orbiter reached Mars, Mariner 9, that scientists finally began to get a balanced view of the planet. Mariner 9 sent back more than 7,000 images of the planet's surface. Scientists were surprised to find a canyon system long enough to stretch from Washington, D.C., to Las Vegas, Nevada! Finally, too, the images had good enough resolution for discovery of real channels, dry riverbeds unrelated to canali, that preserved evidence for ancient liquid water on Mars. Volcanoes as large as the state of Arizona and three times taller than Mount Everest showed that Mars also was not a geologically dead planet. The hope for life still holding out somewhere, in its last stand on Mars, flickered again.

Mariner 9 images revealed clouds, tectonic features, and evidence of past flowing water. Image of a cloud-filled Valles Marineris (left) courtesy of Jet Propulsions Laboratory. The dark spot is the shadow of Mars' moon Phobos. Image of Nirgal Vallis (right) courtesy of Lunar and Planetary Institute.

The most ambitious Mars missions to date were launched in 1975 and reached Mars in summer 1976, Viking 1 and Viking 2. Each Viking has one orbiter and one lander. The lander had experiments to take panoramic images, measure the chemical composition of the soil, and look for chemical signs of life. Humanity reaching out to another world to conduct scientific tests for micro-organisms in the soil represented a momentous achievement. The life detection experiments yielded results that in some ways looked like they were caused by life, but in other ways looked like they were caused by inorganic chemical reactions. However the soil composition experiments suggested that the soil was devoid of organic matter, even below the levels of non-biological organic materials that would result from infall of primitive meteorites. Now Mars looked dead (again). But images and measurements of atmospheric composition from the Viking orbiters provided the best-ever understanding of Mars' surface as it is today. For the first time, changes in weather patterns and humidity of Mars' atmosphere over a Mars year were measured carefully. The images revealed more volcanoes, and smaller and far more abundant ancient dry riverbeds than Mariner 9 had seen. Finally, the modern view of Mars began to emerge: a planet that is cold and dry now, but that in the distant past – 3 to 4 billion years ago – was shaped in part by liquid water. Might life have existed, and might evidence of it be preserved?

The Viking missions took landed measurements and orbital measurement with unprecedented resolution. Viking Lander 2 panorama (left) and orbital image of dry river valleys (right) both courtesy of Jet Propulsion Laboratory.

The renaissance of Mars exploration began in the 1990's, when NASA's philosophy of how to build and launch planetary exploration missions changed. Out was the concept of infrequent, massively complicated and massively expensive missions like Viking; in was the concept of more frequent, less complicated, less expensive missions. The first of these was Mars Pathfinder, which carried a miniature rover, Sojurner, that for the first time measured the composition of rocks - not just soil. The first Mars landing in 20 years whetted the public's appetite for more.

The Mars Pathfinder mission captivated public attention and reinvigorated exploration of Mars.

Soon after, an orbiter – Mars Global Surveyor – arrived on the scene. Its high resolution camera made discoveries that transformed future study of Mars: vast thick layers of sedimentary rock, fan-shaped deposits thought to be river deltas, and gully-like features that were initially hypothesized to have formed by intermittent modern flowing water on Mars surface. The modern view of Mars moved 2 notches into clearer focus: not only was the ancient surface of Mars sculpted by water, but the planet's geology records that period in many, many places. And here and there, drips of activity may occur even now. The perception of Mars moved again, from dead to just mostly dead, and maybe once alive.

Images from the Mars Global Surveyor mission for the first time fully revealed the breadth of water's influence on the Martian surface. Images of an ancient river delta in Eberswalde crater (left), layered sedimentary rocks on the floor of Valles Marineris (center), and gullies thought initially possibly to be carved by intermittent present-day water (right), all courtesy of Malin Space Science Systems.

The stage was set for the missions that explored Mars in the last several years. The U.S. missions are coordinated in their objectives, into what NASA calls the Mars Exploration Program. One mission's results raise new questions, and other missions address those questions. The program is all about the scientific method, creating hypotheses and testing them. (We've come a long way from the days of Percival Lowell!) Three orbiters - Mars Odyssey, the European Space Agency's Mars Express, and Mars Reconnaissance Orbiter - act as "weather satellites" to understand how Mars' atmosphere is similar to and different from Earth's, and scout the mineralogy and landforms that record ancient (and possibly modern) wet environments. In fact, most of our knowledge about composition of the surface comes from these orbiters. Mars Express and Mars Reconnaissance Orbiter, especially, are the missions that have revealed mineral records of past water – oxidized iron minerals, opal, and clay, carbonate, and sulfate minerals – in thousands of locations. The orbiters find key locations to land, where landed measurements put a magnifying glass to key places and times in Mars' history. The Mars Exploration Rovers Spirit and Opportunity and the Mars Science Laboratory Curiosity landed where orbital data pointed to sedimentary rocks or water-formed minerals. These missions pursue four main goals in the exploration of Mars:

  1. Determine whether life ever arose on Mars
  2. Characterize the climate of Mars
  3. Characterize the geology of Mars
  4. Prepare for human exploration

The instruments on these missions each are designed to carry out a specific task, such as study the Mars atmosphere or map the planet's surface. The Compact Reconnaissance Imaging Spectrometer for Mars (better known as CRISM) is an instrument built by the Johns Hopkins University Applied Physics Laboratory (APL). It was launched from Cape Canaveral, Florida onboard the Mars Reconnaissance Orbiter in 2005 and reached Martian orbit in 2006. CRISM takes images in 544 colors of sunlight reflected back to the instruments' two spectrometers to search the planet for signs of past water, to explore the structure of Mass' crust, and to understand how the atmosphere works today. Data collected by CRISM have, and will continue, to help scientists determine where to land with future missions. The exploration of Mars is an ongoing adventure!