The Mars Science Lab rover, Curiosity, carries the most advanced payload of scientific instrumentation ever used on the surface of Mars. Curiosity - and its team of scientists and engineers - is tasked with investigating whether conditions at its landing site have been favorable for microbial life and for preserving clues in the rocks about possible past life. Curiosity will NOT be looking for life; its instruments are not designed to find life. Curiosity will be looking for clues that point to the possibility that life could have existed in the past. Like its predecessors’ landing sites, Curiosity will land in a region with exposed minerals that are formed in wet environments. All of Curiosity’s scientific instruments will work in concert to determine the landing region’s potential for life. In particular, ChemCam will tell mission scientists what the rocks are made of in the rover’s landing region. One of ChemCam’s primary objectives is to determine the compositions of rocks and soil and to identify samples that would be of great interest to scientists for analysis by other instruments onboard Curiosity. Knowing a rock’s composition gives scientists clues as to the environment in which the rock formed. ChemCam will set its sights on the rocks in its landing region, looking for the chemical evidence that water was once abundant there.
Early Fascination and Robotic Exploration of Mars
Prior to the mid-20th century, many people, including scientists and authors, believed Mars was a world capable of supporting life. Authors wrote tales of Martians visiting Earth (Figure 1), sometimes peacefully, but not peacefully in many stories. Early telescopic observations of Mars showed dark surface features that changed over time (Figure 2). Some interpreted these changing patterns as vegetation growing and dying with the seasons on Mars. The first close-up views of Mars from the Mariner 4 spacecraft in 1965 (Figure 3) shattered that belief and other fanciful ideas like Martian plots to destroy humanity.
Figure 1. Artwork for the book "The War of the Worlds" from a 1906 Belgian edition by the Brazilian artist Henrique Alvim Corréa.
Figure 2 Ground-based, telescopic image of Mars showing changes in both dark patterns and the northern polar cap.(Images of Mars are inverted.) Credit: Lowell Observatory
Figure 3 One of the first close-up images of Mars returned by the Mariner 4 spacecraft in 1965. The image shows craters – circular depressions – on the Martian surface. Credit: NASA
Images returned by Mariner 4 revealed a heavily cratered surface with no signs of liquid water or life of any kind. Mariner 4 showed us a Mars that looked similar to our Moon. Two more missions, Mariner 6 and Mariner 7, flew by Mars in 1969 revealing more of the same terrain (Figures 4 and 5) seen by Mariner 4.
Figure 4. Mariner 6 image of the Martian surface. Combined, the Mariner 6 and 7 missions returned only 198 images. Credit: NASA
Figure 5. Mariner 7 image of Mars’ southern polar cap. Temperature measurements from both missions showed the southern cap to be made of carbon dioxide ice. Credit: NASA
In 1971, Mariner 9 became the first spacecraft to orbit the Red Planet. Like its predecessors, Mariner 9 did not find any water or life. However, images from the spacecraft did give scientists their first look at what appeared to be ancient river beds (Figure 6).
Figure 6. Mariner 9 image of ancient, dried-up river beds. Credit: NASA
With the discovery of ancient river beds, the prospects for life on Mars, if even ancient life, reentered the imaginations of scientists and the public, alike. If Mars once had liquid water flowing across its surface, is it also possible life had existed on the surface? In 1976, the twin Viking landers (Figures 7 and 8) touched down on Mars, in two separate locations, while their orbiters remained above Mars, snapping more photos showing evidence for a wet Mars in the past (Figure 9). Fitted with biological experiments, scientists hoped the landers would find evidence of life in the Martian soil. Many scientists believe the results of these experiments do not support evidence for life. The Viking 1 lander proved to be the most robust of the mission’s fleet of spacecraft, ceasing communications with engineers in 1982. It would be 14 years before robotic explorers would return to Mars.
Figure 7. Site of the Viking 1 lander as seen by the lander’s camera. Credit: NASA
Figure 8. Site of the Viking 2 lander as seen by the lander’s camera. Credit: NASA
Figure 9. Long, winding channel in the Martian surface imaged by the Viking 1 orbiter. This channel was carved by water early in Mars history. A detail of the area in the white box can be seen in Figure 11. Credit: NASA
The early years of Mars exploration revealed a planet with characteristics that challenged preconceptions held by many people for many years. However, evidence of liquid water in the past, and the possibility of even simple life, continued to tantalize scientists and stir the public imagination. The science fiction of years past began to seem not so fantastic.
Follow the Water
After the loss of the Mars Observer orbiter in 1992, Mars exploration was given a shot of adrenaline in the latter half of the decade with the successful arrival of the Mars Global Surveyor (MGS) orbiter in 1996 (Figure 10).
Figure 10. Artist’s conception of the Mars Global Surveyor (MGS) spacecraft. MGS forever changed scientists understanding of Mars with its high-resolution imagery of the Martian surface. Communications ceased with MGS in November 2006. Credit: NASA
MGS picked up where the Viking orbiters left off. MGS gave scientists the most detailed images of the Martian surface to date, finding more evidence for a watery past (Figure 11), and evidence of relatively recent activity of liquid water. MGS spotted gullies carved into crater walls (Figure 12). On Earth, gullies typically form from liquid water carving through material as it flows downhill. This finding was significant because later analysis revealed many of the gullies formed relatively recently, possibly within the past few million years. Yes, a few million years is still old. So, why is this a big deal? In the early history of Mars, the atmosphere must have been thicker, providing warmer conditions that would allow liquid water to exist at the surface. This can explain the numerous geologic features on the surface that resemble similar features formed by liquid water on Earth. Currently, Mars has a very thin atmosphere and extremely cold temperatures, conditions that have prevailed for at least the past 3.5 billion years! Under such conditions, water should not exist on the surface as a liquid. If, however, it is possible for liquid water to currently exist, near or on the surface, maybe life currently exists, near or on the surface.
Figure 11. Detail from the Viking 1 Orbiter image in Figure 10. The dry bed of a smaller channel can be seen in the upper-right corner of the image. Credit: NASA/JPL/MSSS
Figure 12. Gullies on the wall of a Martian crater. On Earth, gullies form from water running downhill, wearing away surface material and transporting it downhill leaving an exposed, open channel. Credit: NASA/JPL/MSSS
With the mounting geologic evidence that water once flowed across the surface of Mars, NASA adopted an exploration strategy titled “Follow the Water” for the Mars exploration program:
Following the water begins with an understanding of the current environment on Mars. We want to explore observed features like dry riverbeds, ice in the polar caps and rock types that only form when water is present. We want to look for hot springs, hydrothermal vents or subsurface water reserves. We want to understand if ancient Mars once held a vast ocean in the northern hemisphere as some scientists believe and how Mars may have transitioned from a more watery environment to the dry and dusty climate it has today. Searching for these answers means delving into the planet's geologic and climate history to find out how, when and why Mars underwent dramatic changes to become the forbidding, yet promising, planet we observe today.
- from the Mars Exploration Program Website
The “Follow the Water” strategy has guided every NASA mission to Mars since the year 2000. Water is the common thread that runs through the objectives of the Mars exploration program (Figure 13). The Mars Science Lab is no exception.
Figure 13. Water is the common thread that ties together the objectives of NASA’s Mars Exploration Program. Credit: NASA
Get Down, Get Dirty
The stunning images of the surface of Mars sent back by Mars Global Surveyor, and its predecessors, show convincing large-scale, geologic evidence for past, maybe present, liquid water on Mars. The 2005 Mars Reconnaissance Orbiter has followed MGS with even more incredibly detailed imagery from orbit (Figure 14).
Figure 14. Mars Reconnaissance Orbiter image taken by the Hi-Resolution Imaging Science Experiment (HiRISE) camera. This image shows blocks of bright, layered rock embedded in darker material that are thought to have been deposited by a giant flood. Credit: NASA/JPL/UA
Geologic evidence for water from orbiters is just one piece of the puzzle. Scientists are also looking for chemical evidence. The best way to obtain chemical evidence is to get on the ground and get dirty. Since 1976, NASA has successfully landed six spacecraft on the surface of Mars. These spacecraft have dug up soil and ground up rock looking for the chemical traces of water. The first two were the Viking landers followed twenty years later by the 1996 Mars Pathfinder mission (Figure 15) which featured the first rover on the Martian surface. The Mars Exploration Rovers (MER) Spirit and Opportunity (Figure 16) landed on Mars in 2004. The most recent landing on Mars was by the Mars Phoenix lander (Figure 17). Phoenix touched down on the northern arctic plains of Mars in 2008 at the farthest northern point of any spacecraft to date.
Figure 15. The Sojourner rover examines the rock “Yogi” near the Mars Pathfinder lander. Sojourner, the first rover to operate on Mars, is about the size of a microwave. Credit: NASA/JPL
Figure 16. Artist’s concept of the Mars Exploration Rovers. MER rovers are much larger than Sojourner. The twin “robot geologists” are each about the size of a golf cart. Credit: NASA
Figure 17. Engineers at Lockheed Martin in Denver, CO assemble the Phoenix lander. The Phoenix spacecraft was originally built for the canceled 2001 Mars Surveyor lander. Credit: Lockheed Martin
Pathfinder landed in a region believed to have been altered by a catastrophic flood early in Mars history. Some of the rocks at the Pathfinder landing site show physical signs of alteration by water, but the relatively crude chemical analyses were too simple to determine evidence of alteration by water. The MER rovers hit the jackpot in looking for chemical evidence of the presence of water. Like Pathfinder, the landing sites for both MER rovers were chosen because both sites were believed to have been the locations of abundant liquid water. Chemical analyses of “blueberries” (Figure 18), at the Opportunity landing site reveal they were formed in an environment with abundant water.
Figure 18. “Blueberries” near the Opportunity landing site. “Blueberries” are actually small, spherical concretions of the mineral hematite. On Earth hematite forms in the presence of large amounts of water, but can also be produced volcanically. Credit: NASA/JPL/Cornell/USGS
The abundance of sulfur and the mineral jarosite at the Opportunity landing site are also tell-tale indicators of an environment that was once drenched in liquid water. The Mars Phoenix lander also hit the jackpot at its landing site north of the Martian Arctic Circle. Phoenix touched water! Ice a little below the surface, to be exact, but this was somewhat expected. Data from the 2001 Mars Odyssey orbiter (Figure 19) suggested vast quantities of ice existed just below the surface of Mars in the arctic regions surrounding the northern polar cap.
Figure 19. This map shows relative concentrations of hydrogen in the subsurface of the Martian arctic. The abundance of hydrogen in this area led scientists to believe the hydrogen is locked up in ice. This map was the basis for the 2007 Mars Phoenix mission. Credit: NASA/JPL/UA
Mission scientists knew the ice was there, however, they were not sure how deep it was. Luckily, it turned out to be right below, and in some places right at, the surface (Figure 20).
Figure 20. The Martian surface below the Mars Phoenix lander. The white material at the center of this image was determined to be ice, possibly uncovered by the lander’s decent thrusters. The image was taken by the robotic arm camera mounted on the end of the eight feet long robotic arm as it peered below the deck after landing. Credit: NASA/JPL/UA/Max Planck Institute
Was the ice at the Phoenix landing site ever liquid? The detection of calcium carbonate in the soil led mission scientists to conclude that the site had been wet or damp sometime in the geologic past. The chemical perchlorate was also identified at the landing site. This is significant because perchlorate can lower the melting temperature of water which could allow small amounts of liquid water to form on the surface today.
MSL and ChemCam will Continue to Follow the Water
Did Mars ever have an environment capable of supporting life? Can it still support life? The missions of the past have returned exciting data supporting the existence of a water soaked Mars in the past. Future missions are expected to do the same. ChemCam supports the Mars Science Lab mission as it stands on the shoulders of giants as scientists continue to Follow the Water.