It’s Saturday, March 18th 2011 and I am saddened by a headline on the website Universe Today… “Hopes Dim for Contacting Spirit Rover.”[i] It seems that these two little robots have not only captured the imagination of a nation but their hearts as well. So, what are Spirit and Opportunity? What was their mission and what have they found in their 7½ half year science ‘sojourn’ traversing the Martian landscape?
THE MISSION
The primary mission for both rovers was to search for water, an essential ingredient for the existence of life as we know it. That doesn’t mean that they were designed as robotic dowsers but rather they were crafted to look for scientific evidence that at some point in Mars’ history liquid water was present.One look at the images of the Martian surface from orbit shows that water may once have played an important part in Mars’ geology but it’s not the kind of hard evidence that would prove that it is present now or was there in liquid form in sufficient abundance to make the genesis of life a real possibility. Could this cold and arid planet have a warmer and wetter past?
The surface geology looks remarkably like that of so many deserts here on Earth. The wind driven dunes reminds us of the central Sahara but it’s the Martian canyons that look so remarkably like the water-cut gulleys and ravines here on Earth that have invited further investigation for possible hydrology.
PLANNING
In the last decades of the 20th century we have launched numerous international robotic missions to orbit the red planet on voyages of remote sensing and mapping. We have even landed a few to conduct science in situ. Notably, the Viking landers touched down in 1976. In addition to the array of imagers, the Vikings carried sophisticated chemical laboratories whose principal function was to look for evidence of life. Even though we didn’t see any macrobiotics the results were inconclusive enough on the question of microbiotic life to further stimulate our scientific curiosity. Even though the Vikings performed flawlessly as meteorological stations the fundamental limitation of the geologic mission was that they were limited in their soil analysis to what they could reach.Later missions were designed to take some tentative steps (um, rolls) with the Mars Pathfinder / Sojourner of 1996. [ii] This mission consisted of a lander and a remotely operated vehicle that together would survey the geology and meteorology of the Ares Vallis region on the northern hemisphere of Mars, a region believed to be an ancient floodplain from its appearance.
Sojourner (the rover) carried cameras and an X-ray spectrometer to analyze the soil and rocks it encountered. The mission collected an impressive amount of data before it went dark in late September of 1997. The data was interpreted as showing that indeed Mars had a warmer climate that included substantial quantities of liquid water and a much denser atmosphere. These discoveries set the stage for the coming of the twins less than six years later.
SPRIT AND OPPORTUNITY
Two vehicles were launched during the summer of 2003. The reason that two vehicles were launched was to provide mission redundancy. History has taught us that successfully landing a vehicle on Mars can be an iffy proposition. Should one of the vehicles miss its destination or worse still, crash, because someone botched a metric conversion then at least the whole mission would not have been a total failure.Landing sites were chosen on opposite sides of the planet Mars because of the differences in their terrain and geology. Both were likely to hold evidence for the past existence of water. The two rovers, named Spirit and Opportunity, were nearly identical twins. Interestingly, one of the most significant differences between the two was in their software programming. Spirit was designed to be a bit more of a risk-taker when operating autonomously. Opportunity was the more cautious brother. Otherwise, these 180 kg rovers carried essentially identical instrument packages. These instruments included a panoramic camera, and alpha particle x-ray spectrometer, a Mossbauer spectrometer, a thermal emission spectrometer, a microscope, magnets to detect the presence of iron and iron containing minerals, and my favorite, the RAT, the rock abrasion tool. It is used to scrape away samples of rock in order to analyze its physical and chemical composition.
These instruments working independently or together were designed to analyze the soil, rocks, and airborne dust to determine their composition, and importantly, how they were formed in the first place.
Figure 1: Spirit, the adventurous one
Landings
After a relatively brief journey of just over six months, both Spirit and Opportunity approached Mars and prepared to land using the method pioneered by the Pathfinder mission in 1996. These vehicles entered the Martian atmosphere, using aerodynamic braking provided by their ablative aeroshells, followed by the deployment of parachutes. Just before touchdown, retro-rockets would fire and a tether holding the landers dropped. Next, a protective cocoon of airbags inflated around the landers and the tether cut. The last few meters of the landers’ flight was spent in freefall. Moments later, the landers impacted the Martian surface, bouncing to a stop protected by their balloon armor. There were breathless moments in mission control waiting for the first signal indicating that the landers had arrived safely, and the airbags deflated. That signal for both came in January of 2004.[iii] It wasn't long after that we glimpsed the alien Martian landscape complete with its red skies and blue sunsets.[iv]
Figure 2: A blue Martian sunset
THE SEARCH FOR WATER
NASA clearly articulated four principal mission goals. They include determining whether or not life has ever existed, or exists, on Mars as well as characterizing the Martian geology and the climate. An even longer-term goal is to gather the data necessary to prepare for human exploration.I remember, shortly after the landings some remarkable images being beamed back to Earth. These were images of little nodules that were scattered about, and nicknamed "blueberries." They weren't blue. They were gray, but they were round beads and just the right size.
Figure 3: Opportunity's blueberry patch.
It turns out that these “blueberries” have a terrestrial analog. They closely resemble hematite concretions , which are usually precipitated from aqueous solutions.[v] [vi]
These closely resemble the round rocks of hematite in southern Utah, whose diameters range from a quarter inch to 8 inches or more. They usually form in the presence of water. A subsequent chemical analysis of said “blueberries” by Opportunity’s spectrometer indicated the presence of hematite. [vii] This is strong evidence for water at the time of formation. These minerals had precipitated.
Meanwhile, on the other side of the planet, Spirit exited the Gusev crater, its landing site. It came across some low hills, which exhibited fine sedimentary layering. But how could you be sure that this was truly sedimentary and not a layering due to some other geological process? One indicator was that some of these deposits included rocks and particles of varying sizes were cemented together. The conclusion was that these were deposited from a fluid. This is as opposed to ejecta from a meteoric impacts or ash falls from a volcano.[viii]
Figure 4: Spirit's sedimentary layering
Not wanting to be outdone, Opportunity found layering of its own. In the Eagle Crater Opportunity came across multiple examples of crossbed layering. It is a type of sedimentary deposition. Its geometry indicated that it was deposited in fast flowing water. If you look at the image in figure 5 you see blueberries, physically embedded in the layers. So very cool!
Figure 5: Crossbed layering near Eagle Crater with “blueberries” in situ.
The crossbedding coupled with the hematitic blueberries, is nearly irrefutable evidence for the past presence of not just water, but substantial amounts of water.[ix] And that is exciting because it also implies an environment that may have been hospitable to early life![x]
ACCESSIBLE LIQUID WATER TODAY
Figure 6: Alluvial flow of water
Examine figure 6 (above), these are two before and after images taken from orbit of what certainly appears to be an alluvial flow of liquid water.[xi] These images were captured by the Mars Surveyor spacecraft. This may have been flowing water. If so, this would be the first direct evidence of liquid water on the planet's bitterly cold surface. At this particular site temperatures do rise higher than 0 C. Still, at surface pressures as low as they are, liquid water cannot exist for long. So these two images cannot be considered conclusive evidence that liquid water is flowing on Mars. But it is indicative that something that certainly appears to be a liquid is flowing although water is not the only possibility. I will speculate that there could be a subsurface geologic process that is producing enough heat to maintain water in a liquid state. When it flows out through the crater wall it rapidly evaporates? Certainly there is reason to continue the search.
Figure 7: Frozen water uncovered by Phoenix under loose "topsoil"
Later, the Mars Phoenix lander too confirmed the presence of water, although this water was in a frozen state. See figure 7 above.[xii]
Regardless, this is an extremely exciting development. Recall that one of NASA's principal objectives for launching the rover missions was to determine the viability of human exploration on the red planet. What these discoveries tell us is that there is some surface water on Mars. And it's not just subsurface water. It is accessible water. Where there is water there is the capacity to sustain a manned mission, if not human colonization of some point in the future. Water isn't just necessary to drink or to grow food, but it can be electrolyzed into oxygen for breathing and hydrogen for fuel.
THE MARTIAN CLIMATE
Spirit and Opportunity have characterized the Martian climate as being cold, very dusty and thin. Opportunity recorded frost collecting on the rover's instruments. It also photographed clouds and the Martian sky. See Figure 8 below.
Figure 8: Clouds in the Martian sky.
These clouds are composed of particles of water ice.[xiii] On the other side of the planet Spirit found that dust devils are a daily occurrence in the Martian spring. These dust devils are also thought to be responsible for producing a hydrogen peroxide snow through the static discharges they produce. This could have disturbing consequences for existing life since it is a powerful oxidant.[xiv]
Atmospheric pressure is roughly the equivalent of Earth's at an altitude of 70,000+ feet or about 21 km. That's twice the cruising altitude of commercial airliners. Surface temperatures range from -87 to -5 C.
95% of the thin Martian atmosphere consists of carbon dioxide. The rest is nitrogen and argon. Oxygen could be considered a trace gas at just .13%.
PRE-ROVER VIEW AND NOW
NASA set out four scientific goals for the rover missions. Three of those goals have been met and the missions have been unqualified successes. Only the first is still unanswered; is there life on Mars? We just don't know. What we do know is that at one time, Mars was aReferences and endnotes:
[i] http://www.universetoday.com/84204/hopes-dim-for-contacting-spirit-rover/#more-84204
[ii] http://www.nasa.gov/mission_pages/mars-pathfinder/
[iii]http://www.msnbc.msn.com/id/4054530/ns/technology_and_science-space/
[iv] Rayleigh scattering of CO2
[v] http://www.lpi.usra.edu/meetings/metsoc2005/pdf/5051.pdf
[vi] http://www.unews.utah.edu/releases/04/jun/marsmarbles.html
[vii] http://www.jpl.nasa.gov/releases/2004/88.cfm
[viii] http://www.geotimes.org/dec04/WebExtra121504.html
[ix] Squyres, S (and 18 others), 2004, In situ evidence for an ancient aqueous environment at Meridiani planum, Mars, Science, v. 306, p. 1709-1714 (December 2004)
[x] http://records.viu.ca/~earles/mars-sediments-dec04.htm
[xi] http://physicsworld.com/cws/article/news/26573
[xii] http://www.jpl.nasa.gov/news/news.cfm?release=2008-113a
[xiii] http://marsrover.nasa.gov/science/goal2-results.html
[xiv] http://news.nationalgeographic.com/news/2006/08/060807-mars-snow_2.html
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