On sol 159, the indefatigable rover Spirit reached its first stop at the Columbia Hills. It had been dawdling, pursuing its scientific endeavors, at Lahonten Crater since sol 118, driving around the rim of the two-hundred-foot depression. Then the long drive to the hills began.
At the base of the hills, it spent a full twenty-three sols studying a feature known as Hank's Hollow, and in particular an odd-looking rock named Pot of Gold (there was no lack of imagination within the MER team). Lo and behold, there be hematite in these hills—Spirit was catching up with the discoveries of its precocious twin, Opportunity. The rock was described as looking “as if somebody took a potato and stuck toothpicks in it, then put jelly beans on the ends of the toothpicks”; a colorful way to describe the oddly shaped, softball-sized rock. The process of formation was thought to be water based, especially because hematite is usually found in the presence of water. But there are chemical processes that could result in such a feature, so it was studied exhaustively.
After investigating with its “sniffer,” the rover repositioned itself (which took several sols, due to the slippery and treacherous nature of the surrounding soil) and ground away at another side of the rock with the RAT. Whatever formed this rock took a long time and was not a mild process. It was harshly eroded and knobby, and the nodules on the rock appeared to be cousins to the “blueberries” found by Opportunity, half a planet away.
Spirit left Pot of Gold to investigate more strange-looking rocks nearby. Dubbed Cobra Heads, these rocks, as did Pot of Gold, appeared to have come down from higher up in the Columbia Hills. The Cobra Heads were probably the cores of more normal-looking rocks that were left behind when the softer surrounding layers wore away to leave the harder interior behind, silently hissing at Spirit.
At this point, while still very capable, Spirit's spirit was being dampened by some problems. The mechanical arm was suffering from a bit of dysfunction in one of the motorized joints. The radio was having issues, making the reception of commands sent from Earth difficult. And the front right wheel was draining far more electrical current than the other five, and this drain was increasing. It was an indication that the gearing that drove that wheel was failing, probably due to sand, dust, and grit having crept inside. To make matters worse, the season was advancing and Spirit was receiving less sunlight on its solar panels every day.
Still, the rover continued its labors, taking a northerly course along the base of the hills, investigating as many rocks as it could along the way. Then, on sol 239, the rover powered down for solar conjunction, when Mars is on the far side of the sun from the Earth and out of communication with JPL.
When we last left Opportunity, it was embarking upon the long and lonely journey to Endurance Crater. On April 30, 2004, the mammoth feature loomed into view. About 430 feet across, Endurance Crater is named after the ship captained by Sir Ernest Shackleton on his epic voyage to Antarctica. It was a fitting name for a landmark so prominent in Opportunity's life.
The rover paused at the lip of the crater, taking its time to survey the sixty-foot-deep interior. As with most impact craters, Endurance ripped through the surface of the planet, exposing the strata below. Finding a suitable point of entry, Opportunity, shaking its metal arm in the Great Galactic Ghoul's odious face, took the plunge and began the risky descent into the crater. The point of entry was named Karatepe (“Black Hill” in Turkish), and the rover took a tentative drive inside the crater rim, then stopped and backed out, just to make sure that it could. It was a bit like watching Neil Armstrong on that July night in 1969, coming down the ladder of the Lunar Module, then before setting foot on the moon, hoisting himself back up the rungs just to be sure that he could.
Nobody wanted Endurance Crater to be the final resting place of Opportunity, though everyone involved knew that it might be. As it turned out, the rover spent the next six months driving around Endurance Crater. The first stop was a patch of exposed bedrock strata. Opportunity got up close and personal with the outcrop and sampled each layer of the strata as carefully as possible. It noted that both the texture and the chemistry of the rocks varied with depth, with the lower layers being the oldest, just as on Earth. And both magnesium and sulfur declined in abundance as the rover's sniffer went from the upper (younger) to the lower (older) rocks, once again confirming the presence of (and alteration by) water in the past.
Moving on, more “blueberries” were seen in the rocks and scattered about the crater itself. Soon the same solar conjunction that idled Spirit forced Opportunity to cease operations for two weeks as the two rovers waited out the silence from Earth.
As Mars moved back within range of the ground controllers, Opportunity moved to a rock unlike anything seen in the Meridiani Planum area. Dubbed Wopmay (after a semifamous Canadian bush pilot), its lumpy, serrated surface promised more signs of alteration by water. More “blueberries” were found here as well, and the sightings of these was becoming almost boring…almost. Again, signs of a watery past.
About this time you might well be wondering just how much evidence of water in Mars's past a planetary scientist might need. It's a good question. But with such an arid planet meeting virtually every spacecraft sent there, and the promise of life, past or present, dangling on this finding, there could be no such thing as too much evidence. Not yet anyway.
Tellingly, the investigation of this rock corresponded with the publication in a scientific journal of the results of the MER missions' efforts to find evidence of a watery past on Mars. The paper included the efforts of 122 authors and was confidently published in Science, perhaps the most important journal appropriate for this kind of news. This made it official: there has been large amounts of water on the Red Planet, and the evidence would just keep piling up. Besides the “blueberries” and various chemical residues, there were cavities, the oddly named vugs, in many of the rocks investigated that were indicative of crystals that had been dissolved over time by water as the rocks lay immersed. This finding, wondrous in the extreme, told of not just large amounts of H2O, but also that it had existed as a liquid on the surface of the planet for a long time in the past. And this indicated a thicker, warmer atmosphere way back when.
If there was a downside, it was that this water would have been salty and acidic and not overly friendly to life. But life finds a way, as it has in so many hostile environments on our own planet, so this was far from a deal breaker.
Returning to Spirit and its numbing slog across the Gusev region, the rover had found something interesting (well, it was all interesting, but this was a major find). The mineral in question was goethite, kin to the jarosite found earlier by Opportunity, and it was another sure indicator of water sometime in the past. The beauty of this mineral is that, unlike hematite, which usually forms in a watery environment, goethite forms only within such an environment. Anyone sweating out the declarations in the Science article could take a deep, relieved breath.
Making spirits brighter, to coin a phrase, the worrisome friction buildup in the front right wheel was diminishing. Nobody could be sure why without a mechanic's house call to Mars, but the fact that controllers had been “babying” that wheel may have helped. All they knew for sure was that it was drawing less current, and that was a good sign of mechanical health.
Spirit now ascended a formation known as Husband Hill (named after the NASA astronaut who died in the shuttle Columbia's demise) in an effort to ascertain how high the water might have once stood. The hope was that it was not merely an underground store but might have pooled on the surface for some length of time, as it appeared to have done at the Opportunity site. Confirmation was the name of the game.
Then Opportunity, never content to allow Spirit to bask in the limelight for long, found something wonderful.
Ascending out of Endurance Crater, the rover had spied its own heat shield from its fiery descent onto Mars. Scientists wanted to examine the structure to see how it had been affected by its hot journey through the Martian atmosphere, and the results were interesting, if not remarkable. Nearby was an interesting rock, dubbed, appropriately enough, Heat Shield Rock. As the rover neared this stone, it began to look eerily familiar to some on the ground, and very unlike the others they had been investigating. Soon, it was confirmed. Opportunity had found the very first meteor found on another planet. Where it came from (besides space) and when it arrived would remain a mystery, but finding it was excitement enough. And it was an iron-rich meteorite, not the more common “stoney”-type. Of course, meteorites are found on Earth, so the rock itself was not the real news. What it would reveal about the area on which it sat was the interesting part. It could help to answer the question about whether or not Meridiani Planum was gradually eroding away as so much of Mars is, or if it was still being built up in geological terms.
Soon, Opportunity moved on. The region it was crossing was sufficiently flat that it allowed controllers to test more autonomous driving than before, depending on the rover's onboard computer to avoid hazards and obstacles. So the first part of each drive was based on a course set by ground controllers, then switched over to the onboard navigation system. This was a good test of JPL's software, and the rover passed with flying colors. Up to a quarter mile was covered in this mode, a new record.
Months later, in June 2005, Opportunity had a near-death experience. It had gotten bogged down in a small sand dune, and it struggled for almost five weeks before freeing itself. Had the arm been able, it might have wiped a bit of sweat from its metallic brow, but Opportunity instead took a time-out to access its health before carrying on. At the same time, JPL wanted to know why Opportunity had gotten stuck in an area that looked so much like regions the rover had crossed with ease before.
Far away, in December 2005, Spirit spent some time staying up at night to observe a meteor shower. Mars, like Earth, passes through swarms of cometary debris every year. The comet in question was the famed Halley's Comet, and the resulting show did not disappoint. It was what researchers call “bonus science,” an unplanned addition to the rovers' busy schedules.
By mid-2006, Spirit had to find a place to spend the harsh Martian winter once again. The fact that both machines had performed far beyond expectations, and had vastly exceeded their warranties, was remarkable enough. But nobody wanted to lose one to the winter cold for lack of planning.
After descending from Husband Hill and investigating a formation called Home Plate, Spirit parked itself on a north-facing slope to allow for maximum exposure of its solar panels through the winter months. Spirit would need all the power it could get to keep warm through the long cold period. An added drama was that the front right wheel, the previous source of concern, had finally given up and was no longer functioning. From now on, Spirit would be dragging one foot as it ambled across Mars.
Opportunity had just wrapped up four months at a 1,000-foot wide crater called Erebus, after the volcano in the Antarctic.1 It had found thin, rippled rock indicative of flowing water—another important discovery. Sitting water was turning out to not be unique in Mars's ancient past, but evidence for wind-blown water was still being sought to confirm interactions between weather and standing water.
Soon Opportunity continued on its trek, heading off on a long drive to the largest and most promising crater yet, the half-mile-wide Victoria Crater, named after one of Ferdinand Magellan's ships. Over two hundred feet deep, Victoria had exposed bedrock layers over 130 feet high, and promised a treasure trove of new information. The larger and deeper the crater, the more of Mars's geological past would be revealed.
Opportunity was near the equator, while the ever-unlucky Spirit was in the southern hemisphere, so while Spirit shivered through the winter, Opportunity was still receiving power from the sun. She was ready to tackle her biggest challenge yet.
In October, Opportunity began driving the rim of the vast crater. About this time, the Mars Reconnaissance Orbiter began work overhead, and was able not only to spot tiny Opportunity going about its chores, but also to begin to assist in planning its future by looking inside the crater itself. The ultra-hi-res images MRO sent home gave MER planners much-needed data and allowed them to plan with far greater confidence.
From its vantage point, perched on the rim of the crater, Opportunity had provided researchers with sufficiently detailed images to show that the layers of rock in the crater wall were diverse and promising. There were specific divisions, indicative of environmental change. This is just the sort of thing they had come for and, if the rover stayed cooperative, would investigate.
Besides investigating the crater from a safe vantage point on the rim, Opportunity was seeking a safe entrance for a possible drive down to the interior. This was no small feat. Victoria was not an exact circle like many craters; its sides were heavily scalloped. While this offered a variation in incline for the drive inside, it also made navigating the rim trickier. The machine needed the best possible combination of slope and material composition to risk entering the crater. Too much sand and it could slip. Too many rocks and it could be blocked. Nobody wanted a replay of the “rover-stuck-in-a-dune” drama of a few months back. Caution was the watchword.
After a few months of carefully picking its way around the rim of the crater, Opportunity joined its sister rover, Spirit, in a bit of downtime for a software update. This improved version would allow the rovers to better “think” and take more independent action. One example was the ability to perceive changes in visual and cloud patterns, so that rather than slavishly sending back hours of images of the horizon through which researchers would have to sift to find a change indicating, say, a dust devil, the rover would be able to “sense” the differences and send back to Earth the relevant images, dismissing the rest.
Another new capability was called visual target tracking. Previously, when the rover was driving in autonomous mode, it could not track the location of a boulder or other impediment as it moved. Once the visual aspect of the item in question changed, it became a new item in the rover's mind. This software update allowed the onboard computer to track the offending item as the rover moved—vital for ongoing mission success.
Finally, a new routine had been programmed to allow the rovers more autonomy in the use of their robotic arms. Previously, when the rover neared a target for investigation, ground controllers had to assess safe approaches and give specific instructions to the arm. This new software allowed the onboard computer to determine the risks itself and preselect targets, and approaches, for a closer look.
While these may seem mundane changes to some, the ability to cut down on the twenty-minute delay in communications (each way—so double that and add time to go through an orbiting relay such as MRO) allowed ground controllers and the rovers to accomplish much, much more in a shorter time. And with the clock ticking on each of the rover's mechanical and electronic systems, time was critical.
It was now June 2007. Opportunity had spent a few months driving around a prominence of Victoria called Duck Bay. The rover had investigated what it could around the rim, while at the same time seeking a safe route for descent. By the end of June, it returned to the most promising site. The slope here varied from fifteen to twenty degrees and was mostly exposed bedrock, which, it was hoped, would minimize the dangers associated with slipping on sand or dirt.
At this point, Victoria had been monopolizing Opportunity's attentions for almost two and a half years. Things move slowly in Mars rover-time. It was time to make the leap of faith and head toward the crater floor. If tension could be eaten, flight controllers would have their lunch set out for months to come.
Then, bad weather intervened. Massive dust storms kicked up, raging for over two months. To the rovers, the threat was not just of a loss of solar power to their batteries, but also of reduced visibility and windblown grit. But they hunkered down and sat out the storms. It was a long wait. Some days the solar panels could only generate power equivalent to that needed to light a 100-watt bulb for an hour. They could not operate normally in this condition and went into reduced power mode, restricting activities to a minimum. Even so, there was real concern. At one point, only 1 percent of the normal light reaching the rovers in daytime was available, and the batteries were dying. The skies overhead were almost opaque. And if power was too low, the rovers would “safe” themselves by going into a condition called “Low Power Fault.” This would put the machine to sleep, from which it would rouse itself briefly each day to check out prevailing conditions and available power. The problem was that nobody was entirely confident that the machine would wake up at all if it slept too long. It was a real nail-biter of a time.
But fate smiled upon the mission, and by September the skies had cleared and it was time to commit. The depleted batteries had recharged and were fit for duty. On the eleventh, Opportunity made its first tentative drive into Victoria. Again, it crossed the threshold, moving about thirteen feet downslope. Then it reversed direction to ascertain how much it might slip on the way out and to make sure that it could retreat to safety if it had to. Two days later, the die was cast, and Opportunity headed toward the crater's interior, its home for the upcoming year.
Within Victoria was a bright band of sedimentary material that girdled the crater, which was inevitably dubbed the “bathtub ring.” This turned out to be a water table from a time not long before the impact that created Victoria some millions of years ago. Then, as the rover descended, it began to spot more “blueberries,” the hematite spherules seen across Meridiani Planum. But these were far larger than those investigated earlier, again suggesting that the farther back one looked, the wetter Mars got. It also argued for much of this watery interaction to have occurred underground, rather than on the surface. The mysteries just kept unraveling.
Interestingly, on the way to Victoria, the number and size of the “blueberries” sighted had been decreasing steadily. At the same time, the rover had been gaining elevation, with Victoria sitting about one hundred feet higher than Endurance. But once the rover dove into Victoria Crater and lost elevation, the “blueberries” got larger. Again, water processes were suspected.
As Opportunity drove around the interior of Victoria Crater, it explored many of the bedrock outcrops visually. Most were named after capes and bays from the age of exploration on Earth. As the rover was able to gain more sampling results, what had been observed previously at Endurance and Eagle Craters was confirmed and recognized as regional processes, that is, they took place across Meridiani Planum and not just where they were first seen.
While exploring Victoria, Opportunity began experiencing trouble with its robotic arm again. This was a replay of the problems experienced in 2005. The balky motor was taking more current to work than it should, so engineers had to increase the amount of electricity being used, but not to the point where it could cause a larger problem. It was decided now to change the way they treated the arm while the rover was driving. Previously, the mission rule was to stow the arm under a restraining hook while a rover was moving. But the motor giving trouble was needed to unstow. So rather than end up with a trapped arm, the programmers instructed Opportunity to unstow the arm after each day's work rather than spending the night with the arm under the hook. A small change, but a potentially limb-saving one.
Opportunity's adventures within Victoria Crater continued. The final results would take time to compile, but it was clear that water was in the mix in a profound way. It had come and gone over billions of years. And always there was the wind; in fact, it appeared that most of the scalloping was done by the wind. Opportunity had been able to look at vertical strata of rock over thirty feet high; some was clearly the result of sand dunes compacting into strata layers. Also found outside the crater bowl were bits of meteorite that may have been parts of the body that created the impact crater so long ago.
Other processes and results were noted—most important, the continuing evidence of not just water in the past, but indications of how the water existed as it was affecting the rocks. In this area, the water seemed to have been operating underground more than above, which could allow it to continue to change the rocks long after Mars lost the ability to support water in liquid form on its surface.
While these results were being digested by the investigative teams on Earth, Opportunity made the sometimes-treacherous climb out of Victoria. A year after entering the crater, the rover used the very same tread marks it had left when it entered to chart a safe path out of the depression. At the end of August 2009, the rover completed the one-month drive out of the crater, crossed the sandy barrier at the rim, and was once again on flat ground.
After a brief period of checkouts, it began what would be a multiyear, ten-mile drive to its next objective: the massive Endeavor Crater named after Captain Cook's famous vessel. The journey was expected to take over two years. The adventure in Victoria had consumed almost half of Opportunity's time on Mars. Before departing the area, it investigated more rocks around the edge, targets it had noted but passed by in the drive to get inside Victoria. A further bounty of science resulted, largely confirming what the scientists already knew: Mars was once a wet, wet world.
Endeavor, its next target, was over twenty times larger than Victoria, almost fourteen miles across, and promised an even greater scientific bonanza. As this book goes to press, Opportunity will begin its descent into this larger impact crater to begin a new round of investigations and discoveries for as long as it can manage to do so.
Across the dusty planet, Spirit continued operating, but barely. By late 2008 it had begun showing increased difficulty in operation, and in January of the following year, the onboard computer began to behave erratically. At times it would download driving instructions and not execute them, and at other times it would not fully report the activities of the day. Spirit's health continued to decline; in addition to the front right wheel (which now merely dragged along), its flash memory was becoming faulty. Even so, as recently as January 6, it had stopped to research a rock called Stapledon, which contained high amounts of silica, possibly indicating a former watery hot spring or steam vent.
By April, Spirit's onboard computer was again being testy and began to reboot spontaneously. The flash-memory problems were increasing, resulting in increased amnesia. Then, in June 2009, it became lodged in soft sand; attempts by controllers to unstick the machine were meeting with little success. It had been driving across a crusty surface but broke through into the softer sand below. With winter coming, if the machine could not be moved to another spot that would afford the increasingly dusty solar panels a better angle toward the weakening sun, its future did not look bright.
To make matters more frustrating, it had become clear that the region Spirit had been investigating was not so dull after all. While Opportunity had discovered a formerly very wet and acidic environment in Meridiani Planum, Spirit's home had been found to be a steamy, violent place. More investigation was clearly warranted, but only if the robot could be moved from its current location.
In late March 2010, Spirit stopped talking. The suspicion was that it had entered a hibernation mode, a result of increasingly low energy reserves in its batteries. Already the daily communications with the rover had been cut to once a week in an effort to conserve power. But even that had proved to be too much. As the Mars Odyssey probe wheeled overhead, it listened intently for a signal, but none came.
In late July, controllers began sending commands to Spirit in an attempt to awaken the rover. It was now deemed safe to do so, as the local environment was emerging from the Martian winter and the power drain, if communication occurred at all, would likely not be fatal. But the rover remained silent. It was hypothesized that it may have lost so much internal data that the clock in the onboard computer may not have known what day or time it was. If this had occurred, the computer should reset the clock and begin a sequence of listening for commands for twenty minutes out of every hour of daylight.
By January 2011, Spirit had been off the air for nine months. Still, controllers continued to try to awaken the rover. It was the local Martian spring, and this was the last best hope for communication. Soon solar opposition occurred, and then, once Mars emerged from the temporary blackout, efforts began again. But it was not to be.
On Wednesday, May 25, JPL sent up its last plea for Spirit to phone home. This was greeted with silence. Earth-based radio dishes had double-teamed with the orbiters around Mars in attempts to reestablish communication, but all had been for naught. Spirit, after being stuck in soft sand and enduring the coldest temperatures ever encountered on Mars, had died. After a career of almost seven years on Mars, the rover had succumbed to the harsh elements. Still, having vastly outperformed its initial mission of ninety days, the machine had outdone itself and performed brilliantly.
Opportunity, meanwhile, has completed the drive to its next, and most exciting, target. The rover had covered about fourteen miles since it landed, almost three times as far as Spirit managed. In a fitting epitaph, JPL named Opportunity's point of arrival at Endeavor Crater “Spirit Point.” Who says engineers and scientists are not sentimental?