From the orbits of the inner solar system to the surface of Mars. For Curiosity and Opportunity, 2016 continued the rovers’ revelations of the mysteries of the Martian surface – providing scientists with confirmation of long-held theories while also revealing surprises about Mars’ geologic past and providing new research for the eventual push of human exploration on the red planet. For Opportunity, the year also marked a continued series of longevity firsts and another mission extension for the rover as it now approaches the start of its 13th year on Mars.
Curiosity – four years and counting:
The first few months of 2016 for Curiosity were relatively quiet as the rover and its Earth-based controllers slowly guided the massive, mobile science laboratory through the roughest terrain the rover has thus far encountered during its time on Mars.
In early March, Curiosity successfully completed a climb onto the Naukluft Plateau – a region on the lower elevations of Mount Sharp – following a successful navigation through and investigation of an area of tricky sand dunes.
Scientists specifically commanded Curiosity to drive itself onto the Naukluft Plateau because its sandstone bedrock has been carved by millions of years of wind erosion into ridges and knobs, thus providing an excellent investigative path for Curiosity as the rover made its way toward smoother surfaces beyond the 400 meter (one-quarter of a mile) wide plateau.
When Curiosity arrived on the plateau, the roughness of the ridges caused concern that driving on it could damage Curiosity’s wheels, as terrain Curiosity crossed before reaching the base of Mount Sharp did earlier in the rover’s mission.
Holes and tears in Curiosity’s wheels were first noticed in 2013, causing the rover’s team to adjust the long-term driving route and gain a better understanding of how they assess local terrain and refine how they plan the rover’s drives.
“We carefully inspect and trend the condition of the wheels,” said Steve Lee, Curiosity’s deputy project manager at NASA’s Jet Propulsion Laboratory (JPL). “Cracks and punctures have been gradually accumulating at the pace we anticipated, based on testing we performed at JPL.
“Given our longevity projections, I am confident these wheels will get us to the destinations on Mount Sharp that have been in our plans since before landing.”
In early April, Curiosity performed a self-examination of its wheels after crossing most of the Naukluft Plateau – which revealed no acceleration of damage.
By late April, Curiosity’s odometer reached 12.7 km (7.9 mi).
On 11 May, Curiosity marked the completion of its second full Martian year – signaling the end of data collection of two full cycles of Martian seasons.
Importantly, while each sol (Martian day) is longer than an Earth day, the planet’s axial tilt – at 25.19° – is roughly the same, in astronomical terms, to that of Earth’s, at 23.43°.
The similar axial tilts give both planets a yearly rhythm of four seasons, and because Curiosity is a science platform, it is importantly a permanent, round-the-clock weather station – taking measurements of temperature, pressure, ultraviolet light reaching the surface, and the sparse water vapor in the air at Gale Crater.
“Curiosity’s weather station has made measurements nearly every hour of every day, more than 34 million so far,” said Curiosity Project Scientist Ashwin Vasavada of JPL. “The duration is important because it’s the second time through the seasons that lets us see repeated patterns.”
To date, Curiosity has revealed strong, repeated seasonal changes at Gale crater and has also helped distinguish, with help from the fleet of orbiters around Mars, seasonal climatic shifts v. sporadic weather events.
Specifically, Curiosity registered a large spike in methane in the local atmosphere during the first southern-hemisphere autumn that was not repeated in the second autumn.
For most of the two Martian years, Curiosity measured methane concentrations between 0.3 and 0.8 parts per billion; however, for several weeks during the first autumn, that level spiked to as high as 7 parts per billion.
Curiosity monitored methane concentrations for a repeat of that spike during the second autumn, but all observations returned nominal, lower background levels and no major spike.
“Doing a second year told us right away that the spike was not a seasonal effect,” said JPL’s Chris Webster. “It’s apparently an episodic event that we may or may not ever see again.”
Nonetheless, Curiosity did detect a possible seasonal pattern in the background methane concentration which appears to be lower in autumn than in other seasons.
If this pattern is confirmed, it may be related to the pressure pattern measured by the Rover Environmental Monitoring Station (REMS) or to seasonal changes in ultraviolet radiation, which is measured by the REMS in concert with Mastcam.
“This shows not only the importance of long-term monitoring, but also the importance of combining more than one type of measurement from a single platform,” Webster said.
Moreover, this monitoring of the modern atmosphere, weather, and climate via REMS fulfills one of Curiosity’s primary mission goals: to characterize the current Martian environment.
In its two years of observations, REMS has measured air temperatures from 15.9° C (60.5° F) on a summer afternoon to -100° C (-148° F) on a winter night.
Curiosity’s air-pressure measurements also confirmed a strong seasonal trend previously seen by other missions.
“There are large changes [in air pressure] due to the capture and release of carbon dioxide by the seasonal polar caps,” explained Germán Martínez, a Curiosity science-team collaborator at the University of Michigan, Ann Arbor.
“Most of the Martian atmosphere is carbon dioxide. During each pole’s winter, millions of tons of this gas freeze solid, only to be released again in spring, prompting very un-Earthlike seasonal variations of about 25% in atmospheric pressure.”
Other seasonal patterns measured by Curiosity include a local atmosphere that is clear in winter, dustier in spring and summer, and windy in autumn and a visibility factor in Gale Crater that can be as low as 30 km (20 mi) in summer and as high as 130 km (80 mi) in winter.
Curiosity then departed the Naukluft Plateau in late May and resumed its climb up Mount Sharp’s mudstone bedrock layer after performing its 10th and 11th laser drills into rock formations on the plateau.
The 10th and 11th drill sites – “Lubango” and “Okoruso” – were part of an experiment to compare material within and away from pale zones around fractures on the Murray formation on the lower layers of Mount Sharp.
Once back on the mudstone bedrock, Curiosity performed its 12th drill into Mars – again into the Murray formation, which is about 200 m (one-eighth of a mile) thick and of which Curiosity has thus far examined about one-fifth of its vertical extent.
“The story that the Murray formation is revealing about the habitability of ancient Mars is one of the mission’s surprises,” said Vasavada. “It wasn’t obvious from pre-mission data that it formed in long-lived lakes and that its diverse composition would tell us about the chemistry of those lakes and later groundwater.”
The 12th drill was performed on 4 June at the “Oudam” target, and after this, Curiosity turned south and began to “climb the mountain head-on,” said Vasavada.
Just two weeks after Curiosity began this all-important climb to reach its three “most valuable” science targets, NASA officially approved a two-year extension to the rover’s mission.
The mission extension now takes Curiosity’s planned life out to 30 September 2018.
Then, as July dawned, an unexpected issue arose which caused the rover to enter safe mode for the first time since three safe mode events in 2013.
On 2 July, the rover put itself into safe mode and ceased all operations less “keep alive” activities and a pre-programed, highly prescribed sequence of resuming twice-daily communications with Earth.
By 6 July, Curiosity was back in communication with Earth and was “stable” – with controllers seeing indications that the safe mode, a standard procedural guideline written into all uncrewed spacecraft programming to protect them from unanticipated events, was triggered by a mismatch between camera software and data-processing software in the main computer.
By 9 July, controllers brought Curiosity out of safe mode, corrected the issue, and the rover resumed normal science operations on 11 July.
By the end of July, Curiosity began selecting its own rock targets for its laser spectrometer – marking the first time for any planetary rover that autonomous target selection was an available aspect of its mission.
“This autonomy is particularly useful at times when getting the science team in the loop is difficult or impossible – in the middle of a long drive or when the schedules of Earth, Mars, and spacecraft activities lead to delays in sharing information between planets,” said robotics engineer Tara Estlin.
Using the Autonomous Exploration for Gathering Increased Science (AEGIS) software developed at JPL, Curiosity’s new ability to frequently choose multiple targets per week for a portion of its Chemistry and Camera (ChemCam) instrument greatly aides scientists back on Earth, who still, however, select most of the rover’s targets based on images is sends back to Earth.
The AEGIS software, previously developed for Curiosity’s cousin – the Mars Exploration Rover Opportunity, analyzes images from Curiosity’s stereo Navigation Camera (Navcam) and selects a target using adjustable criteria specified by scientists, such as identifying rocks based on their size or brightness.
The criteria can be changed depending on the rover’s surroundings and the scientific goals of the measurements.
The AEGIS software, once a target is identified, then uses ChemCam’s Remote Micro-Imager to perform image analysis and fine-tune pointing of the laser at these targets.
“Due to their small size and other pointing challenges, hitting these targets accurately with the laser has often required the rover to stay in place while ground operators fine tune pointing parameters,” Estlin said.
“AEGIS enables these targets to be hit on the first try by automatically identifying them and calculating a pointing that will center a ChemCam measurement on the target.”
After demonstrating this new ability, Curiosity celebrated its fourth landing anniversary on 6 August.
By 8 September, the rover was exploring a specific geologic region of which Curiosity returned stunningly detailed color images in the Murray Buttes region on the lower levels of Mount Sharp.
The buttes and mesas rising above the surface at Mount Sharp have been eroded away to their present-day remnants of ancient sandstone that originated when winds deposited sand after lower Mount Sharp formed.
“Studying these buttes up close has given us a better understanding of ancient sand dunes that formed and were buried, chemically changed by groundwater, exhumed and eroded to form the landscape we see today,” said Vasavada.
Curiosity then continued its southward journey, stopping on 9 September for one last drill at the Murray Buttes before continuing its steady climb up Mount Sharp.
The rest of September, October, and November were relatively quiet for Curiosity as it drove itself up the surrounding escarpments.
Then, on 22 November, the rover turned its communications systems not toward Mars Odyssey or the Mars Reconnaissance Orbiter (MRO), but instead toward the recently arrived Trace Gas Orbiter (TGO) from ESA and Roscosmos.
The TGO had successfully inserted itself into orbit of Mars on 19 October and spent the first month in orbit undergoing various checkouts.
On 22 November, a primary aspect of the TGO’s mission – to enhance the telecommunications network at the Red Planet – underwent its first major test: relaying communications and data from Curiosity and Opportunity back to Earth.
For Curiosity, the rover’s signals were received on the TGO via one of the orbiter’s twin Electra radios.
The TGO’s main radio antenna then transmitted the signal to Earth.
The event marked a strengthening of international telecommunications at Mars and served as a reminder of the importance of such international cooperation for exploration at Mars.
“The arrival of ESA’s Trace Gas Orbiter at Mars, with its NASA-provided Electra relay payload on board, represents a significant step forward in our Mars relay capabilities,” said Chad Edwards, manager of the Mars Relay Network Office within the Mars Exploration Program at JPL.
“In concert with our three existing NASA orbiters and ESA’s Mars Express orbiter, we now have a truly international Mars relay network that will greatly increase the amount of data that future Mars landers and rovers can return from the surface of the Red Planet.”
More concretely, the test paved the way for permanent use of the TGO as an enhanced relay for Curiosity – with the rover scheduled to start using the TGO for regular communications with Earth in 2017.
Specifically for the Electra radios, the devices can maximize data volume by actively adjusting the data rate to be slower when the orbiter is near the horizon from the rover’s perspective and faster when the orbiter is overhead.
Due to improvements in the newest Electra radios and reduced interference levels, TGO’s relay radios are anticipated to offer performance about double that of MRO’s.
Then, on 30 November, Curiosity’s teams commanded the rover to drill into the seventh sample-collection site of the four-year mission.
On 1 December, Curiosity relayed information back to Earth that it had not performed the drill after detecting a fault: the drill feed mechanism did not extend the drill to touch the rock target with the bit.
The drill feed mechanism pushes the front of the drill outward from the turret of tools at the end of Curiosity’s robotic arm. The drill collects powdered rock that is then analyzed by laboratory instruments inside the rover.
“We are in the process of defining a set of diagnostic tests to carefully assess the drill feed mechanism. We are using our test rover here on Earth to try out these tests before we run them on Mars,” said Curiosity Deputy Project Manager Steven Lee on 5 December.
“To be cautious, we want to restrict any dynamic changes that could affect the diagnosis. That means not moving the arm and not driving, which could shake it.”
By 5 December, as the rover held its position pending resolution of the drill issue, Curiosity’s odometer stood at 15.01 km (9.33 mi) – having driven more than 840 m (more than half a mile) since late September and having climbed more then 165 m (541 ft) in elevation since landing, including 44 m (144 ft) since late September.
Curiosity’s teams eventually determined the cause of the failure to be a brake on the drill feed mechanism that did not disengage fully.
A fix was implemented and transmitted to the rover with initial success; however, the problem cropped up again soon thereafter, leading mission engineers to realize that the issue is recurring.
As of writing, Curiosity’s drill is still sidelined and the rover remains stationary.
Nevertheless, Curiosity is continuing to return valuable scientific data about the change in geologic conditions as it climbs Mount Sharp.
Specifically, Curiosity has returned evidence of how ancient lakes and wet underground environments changed billions of years ago to create more diverse chemical environments that affected the favorability for microbial life.
Specifically, hematite, clay minerals, and boron have all been either out-right discovered or found in higher quantities in newer layers of rock at higher elevations than the lower, older layers examined earlier in Curiosity’s mission.
“We are well into the layers that were the main reason Gale Crater was chosen as the landing site,” said Curiosity Deputy Project Scientist Joy Crisp of JPL.
“We are now using a strategy of drilling samples at regular intervals as the rover climbs Mount Sharp. Earlier, we chose drilling targets based on each site’s special characteristics. Now that we’re driving continuously through the thick basal layer of the mountain, a series of drill holes will build a complete picture.”
To this end, four recent drill sites, from “Oudam” in June to “Sebina” in October were each about 25 m (80 ft) apart in elevation – allowing scientists to sample progressively younger layers that reveal Mount Sharp’s ancient environmental history.
One clue to the changing ancient conditions is the mineral hematite, which has replaced the less-oxidized magnetite as the dominant iron oxide in rocks Curiosity has drilled recently, compared with the site where Curiosity first found lakebed sediments.
“Both samples are mudstone deposited at the bottom of a lake, but the hematite may suggest warmer conditions, or more interaction between the atmosphere and the sediments,” said Thomas Bristow of NASA’s Ames Research Center.
Furthermore, an element Curiosity out-right discovered on Mars in recent measurements is boron, which ChemCam detected within mineral veins primarily composed of calcium sulfate.
“No prior mission has detected boron on Mars,” said Patrick Gasda of the U.S. Department of Energy’s Los Alamos National Laboratory. “We’re seeing a sharp increase in boron in vein targets inspected in the past several months.”
On Earth, Boron is primarily associated with arid sites from which a great deal of water has evaporated.
However, environmental implications of the minor amount of boron found by Curiosity are less straightforward than the increase in hematite.
“Variations in these minerals and elements indicate a dynamic system,” Grotzinger said. “And the more complicated the chemistry is, the better it is for habitability.”
Opportunity – the “rover that could” nears 13 years on Mars:
The mission, which was only supposed to last 92.5 days (or 90 sols), has, as of writing, operated for a total of just under 4,590 sols (or 4,716 days) and traversed 43.7 km (27.15 miles) across the Martian surface.
For 2016, Opportunity began the year in one of its more precarious situations: Martian winter.
Because Opportunity, unlike Curiosity, is a solar-power rover, it relies on the sun to generate power via its dust-covered solar panels.
The dust reduces the amount of sunlight that reaches the rover’s solar panels, and this – coupled with Martian winter – can create a dangerous power generation issue for the longest-serving rover on the surface of any celestial body other than Earth.
But the winds of Mars once again favored Opportunity, sweeping the rover with wind storms that took a portion of the dust off the solar panels – essentially cleaning them – and allowing the rover to generate enough daily power to remain continuously active throughout the winter months.
Not only did the rover remain active through the lowest solar-energy days, it continued to use its diamond-toothed rock grinder and drive itself across the Martian surface.
Normally, Opportunity has to hunker down and remain stationary on a portion of terrain tilted toward the sun during winter. But not this year.
“Opportunity has stayed very active this winter, in part because the solar arrays have been much cleaner than in the past few winters,” said Mars Exploration Rover Project Manager John Callas.
By mid-January, the solar panels were generating more than 460 watt hours per day, up 40% from earlier in the Martian winter.
By contrast, during Opportunity’s first Martian winter at its present location, power generation dipped below 300 watt-hours for more than two months, and the mission refrained from driving or rock-grinding during that time.
With the passing of the winter solstice on 2 January, the amount of sunlight available to Opportunity increased throughout 2016.
On 25 January, Opportunity marked the 12th anniversary of its arrival on Mars as the rover explored rocks on the southern side of Marathon Valley on the western rim of Endeavour crater.
In late-January, Opportunity used its rock abrasion tool to remove surface crust from a rock target called “Private John Potts.” The composition and texture of the rock’s exposed interior was then examined with instruments on Opportunity’s robotic arm.
By the end of March, Opportunity had completed the steepest climb it had thus far attempted.
During the climb, Opportunity’s tilt measured 32° as the rover climbed toward at target on Knudsen Ridge.
Mission planners had anticipated that Opportunity’s wheels would slip quite a bit during the climb, but the slippage was so great that the number of wheel rotations, which should have carried the rover 20 m (66 ft), only carried it 9 centimeters (3.5 inches).
Opportunity’s teams tried two more times to get the rover to its target on Knudsen Ridge, but when the third attempt came up short, the team decided it was too dangerous to continue and instead backed Opportunity down the ridge.
On 31 March, Opportunity captured an image of a nearby Martian dust devil – an uncommon site for Opportunity (though one that was quite usual for its twin rover, Spirit).
In the middle of the year, NASA formally approved a two-year extension to Opportunity’s mission – taking the rover, barring mechanical failure – out to 30 September 2018.
This was followed by the formalization of plans to drive Opportunity into a gully – about 219.5 m in length -carved millions of years ago by a fluid that could have been water.
This new mission target meant Opportunity would now have the ability to visit the interior of Endeavour Crater, which it has worked beside for the last five years.
The plans also came at a time when Opportunity exceeded its planned mission by more than a factor of 50.
“Milestones like this are reminders of the historic achievements made possible by the dedicated people entrusted to build and operate this national asset for exploring Mars,” said Callas.
For its extended mission, the gully chosen as the next major destination for Opportunity lay less than a kilometer (about half a mile) south of the rover’s then-location.
“We are confident this is a fluid-carved gully, and that water was involved,” said Opportunity Principal Investigator Steve Squyres. “Fluid-carved gullies on Mars have been seen from orbit since the 1970s, but none have been examined up close on the surface before.
“One of the three main objectives of our new mission extension is to investigate this gully. We hope to learn whether the fluid was a debris flow, with lots of rubble lubricated by water, or a flow with mostly water and less other material.”
The team intends to drive Opportunity down the full length of the gully, onto the crater floor, leading to the second goal of the extended mission: to compare rocks inside Endeavour Crater to the dominant type of rock Opportunity examined on the plains above.
“We may find that the sulfate-rich rocks we’ve seen outside the crater are not the same inside,” Squyres said. “We believe these sulfate-rich rocks formed from a water-related process, and water flows downhill. The watery environment deep inside the crater may have been different from outside on the plain – maybe different timing, maybe different chemistry.”
Nonetheless, the mission extension and its goals do pose some challenges to keeping Opportunity active for another two years.
Most mechanisms onboard the rover still function well, but motors and other components have far exceeded their life expectancy.
Additionally, Opportunity will face its eighth Martian winter in 2017, and with use of Opportunity’s non-volatile flash memory for holding data overnight discontinued last year, results of each day’s observations and measurements must be transmitted that day or lost.
Overall though, Opportunity remains in excellent condition relative to its age and stands poised to enter its 13th year on Mars ready for a challenge no other rover has face to date.
(Part 3 – Dawn, Juno, and Cassini – of NASASpaceflight.com’s 5 part Year In Review will be published in the coming days)
(Images: NASA, ESA)