Year In Review, 2017 (Part II): Rovers, orbiters peel away Mars’ secrets & reveal new mysteries

by Chris Gebhardt

For solar and space weather science, 2017 was a major boon to understanding how Coronal Mass Ejections form, travel through space, and affect Earth.  But Earth wasn’t the only planet this year that gave scientists a front row seat to solar storms.  Thanks to the fleet of orbiters and rovers at Mars, the Red Planet provided a host of data for NASA – and by extension SpaceX – that will greatly help refine models for radiation exposure levels and efforts to protect future robotic and human missions to the planet we hope to colonize in the near future.

On 11 September 2017, the Sun threw off a major Coronal Mass Ejection – sending a stream of radiation and charged particles toward Mars.  As the wave front blasted across the Martian atmosphere, and international fleet of spacecraft were there to detect the storm’s passage and monitor – as never before – the radiation’s effect to Mars’ atmosphere and at the planet’s surface.

All told, four Martian orbiting satellites detected the event – NASA’s Mars Odyssey, Mars Reconnaissance Orbiter, and MAVEN spacecrafts as well as the European Space Agency’s (ESA’s) Mars Express.  It’s unknown at the time of writing whether the Indian Space Research Organisation’s MOM spacecraft observed the event as well.

This animation shows the sudden appearance of a bright aurora on Mars during a solar storm. The purple-white color scheme shows the intensity of ultraviolet light over the course of the event, from observations on Sept. 12 and 13, 2017, by the Imaging Ultraviolet Spectrograph on NASA’s MAVEN orbiter. Credits: NASA/GSFC/Univ. of Colorado

On the surface, NASA’s Curiosity rover was also primed for the event.  And the results represented the first time a distributed set of instruments observed such a powerful solar storm’s interaction with Mars.

The solar event was so massive that it: was detected at Earth, even though Earth was on the opposite side of the Sun from Mars, sparked a global Mars aurora – lighting up the whole planet in ultraviolet light – more than 25 times brighter than any previously seen by MAVEN, and produced radiation levels on the surface more than double any previously measured by Curiosity’s Radiation Assessment Detector (RAD) since that mission’s landing at Gale crater in August 2012.

The high readings lasted more than two days and, according to RAD Principal Investigator Don Hassler of the Southwest Research Institute in Boulder, Colorado, is “exactly the type of event both [MAVEN and Curiosity] were designed to study.  [These measurements] will improve our understanding of how such solar events affect the Martian environment, from the top of the atmosphere all the way down to the surface.”

Energetic particles from a large solar storm in September 2017 were seen on the surface of Mars by NASA’s Curiosity rover. Credits: NASA/GSFC/JPL-Caltech/Univ. of Colorado/SwRI-Boulder/UC Berkeley

Curiosity’s RAD instrument has been steadily monitoring the radiation environment at Mars’ surface for more than five years, returning data points that will strengthen our understanding of radiation’s impact on Mars habitability.  Highly energetic solar events – such as the one seen in September – can significantly increase the amount of radiation present at Mars’ surface.  As this extra kick of radiation passes through Mars’ atmosphere – which is much thinner than Earth’s – it produces secondary radiation particles which need to be understood and shielded against to ensure the safety of future human explorers.

Over the course of billions of years, Mars’ atmosphere has been steadily lost to space due to the planet’s lack of a strong magnetic field.  MAVEN’s primary mission is to examine the processes that continue to lead to the loss of the Martian atmosphere, and major solar storms like this provide a wealth of data for this mission objective – which also helps scientist understand what Curiosity is finding on the Red Planet’s surface.

Mars was once quite wet – this much is fact.  This year, as Curiosity entered its fifth Earth year of surface operations on Mars, the rover and its teams made several discoveries about Mars’ watery past that continue to reveal just how dynamic Mars’ climate once was.

The network of cracks in this Martian rock slab called “Old Soaker” may have formed from the drying of a mud layer more than 3 billion years ago. The view spans about 3 feet (90 centimeters) left-to-right and combines three images taken by the MAHLI camera on the arm of NASA’s Curiosity Mars rover. Credits: NASA/JPL-Caltech/MSSS

Based on lower Mount Sharp at a rock dubbed “Old Soaker”, Curiosity found a series of slabs with cross-hatched shallow ridges that likely originated as cracks in drying mud.  If this interpretation holds to scrutiny, these would be the first mud cracks – called desiccation cracks – confirmed by the rover and would be concrete evidence that the ancient era when these sediments were deposited included some drying after wetter conditions.

The cracked areas investigated by Curiosity formed more than 3 billion years ago and were found to be filled with windblown dust or sand and minerals delivered by groundwater circulating through the cracks (in this case, bright veins of calcium sulfate).

The presence of both of these types of crack-filling material may indicate multiple generations of fracturing: mud cracks first, with sediment accumulating in them, then a later episode of underground fracturing and vein forming.  “If these are indeed mud cracks, they fit well with the context of what we’re seeing in the section of Mount Sharp Curiosity has been climbing for many months,” said Curiosity Project Scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory in Pasadena.

An artist’s rendition depicts a solar storm hitting Mars and stripping ions from the upper atmosphere. Credit: NASA GSFC

But as more information came in from Curiosity this year, the more Mars climate scientist began scratching their heads.  Ample evidence says ancient Mars was sometimes wet, with water flowing and pooling on the planet’s surface.  Yet the ancient Sun was about one-third less warm than it is today, and climate modelers have struggled to produce scenarios that get the surface of Mars warm enough to keep water unfrozen.

Until this year, climate modelers theorized that Mars’ ancient atmosphere must have contained high concentrations of carbon dioxide that caused a global warming/greenhouse gas effect that kept the planet’s surface warm.  But Curiosity now appears to have nixxed that theory.

According to a new analysis of data from Curiosity, Mars had far too little carbon dioxide 3.5 billion years ago to provide enough greenhouse-effect warming to thaw water ice.  The same Martian bedrock in which Curiosity found sediments from an ancient lake where microbes could have thrived is the source of the evidence adding to the quandary about how such a lake could have existed.

Artist’s impression of present-day Mars v. what the planet might have looked liked 3.5 billion years ago. Credit: NASA GSFC

Curiosity detected no carbonate minerals in the samples of the bedrock it analyzed, and a new analysis concludes that the dearth of carbonates in that bedrock means Mars’ atmosphere when the lake existed could not have held much carbon dioxide.  “It would be really hard to get liquid water even if there were a hundred times more carbon dioxide in the atmosphere than what the mineral evidence in the rock tells us,” said Thomas Bristow of NASA’s Ames Research Center.  

With these new findings, Curiosity adds to the growing dilemma of two seemingly incompatible facts with ancient Mars: it’s atmosphere wasn’t thick enough and didn’t contain the right material to keep the planet’s surface warm enough for liquid water to exists… yet liquid water existed in abundance.

Despite clues such as isotope ratios in today’s Martian atmosphere that indicate the planet once held a much denser atmosphere than it does now, theoretical models of the ancient Martian climate struggle to produce conditions that would allow liquid water on the Martian surface for many millions of years.  Not deterred, researchers are evaluating multiple ideas for how to reconcile the dilemma.

“Some think perhaps the lake [Curiosity’s investigating at Gale Crater] wasn’t an open body of liquid water.  Maybe it was liquid covered with ice,” said Robert Haberle, a Mars climate scientist at NASA Ames.  “You could still get some sediments through to accumulate in the lakebed if the ice weren’t too thick.”

A drawback to that explanation, however, is that Curiosity has sought but not found any evidence one would expect from ice-covered lakes, such as large and deep cracks called ice wedges, or “dropstones,” which become embedded in soft lakebed sediments when they penetrate thinning ice.

As with all things science, when two lines of scientific evidence appear irreconcilable, the scene may be set for an advance in understanding of why they are, in fact, both correct and open our eyes to what we’re not yet understanding of how water, the Sun, and atmosphere all interact on celestial bodies other than Earth.

Nonetheless, Curiosity’s examination of Mars’ past continued unabated as the rover’s suite of powerful instruments revealed more information on the lake that once existed at Gale Crater.  New analyses of Curiosity data revealed that the lake was stratified – exhibiting sharp chemical or physical differences between deep water and shallow water.  Gale Crater’s lake is now known to have had shallow water that was richer in oxidants than its deeper water.

This diagram presents some of the processes and clues related to a long-ago lake on Mars that became stratified, with the shallow water richer in oxidants than deeper water was. Credits: NASA/JPL-Caltech/Stony Brook University

“These were very different, co-existing environments in the same lake,” said Joel Hurowitz of Stony Brook University, lead author of a report on the findings.  “This type of oxidant stratification is a common feature of lakes on Earth, and now we’ve found it on Mars.  The diversity of environments in this Martian lake would have provided multiple opportunities for different types of microbes to survive, including those that thrive in oxidant-rich conditions, those that thrive in oxidant-poor conditions, and those that inhabit the interface between those settings.”

Whether Mars has ever hosted any life is still unknown, but seeking signs of life on any planet begins with reconstructing the past environment to determine if it was capable of supporting life.  Curiosity’s primary goal was to determine whether Mars ever offered environmental conditions favorable for microbial life, and in its first year on the Gale Crater floor at Yellowknife Bay the rover found evidence of ancient freshwater river and lake environments with all the main chemical ingredients and a possible energy source for life.

This map shows the route driven by NASA’s Curiosity Mars rover, from the location where it landed in August 2012 to its location in July 2017 (Sol 1750), and its planned path to additional geological layers of lower Mount Sharp. Credits: NASA/JPL-Caltech/Univ. of Arizona

In addition to revealing new information about chemical conditions within the lake, Mr. Hurowitz and his co-authors also documented fluctuations in the climate of ancient Mars based on comparing differences in the chemical composition of layers of mud-rich sedimentary rock that were deposited in quiet waters in the lake.

These rock-based investigations revealed that 3 billion years ago when the lake was present, Mars’ atmosphere changed from cold and dry to warm and wet – a short-term fluctuation that took place within a longer-term climate evolution from the ancient warmer and wetter conditions that supported lakes to today’s arid Mars.

Understanding how Mars’ climate changed over time – including momentary swings opposite the general trend – help scientists unlock more of Mars’ mysteries in preparation for the first human missions to the Red Planet and also provide data points Earth climate scientists can use to understand how some climate processes we’ve seen play out on our own planet relate to the larger picture of known planetary and climate development models.

Meanwhile, on the other side of the planet, NASA’s Opportunity rover marked the beginning of its teenage years in January as the rover passed the 13 year mark since its successful landing on the surface of Mars for what was anticipated to be a 90-day mission.

Overall, 2017 was a quiet year for Opportunity, spent mainly at its winter energy refuge in Perseverance Valley on the western rim of Endeavour crater.  Because Opportunity functions from the energy received by its solar panels, the rover’s teams must position the intrepid little craft on north-facing slopes during the Southern Hemisphere winter so that as much of Opportunity’s solar panels as possible are aimed toward the Sun – which is low in the northern sky during Opportunity’s winter.

As of 18 December 2017, Opportunity has driven 28.01 miles (45.08 km) across the Martian surface since landing.  At the time of publication, the rover was in its 4,952nd Martian day for operation – more than 4,860 Martian days beyond its originally planned mission duration.

Fortunate for Opportunity, a series of wind-based solar panel dust cleaning events have recently left the rover’s panels condition in excellent shape as it heads toward its 14th landing anniversary and into the southern hemisphere spring and summer months – the time of the Martian year when the probability of global dust storms peaks.

Opportunity’s solar panels in 2014, before and after a wind cleaning event. Credit: NASA

The last global Martian dust storm was more than a decade ago, during which both Opportunity and its companion rover, Spirit, came precariously close to not receiving enough power through their solar arrays to continue their missions.  With Opportunity’s arrays now in good condition, the odds of the vehicle surviving a global dust storm this year – should one occur – are higher than they were a few months ago, when team members worried about the rover’s power generating capabilities through such an event.

Any human mission and colonization effort to Mars will have to contend with the reality of sooner or later having to weather a global Martian dust storm.  While a global dust storm has not occurred since 2007, the international fleet of orbiters and NASA’s rovers on the surface have been dutifully gathering data about Martian atmospheric conditions during which those dust storms did not occur – allowing scientists to compare that information to the available atmospheric data from the 2007 global event.

The information returned by both Spirit and Opportunity as well as the fleet of orbiters present at Mars during the 2007 global dust storm contained a treasure trove of information for future human and robotic Mars exploration planners.

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