The Opportunity rover stopped communicating with Earth when a severe Mars-wide dust storm blanketed its location in June 2018. After more than a thousand commands to restore contact, engineers in the Space Flight Operations Facility at NASA’s Jet Propulsion Laboratory (JPL) made their last attempt to revive Opportunity Tuesday, to no avail. The solar-powered rover’s final communication was received June 10.

“It is because of trailblazing missions such as Opportunity that there will come a day when our brave astronauts walk on the surface of Mars,” said NASA Administrator Jim Bridenstine. “And when that day arrives, some portion of that first footprint will be owned by the men and women of Opportunity, and a little rover that defied the odds and did so much in the name of exploration.”

Designed to last just 90 Martian days and travel 1,100 yards (1,000 meters), Opportunity vastly surpassed all expectations in its endurance, scientific value and longevity. In addition to exceeding its life expectancy by 60 times, the rover traveled more than 28 miles (45 kilometers) by the time it reached its most appropriate final resting spot on Mars — Perseverance Valley.

“For more than a decade, Opportunity has been an icon in the field of planetary exploration, teaching us about Mars’ ancient past as a wet, potentially habitable planet, and revealing uncharted Martian landscapes,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate. “Whatever loss we feel now must be tempered with the knowledge that the legacy of Opportunity continues — both on the surface of Mars with the Curiosity rover and InSight lander — and in the clean rooms of JPL, where the upcoming Mars 2020 rover is taking shape.”

The final transmission, sent via the 70-meter Mars Station antenna at NASA’s Goldstone Deep Space Complex in California, ended a multifaceted, eight-month recovery strategy in an attempt to compel the rover to communicate.

“We have made every reasonable engineering effort to try to recover Opportunity and have determined that the likelihood of receiving a signal is far too low to continue recovery efforts,” said John Callas, manager of the Mars Exploration Rover (MER) project at JPL.

Opportunity landed in the Meridiani Planum region of Mars on Jan. 24, 2004, seven months after its launch from Cape Canaveral Air Force Station in Florida. Its twin rover, Spirit, landed 20 days earlier in the 103-mile-wide (166-kilometer-wide) Gusev Crater on the other side of Mars. Spirit logged almost 5 miles (8 kilometers) before its mission wrapped up in May 2011.

From the day Opportunity landed, a team of mission engineers, rover drivers and scientists on Earth collaborated to overcome challenges and get the rover from one geologic site on Mars to the next. They plotted workable avenues over rugged terrain so that the 384-pound (174-kilogram) Martian explorer could maneuver around and, at times, over rocks and boulders, climb gravel-strewn slopes as steep as 32-degrees (an off-Earth record), probe crater floors, summit hills and traverse possible dry riverbeds. Its final venture brought it to the western limb of Perseverance Valley.

“I cannot think of a more appropriate place for Opportunity to endure on the surface of Mars than one called Perseverance Valley,” said Michael Watkins, director of JPL. “The records, discoveries and sheer tenacity of this intrepid little rover is testament to the ingenuity, dedication, and perseverance of the people who built and guided her.”

More Opportunity Achievements

• Set a one-day Mars driving record March 20, 2005, when it traveled 721 feet (220 meters).
• Returned more than 217,000 images, including 15 360-degree color panoramas.
• Exposed the surfaces of 52 rocks to reveal fresh mineral surfaces for analysis and cleared 72 additional targets with a brush to prepare them for inspection with spectrometers and a microscopic imager.
• Found hematite, a mineral that forms in water, at its landing site.
• Discovered strong indications at Endeavour Crater of the action of ancient water similar to the drinkable water of a pond or lake on Earth.

All of the off-roading and on-location scientific analyses were in service of the Mars Exploration Rovers’ primary objective: To seek out historical evidence of the Red Planet’s climate and water at sites where conditions may once have been favorable for life. Because liquid water is required for life, as we know it, Opportunity’s discoveries implied that conditions at Meridiani Planum may have been habitable for some period of time in Martian history.

“From the get-go, Opportunity delivered on our search for evidence regarding water,” said Steve Squyres, principal investigator of the rovers’ science payload at Cornell University. “And when you combine the discoveries of Opportunity and Spirit, they showed us that ancient Mars was a very different place from Mars today, which is a cold, dry, desolate world. But if you look to its ancient past, you find compelling evidence for liquid water below the surface and liquid water at the surface.”

All those accomplishments were not without the occasional extraterrestrial impediment. In 2005 alone, Opportunity lost steering to one of its front wheels, a stuck heater threatened to severely limit the rover’s available power, and a Martian sand ripple almost trapped it for good. Two years later, a two-month dust storm imperiled the rover before relenting. In 2015, Opportunity lost use of its 256-megabyte flash memory and, in 2017, it lost steering to its other front wheel.

Each time the rover faced an obstacle, Opportunity’s team on Earth found and implemented a solution that enabled the rover to bounce back. However, the massive dust storm that took shape in the summer of 2018 proved too much for history’s most senior Mars explorer.

“When I think of Opportunity, I will recall that place on Mars where our intrepid rover far exceeded everyone’s expectations,” Callas said. “But what I suppose I’ll cherish most is the impact Opportunity had on us here on Earth. It’s the accomplished exploration and phenomenal discoveries. It’s the generation of young scientists and engineers who became space explorers with this mission. It’s the public that followed along with our every step. And it’s the technical legacy of the Mars Exploration Rovers, which is carried aboard Curiosity and the upcoming Mars 2020 mission. Farewell, Opportunity, and well done.”

Mars exploration continues unabated. NASA’s InSight lander, which touched down on Nov. 26, is just beginning its scientific investigations. The Curiosity rover has been exploring Gale Crater for more than six years. And, NASA’s Mars 2020 rover and the European Space Agency’s ExoMars rover both will launch in July 2020, becoming the first rover missions designed to seek signs of past microbial life on the Red Planet.

JPL managed the Mars Exploration Rovers Opportunity and Spirit for NASA’s Science Mission Directorate in Washington. For more information about the agency’s Mars Exploration program, visit: https://www.nasa.gov/mars

A team of British and American researchers, co-led by the University of Cambridge, has measured how much the McMurdo ice shelf in Antarctica flexes in response to the filling and draining of meltwater lakes on its surface. This type of flexing had been hypothesised before and simulated by computer models, but this is the first time the phenomenon has been measured in the field. The results are reported in the journal Nature Communications.

The results demonstrate a link between surface melting and the weakening of Antarctic ice shelves and support the idea that recent ice shelf breakup around the Antarctic Peninsula may have been triggered, at least in part, by large amounts of surface meltwater produced in response to atmospheric warming.

As the climate continues to warm, more and more ice shelves may become susceptible to flex, fracture and break up over the coming century.

Most of the Antarctic continent is covered by the Antarctic Ice Sheet, which is up to four kilometres thick and contains enough ice to raise global sea levels by about 58 metres. Over most of the continent and for most of the year, air temperatures are well below zero and the ice surface remains frozen. But around 75% of the ice sheet is fringed by floating ice shelves, which are up to a kilometre thick, mostly below sea level, but with tens of metres of their total height protruding above the water. In the summer months, when air temperatures rise above freezing, the surfaces of these ice shelves are susceptible to melting.

“Surface water on ice shelves has been known about for a long time,” said co-author Dr Ian Willis from Cambridge’s Scott Polar Research Institute. “Over 100 years ago, members of both Shackleton’s Nimrod team and the Northern Party team of Scott’s British Antarctic Expedition mapped and recorded water on the Nansen Ice Shelf, around 300 kilometres from where we did our study on the McMurdo Ice Shelf. For the last few decades, it has also been possible to see widespread surface meltwater forming on many ice shelves each summer from satellite imagery.”

What is not fully known is the extent to which surface water might destabilise an ice shelf, especially in warmer summers when more meltwater is produced. If the slope of the ice shelf is sufficiently steep, the water may flow off the ice shelf to the ocean in large surface rivers, mitigating against any potential instability.

The danger comes if water pools up in surface depressions on the ice shelf to form large lakes. The extra weight of the water will push down on the floating ice, causing it to sink a bit further into the sea. Around the edge of the lake, the ice will flex upwards to compensate. “If the lake then drains, the ice shelf will now flex back, rising up where the lake used to be, sinking down around the edge,” said lead author Dr Alison Banwell, also from SPRI. “It is this filling and draining of lakes that causes the ice shelf to flex, and if the stresses are large enough, fractures might also develop.”

Banwell and co-author Professor Doug MacAyeal from the University of Chicago had previously suggested that the filling and draining of hundreds of lakes might have led to the catastrophic breakup of the Larsen B Ice Shelf 2002 when 3,250 square kilometres of ice was lost in just a few days.

“We had been able to model the rapid disintegration of that ice shelf via our meltwater loading-induced fracture mechanism,” said Banwell. “However, the problem was that no one had actually measured ice shelf flex and fracture in the field, and so we were unable to fully constrain the parameters in our model. That’s partly why we set out to try to measure the process on the McMurdo ice shelf.”

Using helicopters, snow machines and their own two feet, the researchers set up a series of pressure sensors to monitor the rise and fall of water levels in depressions which filled to become lakes, and GPS receivers to measure small vertical movements of the ice shelf.

“It was a lot of work to obtain the data, but they reveal a fascinating story,” said MacAyeal. “Most of the GPS signal is due to the ocean tides, which move the floating ice shelf up and down by several metres twice a day. But when we removed this tidal signal we found some GPS receivers moved down, then up by around one metre over a few weeks whereas others, just a few hundred metres away, hardly moved at all. The ones that moved down then up the most were situated where lakes were filling and draining, and there was relatively little movement away from the lakes. It is this differential vertical motion that shows the ice shelf is flexing. We’d anticipated this result, but it was very nice when we found it.”

The team hope that their work will inspire others to look for evidence of flex and fracture on other ice shelves around Antarctica. Their work will also help in developing ice sheet scale models that could be used to predict the stability of ice shelves in the future and to understand the controls on ice shelf size since they act as buffers against fast-moving ice from the continent. As ice shelves shrink, glaciers and ice streams behind them flow more rapidly to the ocean, contributing to global sea level rise.

The work was funded by the US National Science Foundation, the Leverhulme Trust, NASA, and CIRES, University of Colorado, Boulder.

These findings are part of a long-term collaborative study between shark researcher Neil Hammerschlag from the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science, and wildlife naturalist Chris Fallows from Apex Shark Expeditions.

The research focused on the waters surrounding Seal Island in False Bay, South Africa, a site well known for its “flying” great white sharks that breach out of the water when attacking Cape fur seals. Since the year 2000, the research team has spent over 8,000 hours observing great whites from boats, during which they recorded 6,333 shark sightings, and 8,076 attacks on seals. These data revealed that for more than a decade, great white numbers were relatively stable, but in 2015 sightings began to drop off steeply.

“In 2017 and 2018, their numbers reached an all-time low, with great whites completely disappearing from our surveys for weeks and months at a time,” said study lead author Neil Hammerschlag, a research associate professor at the UM Rosenstiel School and Abess Center for Ecosystem Science & Policy. “While the reasons for their decline and disappearance remains unknown, it provided a truly unique opportunity for us to see what happens to an ocean ecosystem following the loss of an apex predator.”

“In 18+ years of working at Seal Island, we had never seen sevengill sharks in our surveys,” said co-author Chris Fallows. “Following the disappearance of white sharks in 2017, sevengill began to show up for the first time and have been increasing in number ever since.”

During periods of great white absence in 2017 and 2018, the researchers documented 120 sevengill shark sightings and even witnessed an individual attacking a live seal. In South African waters, sevengill sharks have no equal in the food web with the exception of the great white and orca whale. Historically, the only well-known aggregation site for sevengills in False Bay was located 18 km away from Seal Island within inshore kelp beds. The study suggested that the appearance of sevengill sharks at Seal Island was due to the disappearance of great whites, thereby allowing sevengills to exploit the area without risk of predation from great white sharks or competition with them for shared prey.

This 18-year study provided new insights into the diverse ways that a marine ecosystem can be altered following the loss of an apex predator.

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Earlier research, which used a worldwide sample of fossils, suggested that the 50-foot-long, giant shark Otodus megalodon went extinct 2.6 million years ago. Another recent study attempted to link this extinction (and that of other marine species) with a supernova known to have occurred at about this time.

However, a team of researchers led by vertebrate paleontologist Robert Boessenecker with the College of Charleston, Charleston, South Carolina, noted that in many places there were problems with the data regarding individual fossils in the study estimating the extinction date.

In the new study, the researchers reported every fossil occurrence of O. megalodon from the densely sampled rock record of California and Baja California (Mexico) in order to estimate the extinction.

Besides Boessenecker, the research team included Dana Ehret, of New Jersey State Museum; Douglas Long, of the California Academy of Sciences; Morgan Churchill, of the University of Wisconsin Oshkosh; Evan Martin, of the San Diego Natural History Museum; and Sarah Boessenecker, of the University of Leicester, United Kingdom.

They found that genuine fossil occurrences were present until the end of the early Pliocene epoch, 3.6 million years ago. All later fossils either had poor data provenance and likely came from other fossil sites or showed evidence of being eroded from older deposits. Until 3.6 million years ago, O. megalodon had a continuous fossil record on the West Coast.

“We used the same worldwide dataset as earlier researchers but thoroughly vetted every fossil occurrence, and found that most of the dates had several problems-fossils with dates too young or imprecise, fossils that have been misidentified, or old dates that have since been refined by improvements in geology; and we now know the specimens are much younger,” Boessenecker said.

“After making extensive adjustments to this worldwide sample and statistically re-analyzing the data, we found that the extinction of O. megalodon must have happened at least one million years earlier than previously determined.”

This is a substantial adjustment as it means that O. megalodon likely went extinct long before a suite of strange seals, walruses, sea cows, porpoises, dolphins and whales all disappeared sometime about 1-2.5 million years ago.

“The extinction of O. megalodon was previously thought to be related to this marine mass extinction-but in reality, we now know the two are not immediately related,” Boessenecker said.

It also is further unclear if this proposed mass extinction is actually an extinction, as marine mammal fossils between 1 and 2 million years old are extraordinarily rare-giving a two-million- year-long period of “wiggle room.”

“Rather, it is possible that there was a period of faunal turnover (many species becoming extinct and many new species appearing) rather than a true immediate and catastrophic extinction caused by an astronomical cataclysm like a supernova,” Boessenecker said.

The researchers speculate that competition with the newly evolved modern great white shark (Carcharodon carcharias) is a more likely reason for megalodon’s extinction.

Great whites first show up with serrated teeth about 6 million years ago and only in the Pacific; by 4 million years ago, they are finally found worldwide.

“We propose that this short overlap (3.6-4 million years ago) was sufficient time for great white sharks to spread worldwide and outcompete O. megalodon throughout its range, driving it to extinction-rather than radiation from outer space,” Boessenecker said.

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