The Rover “Opportunity” Reaches Its 5,000th Martian Dawn
By Judith E Braffman-Miller
No other major planet in our Solar System has captured the collective imagination of our species more than the Red Planet Mars. This is because it has traditionally been considered the most likely abode of life beyond our Earth–where “little green men” frolic on a rust-red planet, under a peach-ice-cream colored sky. While such a form of intelligent alien life has not been found on Earth’s neighboring planet, Mars still captivates us–and, as a result, it has been extensively explored by both Earth-bound telescopes and visiting spacecraft, as well as wandering rovers that explore its fascinating surface. In February 2018, NASA’s highly successful solar-powered Mars rover, dubbed Opportunity, witnessed for the 5,000th time a Martian dawn. This sturdy little traveler, that Earthlings have dispatched to Mars, still keeps providing scientists with surprising revelations about the Red Planet–most recently with observations of possible “rock stripes”.
“Five thousand sols after the start of our 90-sol mission, this amazing rover is still showing us surprises on Mars,” commented Dr. John Callas in a February 15, 2018 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Callas is Opportunity Project Manager at the JPL in Pasadena, California. A Martian day is called a sol.
A Martian sol lasts for approximately 40 minutes longer than a day on Earth, and a year on Mars lasts as long as nearly two Earth-years. Opportunity’s first sol was on the day it landed on the Red Planet–which was on January 25, 2004, Universal Time. However, it landed on January 24, 2004, California time–where the JPL is located.
Opportunity’s primary mission was planned to last 90 sols, and NASA did not expect this surprisingly sturdy little rover to survive through an entire, very cold, Martian winter. Sol 5,000 happened early Friday, February 23. 2018, Universal Time. Opportunity’s 4,999th Martian dawn happened a few hours later. In fact, Opportunity has succeeded in working quite well through the lowest-energy months of its frigid eighth Martian winter.
Seeing this milestone event from the rover’s perspective–from where it stood situated on the inside slope of the western rim of Endeavor Crater–its 5,000th Martian sunrise appeared over the basin’s eastern rim, approximately 14 miles away. Opportunity has traveled over 28 miles from its landing site to its current location, which is about one-third of the way down Perserverance Valley. Perserverance Valley is a shallow channel cut from the rim’s crest of the crater’s floor. Opportunity has dispatched back to Earth about 225,000 images–all of which have been made available to the public online.
“We’ve reached lots of milestones, and this is one more, but more important than the numbers are the exploration and the scientific discoveries,” commented Dr. Callas in the February 15, 2018 JPL Press Release.
The plucky little rover made headlines during its first months on the Red Planet, when it provided evidence that groundwater and surface water environments once existed on ancient Mars. The presence of liquid water is necessary for the emergence of life as we know it. This means that Mars may have once been, very long ago, the home of primitive forms of life–and it may still host primitive life. However, no life forms have been discovered as yet on the Red Planet.
Opportunity wandered to increasingly larger craters in order to peer deeper into Mars, as well as farther, and farther back into Martian history–finally reaching Endeavor Crater in 2011. Planetary scientists are currently using the rover to study the processes that shaped Perseverance Valley.
Uncovering An Ancient Martian Mystery
Mars is located close to our Sun in the brightly-lit, and comparatively warm, inner region of our Solar System. Even though Mars is the fourth major planet from our Star, as well as Earth’s neighbor, it has still managed to keep many of its secrets to itself. For example, in March 2016, astronomers announced their discovery that the surface of Mars was tilted by 20 to 25 degrees, approximately 3 to 3.5 billion years ago. This disaster occurred in our Solar System’s youth–our Sun and its family formed approximately 4.56 billion years ago–and it was probably caused by a behemoth of a volcanic structure. This enormous volcano, dubbed the Tharsis volcanic dome, is the largest of its fiery, eruptive kind in our entire Solar System. Because of the immense mass of this volcanic structure, it caused the outer layers of Mars–its crust and mantle–to rotate around its core!
This bizarre shift is believed to have occurred at a time when life may have first formed out of a primeval Martian soup of non-living ingredients. It also provides an answer to three nagging mysteries: why Martian rivers formed where they are today; why underground reservoirs of water ice are located far from the poles of Mars, and why the Tharsis dome is located on the Martian equator.
Mars has not always looked the same as it does today. The Red Planet suffered its catastrophic great tilt billions of years ago. It was not the rotation axis of Mars that shifted, as the result of a process called a variation of obliquity. Instead it was the mantle and the crust that rotated with respect to the Red Planet’s inner core.
Mars is called the Red Planet because of the great quantities of iron oxide that exist on its surface, which bestows upon it a rusty-reddish hue. The Red Planet is an Earth-like rocky world that is veiled by only a very thin atmosphere, and it is pockmarked with numerous surface features that are similar to the impact craters observed on Earth’s Moon. Mars also displays valleys, polar ice caps, deserts, and volcanoes that resemble those seen on our own little blue planet. The Red Planet is a member of the toasty quartet of solid inner Solar System worlds, called terrestrial planets, along with Mercury, Venus, and Earth. Adding to this significant family resemblance, Mars also sports a rotational period, and changing seasons, similar to those of Earth.
Unlike our own planet’s lovely, large lunar companion, Mars is circled by a duo of deformed, very small moons, dubbed Phobos and Deimos. These interesting potato-shaped moons are believed to have originally been asteroids inhabiting the Main Asteroid Belt between Mars and Jupiter. The tiny objects probably escaped from the Main Asteroid Belt in the early days of our Solar System, only to be snared by the intense gravitational grip of Mars. Thus, the duo experienced a sea-change, morphing from traveling asteroids into moons of one of the major planets.
Since the year 2000, cameras in orbit around Mars have sent back to Earth a remarkable treasure trove of revealing images. These pictures have shown a Martian surface that is etched by small valleys carved into slopes, that are eerily similar in shape to gullies formed by gushing water on Earth. The Martian gullies are believed to be less than a few million years old–only a brief moment on geological time scales. Indeed, some of the gullies are apparently considerably younger than that. These observations whisper tantalizing hints that great quantities of liquid water may still be sloshing around on the Red Planet today–and that this flowing Martian water might be responsible for carving the gullies.
This particular model has been questioned by some planetary scientists as a result of frequent monitoring of the Martian surface by the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter (MRO). Some MRO observations indicate that the formation of the Martian gullies is still going on today–occurring during seasons when the surface environment of Mars is too cold for liquid water to flow. However, the observed gully activity seems to occur when CO2 ice (dry ice)–which condenses from the Martian atmosphere during the winter–is melting on the warming surface of Earth’s neighboring world.
The seasonal cycle of frosting and defrosting dry ice results in the appearance of numerous features on the Martian surface, such as dark dune spots with spider-like rivers or channels lurking below the ice–where spider-like radial channels are carved between the dry ice and the ground. This is what creates the interesting appearance of spider webs. The pressure that builds up in the hidden interior of these Martian features shoots gas and dark basaltic sand or dust upward, which is ultimately deposited on the icy surface–forming dark dune spots. This process occurs rapidly, and planetary scientists have seen it happening over brief periods of only a few days, weeks, or months–a growth rate that is somewhat unusual on geological time scales, especially for Mars. It would seem that many years would be necessary to form the larger spider-like channels.
Mariner 4 made the first successful flyby of the Red Planet back in 1965. This early historic visit, that occurred half a century ago, stimulated considerable speculation among scientists that life-sustaining liquid water may once have pooled on the Martian surface. This theory was devised on the basis of observed periodic changes of light and dark regions, particularly in the polar latitudes on Mars–which looked like irrigational channels for streaming water. However, these very interesting and captivating straight lines were eventually determined to be merely optical illusions. Nonetheless, geological evidence continued to indicate to planetary scientists that Mars was once covered by large quantities of liquid water. Back in 2005, information derived from radar hinted that significant amounts of water ice lurked on the Martian poles–as well as at mid-latitudes. Two years later, the Mars rover named Spirit detected chemical compounds that contained water molecules. On July 31, 2008, the Phoenix lander also spotted water ice within shallow samples of Martian soil.
It’s Mysterious; It’s Exciting
The Martian ground texture, as seen in recent images provided by the rover Opportunity, resembles a smudged version of distinctive stone stripes appearing on some mountain slopes on our own planet that are caused by repeated cycles of freezing and thawing of wet soil. However, these features could also be caused by downhill transport, rushing winds, other processes, or even a combination of processes.
“Perseverance Valley is a special place, like having a new mission again after all these years. We already knew it was unlike any place any Mars rover has seen before, even if we don’t yet know how it formed, and now we’re seeing surfaces that look like stone stripes. It’s mysterious. It’s exciting. I think the set of observations we’ll get will enable us to understand it,” commented Principal Investigator Dr. Ray Arvidson in the February 15, 2018 JPL Press Release. Dr. Arvidson is of Washington University in St. Louis.
Another mystery involves the uncertainty about how Perseverance Valley formed in the first place. Rover-team planetary scientists are currently analyzing a number of clues that indicate actions of water, ice, and wind may be responsible. They are also investigating the possibility of a broader range of explanations for the formation of the stripes. It remains uncertain about whether this texture has been caused by processes of a relatively modern Mars or, alternatively, of a much more ancient Mars.
Other lines of evidence have convinced those planetary scientists who study the Red Planet that, over a time-span of hundreds of thousands of years, Mars experiences cycles when the tilt (obliquity) of its axis increases to such a great extent that some of the water that is currently frozen at the poles vaporizes into the thin atmosphere–ultimately becoming snow or frost that accumulates closer to the equator.
“One possible explanation of these stripes is that they are relics from a time of greater obliquity when snow packs on the rim seasonally melted enough to moisten the soil, and then freeze-thaw cycles organized the small rocks into stripes. Gravitational downhill movement may be diffusing them so they don’t look as crisp as when they were fresh,” Dr. Arvidson continued to explain in the February 15, 2018 JPL Press Release.
Dr. Bernard Hallet of the University of Washington (Seattle) agrees that the alignments observed in the images of Perseverance Valley are not as distinctive as the stone stripes he has studied on our own planet. Field measurements conducted on Earth, near the summit of the dormant volcano, Mauna Kea, in Hawaii–where the soil freezes every night but is frequently dry–have documented how those form when ground conditions and temperature are just right. Soils that contain a mixture of sand, silt, and gravel expand more in places where the finer-grained material is most abundant and retains more water. Freezing expands the soil, thus pushing larger particles upward. If the particles travel to one side, as well as down the general slope–as a result wind or gravity–they usually travel away from the finer-grain concentrations and stretch out downslope. Where the larger particles become more concentrated, the ground expands less. The entire process repeats itself hundreds or thousands of times, and the pattern self-organizes into alternating stripes.
Perseverance Valley contains rocks that have been carved by sand blowing uphill from the crater floor, as the result of fierce, roaring Martian winds. Wind might also be the key in sorting larger particles into rows parallel to the slope.
Every sol that the durable little Martian rover manages to observe, contributes more and more valuable information to curious planetary scientists. This treasure chest, filled with new data, helps scientists solve some still well-kept Martian mysteries–as well as unveiling new ones.
Opportunity science-team member, Dr. Robert Sullivan of Cornell University, in Ithaca, New York, commented in the February 15, 2018 JPL Press Release that “Debris from relatively fresh impact craters is scattered over the surface of the area, complicating assessment of effects of wind. I don’t know what these stripes are, and I don’t think anyone else knows for sure what they are, so we’re entertaining multiple hypotheses and gathering more data to figure it out.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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