For about the past 30 months, the Opportunity rover has been exploring Cape Tribulation on Mars, a towering ridge on the rim of Endeavour crater. Now, Opportunity has finally left that location, to continue its journey southward down the western side of the crater rim. The views have been scenic from the top of Cape Tribulation, but now it is time to move on, and head to the next major target, an ancient gully not too far to the south-east, also on the crater rim. This gully is thought to have been carved by running water millions or billions of years ago, so scientists are very interested in examining it up close, and the rover is now almost there.
The gully, called Perseverance Valley, is about the length of two football fields, and cuts through the crater rim. Although it is considered likely to have been formed by water, mission scientists need to study it in detail to determine whether it actually was, or could have been created by ice or wind instead. Opportunity is now closing in on the gully, and as of now is less than four football field lengths away.
“From the Cape Tribulation departure point, we’ll make a beeline to the head of Perseverance Valley, then turn left and drive down the full length of the valley, if we can,” said Opportunity Deputy Principal Investigator Ray Arvidson, of Washington University in St. Louis. “It’s what you would do if you were an astronaut arriving at a feature like this: Start at the top, looking at the source material, then proceed down the valley, looking at deposits along the way and at the bottom.”
Scientists will look at the arrangements of various sizes of rocks and pebbles for clues as to how the gully formed.
According to Arvidson, “If it was a debris flow, initiated by a little water, with lots of rocks moving downhill, it should be a jumbled mess. If it was a river cutting a channel, we may see gravel bars, crossbedding, and what’s called a ‘fining upward’ pattern of sediments, with coarsest rocks at the bottom.”
Principal Investigator Steve Squyres of Cornell University in Ithaca, N.Y. is optimistic that this is indeed a water-carved gully.
“We are confident this is a fluid-carved gully, and that water was involved,” said Squyres. “Fluid-carved gullies on Mars have been seen from orbit since the 1970s, but none had 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.”
What else might be found in the gully?
“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.”
“This gully has been calling us for years now,” Squyres also said. “We saw this thing a long time ago and for the longest time I didn’t even dare talk about it, because it seemed so impossibly far off.”
Such gullies are common on Mars, but there may be a variety of ways that they could form. Many are thought to have been created by liquid water billions of years ago, but have been dry ever since. Others could be explained by dry avalanches or ice. Gullies are also distinct from dark slope streaks, some of which are thought to be dry slides of soil or dust, but others, called Recurring Slope Lineae, may be currently caused by small amounts of liquid water brines which run down slopes and then sublimate in the thin atmosphere. Opportunity’s study of Perseverance Valley will be the first time a gully has been looked at closely, on the ground, by any rover or lander.
While leaving Cape Tribulation, at a ridge called Rocheport, Opportunity took a new color panorama of its surroundings – another stunning view from the edge of the huge 14 mile (22 kilometer) diameter crater.
“The degree of erosion at Rocheport is fascinating,” said Arvidson. “Grooves run perpendicular to the crest line. They may have been carved by water or ice or wind. We want to see as many features like this on the way to Perseverance Valley as we can, for comparison with what we find there.”
Opportunity first reached Endeavour crater 68 months ago, and explored Cape York, Solander Point and Murray Ridge before reaching Cape Tribulation. The rover is now close to Cape Byron, where the gully is located. At the peak of Cape Tribulation, Opportunity was at its highest altitude so far in the mission, with breathtaking views across the crater floor. The rover also found high levels of sulfur in this location, in a scuff in the soil made by one of its wheels.
“In the scuff, we found one of the highest sulfur contents that’s been seen anywhere on Mars. There’s strong evidence that, among other things, these altered zones have a lot of magnesium sulfate. We don’t think these altered zones are where the clay is, but magnesium sulfate is something you would expect to find precipitating from water,” said Squyres. “Fractures running through the bedrock, forming conduits through which water could flow and transport soluble materials, could alter the rock and create the pattern of red zones that we see.”
Another highlight was Marathon Valley, on Cape Tribulation, where the rover studied clay minerals which had first been detected from orbit. These minerals are also evidence of a once wetter environment in this region billions of years ago. Marathon Valley is a regular valley, not a gully.
In just the past few days, Opportunity has also used its Rock Abrasion Tool (RAT) for the first time in hundreds of sols (days). The RAT can grind off a thin layer from rocks for analysis, but it is now aging and used only sparingly.
“What we usually do to investigate material that’s captured our interest is find a bedrock exposure of it and use the RAT,” noted Squyres. “What we didn’t realize until we took a close-enough look is that this stuff has been so pervasively altered, it’s not bedrock. There’s no solid bedrock you could grind with the RAT. Fractures running through the bedrock, forming conduits through which water could flow and transport soluble materials, could alter the rock and create the pattern of red zones that we see.”
It is almost difficult to believe that Opportunity is still alive and roving – this was a mission which had a hoped-for lifespan of three months after first landing in 2004. It has now been more than 13 years! Opportunity celebrated its 13th anniversary on Jan. 24, 2017.
“We have now exceeded the prime-mission duration by a factor of 50,” said Project Manager John Callas of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “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.”
On April 8 of this month, the Mars Reconnaissance Orbiter (MRO) spacecraft took a new high-resolution color image of Opportunity’s landing site, where the rover ended up sitting inside a small crater, called Eagle crater, after a series of planned bounces of Opportunity inside its cushioned airbags. Like a cosmic hole-in one, the rover, on its lander, tumbled into the tiny crater, which is only 72 feet (22 meters) across. MRO has taken images of the rover in various locations since, but this is the first color view of the original landing site. The very first images the rover took showed sedimentary rocks along the inside wall of the crater.
Since then, Opportunity has helped to revolutionize our knowledge about Mars, finding evidence for ancient groundwater and salty playa lakes in the Meridiani Planum region. It has traveled across vast flat stretches of desert sand and navigated steep cliffs on crater rims. It discovered the first concretions on Mars, nicknamed “blueberries,” which are spherical but only a few millimeters across. They are also evidence for water a long time ago. The rover has also seen large dust devils, although not as many as its twin, Spirit, did before it became stuck is sand and never recovered. It has survived several brutal Martian winters (each Martian year being about two Earth years), and is still going. So far this year, the rover has driven about four-tenths of a mile (two-thirds of a kilometer), bringing the total traverse so far to 27.6 miles (44.4 kilometers).
As noted by MER Chief of Engineering Bill Nelson, at JPL, “We who operate her look forward to the new discoveries, new vistas and new milestones she will surely achieve.”
More information about the Opportunity mission is available on the NASA website.