The valley opens to the northeast to a view across the floor of Endeavour Crater.
The images from which it is compiled were taken between 16 April and 15 May 15, 2016 – 28 sols, or Martian days.
The high point in the right half of the image is “Knudsen Ridge”, part of the southern edge of Marathon Valley.
Endeavour Crater, seen in the far distance, is 22 kilometres in diameter.
Opportunity entered Marathon Valley in July 2015.
The name was chosen as Opportunity arrived here just after passing the marathon distance – 42.2 kilometres – covered since it landed on the Red Planet (as of 14 June, Opportunity has driven 42.79 kilometres).
“We are wrapping up our last few activities in Marathon Valley and before long we’ll drive away, exiting along the southern wall of the valley and heading southeast,” said one of the program’s scientists, Steve Squyres of Cornell University.
Opportunity has examined the clay-bearing rocks on the valley floor and revealed streaks of red-toned, crumbly material on the valley’s southern flank.
When analysed, the material was found to have high sulfur content, “one of the highest sulfur contents that’s been seen anywhere on Mars”, said Squyres.
“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.”
He said that fractures running through the bedrock could form conduits through which water transported soluble materials, alter the rock and create the pattern of red zones.
NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, built the rover and manages the mission for NASA’s Science Mission Directorate, Washington.
The project’s NASA site has more information.