NASA's Mars Exploration Rover Opportunity was designed to last just 90 days on Mars. Today, on Jan. 25, it is celebrating
of working on the Red Planet's surface. Though the rover is showing inevitable signs of age,such as flash memory problems
, it is still taking valuable observations and measurements of the Martian surface. Here is a
small selection some of the things it has done since landing at Meridiani Planum in 2004.MORE: Drive With Opportunity on Epic Mars Rover Marathon
Photo: A simulated view of Opportunity overlaid onto an observation of the Martian surface made by the rover.
While Opportunity was not equipped to look for life itself, it can look for the conditions of habitability for life in the past. One of those key elements is water, and Opportunity was lucky enough to stumble upon signs of it shortly after landing in 2004. It found concretions of "blueberries" made of hematite, an element that generally forms in the presence of water. It also found evidence of jarosite, which is created in acidic water. Later in its mission, Opportunity found crossbeds -- small centimeter-size rock layers that overlap each other and show that water once flowed through the area.MORE: Mars Rover Opportunity Suffers Worrying Bouts of 'Amnesia'
Photo: This false-color image taken by Opportunity on Mars shows concretions, nicknamed "blueberries", that formed in the presence of water.
The challenge with Mars is we've only sampled a few locations up close, the majority of our observations from afar, in orbit. This makes it hard to answer questions such as how abundant methane is, because different instruments have recorded wildly different measurements of the molecule (which in some cases points to life processes). One of Opportunity's notable discoveries in this area was helping to create the first atmospheric temperature profile, in concert with NASA's Mars Global Surveyor (a mission that is no longer operating today). This showed how warm different layers of the atmosphere are at that one location, which gave a snapshot of Mars' global weather. Several atmospheric science papers have also been published.MORE: Watching the Sunsets of Mars Through Robot Eyes
Photo: Cirrus clouds spotted by Opportunity in November 2004.
While seeking out the past and present habitable conditions remains the key focus of Martian exploration, we are also interested in other parts of its geology. How long ago did the volcanoes erupt? When were craters formed? What were the conditions that created various rocks on the surface? By comparing with Earth and running models of Martian climate, we can then begin to reconstruct the planet's past. Opportunity's major geological discoveries were water-related, specifically finding the elements hematite and jarosite on Mars. The rover also sends back hundreds of pictures a week of geological features, which are used for long-term studies to build up our understanding of the Red Planet.MORE: Rover Opportunity Has Found Some Odd Mars Rocks
Photo: Opportunity's robotic arm stretches towards a rock dubbed "Tisdale 2" in 2011.
Opportunity was not built to do astronomical observations per se, but it has looked up into the night sky from time to time. One recent prominent example was when Comet Siding Spring came within just 87,000 miles (139,500 kilometers) of Mars. Along with a fleet of other Mars spacecraft, Opportunity took pictures of the celestial visitor to get more information about how comets look and behave. The rover has additionally been used to watch the Martian moons, Phobos and Deimos, transit across the sun. The Earth is also highly visible in the rover's camera.MORE: NASA Spacecraft's Epic 10 Years of Mars
Photo: Opportunity's view of Comet Siding Spring, which passed close to Mars in 2014.
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When the rovers headed out to Mars, NASA was pursuing a Constellation program that was intended to put humans on the Red Planet around the 2030s. While the program was cancelled, NASA maintains that its Orion spacecraft today is still preparing humans for working on the Red Planet. A large part of that will be problem-solving, as was demonstrated in the 2015 movie "The Martian." It's well-known that Opportunity's twin rover, Spirit, died in a sand trap. But Opportunity also got stuck in the sand for a month and with NASA's help, successfully escaped. Engineering information from the Mars rovers is also telling scientists about how well wheels perform in the soil, how lubricants do in very cold temperatures, and the performance of solar panels.MORE: 12 Years On Mars: Opportunity's First Sols
Photo: A scene from the 2015 movie "The Martian."