NASA's Curiosity rover began its two-year mission extension on Oct. 1, moving toward its uphill destinations in Mars' Mount Sharp.

The size of Mount Rainier, Mount Sharp is a layered mound that is being investigated by the rover for evidence of water-rich environments in the past that starkly go against the harsh and dry conditions currently prevailing in the planet. Key exploration sites for Curiosity include a hematite-capped ridge and a bedrock rich in clay beyond the ridge.

"Even after four years of exploring near and on the mountain, it still has the potential to completely surprise us," said Ashwin Vasavada, Curiosity project scientist from NASA's Jet Propulsion Laboratory.

Since it landed on Mars back in August 2012, the Curiosity rover has taken more than 180,000 pictures of the red planet's landscape, the most recent highlights of which are images of a cluster of buttes and mesas of varying shapes. The rover also recently took a self-portrait using the color camera on its arm and a scenic panorama using the color camera on its mast.

The area where the Curiosity rover took its self-portrait is located close to the base of one of the Murray Buttes, the same spot where it drilled the ground for a rock powder sample. After leaving the buttes, the rover used its internal laboratory to analyze the sample.

The Murray formation is the 14th drill site for the rover. Made primarily of mudstone created from accumulated mud from the bottom of ancient lakes, the formation is about 600 feet thick, almost half of which has already been climbed by the Curiosity rover.

In the first half of its mission extension, the rover is expected to investigate the Murray formation's upper half. Vasada said the rover team is looking to determine if Mars' ancient environment changed over time but it's also possible that the evidence they just found will point to something else entirely.

The Curiosity rover is headed for the clay and hematite units above the Murray formation because the area was identified before as a high-priority destination, thanks to images provided by NASA's Mars Reconnaissance Orbiter. Clay and hematite usually form in wet conditions so the rover is traveling to a spot that is different from the environment it has so far encountered in Mars.

Michael Meyer, Curiosity program scientist, looked back at the mission and likened it to "reading the pages of Martian history." According to him, Curiosity's exploration of rock layers has changed what people understand of the red planet and how it evolved.

"Curiosity has been and will be a cornerstone in our plans for future missions," he said.

The rover's original mission was to determine if the Curiosity's landing region was conducive to microbial life. It found proof of ancient lakes and rivers, with all the necessary chemical ingredients to start life, as it is known.