Two spacecraft engineers stand with a group of vehicles providing a comparison of three generations of Mars rovers developed at NASA's Jet Propulsion Laboratory, Pasadena, California. The setting is JPL's Mars Yard testing area. Front and centre is the flight spare for the first Mars rover, Sojourner, which landed on Mars in 1997 as part of the Mars Pathfinder Project. On the left is a Mars Exploration Rover Project test rover that is a working sibling to Spirit and Opportunity, which landed on Mars in 2004. On the right is a Mars Science Laboratory test rover the size of that project's Mars rover, Curiosity, which landed on Mars in August 2012. Sojourner and its flight spare, named Marie Curie, are 2 feet (65 centimeters) long. The Mars Exploration Rover Project's rover, including the "Surface System Test Bed" rover in this photo, are 5.2 feet (1.6 meters) long. The Mars Science Laboratory Project's Curiosity rover and "Vehicle System Test Bed" rover, on the right, are 10 feet (3 meters) long. The engineers are JPL's Matt Robinson, left, and Wesley Kuykendall. The California Institute of Technology, in Pasadena, operates JPL for NASA.

This low-resolution picture of Curiosity's shadow was one of the first it snapped after landing on the dusty planet.

Curiosity charges toward Mount Sharp, which towers at a startling 3.4 miles high (5.5 kilometres)—taller than Mont Blanc, the highest summit in the Alps. Scientists hope to study the mountain's rocky layers to unfold the history of life on Mars.

The rover's track marks along the top of this image include a pattern of lines that is Morse Code for "JPL," or NASA's Jet Propulsion Lab. Interspersed with the zigzag tracks, this pattern was a reference to help the rover navigate.

The rover is packed with sensors and instruments to record the Martian landscape. The pink circle in the centre of this camera is one of the rover's many "eyes."

Curiosity collected its first scoop of Martian soil at the Rocknest dunes, and also found this bit of debris, possibly dropped during the rover's landing. When Curiosity heated the soil in its onboard lab, a bit of water vapour escaped.

This selfie of Curiosity was stitched together from 55 pictures at Rocknest dune—the site where the rover sampled Martian soil for the first time.

This piece of rock, dubbed Tintina, was crushed under one of Curiosity's wheels to reveal a surprisingly white interior. The minerals of the rock contain water, further suggesting the planet was once watery.

The Curiosity rover appears as a tiny blue dot at the lower right of this image. Behind it trails a path of winding tracks back to its landing site, where a scar remains from the landing jets' blast.

On the 535th Martian day, Curiosity traversed the Dingo Gap—a three-foot-tall (one-metre) dune. Scientists aim Curiosity towards features like the dune sands to prevent rapid wearing of the treads on the surprisingly sharp Martian rocks.

Scientists believe that the even layers of this Martian rock were first deposited near the edge of an ancient lake, close to where water flowed into the basin. These formations are one of the many features that signal the planet's watery past.

Scientists believe that the layers of rock on the lower flanks of Mount Sharp will help them untangle Mars' geologic history. Scientists will examine the mountain layer by layer, as if leafing through the pages of a book.

The first drill hole in Mount Sharp was less than an inch wide (1.6 centimetres) but held a wealth of information. The sample contained hematite, an iron-oxide mineral, suggesting Mars may have once had chemistry that could support microbes.

Curiosity got its second bite of Mount Sharp's soil at the Mojave site, surrounded by the Pahrump Hills. Around this time, scientists announced the rover had detected methane in Mars's atmosphere and organic molecules in rocks.

The rover caught this stunning view of the sun setting over the rim of Gale crater on April 15, 2015. Unlike our red-hued sunsets, Martian views are tinted blue thanks to fine particles that allow more blue light into the atmosphere.