The Mars Exploration Rovers, Spirit and Opportunity, embarked on a journey to Mars intending to complete a three month primary mission to learn about the history of water on Mars. Over eight years since they landed, and Opportunity is still making tracks across the red planet. Although Spirit succumbed to the harsh Martian environment in 2010 it still far exceeded expectations by driving 12 times further than was originally envisioned.
Spirit was launched on 10 June 2003 and Opportunity followed on 7 July 2003. The rovers then landed on Mars on 4 and 25 January 2004 respectively. Spirit touched down on Mars at Gusev crater, an impact crater that might have once held a lake. Meridiani Planum, on the opposite side of the planet, was the landing site for Opportunity - chosen because mineral deposits indicated water might have once been present.
Opportunity spent the last Martian winter at Greeley Haven, which has the advantage of a sun-facing slope that is essential for the rover to survive. Opportunity set out for a new season of exploration on 8 May 2012, and its future goals include investigating a bright patch of dust, unusual for the Meridiani region, and searching for ancient clay along the rim of the Endeavour crater.
According to principal investigator Steve Squyres, it is unknown how long Opportunity will continue to function. “The way you predict the lifetime of space flight hardware is by doing tests to failure of critical components. We've never done such testing... there was never any reason to. So Opportunity could fail tomorrow, or could keep going for years... we have no idea”.
The twin rovers are kitted out with two cameras: the Panoramic Camera (Pancam) and the Microscopic Imager, which serve opposite purposes as the names suggest. Three different spectrometers are used for analysing the alien red planet. The Miniature Thermal Emission Spectrometer (Mini-TES) is an infrared spectrometer that lays the ground work by identifying interesting rocks that deserve further inspection. Mini-TES can also look up and discern the temperature of the atmosphere above. The Mössbauer Spectrometer (MB) performs the close up inspections of the mineralogy of iron-bearing rocks and soil, while the Alpha Particle X-Ray Spectrometer (APXS) peeks at the abundances of the rocks and soil.
The rovers are equipped with magnets to help gather magnetic dust particles for analysis by the spectrometers. There is also a RAT on board each rover, in the form of the Rock Abrasion Tool. The RAT is responsible for nibbling away weathered surfaces of rocks to expose the fresh material beneath for analysis.
Power is achieved through the solar panels that have cells stacked in a triple layer on the solar array. This is in order to accumulate more energy than predecessor rovers such as Sojourner. The solar panels on Opportunity are coated in a layer of dust, which blocks some of the sunlight. This is why it was necessary to find a sun facing slope for the winter, which was never needed before as the rover is close to the equator. It is hoped that the Martian wind will help to blow away some of the dust on the solar panels, as it has previously.
Opportunity's self portrait, showing the solar panals covered in dust. Credit: NASA/JPL-Caltech/Cornell/Arizona State Univ
In December 2009 Opportunity was sent a software upgrade for Pancam - the Autonomous Exploration for Gathering Increased Science (AEGIS) system. Previously, interesting geologic targets were selected by scientists looking at the images that the rover produced. The AEGIS system, however, allows for automatic target selection and detection of interesting features such as crater ejecta and outcrops. This negates the need for constant communication with Earth.
The Mars Exploration Rovers have successfully completed their scientific objectives, along with plenty of other unexpected achievements. The main goal was to learn about water in Mars’ history by examining rocks and soil, as well as to discover the distribution of minerals in the localities. Another goal was to determine the geological processes that carved the terrain and to seek out clues as to what conditions persisted when liquid water flowed over the now barren landscape. Meridiani was confirmed to have once had water, perhaps in the form of a shallow sea, and Spirit discovered overwhelming evidence for past hydrothermal activity on Mars, some of which could have occurred relatively recently.
In its travels across the Martian landscape, Opportunity happened upon several large iron meteorites. The rover deduced that these meteorites impacted onto a soft and wet Mars millions of years ago, and the buried meteorites were subsequently exposed by erosion after the water dried up.
The Mars Exploration Rovers have shown that the red planet is still somewhat active. Unlike the footprints of the Apollo astronauts left on the Moon, the tracks left by Spirit and Opportunity fade away as the Martian winds cause the tracks to dissipate. Other changes in the landscape observed by the rovers include a change in the reflectivity of the land as dust settles onto rocks or is eroded from them. Subtle ripples have also been seen to move across the sand.
There is still plenty of work left for Opportunity on Mars according to Squyres. “We've only recently arrived at Endeavour Crater, and there's an enormous amount of possible exploration ahead if the rover holds together.”