Happy landing anniversary to Opportunity and to all who have made this amazing achievement possible. In celebration, here’s a snippet from my book Astrobiology: a brief introduction, describing Opportunity’s first (of many) day at work:
In early 2004, abiding by its Mars creed “follow the water,” NASA landed solar-powered rovers at two seemingly water-modified landscapes in Mars’s northern hemisphere. The first, called Spirit, touched down (bounced to a stop, actually, on airbags) on January 4 in the Guseve Crater. Located at the end of the 900-kilometer-long, 10-kilometer-wide Ma’adim Vallis (valley), Guseve appeared from orbit to have once been a lake. After extensive exploration, however, Spirit found little but the most indirect evidence that the crater was ever filled with water. Some three weeks later, Spirit’s sister craft, Opportunity, bounced to a stop on Meridiani Planum, a smooth, flat plain the size of the state of Colorado that had been selected because spectroscopic investigations from orbit indicated it was decorated with the mineral hematite. Hematite, an iron mineral, can be formed by several mechanisms, but on Earth it is most commonly deposited in aqueous environments. Cushioned by its airbags, Opportunity bounced a dozen times on the Martian surface before finally coming to rest in a small crater. When the rover shook off its landing and turned on its cameras, it found itself staring at thick, layered beds. The Opportunity rover, the fifth successful Mars lander, was the first to find bedrock.
On Earth, the majority of layered rock is sedimentary, laid down over successive seasons or successive floods by liquid water. But successive volcanic eruptions (either as lava flows or as successive layers of ash) can also form layered rock, as can shifting patterns of wind-blown dust. So which was it at Meridiani? The bedrock in Opportunity’s crater showed clear evidence of cross-bedding, layers formed at angles, as often formed in stream beds on Earth due to turbulent flow. Some of the rock surfaces also showed clear signs of polygonal cracks, reminiscent of the hexagons that sometimes form in drying mud. The layers were also filled with the type of small, round rocks that littered the ground at Meridiani. Termed “blueberries” after the fruit they seemed to resemble in shape and size, if not color (somehow “grayberries” just sounds wrong), the small spheres littering the crater are composed of hematite--again, most likely formed in situ in the rocks by the action of water.
Perhaps even more revealing than the gross structures of the strata are their chemical compositions. The rover’s α-particle and x-ray spectrometer (APXS), which can identify the elemental composition of rocks, indicated that the rocks at Meridiani contain large amounts of magnesium, calcium, and iron sulfates, along with traces of chlorine and bromine. On Earth such salts form preferentially by aqueous deposition, and on Mars are a near certain sign that liquid water was once present. Similarly, Opportunity’s Mossbauer spectrometer found evidence that some of the iron at Meridiani is tied up in the mineral jarosite, a hydrated form of potassium iron sulfate that, on Earth, is invariably formed by the aqueous leaching of iron minerals under acidic conditions. So it looks as if we have very firm evidence that at least parts of Mars were wet sometime in the distant past (albeit with rather acidic brine), but that Mars then took a very different environmental turn.
Opportunity looking back at its landing site (NASA).