Alister Cruickshanks (UK)
The discoveries made by Spirit, one of two exploration robots that NASA sent to Mars in 2003, have shown that it is more than likely that the planet once had oceans, lakes and rivers, and that water could still exist at the poles and in deep craters, or in underground reserves. Spirit’s twin, Opportunity, has been exploring the Martian landscape from a different starting point and has also found significant evidence of ancient oceans, since almost immediately after landing. In this article, I will discuss Opportunity’s discoveries and, in particular, what they suggest about the Red Planet’s ancient hydrology.
Welcome to a bizarre landscape
On 25 January 2004, Opportunity returned its first signal to Earth, having arrived on Mars 20 days after Spirit. Immediately, the robot began taking photos of its surroundings. These revealed a surreal, dark and bizarre landscape unlike any ever seen before on Mars. The images were relayed back to Earth via NASA’s Mars Odyssey orbiter.
The area in which Opportunity had landed was in the middle of a small impact crater, 20m across, which NASA named ‘Eagle Crater’. This lies in the Martian region known as ‘Meridiani Planum’ (that is, the Meridiani Plain) and it was hoped that this location would give scientists the opportunity to explore beneath the surface without needing to dig through large accumulations of soil. From information gathered by orbiters and probes during previous studies, the Meridiani Planum region was known to have extensive deposits of the grey mineral, crystalline hematite.
The vertical cross-section of the region, shown in Fig. 1, illustrates why NASA chose this location to explore. The occurrence of grey hematite is seen as important in determining if water was once present on Mars since it is a mineral that, on Earth, typically forms in the presence of liquid water. The hematite-bearing plains form part of an extensive set of deposits located on top of an ancient and heavily cratered terrain.
Opportunity examined soil in Eagle Crater and found that some grains had jagged shapes, while others were strikingly spherical. These can be seen in Fig. 2, which show a magnified view of the soil. Here, coarse grains can be seen sprinkled over a layer of fine sand. The spherical particles could have been formed by a variety of geological processes, including the cooling of molten lava droplets or accretion of concentric layers of material around a nucleus.
The diversity of the shapes and colours of the grains suggests that they came from a variety of sources. Mineralogical analysis of a portion of the crater (Fig. 3) shows that concentrations of coarse-grained hematite vary over its area. The soil patch Opportunity examined was low in the mineral.
Opportunity’s Mössbauer spectrometer, an instrument on the rover’s robotic arm designed to identify the types of iron-bearing minerals in a target, detected a strong signal for olivine in the soil patch (Fig. 4). This shiny green mineral is often associated with finely grained basalt.
To examine the solid geology of the dust-covered crater floor, NASA rotated one of the robot’s wheels, while applying brakes to the other three. In doing so the rover managed to create quite a deep trench (Fig. 4).
It soon became clear that the sand of the crater floor was hiding some important geological features, as shiny, fine, round grains were soon identified (Fig. 6). However, the robot had difficulty examining these using its on-board microscope, as light reflected strongly off their surfaces.
After assessing the best rock to examine, the robot explored the nearby rock outcrop dubbed ‘El Capitan’ on 23 February 2004; sol 29 (Fig. 7).
This was close to its landing site, on the inner slope of Eagle Crater. The outcrop was seen as the perfect location to test the rover’s onboard rock abrasion tool, since an initial examination of the structure showed it to have layers and to possibly be a type of sedimentary rock. El Capitan is located within a larger outcrop nicknamed ‘Opportunity Ledge’, named after a mountain in Texas – although, on Mars, its namesake stands just 10cm high.
After making several small holes, Opportunity used its microscopic imager and two spectrometers to examine the details of the freshly exposed, clean surfaces. The data obtained indicated that the composition of these rocks included types of sulphates that could have only been created by liquid water flowing through them and depositing the chemicals., including magnesium, iron and other sulphate salts. Elements that can form chloride, or even bromide salts, have also been detected.
This was an exciting discovery since similar geohydrological conditions involving the same chemicals can be found at a few locations on Earth, such as Rio Tinto in Spain. Here, micro organisms live and thrive in tiny air spaces in these mineral-rich rocks. The fact that scientists have now found evidence of a similar relationship between water and rock on Mars does not necessarily mean that life exists on the planet or ever did, but it does bring the possibility one step closer to reality.
At the same location, the rover’s Mössbauer spectrometer detected a hydrated iron sulphate mineral called jarosite. The presence of this mineral may suggest that the surrounding rock’s history included time in an acidic lake or an acidic hot springs environment. It also suggests that these rocks have been highly altered by long exposures to water.
The physical appearance of the rocks provides further evidence of the past presence of water. This is demonstrated by three geological features, shown in Fig. 8.
Crossbedding (bedding planes that are inclined and often cross and terminate each other as a result of the action of wind or water) can be seen to be quite prominent within the rock of this outcrop. It is believed in this case, that the bedding was caused by water action, due to the identification of possible concave patterns, which would have been created by sinuous crestlines of underwater ridges.
Vugs can also be identified as voids left behind in the matrix when crystals of salt minerals that formed in rocks while immersed in briny water later disappeared due to erosion, or through dissolution in less-salty water. The third geological feature that can be seen are spherules (Fig. 9).
These are small, round particles, about three millimetres in diameter, which can be seen embedded in the outcrop. From shape alone, the spherules could have been formed by volcanic eruptions, by the lofting of molten droplets produced by a meteor impact, or by the accumulation of minerals precipitating out of solution inside a porous, water-soaked rock. However, the spherules are not concentrated within particular layers of the outcrop, a fact which weighs against, but does not completely rule out, the first two origins.
On Earth, as water precipitated mineral concretions grow in close proximity, their outer edges often intersect each other, creating arrangements that look similar to soap bubbles. Spheres formed by impacts or volcanoes do not tend to mould together like those seen in the figure. Similar structures produced by impacts are usually round or teardrop shaped from flying through the air and freezing before hitting the ground. Any droplets of magma that combine together usually grow into a single mass in a spherical, dumbbell, or teardrop shape.
Further analysis of the outcrop revealed an area full of the spherules, which offered an ideal opportunity to examine the structures in more detail. This rock, within the main outcrop, was called ‘Berry Bowl’ (shown by the arrow in Fig. 10) after the NASA team noted that the collection of spherules within was reminiscent of a bowl of blueberries.
Opportunity’s Mössbauer spectrometer found a large difference between the ferric chemistry of the spherules and a ‘berry-free’ area of the underlying rock. It also revealed that the major iron-bearing mineral within the structures is hematite. Further evidence of a wet past came from comparing the crystalline grain size of the hematite of the spherules with the grain size of the mineral when formed in a wet environment on Earth.
Upper Dells, Last Chance and Slickrock
After assessing the distribution of the spherules, the rover studied a few more rocks within the same outcrop, nicknamed ‘Upper Dells’, ‘Last Chance’ and ‘Slickrock’. Here, it found deposits (Fig. 11), which can be shown to have formed at the bottom of a body of gently flowing saltwater. In the picture, black lines show where fine layers (laminae) are truncated, discordant and at angles to each other (cross-laminations).
The characteristics of these deposits indicated that Opportunity was positioned on what was once the shoreline of a salty sea or a salt flat (sometimes covered by shallow water and sometimes dry). The direction of the ancient flow in this photograph would have been toward or away from the viewer. In addition, the bedding patterns in some of the finely layered rocks indicate that the sand-sized grains of sediment that eventually bonded together to make them were shaped into ripples by water at least 5cm deep, possibly much deeper, and flowing at a speed of 10 to 50cm per second. The blue lines point to boundaries between possible sets of cross-laminae.
Cross-laminae can also be seen in Last Chance (Fig. 12), marked by a series of red arrows.
The rock layers identified by the middle red arrow are suggestive of cross-lamination produced when flowing water shaped sinuous ripples and caused them to migrate in a single direction. The course of the ancient flow would have been either toward or away from the viewer. The lower and upper red arrows point to sets of cross-laminae that are consistent with underwater sediment ripples moving in water that was flowing left to right.
The yellow arrows indicate points that correlate with the rover’s microscope’s examination of the same rock, pictured in Fig. 13. In this magnified view, cross-lamination is expressed by lines that trend downwards from left to right, which are traced with black lines in the overlay. The orientation of these laminations is consistent with dipping planes that would have formed surfaces on the down-current side of migrating ripples. Blue lines indicate boundaries between possible sets of cross-laminae.
The third rock, Slickrock, has a more variable grain size within a parallel-stratified portion of its composition. These varying grain sizes differentiate fine layers, or laminae, with the variability in grain size within each lamina being small compared to the variability of grain sizes between laminae. Some of the layers have mostly smaller grains while others have mostly larger grains. Red arrows and labels point to a representative large grain (0.8mm) and a representative small grain (0.3mm).
Slickrock also contains broader stratigraphic units, as seen in Fig. 14. In the left hand photo, blue lines indicate boundaries between the units. The upper blue line may coincide with a scour surface. The lower and upper units have features suggestive of ripples or early soft sediment deformation. The central unit is dominated by fine, parallel stratification that could have been produced by wind-blown ripples. Features labelled with red letters are shown in an enlargement of a portion of the image.
‘A’ is a scour surface characterised by truncation of the underlying laminae. ‘B’ is a possible soft sediment buckling characterised by a teepee-shaped structure. ‘C’ shows a possible ripple beneath the arrow and a possible ripple cross-lamination to the left of the arrow, along the surface that the arrow tip touches. ‘D’ is a scour surface or ripple trough lamination. These features are consistent with sedimentation on a moist surface where processes driven by wind may also have occurred.
A NASA team of geologists also identified festooning (layers that have smile-shaped curves produced by shifting of a loose sediment’s rippled surface under a current of water). On Earth, environments found either at the edges of oceans or in desert basins can have currents of water that produce the same type of ripples.
Findings of chlorine and bromine in the rocks also support this type of environment. However, the data collected did not indicate how long ago liquid water covered the area. To gather this evidence, the rover would need to continue to its main mission objective, climbing out of the small crater and driving 750m further, towards a thicker exposure of bedrock layers that form the walls of the nearby large crater, ‘Endurance’.
Investigating soil patches
Before leaving Eagle Crater, Opportunity inspected the soil at five sites near the El Capitan outcrop. The target patches showed a variety of particle sizes and shapes on the surface, indicating the effects of differing wind speeds. Spherules were more plentiful in soil patches that were higher on the inner slope of the crater than those nearer the centre of the crater.
A reading by the rover’s Mössbauer spectrometer from one of the higher patches (named ‘Punaluu’) found the highest concentration of hematite encountered to date by either rover. This can be seen in the spectra comparing hematite concentration, together with a photomicrograph of the patch, in Fig. 15.
Finally, at the end of March 2004, the robot climbed out of its small impact crater and headed towards the larger crater, Endurance. By the completion of its first mission set by NASA, Opportunity had driven 811m and sent home 15.2 gigabytes of data, including 12,429 images.
Preparing to enter Endurance
On the way to Endurance, the robot passed a small crater dubbed ‘Fram’ (Fig. 17).
Here, it used its rock abrasion tool to grind a hole in a rock named ‘Pilbara’ to allow examination of its interior. This was found to have small, iron rich spherules similar to those found near the rover’s landing site, that provided evidence for a body of salty water covering the area in the past.
On 6 May 2004, scientists and engineers celebrated when they saw the first pictures sent back from the rim of the stadium-sized crater, Endurance (Fig. 18), that the rover had reached after a six-week trek.
These photos showed multiple layers of exposed bedrock, in tall cliffs of five to ten metres high. Endurance has a slope angle of around 20o. In the past, when NASA sent astronauts to the lunar surface more than 30 years ago, it was decided not to allow them to enter craters as fresh and steep as Endurance, but Opportunity would attempt to do what no human has done on another celestial body.
The maximum slope angle that the Mars rovers can descend is 25o (Fig. 19), provided their wheels are on a rock surface and not loose sand. However, portions of the route to an outcrop of interest could turn out to be between 25o and 30o, potentially causing a premature end to Opportunity’s mission. The team decided that the scientific value of entering this crater at the risk of losing the robot was far more important that aborting the objective altogether.
Rocks from the same layer could also be seen scattered around the rim of the crater, ejecta from the impact of the meteorite or comet that created it. One of these rocks, named ‘Lion Stone’ (Fig. 20), is 30cm long, again with fine layering, a sulphur-rich composition and spherical concretions, and likely formed under wet conditions.
It varies slightly to those rocks found in Eagle Crater, as it has a slightly different mineralogy and colour. Another rock, called ‘Pyrrho’ (Fig. 21), has a braided ripple pattern, again suggestive of past aqueous processes on Mars.
Opportunity drove around the rim counter-clockwise (fig. 21), stopping at locations nicknamed ‘Karatepe’ and ‘Burns Cliffs’ to assess possible routes of entry into the crater.
Potential access points were named ‘Kalahari’ and ‘Namib’. While it was circumnavigating the rim, and by using the rover’s onboard miniature thermal emission spectrometer, it was possible to determine the presence and concentrations of hematite and basalt within the crater.
The colour-coded interpretation of this data has been overlain onto a photograph of the crater taken by Opportunity’s panoramic camera (Fig. 23).
The colour green, as seen on some slopes, indicates material rich in hematite. Blues and purples, seen for example on some cliffs of exposed rock, indicate the presence of basalt that may have been broken down into sand by weathering and then re-deposited by wind or water. This mode of deposition is likely because the thin layers do not appear to have a volcanic origin. Reds and yellows indicate areas covered by Martian dust.
The descent into Endurance
On 5 July 2004 (sol 159), Opportunity found a suitable access point at Karatepe (Fig. 24).
The robot then started descending into the crater, examining rocks along its route. Seven holes were initially drilled during the descent (Figs. 25 and 26): the first into a flat-lying stone, about 36cm by 15cm across, named ‘Tennessee’.
An initial examination showed that it contained the same rich materials as those found in Eagle Crater, but further holes named ‘Ontario’, ‘Manitoba’ and ‘Millstone’, differed significantly.
In fact, there were at least three lower, older layers that became more consolidated as the robot descended the slope. Furthermore, the lowest part of the slope showed strange sharp, teeth-like features in layers that NASA nicknamed as a rock-type called ‘Razorback’ (Fig. 25). This generated major scientific interest because, based on their understanding of processes on Earth, scientists believe these features may have formed when fluids migrated through fractures, depositing minerals. Minerals that filled fractures would have formed veins that eroded more slowly than the surrounding rock slabs.
More holes were drilled further down the slope (Figs. 26 and 27).
These revealed that chlorine concentration increased by up to three times in the middle layers, referred to as ‘London’ and ‘Virginia’. Magnesium and sulphur declined at similar rates to each other in older layers, suggesting those two elements may have been dissolved and removed by water. In addition to this, chlorine levels rose dramatically in the deeper rocks lining the walls of the crater.
Areas tested for this element are labelled as layers ‘A’ to ‘F’ in the same figure, where ‘A’ contains the least chlorine and ‘F’ the most. At the lowest accessible outcrop, ‘Axel Heiberg’, the rocks have a rougher texture compared to those above, vary more in size and have a reddish tan, rather than the grey of the upper rocks.
At the base of the crater, dunes of accumulated dust could be seen (Fig. 29).
This dust obscured potentially interesting older, underlying bedrock. A blue tint was evident on the dunes’ flat surfaces that resulted from the accumulation spherules containing hematite. Sinuous tendrils of sand, less than one metre high, extended from the main dune field toward the rover.
However, for NASA to examine the dunes, Opportunity would have to descend all the way to the bottom of the crater, something that the robot would ultimately achieve and which will be discussed in the next part of this article. I will also discuss whether Opportunity became trapped inside the crater, or if NASA navigated a route out. One thing is for sure, the robot could not come out the same way it went in, since the slopes were too steep.