The Geology of Mars: Discoveries by Spirit and Opportunity – Part 2

Alister Cruickshanks (UK)

In this second part of my article, I continue to follow the journey of Spirit, one of NASA’s twin exploration robots, in its exploration of the geological features of Mars up to its present position. The opportunity for the robot to explore has been curtailed during the second part of its journey as a raging global dust storm and harsh winter conditions have forced NASA to limit power usage by putting both Spirit and Opportunity into hibernation. In addition, most of the detailed chemical and mineral data for rocks found from ‘Bright Soil’ (see below) has yet to be analysed and released by NASA. I will, therefore, confine myself to examining the basic geological findings by Spirit during this part of the robot’s journey.

Bright Soil

I discussed Spirit’s findings in the last article, from its landing point up to ‘Home Plate’, and I will now continue from Home Plate to the robot’s current resting place – Bright Soil near McCool Hill – its winter hideaway. While driving eastward toward the north-western flank of McCool Hill on sol 787, Spirit’s wheels churned up the largest amount discovered so far of what is now called ‘bright soil’. This was a pleasant surprise for NASA’s scientists as it enabled them to study this material in more detail. They named the location ‘Bright Soil’ after the discovery.

Just a few days earlier, Spirit’s wheels had unearthed a small patch of light-toned material informally named ‘Tyrone’. The robot’s panoramic camera showed that this material strongly resembled previously discovered light-toned soils called ‘Arad’ and ‘Paso Robles’, discovered while the rover was climbing Cumberland Ridge in 2005. The same material was also found in early January 2006 on the basic floor just south of Husband Hill. At the time, Spirit’s instruments confirmed that the soil had a salty chemistry dominated by iron-bearing sulphates.

These discoveries indicate that light-toned soil deposits might be widely distributed on the flanks and valley floors of the Columbia Hills region in the Gusev Crater. NASA currently believes that the salts may evidence the past presence of water, as they are easily mobilised and concentrated in liquid solution as the water in which they were dissolved evaporated.

Fig. 2. A patch of bright-toned soil so rich in silica that scientists propose water must have been involved in concentrating it. The silica-rich patch is informally named ‘Gertrude Weise’. One of Spirit’s six wheels no longer rotates, so it leaves a deep track as it drags through soil. Most patches of bright soil that Spirit had investigated previously are rich in sulphur, but this one has very little of this element and is about 90 percent silica. ©Courtesy NASA/JPL-
Caltech.

Winter Haven at Low Ridge

Fig. 3. Winter panorama. View from Spirit’s winter resting place showing nearby rocks, landscape features and positions of previously visited locations.

During the Martian winter of 2006, the robots where put into a state of near-hibernation to ensure onboard instruments were kept warm and to preserve energy. Spirit was parked on a north-facing slope in the Columbia Hills, known as ‘Winter Haven’ near to a slope called ‘Low Ridge’. Its solar panels were tilted to receive maximum light. In fact, the area was carefully chosen to optimise solar power and maximise the vehicle’s ability to communicate with the NASA Odyssey obiter. NASA took this opportunity to study images and data from Spirit’s onboard instruments, while the robot slowly studied the soil surrounding its resting place.

During the time that Spirit has been exploring on Mars, the grinding teeth on the rock abrasion tool have slowly worn away, slowing down the progress of sample taking. Spirit has used the tool five times more than it was designed for but it still had useful wire bristles for brushing targets. Layer by layer, the rover brushed away sediments using these brushes. At each level, it investigated the mineral and chemical properties and examined the physical nature of the material (such as grain size, texture and hardness), using the Athena scientific instruments on its robotic arm. In particular, Spirit wanted to look at vertical variations in soil characteristics that may indicate water-related deposition of sulphates and other minerals.

While studying the soils, Spirit took high-resolution photographs of the rocks nearby (Fig. 4).

Fig. 4. At least three different kinds of rocks can be seen in this image.

One of these images contains two suspected iron meteorites. The rock in the foreground of the photo was informally named ‘Allan Hills’ after a site where meteorites are frequently collected in the Antarctic. The most famous meteorite found here was named ‘ALH84001’. This achieved notoriety in 1996 when NASA scientists suggested that it might contain evidence of fossilised extraterrestrial life.

The second possible meteorite in Fig. 4 was named ‘Zhong Shan’ after an Antarctic base established by China in 1989. This rock is just out of view to the left [in the picture shown]. Both rocks have a smoother texture and lighter tone than other rocks in the area. Observations using Spirit’s miniature thermal emission spectrometer indicate that they are very reflective.

These are the first probable meteorites found by Spirit so far on this mission. In addition to the meteorites, paper-thin layers of a light-toned, jagged-edged rock can be seen protruding horizontally from beneath small sand drifts. On top of these sand drifts, a light grey rock with smooth, rounded edges can be identified. Several dark grey to black, angular rocks with vesicles (small holes), typical of hardened lava, also lie scattered across the sand.

Spirit also photographed a dark boulder with an interesting surface texture using its 360^o panoramic camera. This boulder sits about 40cm tall on a Martian sand dune about 5m away from the robot. It is one of many local, dark, volcanic rock fragments that litter the slope of ‘Low Ridge’. Most are pockmarked with vesicles. The rock surface facing the rover is similar in appearance to the surface texture of lava flows on Earth.

Fig. 5. Dark boulder, just five meters away from Spirit.

There are several continuous rock layers making up the ridge (‘Low Ridge’), in front of ‘Winter Haven’. Some of these layers form fins, which protrude from the other rocks, suggesting that they are resistant to erosion. Spirit found a small fragment of light-toned material that was broken away from these layers by its wheels when they drove over it. The rock has been named ‘Halley’. The light-toned soils in the bottom centre and the top centre of the image correspond to small, bright, bluish-white deposits, just to the right of the rover’s tracks.

First analyses of Halley showed it to be unusual in composition, containing a lot of the minor element zinc, compared to the soil around it and having much of its iron tied up as the mineral hematite. When scientists placed the scientific instruments on Spirit’s robotic arm on a particularly bright-looking part of Halley, they found that the chemical composition of the bright spots was suggestive of a calcium sulphate mineral. This was interesting as the bright soils that Spirit examined earlier in the mission contain iron sulphate.

Fig. 6. ‘Halley’. Spirit studied a fragment of this rock that broke away when its wheels ran over it.

On sol 1,031, Spirit examined a rock target called ‘King George Island’, after soil was brushed away using the rock abrasion tool and the rock below was exposed. The robot took an image that showed that the exposure had a granular nature, the grains being typically about one millimetre wide. Data from the rover’s Mössbauer spectrometer provide evidence that this rock has an enhanced amount of the mineral hematite compared to surrounding soils.

Fig. 7. King George Island. The grains are approximately one millimetre wide.

Spirit examined its final rock before entering its semi-hibernation state at Winter Haven on sol 1,177. It used the rock abrasion tool to brush away dust covering the surface of a small, nodule-rich outcrop nicknamed ‘Slide’. The NASA science team has a special interest in the rocks in this general area, some of which show the highest silica content yet measured on Mars.

Homeplate and the storm

After the long, cold Martian winter months, Spirit was finally able to explore again, with its batteries fully charged.

Fig. 8. Having endured two Martian winters (May-September, 2004 and April-November, 2006), Spirit’s handlers have already started scouting out potential locations within driving distance where the rover might survive another southern-hemisphere Martian winter (March-October, 2008). Getting to one of them will take some careful navigation since many of the slopes leading down from the top of Home Plate are too steep for the rover to cross safely with its dragging right front wheel. ©Courtesy NASA/JPL-Caltech.

It was now back to ‘Homeplate’ to conduct some unfinished research. On the way back, a surprising discovery was made. A shallow trench made by the rover’s dragging right front wheel uncovered some of the best evidence Spirit has found to date for ancient water-rich environments in Gusev Crater. Bright patches of almost pure, fine-grained silica (SiO₂) were unearthed. On ancient Earth, warm, evaporating coastal waters deposited fine silica in shallow sediments. Today, in Yellowstone National Park, hot, mineral-laden waters currently deposit similar silica around geysers and hot springs. Therefore, the discovery of silica-rich deposits on Mars adds compelling new evidence of ancient environments that might have been favourable for life.

Fig. 9. Spirit finds compelling new evidence of ancient environments favourable for life.

Shortly after making this spectacular discovery, a violent dust storm blocked vital light from reaching Spirit’s solar panels. I briefly mentioned this in the first part of this article as, at that time, the fate of the robots was not known. The dust storm raged for weeks and NASA’s engineers had to put the robots into a state of hibernation, using only the most basic power to keep them warm to survive.

For the twins, their lives had already been extended well beyond their original goals and expectations, and had already surprised NASA’s team who had fulfilled many more research missions than first planned. Each time it looked like they were nearing the end of their battery life, a sudden gust of wind swept away dust from their solar panels and gave them a sudden boost of energy. It was thought that it was unlikely Spirit and Opportunity would survive this time but, on 7 August 2007, the dusty Martian skies started to clear again.

After waiting out a long cold winter, and following this global dust storm, Spirit finally now had enough power to explore Home Plate in the way scientists had been longing to do since they first saw its layered terrain. NASA saw this as an important goal since there are three tests to determine if water was present in the past. The first is to look for salts left behind from salty water that has evaporated. The second is to look for rocks that show evidence of having been originally laid down in wet conditions and the third is to look for signs of volcanic explosions that would have occurred as hot, molten rock came into contact with water. At ‘Home Plate’, scientists have found all three, making it almost inevitable that water was once present.

At the time of writing, the robots were entering their third Martian year and discoveries are still being made. NASA knows that, at any time, the robots could finally give up their heroic struggle and their mission would be over. However, having survived a bitterly cold winter followed by a violent dust storm, they still keep going.

Gusev Crater

While the robots have been braving out the winter and the storms, NASA has concentrated its efforts on identifying rocks previously analysed by Spirit and Opportunity and comparing them to rocks on Earth. Their main focus has been on those found at the Gusev Crater. Here, Spirit documented the first example of a particular kind of volcanic region on Mars known as an ‘alkaline igneous province’. Until now, all Martian rocks including Martian meteorites found on Earth and the rocks analysed by the Mars Pathfinder rover in 1997, have been subalkaline. This means that they contain high levels of silica.

When scientists used Spirit to examine rocks in the Gusev Crater area, all of these relatively unaltered rocks (those least changed by wind, water, freezing or other weathering agents) were found to be igneous. These include the ‘Adirondack Class’ that litters the Gusev plains and the Backstay, Irvine and Wishstone class rocks that occur as loose blocks on the north-west slope of Husband Hill as well as outcrops of Algonquin Class rocks protruding in several places on the south-east face. These rocks have been found to contain low levels of silica, which is surprising since silica is the most abundant rock-forming compound in the Earth’s crust.

One way that geologists classify groups of igneous rocks is by looking at the amount of potassium and sodium contained in them relative to the amount of silica. In this way, scientists can determine the type of volcanism that gave rise to them and gain clues about their history. Rocks with a higher silica content tend to erupt explosively, while rocks with higher levels of potassium and sodium indicate partial melting of magma at greater pressure. Rocks at the Gusev Crater contain high levels of potassium and sodium, which means that these rocks originated from deep within the Martian mantle. They also contain minerals formed when such rocks contain enough potassium and sodium to bond with the silica.

Geologist, Harry McSween from the University of Tennessee, plotted these rocks on a diagram to compare alkalis versus silica, enabling NASA to determine that the Gusev rocks define a new chemical category not previously seen anywhere else on Mars.

Fig. 10. Graph comparing alkalis versus silica. ©NASA/JPL-Caltech/University of Tennessee.

In the graph in Fig. 10, the abbreviations ‘Na₂O’ and ‘K₂O’ refer to oxides of sodium and potassium. The abbreviation ‘SiO₂’ refers to silica. The abbreviation ‘wt. %’ indicates what percentage of the total weight of each rock is silica (on the horizontal scale) and what percentage is an oxide of sodium and potassium (on the vertical scale). The thin lines separate volcanic rock types identified on Earth by different scientific names such as ‘foidite’ and ‘picrobasalt’. Various classes of Gusev rocks [(see box in upper right)] all appear either on, or to the left of the green lines, clearly showing that they fall within the ‘alkaline’ and ‘subalkaline’ categories.

Finally, the landing site in the Meridiani Planum region provided an opportunity to study hematite-bearing rocks, in a heavily cratered terrain. On Earth, grey hematite is an iron oxide mineral that typically forms in the presence of water. The rover set out to determine whether water was present in the past when rocks were being formed and these hematite-bearing rocks provide further evidence that there was.

Fig. 11. The landing site for the second Mars Exploration Rover mission was Meridiani Planum, seen here in its geological context, from NASA Viking images. ©Courtesy NASA/JPL-Caltech.

In the third part of this article, I will continue follow Opportunity’s journey.

Fig. 12. NASA engineers building Spirit. ©Courtesy NASA/JPL-Caltech.