The Geology of Mars: Discoveries by Spirit and Opportunity – Part 1

Alister Cruickshanks (UK)

Fig. 1. View of Mars from the Hubble Space Telescope. Image courtesy of © NASA/JPL-Caltech.

It is perhaps one of the most exciting explorations in recent years – NASA’s Mars Exploration Rovers have changed our views of the red planet and re-written textbooks. In the past, researching and mapping the geology of Mars has seemed something that geologists could only dream of doing.

However, while geologists around the world have been busy studying rocks here on Earth, two robots have been busy at work, carrying out their own studies on Mars. Their findings have confirmed a previous theory that Mars had an active geological past and have provided evidence of water – a fundamental building block of life.

In the first and second parts of this article, I will examine the geological findings by one of the robots, Spirit. In the third and fourth parts, I will examine findings by Spirit’s twin, Opportunity.

Facts about Mars

Before looking at their recent geological discoveries, it is worth looking at the basic facts about Mars and the two robots that keep changing our understanding of the planet. Mars is the fourth planet from our Sun and is, on average, 78.3 million km from Earth and 227.9 million km from the sun. This might seem like an extraordinarily large distance but, as space goes, it is actually very close.

It has an egg-shaped orbit and, at its nearest to the sun, is 206.6 million km away and, at its furthest, is 249.2 million km away. This means that its seasons are more variable than our own. It travels round the sun every 687 days, compared with 365 for our Earth and has a day (a ‘sol’) 39 minutes longer than Earth’s. Mars is also roughly half the size of Earth, having a diameter of 6,794km and its the average surface temperature is a bitterly cold -63oC.

Because the gravity of Mars is just one-third as strong as that on Earth, most of the Martian atmosphere has gradually disappeared into space. The atmosphere on Mars is mostly carbon dioxide: 95.32% compared to Earth’s 0.035%. Our atmosphere is also made up of 20.95% oxygen, but Mars’ atmosphere has only 0.13% making it impossible for the air to be breathable by humans. The deadly gas, carbon monoxide, also makes up 0.08% of the Martian atmosphere but, here on earth, it is only present as a trace.

Most people are surprised that there are clouds on Mars. However, they mostly form on the great volcanic peaks in summertime when the warmer air flows upwards and cools. Clouds also form over the polar caps at high altitudes. These clouds consist of both water ice (found at an altitude of 12 to 18 miles) and carbon dioxide (found an altitude of 30 miles). However, since Mars is very dry and cold, it never rains. During the winter months, frost and possibly snow accumulates at the polar regions.

Mars once had a milder climate and scientists believe it had rivers flowing into lakes and seas when Mars’ axis was tilted more towards the sun. In 2004, evidence of this was found by Opportunity with the discovery of sedimentary rocks that had been laid down in the presence of liquid water. Soil and debris would have also been found at any river mouth or flood plain but would have since been wind swept away by frequent and intense dust storms across the planet. The atmosphere would have probably been very different to that of today and, perhaps in the future, this will be confirmed through ice-core research, drilled at the polar regions.

Fig. 2. Topography of Mars by the Mars Orbiter Laser Altimeter. ©Courtesy NASA/JPL-Caltech.

The differences between Earth and Mars are quite apparent but so are those of Mars’ past compared to its present, which makes the planet such an interesting object of study. With different elements, a different structure and a different past, it gives a chance to examine geological processes that differ to those here on Earth, and perhaps our understanding of Mars may enable us to think more carefully about our own Earth, and to protect it for the future generations.

Here on earth, it has been shown that bacteria and algae can live in extremely harsh conditions: inside volcanoes, in the icy depths of Antarctic and in the murky-deep dark waters below our oceans. Life has also been found to exist without the need for oxygen. So, if life can be supported in these conditions and, if Mars had a very different past with rivers and soil, it is very feasible that life could once have existed there or even still exist today. Spirit and Opportunity are still on the case.

Spirit and Opportunity

Fig. 3. NASA engineers building Spirit. ©Courtesy NASA/JPL-Caltech.

NASA’s twin robot geologists – the Mars Exploration Rovers – left Earth on an incredible one-way journey to the red planet on 10 June 2003 and touched down on Mars on 3 and 24 January 2004. Their aim is to search for, and characterise, a wide range of rocks and soils that hold clues to past water activity and geological features on Mars. The landing sites of the Gusev Crater and Meridiani Planum were carefully chosen because scientists believed these provided the best chance of finding evidence of water. The Gusev Crater is believed to have been a former great lake in a giant impact crater and the Meridiani Planum is believed to be rich in minerals.

Fig. 4. Stowed in the nose cone of this Delta II rocket, the Mars Exploration Rover, blasts off from the Kennedy Space Center in Florida. Its destination: the planet Mars. ©Courtesy NASA/JPL-Caltech.

Spirit and Opportunity were armed with highly sophisticated tools including:

A rock abrasion tool: this is effectively a geological hammer for removing dusty and weathered rock surfaces and exposing fresh material for examination by onboard instruments. A special magnet was added to each rover for collecting magnetic dust particles.

Fig. 5. Rock Abrasion Tool in use. ©Courtesy NASA/JPL-Caltech

Spirit’s rock abrasion tool in use.

Three spectrometers: a Mössbauer spectrometer for close-up investigations of the mineralogy of iron-bearing rocks and soils; a miniature thermal emission spectrometer for identifying promising rocks and soils for closer examination and the processes that formed Martian Rocks; and an alpha particle X-ray spectrometer for analysis of the abundances of elements that make up rocks and soils.

A powerful microscope: each rover was also fitted with a small but powerful microscope. This, of course, is the geologist’s field lens, but is much more powerful, enabling high-resolution images to be beamed back to Earth.

The original goal set by NASA was for each robot to drive up to 40m in a single day for a total lifetime journey of up to 1km, but both of these goals have already been far exceeded. Every time it looks like the battery life of Spirit and Opportunity is starting to decline, they receive a sudden power boost, believed to be caused by winds blowing dust off their solar panels, resulting in the recharging of their batteries. However, the presence of huge dust storms during July and August 2007 blocked sunlight from the robot’s solar panels and NASA has become worried that they will lose the ability to keep themselves warm. This could result in damage to the electrical components by freezing. However, while waiting for the dust to clear, Spirit witnessed ripples forming in the sand beneath its robotic arm.

Gusev Crater

Spirit’s landing site was on the plains of the Gusev Crater area. This crater is a 90-mile wide hole in the ground that probably formed three to four billion years ago. It is believed that an asteroid crashed here, just south of Mars’ equator. A channel system drains into it, which scientists believe probably carried liquid water, or water and ice, into the crater. Within the crater area, Spirit has identified and studied several different rock types. Broadly, the rocks making up the Gusev Plains are weakly altered olivine basalts. At the Columbia Hills, rocks are more extensively altered and cover a much wider range of rock types and geochemical classes. These are discussed below.

Gusev Plains

The Gusev Plains is an extensive area derived from the breakdown and weathering of basaltic flows. They originate from the volcano, Apollinaris Patera, 600km to the north of the crater. Spirit traversed the sandy Martian terrain at the Gusev Crater to arrive in front of a football-sized rock on Sunday, 18 January 2004, just three days after it successfully rolled off the lander. The rock was selected as Spirit’s first target because its dust-free, flat surface is ideally suited for grinding. Clean surfaces also are better for examining a rock’s top coating. Scientists named the angular rock after the Adirondack mountain range in New York (the Adirondack Class). This class is a typical basalt rock found in this area. They are also the youngest rocks found so far at the Gusev Crater.

Fig. 6. Spirit’s first target rock, Adirondack. ©Courtesy NASA/JPL-Caltech.

The nearby ‘Humphrey Rock’ was discovered after Spirit ground into the dusty surface with the rock abrasion tool located on its robotic arm. After the discovery of further occurrences of this rock, it was determined that this is the most common rock found at the Gusev Plains. It is a very primitive olivine basaltic flow with weak alteration along fractures. This rock contains 26% to 32% olivine and 17% to 24% pyroxene. Spirit used a mosaic of four individual frames to compile one of the largest microscopic photographs from space ever taken. The unprecedented detail enabled scientists to determine that the rock has dark phenocrysts in a fine-grained, grey matrix containing 38% to 45% feldspar and 4.5% to 4.75% iron oxide, in the form of magnetite.

Fig. 7. A mosaic of four individual frames taken by the microscopic imager that have been very carefully stitched together to reveal the entire 5cm diameter hole left on the rock dubbed ‘Humphrey.’ The holes were created by the Mars Exploration Rover Spirit’s rock abrasion tool. ©Courtesy NASA/JPL-Caltech.

Near to the landing site, a smaller crater, known as the ‘Bonneville Crater’, contained another rock type of the Adirondack Class. ‘Mazatzal’ is similar to Humphrey but has been previously buried by soil and exposed to low-level, subsurface aqueous alteration to the rock surface and also within fractures. These rocks are lighter and contain high levels of chlorine, sulphur, bromine and oxidized iron. They also contain 10% hematite and traces of nickel and zinc on the surface.

Fig. 8. View of the Bonneville Crater (shown to the right of the image). Image courtesy of ©NASA/JPL-Caltech.

West Spar

Sprit had to travel over 2km eastward and upward onto the Columbia Hills before the rocks started to change. It took the rover 200 sols to reach its destination. At the foot of the hills, at a location known as ‘West Spar’, rocks were older and more altered but also softer and easier to drill due to extensive weathering.

The class of rock found here has not been officially named, although they have been referred to as ‘Pot of Gold’. NASA has unofficially and less flamboyantly called them ‘Rotten Rocks’! These types of rocks are volcaniclastic and have been altered by sulphate rich liquids. Wind erosion has removed the softer, non-cemented interior portions of rock. They contain erosion resistant nodular protrusions along softer layers. Under Spirit’s microscope, the rocks are made up of fine layers and protrusions comprised of medium and coarse subangular to rounded clasts. They are rich in the mineral olivine and have traces of pyroxene and hematite, and are composed of high amounts of titanium and phosphorus.

The nearby north-western flank of West Spur also has an unnamed class type and rock, unofficially known as ‘Wooly Patch Class’. This is a very soft rock that is flat lying and possibly layered. Two small drill holes enabled Spirit to determine that this rock may have been modified by water. Small cracks in the surface outside the drill holes may be the result of interactions with water-rich fluids.

Fig. 9. Wooly Patch Class with holes created by the Mars Exploration Rover Spirit’s rock abrasion tool. ©Courtesy NASA/JPL-Caltech.

Further up the Columbia Hills, Spirit examined rocks at the upper level of West Spar. Here, a meteorite impact or multiple meteorite impacts into the basaltic source rocks have created ejecta deposits that have subsequently been strongly altered by aqueous process. This rock, called the ‘Clovis Class’ rock, required close attention, and so Spirit used its rock abrasion tool to drill a hole 9mm deep in order to extract small samples.

These samples were then analysed by the rover’s Mössbauer spectrometer and alpha particle X-ray spectrometer. The conclusion was that this rock is massively to finely layered and very soft. The rock is full of the minerals hematite, pyroxene, goethite, magnetite, calcium sulphate, calcium phosphate and secondary aluminosilicates. Its geochemistry is highly rich in magnesium, sulphur, chlorine, bromine, zinc and traces of calcium, phosphorus, nickel, titanium and potassium. When Spirit examined this rock under a microscope, its appearance had poorly sorted clasts in fine-grained matrix.

Fig. 10. The rock abrasion tool cut a 9mm hole into a rock called ‘Clovis’ during the rover’s 216th Martian day. Scientists used Spirit’s Mössbauer spectrometer and alpha particle X-ray spectrometer to look for iron-bearing minerals and to determine the chemical composition of the rock. ©Courtesy NASA/JPL-Caltech.

Husband Hill

Climbing further, Spirit reached the area of known as ‘Husband Hill’, 331 sols after landing. At the north-western flank, an outcrop of pyroclasic rock was discovered. This rock, known as the ‘Wishstone Class’, is very knobbly and bumpy, and also has a rough surface. Wishstone Class is believed to have been moderately altered. Minerals found in this rock have high amounts of plagioclase, 5% calcium phosphate, limonite, magnesium-sulphate and some aluminosilicates, and are low in olivine and pyroxene. It is also rich in titanium and phosphorous and has traces of magnesium and iron. Under the microscope, it appears to contain light and dark angular clasts in a fine-grained matrix with a possible alteration rim around some clasts.

Fig. 11. Wishstone. ©Courtesy NASA/JPL-Caltech.

At the north-western flank of Husband Hill, Spirit found evidence of altered sandstone consisting of ultramafic grains deposited by wind and then subsequently exposed to water. These rocks have been classified as part of the ‘Peace Class’. They contain the minerals olivine, pyroxene, feldspar, apatite, halides and secondary aluminosilicates. They also contain a high concentration of magnetite. Their appearance under the microscope consists of black grains cemented in a light coloured matrix. These rocks contain high amounts of sulphur and magnesium. Further research has concluded that the ultramafic grains are unaltered, suggesting that any exposure to water was brief.

The northern slope of Husband Hill contains a ridge known as ‘Cumberland Ridge’. Here, rocks classified as the ‘Watchtower Class’ are variably massive and have a rough, bumpy surface texture. Spirit has also noted some evidence of layering. It is believed that these rocks are an impact ejecta deposit created by multiple impact events into Wishstone Class material at the base of Husband Hill. Therefore, the rock is very similar to the Wishstone Class, except that it has a much higher concentration of magnesium, tin and potassium, and the iron has also been highly oxidised. Further research has determined that the rocks were subsequently subject to hydrothermal alteration with a low water-to-rock ratio likely for a brief period of time.

On Husband Hill, there is a rock outcrop known as ‘Jibsheet’. The slope of Cumberland Ridge continues up to the south-east side of this outcrop. Here, 479 sols after landing, Spirit found massive unweathered, dark grey-black basalt rocks with minor pits and bumps visible on the surface. They represent unaltered, localised, intrusive igneous rocks that were laid down from an uplift of the rocks at Husband Hill. They have been classified as the ‘Backstay Class’. The rocks are actually a form of alkaline trachybasalt and contain the minerals orthopyroxene, magnetite and small amounts of olivine. They also contain high amounts of sodium and potassium and low traces of iron.

At the upper western slope of Husband Hill, a class of rock called ‘Independence’ can be recognised by its light toned, rough surface that is weakly to moderately layered. This rock has had significant aqueous alteration and, possibly, the source of the rock is basaltic ash. The alteration has caused a low iron and high aluminium/silicon content. The chemical composition is consistent with phyllosilicate (clay) and most closely corresponds to a class of rocks referred to as the ‘Montmorillonite Group’. It is believed to be roughly the same age as Wooly Patch Class found at the north-western flank of West Spur.

In addition to Independence Class rocks, a brecca rock called ‘Descartes Class’ can also be found in the western slope of Husband Hill. This rock is a layered outcrop with centimetre-sized rounded to angular pebbles in fine-grained matrix. It is believed that this rock has been altered by a meteorite impact consisting of Wishstone clasts in fine-grained matrix of Clovis material. It is high in magnesium oxide and sulphur oxide and is extremely high in chromium. Its microscopic appearance is fine to medium grained groundmass containing lithic fragments.

The oldest rock found by Spirit, at the time of writing this article, has also been found on the western slope, not far from the ‘Descartes Class’. This layered rock, called ‘Seminole Class’, can be found in terraces up to 28m high. This relatively unaltered mafic to ultramafic magmatic rocks contain up to 50% olivine. Minerals consisting of plagioclase and orthoclase decrease while calcium pyroxene increase further down the slope.

On route to Home Plate, NASA’s Mars Exploration Rover Spirit used its microscopic imager to capture a spectacular, jagged mini-landscape on a rock that has been named ‘GongGong’. The mini-landscape measured only 3cm across but had an incredible amount of detail and importance. The mini-landscape records two of the most important and violent forces in the history of Mars: volcanoes and wind.

Fig. 12. ‘GongGong’ – a miniature landscape formed by volcanoes and wind.

GongGong formed billions of years ago in a stirring mass of molten rock. The rock captured bubbles of gases that were trapped at great depth but had separated from the main body of lava as it rose up to the Martian surface. The molten rock was deformed as it moved across the landscape and, as it did so, the shape of tiny bubbles of gas was deformed, in particular, becoming elongated. When the molten lava solidified, the rock looked like a frozen sponge. This newly formed rock then withstood billions of years of pelting by small sand grains carried by Martian dust storms. The sand eroded the surface until, little by little, the delicate strands that enclosed the bubbles of gas were breached and the spiny texture that can be seen in the photo below emerged.

Similar rocks can be found on Earth where the same complex interplay of volcanoes and weathering occurs, whether it be the pelting of rocks by sand grains in the Mojave Desert or by ice crystals in the frigid Antarctic.

Home Plate

Home Plate is a roughly circular feature about 80m in diameter. After scientists had identified Home Plate from orbit, they had many theories about what it could be and what they might see. But when Spirit’s panoramic camera took photographs, NASA scientists where stunned to find the best evidence of layering so far. The ‘Home Plate Class’ is divided into two groups: the lower unit and the upper unit. At the north-west corner of Home Plate, the lower unit of rocks is part of the Gusev Plains magmas, except that the Home Plate magmas intruded through the Columbia Hills.

These magmas interacted with groundwater at depth, heating and boiling the groundwater and producing explosive ash deposits on the surface. Therefore, the ‘Home Plate Class – lower unit’ is eroded remnant tuff and contains well-layered, coarser beds at the base with finer-grained beds at the top. Spirit found 1cm-sized bomb clasts when sampling. Both the upper and lower units are rich in magnetite and contain high amounts of potassium, titanium, phosphorous and sodium. The difference between the upper and lower units is that the upper unit is a possible cross-stratified base surge pyroclastic deposit or, maybe, even an aeolian deposit derived from erosion and reworking of Lower Home Plate Unit.

A class of rock called the ‘Halley Class’ has also been documented in the Home Plate area and this rock is found at the broken piece of outcrop from northern slope of Low Ridge. It is thinly layered, with harder portions sticking out as fins due to differential erosion. Bright blue patches on this rock are rich in calcium sulphate. The rock itself is rich in zinc. It is believed that this rock was formed by aqueous alteration. It is possible that, for Home Plate-type layers, calcium sulphate was precipitated out as groundwater percolated through the layered units.

Other rocks in the area are informally named for Josh Gibson, ‘Bullet Joe’ Rogan, and Cumberland Posey, but by far the most interesting type of rock found in this area was discovered just before departing Home Plate. Spirit took an image showing some of the most complex layering patterns seen so far at this location.

Fig. 14. True colour image showing complex patterns of alternating erosion and deposition. ©Courtesy NASA/JPL-Caltech.

The layered nature of these rocks presents new and exciting questions for the rover team. The rocks identified by Spirit have recorded a detailed history of the physical properties that formed them. In the black box highlighted in Fig. 15, one group of layers slopes downward to the right, while the strata around this group slope downwards to the left. The layers above and below this group are more nearly horizontal. Where layers of different orientations intersect, other layers are truncated. This indicates that there were complex patterns of alternating erosion and deposition occurring as these layers were being deposited.

Fig. 15. False colour image showing complex patterns of alternating erosion and deposition. ©Courtesy NASA/JPL-Caltech.

You can find similar patterns in some sedimentary rocks here on Earth. Scientists now suspect that the rocks at Home Plate were formed in the aftermath of a volcanic explosion or impact event, and they are currently investigating the possibility that wind may also have played a role in redistributing materials after this event.

This article will be continued in the next issue of Deposits and will continue to follow the journey of Spirit from Bright Soil near ‘McCool’ Hill to its present position.