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The search for water on Mars
The search for water on Mars

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4.9 Rolling on the ground again: MSL

On 6 August 2012, NASA’s Mars Science Laboratory (MSL) rover Curiosity landed in Gale crater at a location called ‘Bradbury Landing’. Gale crater was chosen as Curiosity’s landing site because orbital data, e.g., from the CRISM instrument, had suggested water might once have been present there. Curiosity was fitted with an array of instruments with which to investigate the martian surface and atmosphere, and these have made a number of discoveries relating to water on Mars.

Firstly, Curiosity confirmed the presence of water frost also seen by Viking and Phoenix, and clouds observed by Pathfinder (Figure 45).

Download this video clip.Video player: Figure 45
This figure is an animation composed of photographs taken of thin, streaky clouds in the martian atmosphere. The photographs are grey scale. The clouds move from right to left across the images.
Figure 45 Clouds moving across the horizon just at sunset in Gale crater observed by Curiosity. Those are either made of carbon dioxide or water, depending on the height they occur. The clouds in this GIF are about 19 km above the surface and likely to be water clouds. Image credit: NASA/JPL.
Interactive feature not available in single page view (see it in standard view).

Secondly, Curiosity found evidence that water had once existed in Gale crater itself. Very early in its mission, it found conglomerates – rocks made up of rounded pebbles that were transported by water (Figure 46). Careful investigation of the size, rounding and distribution of the pebbles led the mission team to conclude that they were deposited by a stream that was knee to hip deep and probably flowed at an average velocity of 0.20 to 0.75 m s−1. In addition to these conglomerates, sandstones and mudstones were found. The fine-grained nature of the mudstones showed that they settled out from a body of water after coarser grained material had settled out (Figure 47).

This figure is a photograph of the martian surface taken by the Curiosity rover. It shows a rock outcrop covered by a thin layer of dust. The dust is a red-brown colour, and the rock outcrop is green-orange.
Figure 46 Conglomerate at the site called ‘Hottah’ encountered on sol 39 (a sol is a martian day, 24 hours 39 minutes) of the Curiosity mission at Gale crater, Mars. This and two similar sites allowed calculation of the water depth and flow-speed of the stream that transported those 2-40 mm sized pebbles to this site. Image credit: NASA/JPL.
This figure is a photograph of the martian surface taken by the Curiosity rover. In the foreground is an area of fine grained martian dust with some larger pebbles. Behind this is a rock outcrop covered by a thin layer of dust. The dust is a red-brown colour, and the rock outcrop is green-orange. The rock has horizontal, parallel beds.
Figure 47 Sandstones at the location ‘The Kimberly’ at Gale crater, Mars. Those sandstones occur together with conglomerates in the sequence of deposits in the crater. The third water-laid type of rock are mudstones. The black, triangular rock in the middle of the image is about 80 cm wide. Image credit: NASA/JPL.

Take a look at Figure 48. This shows the minerals found within rocks at Gale crater expressed as pie charts, where the larger the wedge of the pie, the more of that mineral present. The first two pies (lower left of the image, labelled JK and CB) and the last three (upper right of the image, labelled MB, QL and SB) are mudstones.

This figure is an illustration showing ten pie charts. Each pie chart represents the composition of the mudstone analysed along the Curiosity rover’s traverse along the martian surface and they are arranged along a line representing that traverse. The locations marked on the line, from left to right, are Yellowknife Bay, Pahrump Hills, Marias Pass, Base of Naukluft, and Murray Buttes. The minerals present are represented by colours: Feldspar is dark grey, mafic minerals are light grey, magnetite is black, haematite is red, clay minerals are green, crystalline silica is blue, jarosite is purple, fluorapatite is orange and calcium sulfate is yellow. All pie charts have a large wedges of dark grey and light grey. All bar two have a wedge of green, and all bar four have wedges of red and yellow. Blue, purple and orange wedges are much less common.
Figure 48 Mineralogical composition of rocks at Gale crater, Mars drilled between 2012 and 2016. Image credit: NASA/JPL.
  • Excluding mafic minerals and feldspar, which are common minerals, what is the most common type of mineral shown on the mudstone pie charts?

  • In almost all of the mudstone pie charts, clay minerals (green wedge) are the most dominant type of mineral. Haematite (red wedge) is also prominent in some of the charts, as is calcium sulfate (yellow wedge).

  • From what you have learned so far, what has led to the presence of clay minerals, haematite and sulfates?

  • Clay minerals, haematite and sulfates form in the presence of water by the alteration of existing minerals. Therefore, these suggest there has been water in the Gale crater area.

You will recall that the Spirit rover observed sulfates that were finely dispersed in soils and present as veins within rocks. Curiosity also found these forms of sulfate (e.g., Figure 49), but also observed rocks that contained sulfate as cement holding the grains together. In addition, the ChemCam instrument also found trace elements such as boron and chlorine in some of the sulfate veins, giving further information about the chemistry of the water that formed them (Figure 50).

This figure is a photograph taken of a martian rock by the Curiosity rover. It covers approximately 180 cm from left to right. The rock is shades of dark brown, but there are criss-crossing white lines across its surface.
Figure 49 Calcium sulfate veins (white) at a location called ‘Garden City’. Note the scale bar in the centre of the image. Image credit: NASA/JPL.
Figure 50 is divided into two parts. On the left hand side is a photograph of the surface of a martian rock outcrop. The rock is mainly brown, but there is a white area in the centre. This has a red oval drawn around it. On the right hand side of the image is a close-up photograph of the area within the red oval, approximately 7cm in length (left to right). This is a light brown rock, with a white area in the centre. Overlain on this are ten bar charts showing the relative amounts of sodium, chlorine and boron. The bar charts are overlain where the rock was analysed to show the distribution of elements across the white area. The details of the bar charts is not relevant.
Figure 50 Sodium, chlorine and boron analysis as seen by ChemCam. There are ten investigation points in this image, and for each of them the bar height indicates the amount of the three elements found. Image credit: NASA/JPL-Caltech/MSSS/LANL/CNES/IRAP/LPGNantes/CNRS/IAS.

The results obtained by Curiosity have allowed the science team to reconstruct some of the history of Gale crater. It is probable that the crater was once filled by a lake for an extended period of time. Fresh water flowed into the lake, occasionally from fast flowing streams, which deposited the conglomerates and sandstones. Mudstones were deposited in quieter periods. The lake level fluctuated, and in places there is evidence (e.g., desiccation or mud cracks) that it dried out (Figure 51). This reconstruction allows scientists to understand the changes in climate that might have taken place in Mars’ history and has enabled them to create an animation (Video 3) of the transition between the warmer and wetter Mars evidenced at Gale crater, to the cold and dry Mars that we observe today.

This figure is a photograph taken by the Curiosity rover looking down on the top of a rock outcrop on the martian surface. The rock outcrop is surrounded by fine grained, dark grey soil. The surface of the rock appears to be cracked.
Figure 51 Possible mud (dessiccation) cracks at a location ‘Old Soaker’ at Gale crater. Mud cracks form when fine grained mud is repeatedly dried out and becomes wet again. This is evidence for fluctuating levels of the lake. Image credit: NASA/JPL.
Download this video clip.Video player: Video 3
Video 3 Animation of the drying out lake at Gale crater, Mars. Image credit: ASU Knowledge Enterprise Development (KED), Michael Northrop. (Please note, this video has no sound.)
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Activity 5: Curiosity on Mars

Timing: 10 minutes

To date, the Curiosity rover mission has exceeded 3000 sols on Mars, and collected a cornucopia of evidence for past and present water at Gale crater.

Since landing at Bradbury Landing in Gale crater, it has travelled over 20 km across the crater floor, passing by several landmark features such as the Bagnold Dunes and Vera Rubin Ridge. If you would like to find out more about the rover’s traverse and track its current location, you can access this via NASA’s interactive map [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] (Figure 52).

This figure is a screenshot of the NASA interactive map, showing a satellite, greyscale image of the martian surface near a crater edge. The traverse of the Curiosity rover is overlaid as a white line, moving from inside the crater to the rim.
Figure 52 Screenshot of NASA’s rover location tool