Scientific results of the Mars Exploration Rover Mission
The two Mars rovers " Spirit " and " Opportunity " have found several clues during their long mission that point to the former existence of water on Mars (planet) . Some discoveries have already been prepared by Mars orbiters, while others have only been made by analyzing the rocks on site. Spirit explored a hill and a former volcanic plateau called "Home Plate" in Gusev Crater during his missionary time . Opportunity found various indications of a previously wet environment of the Martian surface in the Meridiani level .
The two landing sites (Gusev Crater and Meridiani Plain) were selected because there were already indications from orbit that water must have played a role there. The instrumentation of the rovers with the two spectrometers ( alpha particle X-ray spectrometer and Mössbauer spectrometer , the panoramic camera with different filters and the mini-TES) is designed to analyze the rocks found according to mineralogical criteria.
Water and salts
Salts are the products of a reaction of acid with a base (or alkali). The result is a neutral substance, and usually water. The chemical reactions that produce salts require that the two components come together in an aqueous solution. Spirit and Opportunity discovered that salts are commonly found on Mars.
Towards the end of his mission, Spirit got stuck on the slope of the Home Plate plateau in a crater filled with sand. When an attempt was made to free the stuck rover from the sand, the ground was partially dug up. The driving tests uncovered light ground around the left front wheel. This place was called 'Ulysses'. Sulphates emerged buried just below the surface. By combining the measurements of the Mössbauer spectrometer, the APX spectrometer and the data from the PanCam, a consistent conclusion could be drawn that Ulysses contains moderately hydrogenated iron sulfates. These sulfates could have been created by a volcanic steam source. The thin, encrusted soil below the surface, which emerged on the eastern edge of Ulysses, is enriched with hematite and iron sulfates. The most likely explanation for the occurrence of iron sulfates and basaltic sand within the crater is that these minerals were blown here from other locations and accumulated as wind deposits. Then episodically neutral to slightly acidic water appeared in the soils in the valleys around home plate. A water-containing solution then seeped under the surface and left layers containing iron sulfate just a few centimeters below the surface. The sulphate-rich deposits and the encrusted soils follow the topography, this gives an indication of the chronological sequence of the deposits. A conclusion can be drawn that the formation of frost and snow provided the necessary amount of water (e.g. during a time when the Martian axis was more inclined than it is today).
Spirit had partially dug up the ground with his blocked bike during a trip east of home plate, in the so-called Eastern Valley , on Sol 1148. A white substance appeared at this place, which was named "Gertrude Weise". Infrared spectra of this substance, which were recorded with the Mini-TES, showed a signature that is typical of fine-grained, granular, relatively pure amorphous, water-containing silicon dioxide ( opal ). The most likely scenario for the formation of this material is assumed to be the leaching of a basaltic rock by hot sulfuric acid aqueous solutions of volcanic origin, as a result of which the silicon dioxide, which is sparingly soluble under these conditions, is locally concentrated. Such formations are known from Earth in the vicinity of fumaroles in Hawaii.
The discovery of such occurrences provides both evidence of the former presence of liquid water on Mars and indications that there might have been living conditions for sulfur-loving microorganisms such as those found at hot springs on Earth. In addition, fossil evidence of such microbes may be found in the silica-rich materials, which is why they are proposed as preferred candidates for sampling on missions bringing Martian rocks to earth.
Blueberries: Minerals that formed in acidic water
In Meridiani Planum, where Opportunity landed, small globules were soon discovered that were nicknamed "Blueberries". These are partly loosely distributed over the surface, sometimes also firmly embedded in individual stone layers. The distribution of the blueberries in the rock is more even than random.
Measurements with the Mössbauer spectrometer could show that these consist mainly of hematite . On earth, hematite mostly forms in water. Water provides oxygen atoms here, which bond with the iron atoms in the mineral. On Mars it is likely that (acidic) groundwater had dissolved iron. As it flowed through the sandstone layers of the Meridiani level, the iron was deposited and these small spheres were formed. This must have happened in stagnant or slowly flowing groundwater so that enough hematite could precipitate out of the water. The globules are larger at greater depths than those at higher altitudes, suggesting that the impact of the groundwater increased with increasing depth.
Sulfates
For their formation, sulphates only require concentrated concentrations of salty water, which, moreover, need not have appeared on the planet's surface over a long period of time.
Small fissures and cracks often form polygonal structures in the exposed surfaces of the Meridiani rocks. These cracks also run through the existing rock stratification, so that it can be assumed that the cracks only emerged after the deposition. When sulfates dry out, they lose a significant part of their volume. The crevices that were found could have been caused by the fact that the sulfate minerals reduced their volume due to water loss when the ambient conditions became drier.
Jarosite
In addition, Opportunity found another mineral called " Jarosite " right at its landing site in Eagle Crater . Like hematite, jarosite forms in acidic water. In jarosite, water has a weight percentage of 10%. The mineral is therefore mineralogical evidence of the existence of water on Mars. It also proves that in earlier times processes took place with acidic water that produced jarosite under oxidizing conditions.
The adjacent spectrum, which was created by the Mössbauer spectrometer, shows iron-containing jarosite minerals in the rock outcrop called "El Capitain". The two yellow tips in the diagram show jarosite, which contains water in its structure in the form of hydroxyl. The data show that water-powered processes exist or have existed on Mars.
Traces of dissolved minerals
Images from Opportunity's PanCam and the Microscopic Imager show that the "El Capitain" outcrop is littered with elongated cavities (vugs) approx. 1 cm long and 1 to 2 mm wide in different orientations. The cavities are unevenly distributed in the Eagle Crater. In the rocks in which they occur, they cover about 5% of the surface. Geologists are aware of this particular pattern of places where salty minerals are deposited within rocks that are in salty water. When the crystals later dissolve again in not quite so salty water, these cavities ( drusen ) remain .
Clay minerals
At the edge of the Endeavor Crater, Opportunity was looking for clay minerals whose signatures can be seen in spectral images from orbit. The sulphate minerals found so far were formed in a mix of minerals and acidic water. Such an acidic environment represents a rather poor livelihood.
The clay minerals on Mars, however, formed in pH-neutral water. This find suggests a more life-friendly environment at times. For the formation of clay minerals, the surface of Mars must have interacted with water over a long period of time.
Carbonates
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In a rock outcrop called "Comanche", which was examined by Spirit in December 2005, minerals were also found that had formed in neutral water. Spirit examined this rock with the Mössbauer spectrometer, the MiniTES and the APXS spectrometer. In 2010, scientists were able to use this data to show that a quarter of the rock consists of magnesium iron carbonate. This concentration is up to 10 times higher than for any stone previously examined.
Carbonates are formed under wet, almost neutral conditions, but soon dissolve again in acid. This find is the first clear indication of the two rovers that the surface of Mars in earlier times was more liveable than the acidic environments that have been found so far.
The carbonates in Gusev Crater were likely deposited from carbonate solutions under hydrothermal conditions at a near-neutral pH, combined with volcanic activity during the Noachian era . The discovery of high concentrations of carbonates in the Comanche exposure serves as the basis for climate models that include CO 2 as a greenhouse gas for a wet and warm Mars and predict subsequent deposition of at least parts of the atmosphere in carbonate minerals.
plaster
The Endeavor Crater is 4 billion years old and 22 km in diameter. Opportunity explored the Odyssey Crater near the southern end of Cape York. This is surrounded by a field of rocks that were thrown out during the impact. A rock called "Tisdale" was examined here in detail as it gave access to older rock. APXS measurements showed that one of the highest concentrations of zinc occur here that has been measured on Mars to date. The heating of the soil from the impact, which created a crater the size of Endeavor Crater, was sufficient to cause hydrothermal activity when water is present. In the Tisdale stone, Opportunity found evidence that the original impact heated groundwater that this zinc accumulation had left in the stone.
Cape York is surrounded by a gently sloping terrace that is approx. 6 m wide on the western side and approx. 20 m on the eastern side facing the Endeavor crater. The outer parts of the terrace on the western side are exposed, light-colored thin layered sandstones, the stratification of which slopes gently towards the plain. These sandstone layers lie directly over darker granular sedimentary rocks that form the inner edge of the terrace. This arrangement is interpreted in such a way that the "Burns" formation rests on the older sediment material of the "Shoemaker" formation. The inner edges of the terraces overlap the Noachian Age breccias that form the inner slopes of Cape York.
The rocks of the terraces are criss-crossed in many places by bright linear veins. These are easily visible in the protruding dark sediment material on the inside of the terraces. However, they also occur in the bright outcrops of the sandstone of the Burns Formation. Measurements of 37 veins showed an average width of 2 cm and an average visible length of 33 cm.
Opportunity discovered these striking bright rock lines (veins) on the edge of Cape York, which were examined more closely. The vein "Homestake" z. B. forms a discontinuous edge with a width of 1 to 1.5 cm and is approx. 50 cm long. It stands approx. 1 cm above the surrounding rock. So it seems to be more resistant to erosion than the rock in which it is embedded.
These veins may have formed after the impact when colder water flowed through crevices in the subsurface near the crater. Plaster of paris was deposited, which formed as veins along the crevices. Gypsum is the most abundant sulfate mineral on earth and is often left as a residue when the water has evaporated. These bright mineral veins have never been found anywhere on Mars.
Rippel: Petrified shore sand
In the rocks in the Eagle Crater and in the Erebus Cat, garland-like, ripple-shaped sloping layers are visible. These layers, only 8 to 18 mm thick, are probably the result of running water. The existing arrangement of layers indicates the transport of sediments through ripples below the water surface. It is known that such deposits of this size only form under water and at flow speeds of a few decimeters per second. Such wave-like ripple structures form on earth z. B. on sandy beaches. The Martian gravity, which is lower than that of Earth, has only a minor effect on sediment transport and the size of the structures found.
These water currents, which are necessary for this, probably not canalised, could have been driven by the force of gravity on Mars. These currents were created by recurring flooding of the salt flats that covered the Meridiani surface.
Individual evidence
- ^ Signs of Water. (PDF) NASA, accessed March 1, 2014 .
- ↑ a b c R. E. Arvidson, et al .: Spirit Mars Rover Mission: Overview and selected results from the northern Home Plate Winter Haven to the side of Scamander crater . In: J. Geophys. Res . tape 115 , 2010, doi : 10.1029 / 2010JE003633 (American English, web.mit.edu [PDF]).
- ↑ a b S. W. Squyres et al .: Detection of Silica-Rich Deposits on Mars . In: Science . tape 320 , no. 5879 , May 23, 2008, p. 1063-1067 , doi : 10.1126 / science.1155429 (American English).
- ^ RV Morris, T. Graff, MD Lane, DC Golden, CS Schwandt, TD Shelfer, DW Ming, SA Mertzman, JF Bell III, J. Crisp, PR Christensen: Acid Sulfate Alteration Products of a Tholeiitic Basalt: Implications for Interpretation of Martian Thermal Emission Spectra . In: 31st Annual Lunar and Planetary Science Conference, March 13-17, 2000, Houston, Texas . Abstract no. 2014, 2000 (American English, usra.edu [PDF]).
- ↑ a b c S.W.Squyres et al .: In Situ Evidence for an Ancient Aqueous Environment at Meridiani Planum . In: Science . tape 306 , no. 5702 , December 3, 2004, p. 1709–1714 , doi : 10.1126 / science.1104559 (American English).
- ↑ a b S.W.Squyres et al .: Opportunity Rover Two Years at Meridiani Planum: Two Years at Meridiani Planum: Results from the Opportunity rover . In: Science . tape 313 , no. 5792 , September 8, 2006, p. 1403-1407 , doi : 10.1126 / science.1130890 (American English).
- ^ SW Squyres et al .: Exploration of Victoria Crater by the Mars Rover Opportunity . In: Science . tape 324 , no. 5930 , May 22, 2009, p. 1058-1061 , doi : 10.1126 / science.1170355 (American English).
- ^ G. Klingelhöfer et al .: Jarosite and Hematite at Meridiani Planum from Opportunity Mössbauer Spectrometer . In: Science . tape 306 , no. 5702 , December 3, 2004, p. 1740–1745 , doi : 10.1126 / science.1104653 (American English).
- ↑ a b c Richard V. Morris et al .: Identification of Carbonate-Rich Outcrops on Mars by the Spirit . In: Science . tape 329 , no. 5990 , July 23, 2010, p. 421-424 , doi : 10.1126 / science.1189667 (American English).
- ↑ a b S.W.Squyres et al .: Ancient Impact and Aqueous Processes at Endeavor Crater, Mars . In: Science . tape 336 , no. 6081 , May 4, 2012, doi : 10.1126 / science.1220476 (American English).
- ↑ jpl.nasa.gov
- ↑ JP Grotzinger: Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars . In: Earth and Planetary Science Letters . tape 240 (2005) . Elsevier, September 22, 2005, p. 11–72 , doi : 10.1016 / j.epsl.2005.09.039 (American English, gps.caltech.edu [PDF; accessed March 1, 2014]).
