From a long distance, the surface of Mars appears reddish-red due to the red dust contained in the atmosphere . Nearby, the color is yellowish brown with an admixture of golden, brown, tan and even green, depending on the color of the minerals on the planet . In ancient times, people easily distinguished Mars from other planets, and also associated it with war and composed all kinds of legends . The Egyptians called Mars "Har Decher," which meant "red." In India, the planet was known by the names of Angarak or Lohitang, both names implying a bright red color that could be seen with the naked eye. Modern research has shown that not only the surface, but also the sky of the planet is red.
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Reason for Red Color
Currently [ when? ] scientists [ who? ] believe that only the upper layer of the surface of Mars is red. The planet looks reddish-red mainly due to the ubiquitous layer of dust (particles 3 - 45 μm in size), whose thickness is only a few millimeters .
Even in places such as the Farsid Highlands , where the layer of red dust is thicker than in other parts of the planet, it still does not reach a depth of more than 2 meters . Thus, red dust, in fact, is the thinnest shell of Mars and does not extend to deeper layers of Martian soil .
The color of Mars is red due to the optical properties of nanoscale iron oxides (npOx), which prevail in the visible region of the spectrum . Thanks to the infrared remote sensors of the OMEGA spectrometer of the automatic space station of the European Space Agency, Mars Express managed to find out that there are various npOx minerals on the planet, but nanocrystalline red hematite (α-Fe2O3), which extends 100 μm in depth, predominates. The rest of the iron particles in the dust (possibly about 50% of the total mass) can be in magnetite enriched in titanium (Fe3O4). Magnetite is usually black, so it has little effect on the red color of the soil .
The mass fraction of chlorine and sulfur in Martian dust is actually higher than expected before the expedition Spirit and Opportunity Rovers , during which samples were taken of the soil of Gusev Crater and the plain of the Meridian Plateau . Sulfur interacts well with npOx. This suggests that minor changes in the chemical composition of a thin layer of salt solution , which is located in Martian dust and contributes to the formation of frost from atmospheric H2O , can lead to the formation of npOx. In addition, a remote study of atmospheric dust (which in composition and size differs little from dust that is on the surface of the planet) indicates that the dust mainly consists of feldspars of the plagioclase , zeolite subgroup, as well as a small amount of pyroxene and olivine . Such material can easily be formed in the process of mechanical erosion of basalt , which contains feldspar and has much in common with the Martian rock of the southern part of the planet. Together, all the data collected showed that under the influence of water, chemical changes in the composition of Martian dust are very insignificant.
The occurrence of npOx in dust
During some processes, npOx are able to oxidize without the participation of free O2 , several similar processes occur on Mars, since atmospheric changes over time indicate that free O2 (which appeared mainly through H2O photodissociation ) may have always existed on the planet as a microcomponent with a partial pressure of not more than 0.1 µPa. One of these processes involves the direct chemical reaction of Fe2 + (occurring in magmatic minerals ), or the reaction of Fe with H2O during which Fe3 + (aq) is obtained, which, due to certain conditions, in turn leads to the formation of a hydroxide ion , for example, goethite (FeO • OH). Despite the fact that the reaction with H2O does not work well from the point of view of thermodynamics , nevertheless, it is still possible during a sharp loss of H2 by-product. Dissolved CO2 and SO2 can also contribute to the reaction.
However, the decomposition of Fe3 + iron metahydroxides , for example, the decomposition of goethite into hematite, requires high temperatures (300 ° C). Similar processes occur during the formation of tephra , consisting of palagonite, on the tops of the slopes of the Mauna Kea volcano , since there are some spectral and magnetic similarities with tephra and Martian dust. Despite the fact that kinetic conditions are necessary for such reactions, prolonged droughts together with a low hydrogen index on Mars can lead to the conversion of goethite to hematite.
Fe and Fe2 + can also be oxidized with hydrogen peroxide (H2O2). Despite the fact that hydrogen peroxide is very small in the atmosphere of Mars, it is a much more stable and strong oxidizing agent compared to H2O .
There is evidence that hematite can form from magnetite during erosion processes . The experiments conducted at the Laboratory for Modeling Martian Conditions at Aarhus University in Denmark showed that if you mix a mixture of magnetite and quartz sand together with particles of quartz dust in one test tube, part of the magnetite turns into hematite and the resulting mixture turns red. A similar reaction occurs because the chemical bonds of quartz are destroyed, and upon contact with magnetite , oxygen atoms pass from quartz to magnetite, forming hematite.
Red Sky on Mars
The color of the sky on Mars can be perceived by the human eye as red. This fact was established thanks to images taken during the Mars exploration programs Mars Pathfinder and Mars Exploration Rover . Dust particles on the surface of the planet absorb sunlight, which is the main reason that the sky on Mars is red. In addition, an additional effect may be exerted by the fact that photon radiation of 3 μm comes from dust particles.