~Atmospheric refraction of a CO2 atmosphere is probably visible, more dedicated photos are required though.
~First photographic hints at very slight horizon refraction
~Given the thinness of the atmosphere, much of this of course was expected, but further very serious photography should be undertaken.
Mars has a 6 mb atmosphere, 169 times less weighty than on Earth. Not many refraction specialists believe it capable of causing some atmospheric mirages, but near the horizon, the game changes, at that level, we look at a much thicker atmosphere. Consider what we always knew, Mars landscape looked carved by water ever since a closer telescopic look at the planet was achieved. Now that we have access to a probe capable of staying still, by refraction optics we can see if there is any ice permafrost, which can give a spectacular effect similar to sea ice horizons on Earth.
Ice and Martian soil should have different specific heat capacity, ice has most certainly a greater heat capacity, which means that it affects the soil right over it, in particular, when the cloudless sunny sky hits a pure rocky outcrop, the air right above warms up rapidly, this may cause a "road mirage" like on Earth, however subtle likely so on Mars, but none heard of so far. Examples here show no rapid lowering, a sure sign of a permeating factor. Here we study the Martian horizon, by NASA design from a probe not moving around all over the place:
CO2 Martian atmosphere may cause different refraction effects, namely 95% CO2 concentration surely gives a great warming potential, so the surface should warm really rapidly when the sun rises, again a dry rocky substratum horizon should lower at local apparent noon, not take a few hours as on Earth. Which as we know, on any given sunny day without much great weather circulation, noon is not the warmest time of day.
A rudimentary model using refraction index of CO2 gas, incidentally greater than air at standard pressures and temperatures, calculates a very small shrinking of the vertical sun disk equally at the horizon. However, the interplay between Martian soil, permafrost and CO2 atmosphere is novel, more studying is needed. WD December 9 2018
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