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| Two visual Themis images (100
m/pix, 10°N 4.5°E, southwestern Arabia, north of Meridiani) with
different sun elevation. The one in which we see the arrow-marked bright spots (I06466022)
was from the evening close to 6 PM and has a sun elevation of 8° only.
The other one (I01273002) has 38° and was from the afternoon 3 PM. The
evening image with the lower sun is much darker than the other. Using
global histogram adjustments both were matched to about the same
brightness. The bright clouds consist probably of water ice crystals like strathospheric clouds on Earth and are directly illuminated by the sun. The left crater has a diameter of about 19 km. If the darkening inside the upper right crater rim is the shadow of the cloud, the pillar would be about 2 km high. The pillar is created by an upward current probably caused by a thermal heat source at the surface not related to sunlight. The clouds result from the evaporation of hot liquid water due to low atmospheric pressure. The pillars are present during the whole day but only visible as the landscape is considerably darker than the clouds. The originating bright area in the daytime image is probably no ice but a high density near surface cloud, too. Notable are the arrow-marked small bright dots in the evening image. In the context of the large pillars this are probably very small ones only some 10 m diameter and a few 100 m high. Their origin is identical to that of the large pillars: hot springs on the Martian surface. |
The right presentation begins
just 20 km (200 pix) below the left one ends. The evening image is the
same and the afternoon image (I01635005, INA 51°, LST 15.4) has about
the same data like before. The images are globally brightness adjusted to
match each other. Here the small pillars are much more common. Some emanate from very small craters with distinct low albedo surroundings. Some of them are probably pseudocraters in low activity mode. The lower portion of the evening image (which is only in its upper part presented here) is closer to the sun and much brighter. Pillars are no longer visible but some haze seems to cover part of the landscape. Early morning Themis thermal images show hot spots at the location of the pillars. But more notable is considerable haze around the area of the big craters' pillars. This supports the assumption that the pillars are present around the clock and insert considerable amounts of ice crystals into the atmosphere. The thermal imager has difficulties to penetrate this type of clouds. |