Annular Clouds over the North Pole (Natural Colors; credits: NASA/JPL/MSSS)

Caption NASA:"Two annular (---> somewhat circular) clouds are seen in the upper left corner of this mosaic of MOC wide angle camera daily global mapping images. To the right of the picture's center is the Martian North Polar Cap. The image has a scale of about 7,5 Km (about 4,7 miles) per pixel. Annular clouds are common in mid-Northern Summer in the North Polar Region and may result from eddy currents in the lower atmosphere. The appearance of such clouds happens every year; this year they came like clockwork within a two-week forecasted period, based on the previous 4 Martian Years of experience gained from MGS MOC daily global imaging.

Despite their superficial resemblance to Earth-orbiting satellite views of hurricanes, these cloud features are not the result of strong winds and they typically dissipate later in the day. The pictures used to make this mosaic were acquired less than 2 days before the MOC was turned off for MGS's 5th Mars-Earth Solar Conjunction period. During Conjunction, Mars was on the other side of the Sun, relative to Earth, and thus MGS could not transmit data (through the Sun) during the second half of October".