Martian Polar Cap
These images, which seem to have been taken while NASA/ESA's Hubble Space Telescope (HST) was looking directly down on the Martian North Pole, were actually created by assembling mosaics of three sets of images, taken by HST in October 1996, January and March 1997, and projecting them to appear as they would if seen from above the pole.
This first mosaic is a view which could not actually be seen in nature because at this season a portion of the pole would have actually been in shadow; the last view, taken near the summer solstice, would correspond to the Midnight Sun on Earth with the pole fully illuminated all day. The resulting polar maps begin at 50° N latitude and are oriented with 0° longitude at the 12 o'clock position. This series of pictures captures the seasonal retreat of Mars' north polar cap.
October 1996 (early spring in the Northern hemisphere): In this map, assembled from images obtained between 8 and 15 October, the cap extends down to 60° N, nearly it's maximum winter extent. (The notches are areas where Hubble data were not available). A thin, comma-shaped cloud of dust can be seen as a salmon-colored crescent at the 7 o'clock position. The cap is actually fairly circular about the geographic pole at this season; the bluish "knobs" where the cap seems to extend further are actually clouds that occurred near the edges of the three separate sets of images used to make the mosaic.
January 1997 (mid-spring): Increased warming as spring progresses in the northern hemisphere has sublimated the carbon dioxide ice and frost below 70° N latitude. The faint darker circle inside the cap boundary marks the location of circumpolar sand dunes (see March '97 map); these dark dunes are warmed more by solar heating than are the brighter surroundings, so the surface frost sublimates from the dunes earlier than from the neighboring areas. Particularly evident is the marked hexagonal shape of the polar cap at this season, noted previously by HST in 1995 and Mariner 9 in 1972; this may be due to topography, which isn't well known, or to wave structure in the circulation. This map was assembled from WFPC2 images obtained between 30 December 1996 and 4 January 1997.
30 March 1997 (early summer): The cap has fully retreated to its remnant core of water-ice. This residual cap is actually almost cut into two by a large, horn-shaped canyon called Chasma Borealis which is cut deeply into the polar terrain. The HST images also reveal a curious layered terrain which is evidence of past climatic changes on Mars. The sublimation of all of the carbon dioxide has exposed the ring of dark sand dunes which encircle the North Polar Cap. Outliers of ice persist south of the polar sand sea (between the 3 o'clock and 9 o'clock positions). The bright circular features at 3, 6, and 9 o'clock are ice-filled craters.
All images were taken with the Wide Field and Planetary Camera 2. The color is constructed from images taken in red (673 nm), blue (410 nm) and green (502 nm) light. The resolution at the North Pole ranges from about 115 km per pixel in October 1996 to about 45 km per pixel in March 1997.