[Nmcaver] New View of Dark Pit on Arsia Mons

[Lee H. Skinner]

NASA’s Mars Reconnaissance Orbiter's HiRISE camera has a new photo of 
one of the pits on Mars:

Dark pits on some of the Martian volcanoes have been speculated to be 
entrances into caves. A previous HiRISE image, 
<http://hirise.lpl.arizona.edu/PSP_003647_1745> looking essentially 
straight down, saw only darkness in this pit.

This time the pit was imaged from the west. Since the picture was taken 
at about 2:30 p.m. local (Mars) time, the sun was also shining from the 
west. We can now see the eastern wall 
<http://hirise.lpl.arizona.edu/images/2007/details/cut/PSP_004847_1745_cut_b.jpg> 
of the pit catching the sunlight.

This confirms that this pit is essentially a vertical shaft cut through 
the lava flows on the flank of the volcano. Such pits form on similar 
volcanoes in Hawaii and are called "pit craters." They generally do not 
connect to long open caverns but are the result of deep underground 
collapse. From the shadow of the rim cast onto the wall of the pit we 
can calculate that the pit is at least 78 meters (255 feet) deep. The 
pit is 150 x 157 meters (492 x 515 feet) across.