A block glacier is an ice stream with
scree (a large amount of small rocks of assorted sizes). For example, similar glaciers (22,000 years ago) have been instrumental in the creation of the Long Island (my home!) on the Eastern sea-shore of continental USA. Now scientists may have found evidence of such (old) glacial activity on Mars. Images taken by the
High Resolution Stereo Camera (HRSC) on board
ESA's
Mars Express spacecraft, shows flow features most likely formed by 'block' glaciers.
Glacial Imprint on Mars (Courtesy: Mars Express)This unusual 'hourglass'-shaped structure (9 and 17 kilometer wide craters) is located in
Promethei Terra at the eastern rim of the
Hellas Basin (which contains the lowest point on Martian surface), at about latitude 38º South and longitude 104º East.
Features include:
a) Along the crater walls, numerous concentric rings called 'end' moraines (similar to
moraines on Earth, which are depositions of rocks as the glacier retreats; similar effects formed the rocky shores of Long Island),
b) Presence of 'middle' moraines, which are parallel stripe-like structures displaying the direction of the glacial flow,
c) Cracks when the glacier is moving over a steep terrain, similar to those found on Earth-glaciers,
d) Elongated grooves/ridges similar to
drumlins (structures formed beneath ice by glacial flow resulting in compression and accumulation of abraded material) on Earth.
The density of meteor impacts within the craters is quite low. The statistical analysis of these craters shows that part of the surface with its present-day glacial characteristics was formed only a few million years ago. This is extremely young! This means that Mars must have had a thicker atmosphere a few million years ago (since glaciers will not easily form under thin atmospheres), and much of the glacial ice might be now trapped under a layer of dust, thus both protecting it from evaporation, and hiding it from the on-board instruments on the Mars Express.
If the above conjecture is true, this would mean that Martian climate underwent a dramatic change in the last few million years. It could be because of a polar axis shift, or some other process we know nothing about today.
