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Ken Tapping, June 03, 2009
In the sky this week...
> Saturn is high in southern sky during the evening.
> Jupiter rises about 2 a.m., Venus and Mars about 4 a.m.
> The Moon will be full on June 8.
One of Galileo's big moments was when he pointed his crude telescope at the Moon. Instead of the heavenly perfection that was believed at that time to apply to all objects in the sky, the Moon seemed to share the blemishes we find on the Earth; it was covered with mountain ranges and craters. These days any small telescope of reasonable quality will show what Galileo saw, but in far greater detail. The Moon is a place of countless craters, ranging from a few centimetres across, as seen by visiting Apollo Astronauts, up to about 200 k.m. There are ranges of mountains, and great lava plains, that in the past we optimistically called "seas", which cover almost 20% of the Moon's surface.
In these days of plate tectonics and subduction zones, we know pretty well how the mountains on the Earth came about. In addition, here on Earth we have found two sorts of crater: volcanoes and the sites of ancient meteoric impacts. Now, thanks to the Apollo astronauts and spacecraft surveying the Moon from close up, we can compare it with our world, and are finding intriguing differences.
For example, British Columbia, along with the rest of North America is gradually moving westwards, colliding with the bed of the Pacific Ocean and assorted islands. Under the impact the edge of the North American Plate is crumpling into mountains. In addition, some of the Pacific seabed and islands pile up along the west coast, making more mountains. The rest of the material goes down, under the edge of the continent, forming a subduction zone. In the high temperatures kilometres underground, that material melts, forming lava and superheated steam, which then bubbles back to the surface forming volcanoes and huge lava flows covering hundreds of square kilometres. We see the same sort of thing all over the world where there are subduction zones. These are also the main sites of earthquakes.
Water seems to be a key element in plate tectonics and subduction zones. On the Moon there are no large bodies of surface water and as far as we know, no plate motions or subduction zones, and there have been none for billions of years. With no subduction zones there can be none of the volcanoes and mountain building that go with them. It is likely that the Moon is now solid all the way to the centre, unlike our Earth, which is hot and partially molten inside.
On Earth we have lots of volcanoes and few meteoric craters. That's because plate motions tend to make volcanoes, and along with the weather, erase meteor craters. The Moon has no plate motions and no weather, so the craters on the Moon are mostly impact craters accumulated over billions of years. Some impacts in the distant past were huge, liberating enough heat to produce lava lakes many hundreds of kilometres across. The mountains we see today are actually made up of the rims of craters, in some places overlapping to form respectable mountain ranges. Over the last couple of decades we have learned a lot about the geology of the Moon, our Earth and other planets. Examining their similarities and differences teaches us about the fundamental forces that made them.
Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,