Copernicus and Tycho Craters
Published on Jun 10, 2006 at 4:57 pm.
No Comments.
Filed under moon.
People often don’t look very closely at the Moon. So, they don’t realize that you can see a lot on the Moon with just the naked eye. Those that do look at the Moon a little more carefully see that you can see dark markings on the face of the Moon. These are the “seas” on the Moon. They are actually ancient basaltic lava flows. But, that isn’t all that you can see. If you look very carefully, you can see evidence of some craters with your naked eye. Actually, you don’t really see the craters, themselves. What you see are the ejecta from the craters.
When large objects strike something bigger to make a crater, the impacting object is obliterated. The resulting explosion carves out the crater. The ground is pushed up around the crater, forming a wall around the crater. Some of the material that was in the crater, particularly away from the center, is thrown out of the crater to lay around it in what we call an ejecta blanket. Ejecta from somewhat farther inside the crater is often thrown with higher velocity and lands along giant streamers extending away from the crater. We call these rays. Near the center of the crater, if the crater is large enough, the ground can rebound to form a mountain peak, or set of mountain peaks, in the middle of the crater. For sufficently large craters, the ejecta blankets and rays are big enough to see easily from Earth with the naked eye. Now, the Moon has many craters of this size, but only a few have ejecta blankets that you can see. Why is this? Well, the material thrown out by the impact is crystaline in nature, and these crystal faces reflect sunlight. This makes the ejecta blankets bright. But over time, the degradation by the intense ultraviolet light from the Sun, as well as the effects of micrometeorites, destroy the nice smooth reflective surfaces of the crystals, and so the eject becomes darker, eventually returning close to its original coloration. The process takes billions of years, though. But, it does occur. So, to some degree, you can estimate the relative ages of craters by looking at their ejecta.
Well, the two most easily seen crater features from Earth are Copernicus and Tycho craters. Both are quite easy to see near Full Moon, if you wait long enough for the Moon to be high in the sky (several hours after sunset). Full Moon is tomorrow, so you might want to go out and look, say around 11pm or midnight.Â
Copernicus is the crater that will be in the middle of the largest dark patch on the Moon, in the northeast quadrant, as seen from Earth (that will be in the upper left as seen from the Northern Hemisphere, or lower right as seen from the Southern Hemisphere). Copernicus crater lies at the edge of Mare Imbrium (The Sea of Rains) and Oceanus Procellarum (the Ocean of Storms).  Copernicus is about 93 kilometers across and about 3800 meters deep. It has three central peaks a little over a kilometer high near its center. The peaks are dark in color, and seem to be made of largely olivine, a common dark green to black mineral.  The bright ejecta blanket is easy to see in contast with the dark lava flows of the seas. The Apollo 12 mission to the Moon landed on one of Copernicus’ rays. The samples returned to Earth indicate an approximate age of about 800 million years for Copernicus.
Tycho is a large crater in the southern part of the Moon. It is about 85 kilometers across and about 4.8 kilometers deep, with a 1.6 kilometer tall central peak. Tycho has among the brightest and longest rays on the Moon. Some of Tycho’s rays appear to stretch nearly completely across the face of the Moon. These rays are easy to see stretching across the face of the Moon even with the naked eye. Apollo 17, the last manned mission to the Moon, landed where the astronauts could collect samples of one of Tycho’s rays. These samples indicate that as might be expected from Tycho’s brighter ejecta, Tycho is younger than Copernicus. Tycho is believed to be a bit under 110 million years old.
So, if you get some clear skies in the next few days, go look at the Moon and see if you can see these craters.
-Astroprof
| Â |
