November 14, 2011 10:25
Analysis of new images of a curious "hot spot" on the Moon showed that this small volcanic province created by the upwelling of silicic magma. The unusual location of the region and an amazing composition of lava may be the key to the thermal history of the moon.
This area is characterized by high concentration of thorium. It is located between the large ancient impact craters Compton and Belkovich. In 1998, it found a gamma spectrometer space probe Lunar Prospector. The official name of the region — Thorium anomaly Compton — Belkovich.
The location of the anomaly on the lunar map (here and illustrations by work).
Recent observations made by powerful optical camera apparatus Lunar Reconnaissance Orbiter (LRO), have allowed scientists to distinguish volcanic features in the center of the anomaly. Three-dimensional, high-resolution thermal radiometer data Diviner same probe showed signs of not just of volcanism, but it is much less common silicon volcanism.
The very existence of the volcanic province should force scientists to change the view of the volcanic history of the Moon, says Bradley Dzholliff from the University of Washington (USA), who led the team that examines images LRO.
Lunar volcanism is very different from Earth as the Moon — a relatively small body, which is rapidly cooled and never had plate tectonics. It is believed that the moon appeared about 4.5 billion years ago by the collision of Earth with a certain protoplanet the size of Mars. Was thrown into orbit magmatic material about 400-km depth, but since the moon has no atmosphere, it has cooled down for some 100 million years. In the end, light minerals (eg, feldspar) crystallized from a magma and floated to the surface, forming the lunar mountains. On the contrary, more solid rocks containing iron and magnesium, after crystallization fell, reaching the top of the mantle of the Moon.
For the differentiation of the crust and mantle followed by a period of volcanic activity (4-3 billion years ago): basalt lava pour into the lunar surface, filling old dents and other lowlands. One of the mysteries — the unevenness of the basalt floods. Almost a third of the visible side of the Moon is covered with ancient traps, but on the other side, where the crust is thicker, they are much smaller. Moreover, almost the entire lunar basaltic volcanism was, not silicon, that is the lunar lava rich in minerals containing iron and magnesium, and no silicon and aluminum.
For comparison, the Earth's continental crust — the result of active geological processes (subduction, magma intrusion, mountain building) — includes a number of breeds that are in composition intermediate between basalt and silicon. In particular, it is widely used granite. On the Moon, granite is rare.
Up until recently, scholars have identified two types of lunar terrain — dark ("sea") and light ("land" or "alpine"). In 2000, Mr. Dzholiff and his colleagues have proposed three other regions, which corresponds better to the geological history of the Moon. One of them, covering the main part of the "sea" of basaltic volcanism, called the Procellarum KREEP Terrane (PKT). KREEP mean potassium (K), rare earth elements (REE) and phosphorus (P). In this type of surface, according to the expert, there are pockets of thorium, for cooling magma ocean thorium mostly excluded from crystallizing minerals and found himself sandwiched between the crust and the mantle.
In fact, the concentration of the fuel elements under PKT may bear partial responsibility for the intense volcanism. "Seas", explains Mr. Dzholliff, formed due to the fact that the hot radioactive elements melted mantle rock. This led to the formation of basalt lava that poured through the cracks on the surface. More than half of PKT appeared as a result of this process.
Basalt "seas" are visible to the naked eye from Earth as dark spots. Volcanic deposits by lavas with high silicon content, in contrast, are known as "red spots." Often, they have a characteristic "dome" and almost all of them are surrounded by PKT.
Thorium anomaly Compton — Belkovich topics and interesting that occurs away from the PKT: from the nearest zone PKT it separates approximately 900 km. At the center of this formation is a small volcanic complex of 25 to 35 km in diameter (diameter craters Compton and Belkovich — 162 and 214 km, respectively).
"The source images LRO, made a narrow-angle cameras, visible signs of lumpy terrain and collapse of rock, which could be the result of volcanic activity, but it could also be a result of the release of the material during the formation of craters, — says Mr. Dzholiff. — That is why we looked at the other images obtained already after LRO entered the 50-kilometer orbit, with a resolution of 50 cm per pixel. "
The last word in favor of a volcanic region has distinctive dome on a plateau with a broad central depression. Typically, there is a dent in the fact that the magma settles down and retreats.
While the steep slopes of basaltic volcanic moon is 7? less, here the figure is 20-25?. This means that they formed a viscous lava.
Data analysis of the thermal radiometer Divine showed a high concentration of species with a high content of silicon — granite and rhyolite.
|Three-dimensional image of the region. Obvious central depression, which is difficult to explain anything other than volcanism.|
Mr. Dzholliff and his colleagues suspect that the volcanic province is much younger than most of PKT. "Although by the direct analysis of samples of lunar rocks, we know that most of the volcanic maria took place 4.3 billion years ago, the view from orbit suggests that some of the" sea "of basaltic flows formed billions of years ago — he says. — We have no way to directly date the formation of craters near Compton and Belkovich, but since there are relatively few craters, the surface appears to be relatively fresh. Accordingly, if the volcanic province formed very late, it can not be the result of radioactive decay, because by the time the source of heat was gone. "