Craters are common on some planets

Press, 3 August 1976, Page 27

Craters are common on some planets

By ]

F. M. BATESON

It is now well established that craters are not only a common feature on the surface of the Moon, but exist in profusion on both jJlercury and Mars. Even the tiny moons of Mars. Phobos and Deimos, are pockmarked with craters.

It is also reasonable to believe that many of the asteriods have craters caused by collisional impacts. It does not necessarily follow that all craters have been formed in the same way. There is better knowledge of the Moon, because the various Apollo missions brought back samples of lunar rocks. The ages of these rocks range from an average of 4000. M years for those from the highlands to

3500 M years for those from the lunar maria.

The age of the highland rocks indicates that the lunar surface solidified when the Moon only 500 M years old. wheras the rocks from the maria obviously solidified at a later time.

Both types of rocks are basalt, formed by the cooling of lava. They do differ in their chemical composition, those from the highlands being lighter, due probably to an abundance of aluminum wheras the maria rocks contain more iron.

The highland rocks are fragments of older rocks that appear to have been smashed by impacts and then forced together again by further impacts. There are two processes by which heating is possible to form molten lava. One is that radioactive energy released in the decay of radioactive elements melted much of the surface and interior of the Moon.

The lunar rock samples do show a fair amount of radioactivity, but melting by this process is slow. The other alternative is that melting was caused by an intense bombardment of meteorites early in the Moon’s history. At the time the Moon and Earth were formed, along with the other planets, there was probably a lot of debris surrounding them. This debris consisted of rocky fragments ranging in size up to a hundred kilometres in diameter.

This literally rained down on the Moon in such profusion that the energy liberated was sufficent to melt the entire surface. Naturally as a result of this there came a time when the amount of debris had diminished. Then the surface of the Moon cooled and solidified However meteorites, in fewer numbers, continued to crash onto the Moon. Occasionally an exceptionally large meteorite struck and possibly these blasted out the roughly circular maria. Meanwhile the interior slowly heated up under the decay of radioactive material. When the Moon was about IOOOM years old the interior became partly molten, and lava poured across the surface through fractures created by the impact of meteorites.

These fractures would lie beneath the maria so that they became flooded with lava. This period of volcanic activity lasted about 500 M years, but ceased because the Moon, being small, rapidly radiated its heat to space.

The drop in te.mperature that resulted caused the outer crust to harden and eventually it became cold and rigid down to som j 800 kilometres. Possibly a small centra) core is still molten but its lava cannot force its way up through the thick overlying crust.

The surface of Mercury has areas in which craters appear in profusion, whilst much of the surface is comparatetvely smooth where the craters appear to have been flattened. There is sharp disagreement as to which type of formation is the oldest.

However, the evidence does suggest that at least once during its history Mercury suffered an intense bombardment of meteorites. Some investigators think that Mercury suffered a second period of impact cratering. On Mars there appears to be ample evidence to show that many of its craters are the result of impacts, whilst others are due to volcanic action.

There are also many features that strongly suggest that water once flowed on Mars because it is difficult to expalin these features other than as rivers laid down in the past but now dry.

There is no question about the craters on Phobos and Deimos. Both satellites show very clearly the effect of impacts.

In fact. Phobos has sustained some impacts which sheared away portions of the satellite itself Collisonal break up is generally accepted as having taken place amongst the

It is probable that the 1 arth also suffered front an intense bombardment from meteorites when it was young. Although meteor crater* are found on the Earth these have resulted from impacts in

comparatively recent geological times. The effect* of the early bombardment have been obliterated by erosion, crustal movements and changes caused through its continual volcanic activity.

As more knowledge is accumulated on the age of the craters on Mars and Mercury it can be expected that there will come a better understanding of events that occurred during the early history of the Solar System. August Planets Mercury can be well seen during the second half of August in tinnorth western evening sky. It set at 648 pro. on August 1 and will set at 8.20 p.m. on August 31. It will be higher in the sky than the brilliant Venus, and will be close to the Moon on the evening of August 27. Venus will gradually become more prominent this month, from setting at 624 p.m. on August 1 to 7.40 pro. on August 31. Jupiter is conspicuous in the north eastern sky in the mornings before daw'n, rising at 2.45 a.m on August 1 and at 1.03 a.m. on August 31. Mars, low in the evening sky at sunset, is now too far from us to be of any interest even with a telescope. Saturn rose with th<Sun on August 1 and is lost in the glare of the dawn sky all month. Several meteor showers are active this month and will provide many meteors, especially after 11.0 p.m. when the Moon is absent.

The periodic comet D'Arrest is making its thirteenth appearance since its discovery in 1851. It will move rapidly sou '.ward, passing close to Fomalhaut on August 25 and 26. It has never been a bright object, but this year it may become visible in binoculars towards the end of August.