Astronomical events and the Earth

The Earth is still influenced by certain astronomical phenomena, although the influence of some of these is still speculative. The Earth is affected by matter falling onto it and by the type of radiation, and its intensity, shining onto it.

Interplanetary space still contains stony rubble which crashes onto the Earth's upper atmosphere at velocities of tens of kilometres per to second. Fragments with a mass exceeding 1 kg reach the Earth's surface, and about twenty of the se meteorites are recorded each year. Very large meteorites (of 100 or more tonnes) are not significantly braked by the atmosphere and they cause impact craters.

The Arizona meteorite crater, 1.2 km in diameter and 100 m deep, is the best preserved of these, and it is a few thousand years old. The total amount of debris that the Earth sweeps up in a year is 10000 tonnes, most of which comprises small particles weighing 1 g or less. The Tunguska event of 1906, which flattened a huge area of Siberia, is thought to have been a collision of the Earth with a comet, an event that is unlikely to happen more than once in a billion years.

A meteorite crater in Arizona. USA. formed about 50 Ma ago

Our Sun displays highly energetic phenomena, principally flares, in its surface layers. These inject clouds of electrically charged particles into space. As these brush past the Earth they can interact with the upper atmosphere and enhance the aurora. At the same time the ionosphere is temporarily destroyed, which inhibits long-distance radio-wave propagation.

The Sun's average energy production may also vary over millions of years. Any evidence favouring this is almost non-existent. It is an attractive idea for explaining fluctuations in the Earth's climate, but extremely difficult to investigate properly. On short time scales, roughly eleven years, the sunspot density reaches a maximum.

At this time the aurora is prominent, on account of the increased production of charged particles from active regions on the Sun. Perhaps this cycle subtly modulates the Earth's climate and weather; real evidence for this is, at best, weak. It is of course possible that the amount of solar radiation reaching the Earth could vary even though the Sun itself remains constant. This would be the case it Our solar system were to pass through an interstellar cloud, and it is not impossible that this has happened at some stage.

In the realms of speculative catastrophism it can be asked: what would happen if the Earth were close to a supernova or if it were hit by a meteorite? This question may be relevant to the sudden disappearance of perhaps as many as 75 per cent of all the species on this planet about 65 Ma ago. This major extinction event occurred within less than a few hundred thousand years, and possibly in a mere thousand years.

Clearly the biosphere was subject to major stress at that time. It may have been a purely terrestrial event caused by, for example, a great increase in volcanic activity. One candidate for extraterrestrial influence is a nearby supernova. One within thirty light years would bathe the Earth in X-rays and high-energy particles. This would grossly modify the atmosphere and cause a dramatic increase in radiation diseases, a drop in temperature and a worldwide drought.

The Earth could get near to such a supernova when the solar system passes through one of the spiral arms. Another possibility is a collision with an asteroid or very large meteorite. There is evidence in ocean sediments that something odd happened 65 Ma ago. Elements that are more abundant in extraterrestrial matter than in the Earth's crust are commoner in this layer.

This is consistent with a huge collision that would also have clouded the atmosphere with dust, eliminating most of the sunlight. Ideas of this type are, of their nature, hard to confirm, but cosmic catastrophes are attractive as explanations of extinctions such as that of the giant reptiles.