Some unexpected results of contemporary research in the rapidly expanding study of the early history of the Earth as revealed by the application of space technology (PLANETARY GEOLOGY).

 In the 35 years following the second world war the art of rocketry rapidly advanced to become a useful scientific tool, with knowledge and technical expertise progressing side by side as uneasily enforced partners during the course of the "space race" between America and Russia. From the late nineteen forties through to the economies of the seventies the story was a continuous one, beginning with research in the upper atmosphere moving up to orbiting space satellites, and culminating in the manned Moon landings of Apollo 11 to 17.

 The late seventies saw unmanned probes sending back pictures and data from Mercury, Venus, Mars, Jupiter and Saturn, not to mention the amazing moons of the last two. Information from the Voyager mission should continue to be received till the mid eighties, and there is always the projected fly past (and possible landing) on comets Halley and Tempel-2 in 1985 and 1988-89, thus as the results have become available for comparison a new picture of the common history of the Earth and her nearest neighbours has begun tentatively to emerge. In this way nearly 40 years worth of accumulated information gleaned from these historic but expensive research efforts is beginning to be pieced together to give some insights into past events in the development of the Earth itself, and not always in ways that could have been foreseen in 1945.

Automatic satellites are monitoring the Earth's weather, the ocean depths have been surveyed from space stations such as Skylab, as varying vantage points in orbits above the Earth enable patterns to be recorded that are invisible to a ground based observer. Infra-red cameras detect minute variations in vegetation cover and temperature which reveal traces of land use, mineral deposits, or diseased crops, and it is possible to take a global view of climatic and geological systems which previously was possible only in theory.

The influence on the weather of the "solar wind" of electrically charged particles continuously discharged by the sun across what was once thought to be the empty vacuum of space is a discovery which has come as a direct result of space born instrumentation. The Earth and the planets are now known to be surrounded by radiation belts such as the Earth's "Van Allen" belts, and all of these scientific "spin-offs" of space exploration are leading to the restructuring of many ideas as to the past evolution of the Earth and the solar system.

 A recent result of the detailed analysis of the several hundred Kg. of rock samples brought back from the Moon's surface, has been the establishment of a sequence of events in the far geological past, which have caused the familiar pockmarked lunar surface with its circular craters. These periods are named after prominent lunar landmarks or features thought to be contemporary with them. There are three of these and they comprise, firstly the "Imbrian" at 4,000 million years ago and named after the Mare Imbrium a lava-filled impact basin 1,00OKm in diameter, and secondly and thirdly the "Eratosthenian" at 2,000 My and the "Copernican" at 1,000My which are named after the craters Eratosthenes and Copernicus. Geological timescales tend to be so vast as to be meaningless (and can be devalued by fashion) so that it is more relevant to say that the Imbrian was taking place as the first oceans were beginning to form, the Eratosthenian accompanied the earliest cellular life-forms, and the late Copernican was contemporary with the first appearance of living creatures in the fossil record of the dawn of the Cambrian period on Earth.

Each of these periods of events is considered to have consisted of successive phases of intermittent or intensive bombardment of the Moon by meteorites, the earliest being the most violent and prolonged in a series of far-reaching geological shake-ups in which matter (in the form of massive rocks left over from the formation of the solar system) impacted with the Moon's surface at speeds of tens of kilometres per second. It is concluded that the Earth and Mars as well as Mercury and the moons of Jupiter and Saturn, (not enough is known of the surface of Venus) were all affected in a like manner during the same or similar periods.

Some of the last major impacts or crater forming events on the Moon (during the Copernican period of up to 1,000My) were taking place during the epoch known as the Proterozoic in contemporary Earth geology, a period which saw the last general upwelling of the ultra deep level mineral called "Anorthosite", a mineral which has not found its way to the surface since then, but which is found in lava flows on the Moon which are associated with rupturing of the lunar surface due to massive deep impact damage by meteorites. This period also spanned the ages during which the sediments were deposited which (after a phase of heat or metamorphosis some 500My later) were to became the Moine schist series of shield rocks which form so much of the west and north of Scotland and which underly the Caithness flagstone (old red sandstone) series.

The weathering caused by the Earth's water and atmosphere has all but erased the actual craters which must have studded the land surfaces of the youthful planet. Recent research has however begun to produce results which confirm that these ancient impacts had a direct and far reaching effect on the terrestrial shield areas of which the north of Scotland is one. One important aspect of these ancient shield areas with their crystalline metamorphic rocks is the presence there of extensive mineral deposits, and in every part of the world where these rocks are to be found they are the subject of close scrutiny by geologists and mineralogists in the employ of commercial mining enterprises.

During these surveys detailed 3-D relief maps are made as part of the routine process of investigating different geographical sectors. The geological structure of the area under consideration is more vividly displayed when the vertical factor in the map is increased by two to give double the effect of height. These maps have been found to exhibit many interlocking circular structures when photographed or viewed in oblique light. The mineralogist John Saul ("Nature" Vol. 271; 26.1.78; P. 345) has charted many hundreds of these relict structures and noted that numerous mining sites based on mineral deposits lie at overlapping junctions of circle rims. This is because the deeply sited original sources of the mineralising fluids below have had access to the surface (where the minerals have crystallized) by means of vertical faulted or fissured "plumbing systems that have taken the place of the fractured crater rim.

(Residents in Sutherland will be interested to note that the gold deposits at Kildonan are to be found in a ring of gold bearing streams leading in from a roughly circular "zone" round the area of Kildonan, but also outwards away from Kildonan if the streams rise on the other side of the watershed zone where the deposits originate.)

These structures are not thought of as the physical remains of actual craters but more like "after-images" comprised of ridges, hills, river channels, lakes, cliffs and valleys, which have subsequently appeared during earth movements on the site of the original crater limits but in much later sediments overlying or replacing the ancient fractures. The circular structures like the lunar craters are often many hundreds of kilometres in diameter, in fact the "Vredevoort Ring" in South Africa has been calculated to be the outcome of the collision with the Earth of an asteroid or very large meteorite, the dimensions of which would make it at least as vast as a Himalayan sized mountain, and which is thought to have had an impact mass of some thirty thousand million tonnes! In a nuclear age it is more than a little off-putting to discover that the Earth has weathered many such impacts in which most of the mass of both meteorite and the rock excavated to form the crater, would be explosively vapourized on contact, and yet the physics involved (in which the kinetic energy is totally turned to heat) would not have troubled Isaac Newton.

While the scale of both the rings and the means with which they were created, tend to dwarf human explosions, they are not out of line with similar estimates for the Moon's larger craters, and would make even something like the Gulf of Mexico a ready candidate for interpretation as a terrestrial "lunar style feature". Landform structure is visibly character- ised by the repetition of two basic types of shapes, one circular or oval in the form of island arcs and bays, and the other linear in the form of fault boundaries and rifts. The Earth's surface moves sideways or vertically along faults and rifts and sinks or rises in the rounded forms of geo-synclines and anticlines (the last of which are important to the oil industry as they form dome-like traps undergrounds for oil). The rifts seem to be connected with the "land tide" which the Moon causes by means of its gravity, and the slow rise and fall of the basin-like structures to be linked to convection currents beneath the Earth's crust. However, the newly formed science of "Impact Geology" may be able to provide links to the two families of geological forms.