ESA Science & Technology30-Jun-2005 15:12:23



A deserted and cratered world

The Sun and the Moon have always been mankind's guiding lights and yardsticks for everyday life. They have formed a celestial couple, adored and feared, influencing human existence. Since ancient times the Moon has inspired much folklore. How different it would be if Earth had several natural satellites, like Saturn's 33 companions! And if Earth had no Moon, its axis of rotation would wander chaotically, rendering the emergence and survival of life more difficult.

But the Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. It has a diameter of 3476 km, over a quarter of Earth's diameter and an eightieth of its mass. So similar in size, the Earth and Moon have often been considered as a double planet. Surface gravity on the Moon is one sixth of that on the Earth.

How high is the Moon?

The distance from Earth varies, 356 400 km at its closest, 406 700 km at its furthest. It revolves on its slightly inclined axis in 27 days 7 hours and 43 minutes, precisely the same as its orbital period, the time it takes to complete one orbit of the Earth. And turning in the same direction, the Moon always keeps practically the same face towards the Earth, locked in this configuration by gravitational laws. This face changes slightly on its borders as a result of several effects, thus a total of 59% of the Moon's surface can be viewed from Earth at some time or other.

The cycle of lunar phases is a consequence of the continuously changing alignment of the Sun, Earth and Moon over the Moon's orbital period. As days go by, the proportion of the Moon's sunlit side that is visible from Earth changes.

The magnitude of a full Moon never fails to impress, but this brightness is deceptive. This satellite's surface is actually quite dark. Its albedo, the fraction of the total light that is reflected from it, is on average only 0.067, lower than all the planets except Mercury. The Moon has temperatures varying from 123 C in the day to -170 C at night at the equator, and down to -233 C in permanently shadowed polar craters.

Crater filled highlands and waterless maria

The Moon's surface displays two different types of terrain, clearly visible to the naked eye: brighter highlands, 'terrae' or 'continents', that cover 80% of the lunar surface (65% of the near side), and some 20 darker areas improperly called maria since these 'seas' are totally waterless.

The maria are like plains with few large craters. They correspond to the filling of old and large impact basins with basaltic lavas between 3.9 and 3.0 billion years ago. The oldest basins gave irregularly shaped mare (such as Tranquillitatis or Fecundidatis), while the most recent gave more circular mare (such as Crisium, Serentatis and Nectaris). The maria areas are concentrated on the nearside of the Moon, where they cover 35% of the surface. They cover only 5% of the hidden side of the Moon, with the largest Sea of Moscow first seen by the soviet Luna-3 probe.

Highest elevation formations were given the names of terrestrial mountains. One peak, Mount Leibnitz, culminates at 8000 m above its surrounding south pole areas. The highlands are filled with innumerable craters often 50 km in diameter. One estimate by the 19th century astronomer Julius Schmidt put the number of craters at 30 000. Today it is reckoned that there are at least 10 times that number, with a diameter larger than 1 km, on the visible side of the Moon alone. The largest exceed 200 km. Volcanic craters are rare and comparatively small. The lunar impact craters inform us about the history of Earth's bombardment since its birth.

Lunar elementals

As on Earth, oxygen is the most abundant element on the Moon's surface but in the form of oxides. There are lots of silicates everywhere. Magnesium, iron and titanium are enriched in the surface of the maria, as is an abundant quantity of pyroxene. The highland rocks consist mainly of feldspar, enriched with calcium and aluminium. Three minerals unknown on Earth have been discovered on the Moon. There are also traces of sulphur, phosphorus, carbon, hydrogen, nitrogen, helium and neon in the soil. The Moon surface is constantly exposed to the solar wind from which it traps the precious hydrogen and helium (and also the helium-3 isotope that could be used for nuclear fusion energy). The enhancement of hydrogen at the poles could be interpreted as a trapped deposit of water ice. These resources could be exploited in the future.

There is virtually no atmosphere on the Moon. The erosion process is thus from impacts of asteroids and meteorites as the myriad of craters testify. The most violent impacts fractured the lunar crust allowing internal magma to flow out. The surface is so churned by the small impacts that it has been transformed into a soil-like layer, the regolith, that can be 15 m deep.

Below this surface, the interior of the Moon consists of a homogeneous solid crust (50-75 km thick) and a mantle (lithosphere) down to some 800 km, and then an intermediary layer (asthenosphere) halfway to the Moon's centre at which may be a small core consisting essentially of molten iron. Moonquakes can occur, although rare and less powerful than on Earth. There is no significant magnetic field.

Mysterious birth and Earth-Moon evolution

Although the age of the Moon is generally recognised to be 4.6 billion years, the same as the Earth and other Solar System planets, the way it was formed is still an open question. Four models have been advocated in the past: capture, co-accretion, fission or collision.

The Moon may have been formed elsewhere in the Solar System and been captured by the Earth, but this poses dynamical and composition problems. The co-accretion model sees the Moon and Earth forming as a double planet system, but this raises composition problems. Finally the Moon could have been formed by fission of part of the early very fast rotating Earth's mantle to create a new body. Here arise dynamical issues. To date, the first three theories do not fit the evidence that has been accumulated notably as a result of lunar exploration. This has, for instance, clearly shown all the differences in the composition of the Moon and the Earth.

The most widely accepted scenario, is that the Moon is the result of a giant impact of a Mars-sized body, sending ejecta from both the early Earth mantle and the impacting body into orbit, where accretion later formed the Moon. However to narrow down the different collision scenarios and the early Earth-Moon evolution, one needs to measure the global and regional elemental composition of the Moon. This is a key science objective for SMART-1.

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