Hot rocks and big splashes

Heather Couper and Nigel Henbest examine new evidence backing a controversial theory of the Moon's formation

The Moon must be just about the most looked-at astronomical object. After the Apollo missions to the Moon in 1969 to 1972, you would think we must know just about everything there is to know about our fellow world in space. The Apollo astronauts not only subjected the Moon to on-the- spot scrutiny, but brought back one-third of a tonne of moon rocks to be analysed in the lab.

Yet Earth's companion still holds plenty of mysteries: astronomers are not even sure how it was born. That is why they are poring over the results of a recent, little-publicised lunar probe. The tiny unmanned American Clementine spacecraft was designed to test Star Wars technology as it orbited the Moon. But it has also sent back unique scientific data. Its camera has viewed the entire lunar surface in detail. Clementine has also revealed what is happening below the surface. Wobbles in its orbit indicated denser regions of the lunar crust that pulled it from the predicted path.

To astronomers' surprise, Clementine's results show that the highest lunar mountains tower 16,000 metres (52,000 feet). This is about the same height as Mount Everest lies above Earth's deepest ocean trenches - and that on a world only one-quarter Earth's diameter.

The gravity measurements reveal the Moon's crust to be immensely variable in thickness. On some parts of the Moon's far side it is 120 kilometres thick, but the hemisphere facing Earth is much thinner. Under the two dark plains that make up the "eyes" of the Man in the Moon, the crust is only one-tenth as thick.

Clementine has also bolstered support for an exciting but controversial theory about the Moon's origin. From its orbit, the spacecraft measured the amount of iron in the Moon's surface and found considerably less than the abundance of iron in the Earth's crust. So the Moon is not just a piece broken off Earth, nor could it have formed with Earth, from a mix of identical material circling the young Sun.

The discovery supports the Big Splash theory. This says another world - as large as Mars - hit early Earth, splashing hot rocks into space. These drops of incandescent magma solidified into a belt of pebbles around Earth, which then came together to form the Moon. The latter's surface material is thus telling us the abundance of iron in that impacting world, which presumably streaked in from a remote part of the solar system.

The planets

Three planets are well on view this month. First is Mars, low in the west after sunset and fading gradually. The crescent Moon lies near Mars on the evening of 3 July.

Jupiter shines brilliantly, low down in the south during the late evening. With binoculars held steadily, look for its four biggest moons, appearing as small points of light to either side of the planet. They change position nightly in a continuous dance.

These four moons are substantial worlds in their own right, each larger than the planet Pluto. Two of them are bigger than Mercury as well. In the past few months, astronomers have found signs of oxygen on two of these moons, Europa and Ganymede: they are the only worlds we know with oxygen apart from Earth.

Saturn rises just before midnight, lying in the faint stars of Aquarius. Its rings are almost edge-on, as seen from Earth. Without the extra sunlight reflected by the rings, the planet appears slightly fainter than usual.

Venus and Mercury are both morning stars this month, rising about an hour before the Sun. But they will be too low in the morning twilight to be easily seen.

The Stars

Use Jupiter as a signpost to Antares, the red giant star that marks the heart of Scorpius, the scorpion. To its left, and always low in British skies, is another ancient constellation, Sagittarius. To the Greeks, this group of stars represented an unusual archer, a centaur - half horse, half man - wielding a bow. To us, though, it resembles nothing so much as a teapot, complete with a cloud of stars marking the steam rising from the spout.

Sagittarius lies in the direction of the centre of our galaxy, and the Milky Way appears particularly bright here. It is also a jewel-box of star clusters and nebulae. From Britain, these remarkable sky sights are dimmed by mist and haze. Some, indeed, never rise above our horizon. But if you are venturing to more southerly climes this summer, take your binoculars with you and "sweep" just above the teapot of stars. As well as seeing dozens of beautiful star clusters, you may be rewarded by finding the Lagoon Nebula, a large glowing gas cloud that is centred on a gorgeous cluster of young stars, or the enigmatic band of gas making up the visible part of the Omega Nebula, lit up by brilliant stars hidden in a neighbouring dark cloud.

Diary

(all times BST)

4 Earth at aphelion (farthest from Sun)

5 9.02pm Moon at first quarter

12 11.49am full Moon

17 Neptune at opposition

19 12.10pm Moon at last quarter

20 Uranus at opposition

27 4.13pm new Moon

28 Mercury at superior conjunction (behind Sun)