Even the best astronomers get it wrong sometimes

The surface of Venus is a scorching hell, not an oceanic paradise as was once believed
 

By Patrick Moore

Astronomers are not infallible. They make mistakes, but unlike politicians they are always honest to admit them. The classic case, to my mind, goes back over 10 years now, when radio astronomers at Jodrell Bank believed that they had located a planet in orbit round a pulsar. In fact, they had made an elementary mistake in their calculations.

It would have been so easy to blame a computer fault or something of the kind. Not a bit of it. Jodrell Bank’s director, Andrew Lyne, said simply, “We got it wrong.” His frankness further enhanced his already worldwide reputation.

Casting my mind back, I can think of three cases where almost everyone got it wrong. The first concerns Mars. By the end of the War, Percival Lowell’s brilliant, canal-building engineers had been reluctantly consigned to the realm of science fiction, but it was still believed that the dark areas were tracts of primitive vegetation. In 1956 Gerard de Vaucouleurs, one of the leading planetary observers, claimed that when water vapour was released from the Martian polar icecaps, every spring, a “wave of darkening” spread slowly equatorward, as the vegetation was revived by the arrival of moisture. Even the canals had “a basis of reality”.

One comment, made by the Estonian astronomer Ernst Öpik, was particularly persuasive. As we know, Mars experiences frequent dust storms and some of these are global. I remember when Mariner 9 was approaching the planet and I was asked by NASA to go and make some observations with the Innes 26-inch refractor at Johannesburg. On my first view, I saw a virtually blank disc – I likened it to a red Venus. It took weeks for the dust to clear.

Well, said Öpik, if the dust covers the entire planet, the dark regions must be made of plants which can grow and push the dust away, otherwise the surface would stay red all the time. Most agreed. It was only when close-range pictures were obtained that the vegetation theory had to be abandoned. As for the wave of darkening – well, I could see no sign of it, even though I had been making regular observations. I was right; others were not. In a moment I will come to a controversy in which I was badly wrong...

The mystery of Venus
Venus had been regarded as ‘the planet of mystery’. In size and mass it is almost a twin of the Earth, and it is the brightest object in our sky apart from the Sun and the Moon, but its surface lies below its cloudy, obscuring atmosphere. The orbital period was 224.7 days, but what about the axial rotation period – Venus’s ‘day’? Nobody knew.

Efforts were made to measure it from following the drifting of surface features across the disc. This is easy enough with Mars or Jupiter, but the markings on Venus were too vague and impermanent to yield any reliable results. Estimates ranged from around 24 hours up to 224.7 days. If the latter had been correct, Venus would have ‘captured’ or ‘synchronous’ rotation, keeping the same face turned permanently sunward just as the Moon behaves with respect to the Earth. The general feeling was that the period was likely to be ‘a few weeks’, following the photographic results of work by GP Kuiper.

In 1960 I wrote a small book about Venus. In it were listed the rotation periods given by different authorities. There were over a hundred estimates and every one of them turned out to be wrong. The true period is 243 days, which is longer than the orbital period – and even more confusingly the spin is from east to west, opposite in sense to Earth and Mars. To an observer on the surface, the Sun would rise in the west and set in the east, though of course it could never be clearly seen through the clouds. There is no such thing as a sunny day on our sister world.

We were also wrong about the surface conditions. Venus is known today to be a scorching-hot world, with lava flows together with volcanoes that may well be active, but a few decades ago there was wide support for the picture painted by FI Whipple and DH Menzel, according to whom Venus was almost completely covered with water. An oceanic Venus sounded attractive. Life could begin there and evolve just as it presumably did in our seas long ago, so that our first astronauts might be welcomed by an ichthyosaurus or perhaps a pterodactyl.

Lunar blunders
Now for the Moon. Originally it was assumed that the craters were of volcanic origin, because they looked like volcanic calderae and there seemed to be no obvious alternative. A suggestion made in the 1830s by Franz von Paula Gruithuisen that they could be due to falling meteorites was not taken very seriously. However, after the end of the War the impact theory was revived, initially by Ralph Baldwin in America, and this time it took root.

At times the discussions between the ‘vulcanists’ and the ‘meteoricists’ became strangely heated. I was a convinced vulcanist and I put forward my arguments, which I firmly believed to be conclusive. For instance, when one crater breaks into another, it is the smaller formation that intrudes into the larger. This is true in 99.9 per cent of cases, so that the crater distribution is not random, as it would be following a meteoritic bombardment.

Similarly, large structures of obviously different ages are lined up in chains, for example: Petavius; Langrenus; Vendelinus. And when one crater breaks into another, the wall of the broken crater can often remain undamaged right up to the point of junction. There are endless examples of the non-random distribution of the craters. Many rounded peaks have summit craterlets, symmetrically placed twins (Challis/Main), concentric craters (Taruntius); crater valleys (Rheita)... but why go on? Finally, if the Moon has been so pelted, and considering that there is no atmospheric erosion, where have all the meteorites gone? Certainly they are not lying around waiting to be picked up, as in Antarctica.

I think I was one of the last of the vulcanists to admit defeat, but in the end I had no choice. There has undoubtedly been widespread vulcanism on the Moon, but the craters are not calderae, as I thought. To the impact theory – sorry. This was one of my major blunders!

This column appeared in the July 2007 issue of Sky at Night Magazine