The Nearer Planets

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There was a time, not so long ago,when most amateur astronomers concentrated almost exclusively upon the members of the Solar System. This is not true today, but of course the Moon and planets remain favourite targets - and despite the space probes, there is still a great deal about them that we do not know. Even a small telescope will show details on some of them, so let us consider the planets one by one.

The four members of the inner group - Mercury, Venus, the Earth and Mars -are solid, rocky bodies. They are comparable in size, and all have atmospheres of a kind (though that of Mercury is extremely tenuous). These are the only common factors. Otherwise, they are as different as they can be.

Mercury, whirling round the Sun at an average distance of only 36 million miles, is never easy to observe. It always lies somewhat near the Sun's line of sight, and we can well understand why it was named after the elusive, fleet-footed Messenger of the Gods. Moreover it is not much larger than the Moon, and is more than 200 times as distant, so that ordinary telescopes will show little except a tiny disk with its characteristic phase.

Mercury takes 88 days to complete one orbit found the Sun. It used to be thought that the axial rotation period was exactly the same, so that Mercury would always keep the same face turned sunward, just as the Moon does with respect to the

Fig. 9.1. Comparative sizes of the Earth, Mercury and the Moon.

Fig. 9.1. Comparative sizes of the Earth, Mercury and the Moon.

Earth. Over part of the surface there would be perpetual daylight, with a surface temperature exceeding 700 degrees Fahrenheit; over another part there would be everlasting night, so that the surface would be colder even than remote Pluto. Between these two extremes there would be a "Twilight Zone" over which the Sun would rise and set. Mercury's orbit is not circular, and so its velocity varies between 362 miles per second at perihelion and only 24 miles per second at aphelion. This would result in effects analogous to the Moon's librations, so producing the Twilight Zone.

Then, however, it was found that the 'night' hemisphere is not nearly so cold as it would be if it received no sunlight at all. Radar measurements confirmed that the true rotation period is 58.6 days, two-thirds of a Mercurian year. Science fiction writers were disappointed to learn that there is no area of permanent day, no region of everlasting night, and no Twilight Zone.

Our first real knowledge of the surface features was due to the U.S. probe Mariner 10. It was launched in November 1973; in the following February it flew past Venus, using the gravitational pull of that world to direct it in towards a rendezvous with Mercury. The first active pass of Mercury was made in March 1974, and there were two more before the probe finally "went silent". Many hundreds of close-range photographs were obtained, and it became clear that superficially, at least, Mercury is very like the Moon. There are craters, mountains and valleys, and even one huge, mountain-ringed structure - now known as the Caloris Basin - which looks decidedly "lunar".

Less than half of the total surface was surveyed by Mariner 10, but there is no reason to suppose that the remaining areas are fundamentally different. Other interesting facts also emerged. Mercury has a definite magnetic field; it is much weaker than the Earth's, but it exists, and presumably indicates the presence of a large iron-rich core. There is a trace of atmosphere, but the density is so low that it corresponds to what we would normally call a vacuum, and it will be of no use whatsoever to possible astronauts of the future. It is painfully clear that so far as Mercury is concerned, any life of the kind we know is quite out of the question.

I have glimpsed a few patches on Mercury, with a 6-inch refractor, but little can be seen with amateur-owned telescopes. This does not mean that there is no point in looking for Mercury. It is always satisfying to see the strange little world glittering shyly in the late evening or early morning, and on an average it can be seen with the naked eye at least a dozen times each year.

When Mercury is glimpsed without a telescope, it is bound to be near the horizon, so that it will be shining through a deep layer of the Earth's atmosphere, and the image will be unsteady. The best method is to find the planet as early as possible, while it is still fairly high up; for sweeping, it is advisable to use either binoculars or else a low magnification on a small telescope (I have found that a power of 25 on a 3-inch refractor does very well). A drawing can then be made while the sky is still bright.

Telescopes fitted with equatorial mountings and clock drives allow a faint object to be found without any tiresome sweeping. This saves a great deal of time, though Mercury is never easy to locate except with a large instrument. Yearly star almanacs tell where and when it is to be seen, and more detailed tables are given in The Handbook of the British Astronomical Association.

It is also possible to sweep for Mercury in broad daylight, but it is never wise to range about with a telescope until the Sun has set. Moreover, Mercury and the Sun will not be far apart, and there is always the chance that the Sun will enter the field of view during the sweeping, with disastrous results.

For examining the phase and for drawing any visible surface markings, the magnification used should be as high as possible, but the slightest unsteadiness or blurring will be fatal, so that one has to strike a happy mean. All things considered, Mercury is more difficult to study than any other planet, and it is hopeless to expect to see anything spectacular. People who live in or near cities will be lucky to find it at all.

Mercury can sometimes pass in transit across the face of the Sun. The last transit was that of 7 May 2003, and using the projection method I was able to follow it with my 5-inch refractor. Though I should have been prepared, I was surprised to see how small Mercury looked; it was indeed tiny, and much blacker than any sunspots. The next transits will be those of 8 November 2006 and 9 May 2016.

Venus, the second of the inferior planets, is as different from Mercury as it could possibly be. It is almost the same size as the Earth, and is the nearest body in the sky apart from the Moon. It is also more brilliant than any other object than the Sun and the Moon. It is at its very best during the crescent stage, since by the time of "dichotomy" (half phase) it has already drawn away from us, so that its apparent diameter is much less (Fig. 9.2). Altogether, Venus is a most infuriating object.

Moreover, the disk appears virtually blank even with powerful telescopes. Vague, dusky shadings may be seen often enough, but they are not permanent, and are so diffuse that they are hard to define. In fact, we are not looking at the true surface of Venus at all; we are seeing only the upper layers of an obscuring atmosphere.

Before the end of 1962 we knew practically nothing about the surface of Venus, and there were constant references to "the Planet of Mystery". It was known that the atmosphere was rich in carbon dioxide, and since this gas acts in the manner of a greenhouse, the surface had to be hot - but how hot? Even a problem as fundamental as the length of the axial rotation period remained unsolved; various estimates were given, ranging from 22i hours up to 225 days. In the latter case the rotation would be synchronous, so that the same hemisphere would be turned toward the Sun all the time. Opinions as to the nature of the surface fluctuated

Fig. 9.2. Apparent size of Venus at various phases.

Fig. 9.2. Apparent size of Venus at various phases.

wildly between a swampy, tropical hothouse, a planet completely covered by water, and an arid dust-desert without a scrap of moisture anywhere.

Only space research methods could give us the truth. The first successful probe was America's Mariner 2, in 1962, which by-passed the planet at 21,000 miles and proved that the surface really is incredibly hot. (In case you are wondering what happened to Mariner 1, I have to tell you that it fell into the sea as soon as it was launched; apparently someone forgot to feed a minus sign into a computer, which makes quite a difference.) Other missions followed, and in October 1976 two Russian vehicles, Veneras 9 and 10, made controlled landings; each sent back one picture before being put out of action by the hostile environment. Since then the U.S. orbiting Magellan has mapped the whole surface by radar, so at last we know what Venus is like. There are uplands, lowlands and craters; there are volcanoes, presumably active, and lava-flows everywhere. The atmosphere is almost pure carbon dioxide, with a ground pressure 90 times that of the Earth's air at sea-level; the surface temperature is nearly 1000 degrees Fahrenheit, while the clouds contain large amounts of sulphuric acid. Obviously, it is not a good place to visit for a peaceful weekend. There is no detectable magnetic field; the rotation period is 243 days - longer than Venus' "year" - and the planet spins from east to west, in a sense opposite to that of the Earth. The chances of finding life there are effectively nil. So what can telescope studies from Earth tell us?

There is no point in observing Venus when it is shining brilliantly down from a dark sky; the disk will be dazzling, and the image is likely to be violently unsteady. I have found that the best seeing is obtained when the planet can just be detected with the naked eye, shortly after sunset or shortly before sunrise, but observations made in broad daylight are almost as good. Venus is so bright that it can usually be found without much difficulty even when the Sun is above the horizon. In general it will not stand a high magnification, but I have often used over x300 on my 15-inch reflector.

It is worth trying to draw the vague dark patches, and to see how they shift, but well-defined markings are very rare. Bright areas are usually seen near the poles; they are cloud formations and so are completely different from the white caps of Mars. But there is another line of research which is interesting to follow up. This involves the exact moment of "dichotomy", or half-phase.

Since the orbit of Venus is known so accurately, it should be easy to predict the time of dichotomy, but these predictions are usually wrong by several days. When Venus is an evening object, observed dichotomy is always early; with morning elongations, dichotomy is late. The first astronomer to notice this curious disagreement was Johann Schröter, and in writing a paper about it years ago I called it Schröter's Effect", a term which now seems to have been generally accepted.

There is no chance of being out of position, and the effect must be due to the planet's atmosphere. Timing the actual date of dichotomy is therefore valuable. The terminator will appear sensibly straight for several nights in succession, so that a series of observations is necessary. What generally happens, of course, is that clouds intervene at a critical stage, and cause one to miss a vital evening's observation. Filters can be a help, and it is worth using several in turn to check against observations made in ordinary light. (Mercury, incidentally, does not seem to show a Schröter effect, which is understandable in view of its lack of atmosphere. More measurements are needed, but telescopes of considerable power are required, and for this sort of work the refractor has the advantage over a reflector.)

The terminator of Venus shows occasional slight indentations and projections. Schröter believed them to be due to differences in level, and thought that he had charted a mountain 87 miles high (!), but we may now be sure that cloud effects are responsible.

Last, but by no means least, there is the Ashen Light. When Venus is crescent, the night area can often be seen shining faintly, so that the whole disk can be traced. The same appearance can be seen with the crescent Moon,but the cause is different. With the Moon, the glow is due to reflected earthlight, but the Earth is certainly unable to illuminate Venus, and Venus itself has no moon. Weird theories have been advanced to explain the Ashen Light - in 1840, Gruithuisen suggested that it might be due to general festival illuminations lit by the inhabitants of Venus to celebrate the crowning of a new ruler, but since the Light is by no means unusual it would seem that the Government of Venus would be as unstable as that of an African banana state! There have been claims that it is a mere contrast effect, but I am confident that this is not so. I made a special telescope eyepiece with a curved occulting bar, which would cover the bright crescent - and the dimly-lit "night" side could still be seen.

One interesting theory is that it is caused by brilliant aurorae in the upper atmosphere of Venus. Since Venus is closer to the Sun than we are, there is no reason to doubt that aurorae exist, and the explanation is not unreasonable. If we could show that the Light is at its brightest during periods of solar activity, when terrestrial aurorae are frequent, we might be able to clear up the problem. I have made an attempt to analyse the available observations of the Light, but unfortunately the observations themselves are too scattered to be of much use. It must however be added that the "aurora" theory has been weakened by the revelations that Venus has no appreciable magnetic field.

All the markings of Venus are so indefinite that they are hard to record, and there is the added complication that the great brilliancy of the disk tends to produce regular, streaky patterns that do not actually exist at all. The nebulous aspect should be drawn as faithfully as possible, and if the depth or sharpness of a shading is exaggerated - as is sometimes necessary - the observer must always be careful to write an explanatory note beside his sketch. A scale of 2 inches to the planet's diameter is convenient.

Venus, like Mercury, can pass in transit across the face of the Sun, but it does so much less often. Transits occur in pairs separated by eight years, after which there are no more for over a century. The last transits were in 1874,1882 and 2004; the next will be on 6 June 2012, after which we must wait until 11 December 2117. The transit of 8 June 2004 was well seen from my observatory (see Figure 9.3, overleaf) - about a hundred observers joined me there! - and Venus was very evident even with a 'pinhole camera'. We had a splendid view of the Black Drop which had so frustrated earlier observers. Let us now turn to Mars, which is much more rewarding inasmuch as it does show permanent surface markings.

Mars can approach us to a distance of 35 million miles. It is therefore always at least 150 times as remote as the Moon, with a diameter of only 4,200 miles, and it is much smaller than the Earth. Fortunately, it is better placed than either Mercury of Venus. Since it lies beyond the Earth's orbit, it can never appear as a half or a

crescent, while at its most gibbous it looks like the shape of the Moon two or three days from Full.

The main trouble about observing Mars is that it comes to opposition only at intervals of nearly two years, as explained in Chapter 4. Things are at their best for only a few weeks before and after the opposition date, so that the observer has to make the best use of the limited time at his disposal. Not all oppositions are equally favourable, because the orbit of Mars is much more elliptical than ours, and the best oppositions occur when Mars is near perihelion. The opposition of 2003 was much better than that of, say, 2010 (see Figure 9.4). At the end of August 2003 Mars was slightly less than 35,000,000 miles from us - as close as it can ever be. For a few weeks Mars outshone even Jupiter, but when the planet is at its furthest from Earth it is not much brighter than the Pole Star.

Mars has a "year" of 687 Earth days (668 Mars days or 'sols'), and the axial inclination is almost the same as ours, so that the seasons are of the same type. Since the mean distance from the Sun is about 48 million miles greater than that of the Earth, we must expect it to be cool; the maximum summer temperature at the equator can be as high as 50 degrees Fahrenheit, but the nights are much colder than a polar

Fig. 9.4. Oppositions of Mars 1997-2012.

night on Earth. The atmospheric pressure at ground level is below 10 millibars everywhere, and the main constituent is carbon dioxide.

The newcomer to astronomy is apt to be disappointed with his first telescopic view of Mars. Generally there is little to see apart from a tiny disk, reddish in colour, crowned in the north or south by a whitish cap. Seeing much on Mars requires a good deal of practice, because the markings are much less spectacular than the belts of Jupiter, the rings of Saturn or even the phases of Venus. The polar caps, too, are very variable in extent. Moreover, global dust-storms sometime develop which hide the surface details completely.

In 1877 the Italian astronomer G.V. Schiaparelli drew a detailed map of Mars, using a fine 9-inch refractor. He charted the bright areas and the dark patches, and gave them names, most of which are still in use - but he also drew straight artificial-looking lines which he called "canali". This is Italian for "channels", but inevitably it was translated as "canals". What were they?

Schiaparelli kept an open mind, but the American astronomer Percival Lowell did not. He was convinced that the canals were artificial, built by the "Martians" to pump water from the polar snows through to the centres of population in warmer latitudes. Lowell set up an observatory at Flagstaff, in Arizona, and equipped it with a fine 24-inch refractor. From 1895 he produced drawings which showed an intricate canal network, and he also claimed that some of the canals became abruptly double, so that presumably the engineers there opened up new waterways.

Even in Lowell's lifetime there was a good deal of scepticism, but others beside Lowell drew the canals, and almost everyone believed that the dark areas were covered with vegetation. Moreover, one thing was definite. If the canal drawings were accurate, then Mars would be inhabited. A network of that kind could not

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develop naturally. Yet some observers, using telescopes as powerful as Lowell's, could not see the canals at all, or else drew them as disconnected patchy strips.

There was the problem of the "wave of darkening", bound up with the regular cycle of the polar caps, which wax and wane according to the seasons. It was claimed that when a cap shrank, the dark areas near its border became sharper, as though the vegetation were being revived by moisture in the atmosphere. It seemed credible enough even to those who had no faith in Lowell's Martians.

I well remember my first views of Mars through Lowell's telescope, in the late 1940s. Would I see canals, and follow the wave of darkening? I did not, and am very glad that I failed, because the canals do not exist; they were mere tricks of the eye. It is only too easy to "see" what you half expect to see. Much later, I superimposed Lowell's canal network on a modern map of Mars, and found that there was no correlation with any genuine surface features.

From 1965 came the flights of the space-probes, and all our ideas about Mars had to be revised. There are mountains, valleys and craters; there are also huge volcanoes, which may or may not be mildly active. The highest, Olympus Mons, towers to 15 miles, and has 40-mile caldera at its summit. (I remember seeing it with my modest 15-inch reflector, but only as a tiny speck, and I had no idea of its true nature.) There are also basins, such as Hellas, which can be cloud-filled and then masquerades as an extra polar cap. One striking feature is the Mariner Valley, which is 2000 miles long and up to five miles deep, with raised, ragged rims; it makes our Grand Canyon of the Colorado seem very puny. The Martian landscape is nothing if not dramatic, and most people will have seen the pictures sent back by space-craft which have made controlled landings there.

Is there life on Mars? There may be - perhaps surviving in underground lakes or seas, but we can expect nothing more advanced than tiny, single-celled organisms. There can be nothing so elaborate as a blade of grass.

In view of all that we have found out from the space-probes, is there any point in looking at Mars through a telescope, apart from the sheer enjoyment of it? The answer is: "Yes". Mars has atmosphere, and not a static world. Studies of dust-storms are useful, and the same is true of the ice-crystal clouds, which shift and tell us a great deal about the Martian wind systems. We can also follow the changes in the polar caps. (Note, incidentally, that they do not 'melt'; liquid water cannot exist on the surface of the Mars, because the atmospheric pressure is too low. A shrinking cap "sublimes" - that is to say the material changes directly from a solid into gas.)

Mars is a difficult object to study with a small telescope, even when near opposition, because a reasonably high magnification is needed. A 3-inch refractor will show the polar caps and the main dark areas, but do not expect too much unless your telescope has an aperture of at least 6 inches. For a drawing, a scale of 2 inches to the planet's diameter is customary; when the phase is evident, the disk should always be drawn to the correct shape. The map given in the Appendix 5 (page 175) shows the features I have been able to see with a 12i-inch reflector. (I have retained the older names; they have been revised in view of the space-craft results so that, for example Mare Acidalium has become Acidalia Planitia.) Much the most prominent of the dark features is the Syrtis Major, now known to be a high plateau from which the dusty layer has been blown away, exposing the darker rock beneath (Fig. 9.5).

Fig. 9.5. Mars. Note the V-shaped Syrtis Major. (Paul Doherty, 12 April 1982, 2150 GMT, 419 mm reflector, x248.)

Fig. 9.5. Mars. Note the V-shaped Syrtis Major. (Paul Doherty, 12 April 1982, 2150 GMT, 419 mm reflector, x248.)

At the start of an observing session, begin by looking at Mars for some time until your eye has become thoroughly dark-adapted. Then sketch in the main details, using a moderate power; next, change to a higher power and add the more delicate detail. As soon as this has been done, put the data in your sketch: time (GMT), power seeing conditions and any exceptional circumstances. Mars spins on its axis in 24 hours 37 minutes, so that the drift of the markings across the disk becomes noticeable over even a short period. Obviously, any particular marking will pass the central meridian about half an hour later each night; tables given in publication such as the Handbook of the British Astronomical Association make it easy to work out the longitude of the central meridian for any particular moment.

Owners of larger instruments may care to look for the two tiny moons, Phobos and Deimos. Both are veritable dwarfs less than a dozen miles in diameter, so that even when Mars is near opposition they are difficult to glimpse. I have seen them both with a 15-inch reflector, and keener-eyed observers should catch sight of them with a 12-inch when conditions are first-class.

A rather stupid mistake on my part may serve to show that it is not wise to reject an observation because it does not "fit in"with what is expected. I was once observing Mars with my 122-inch reflector, when I recorded a minute starlike point, clearly visible only when Mars itself was hidden by an occulting bar, which I took to be Phobos. I then consulted my tables, and found that Phobos was not in fact anywhere near the position recorded. I therefore dismissed the observation, as either a mistake or else an observation of a faint star. It was only on the following day that I found that the observation itself was perfectly correct; I had made a slip in my calculations.

Phobos is a peculiar little body. It whirls round Mars at a distance of only 3,800 miles above the surface, about as far as from London to Aden, and it completes one revolution in only 71 hours. So far as Phobos is concerned, the "month" is shorter than the "day", and to a Martian observer Phobos would seem to rise in the west, gallop across the sky - taking only 42 hours to pass from horizon to horizon - and set in the east. Neither it nor Deimos would be of much use as a source of moonlight, and Deimos would indeed look like a large, dim star.

Both satellites were photographed from Mariner 9 and the Vikings. Each is irregular in shape, and each is pitted with craters. Phobos and Deimos are quite unlike our own Moon, and probably they are nothing more than ex-asteroids which were captured by Mars in the remote past, Iosif Shklovsky, a famous Russian astronomer, once suggested that they were nothing more nor less than hollow space-stations, launched by the Martians for reasons of their own; but I fear that the latest probes have put paid to this attractive, if somewhat remarkable, idea!

To many people, Mars seems to be the most intriguing world in the entire Solar System. If all goes well, it should be reached by men before the end of the present century. Whether any trace of life will be found is a question which ought to be solved within the next few decades; at present, the jury is still out.

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