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BAA Tutorials Starting out

Venus as a planet

Figure 1. Venus seen at left in a false colour radar image compared to the Earth at the same scale. Earth image: NASA/Apollo 17 crew, Venus image: NASA.

Introduction
This short tutorial discusses the planet Venus as a body in its own right rather than as a target for observation. However, Venus is an interesting though often challenging object through a telescope. For more on what can be seen visit the Mercury and Venus Section webpage of the BAA.

Venus in the solar system
Figure 2. The position of Venus’ orbit in the inner solar system. Note how circular this is in comparison to that of Mercury.
Venus is the second planet from the Sun, orbiting between Mercury which is closer in than Venus and the Earth which is farther out. Like the other planets the orbit of Venus is not exactly circular but slightly elliptical. However its orbit is the most circular of all the planets in our Solar System (Figure 2) and its mean distance from the Sun is 108,208,00km.

Venus is one of the four terrestrial or rocky planets, the others being Mercury, the Earth and Mars. Superficially Venus is a close twin of the Earth with a diameter of 12,104km or about 95% that of our own planet. Like Mercury and unlike the Earth and all the other major planets Venus has no moon.

Venus takes 224.7 days to orbit the Sun (its year) and 243 days to rotate through 360o around its axis (its day). Curiously this latter rotation of the planet is the ‘wrong way’ in that it is from east to west rather than west to east which is the norm for the majority of the major planets of the solar system. This type of ‘backward’ rotation is properly known as retrograde.

It is often said that Venus’ day is longer than its year and indeed from these numbers that is certainly true given the technical definition of a day as being the time taken to rotate through 360o. However, the more popular understanding of the length of a day is related to the apparent position of the Sun in the sky (say from noon to noon) and the lengths of daylight and night-time combined to make up the Solar Day. This is given by the interaction of a planets true rotation (day) and its journey around the Sun (year). On Venus these two periods combine to give a Solar Day of around 117 days, much shorter than the year but still very long in Earth terms. What’s more, because of the retrograde rotation, the Sun, were it visible through Venus’ dense atmosphere, would appear to rise in the west and set in the east, the opposite of what we are used to here on Earth!

For centuries Venus was considered likely to be a somewhat warmer near twin of our Earth. Even as late as the 1960's the planet was thought to possibly be a warm, swampy, humid place perhaps akin to the Earth in the Carboniferous Era. All this seemed eminently reasonable, after all Venus is similar in size to the Earth and not that much closer to the Sun. However, as we shall see these ideas proved to be very wide of the mark indeed!

Venus as seen from Earth
Figure 3. Venus, here seen above the Moon can be a striking object even in twilight. Image courtesy Andrew Paterson
From Earth Venus can appear as the third brightest object in the sky after the Sun and Moon (Figure 3). Indeed it becomes so bright that at times it can be seen in broad daylight if you know just where to look. There are two reasons for this brilliance. Firstly it can come closer than any other planet and secondly it is highly reflective. In fact it reflects around 65% of the light falling on it; compare this to the Moon which only reflects 12%. What is it that makes Venus so reflective? The answer is that Venus is shrouded in bright clouds, covering the entire surface and never clearing to reveal the planet beneath.

Being closer to the Sun than the Earth, Venus alternates between appearances as a ‘morning star’ and an ‘evening star’. It first appears close to the horizon just before sunrise or after sunset. As time passes it moves further and further out until it is 47o from the Sun. It then swings back in close to the Sun, becoming lost in its glare before emerging on the other side in the twilight. Once more it swings out to 47o before returning to the Sun's glare to repeat the whole process again.

Throughout these motions Venus, when seen through a telescope, exhibits a full set of phases as described here.

Atmosphere
Figure 4. The atmosphere of Venus as seen in an enhanced image taken by the Mariner 10 probe. Image, NASA/JPL-Caltech.
Venus is swathed in a dense, unbroken and visually impenetrable atmosphere. Its clouds never part to reveal the planet’s surface beneath.

Through a telescope Venus appears as white/pale yellow disc. Visually little can be seen from Earth apart from vague diffuse shadings often at the limit of visibility. The position is somewhat clearer in both ultraviolet and infrared light and of course from space probes. Here markings are quite often seen and from spacecraft considerable detail can be discerned indicating clouds driven by winds in the planet’s atmosphere (Figure 4).

What we are seeing are reflective clouds high in Venus’ atmosphere. Surprisingly these are mainly composed of sulphuric acid and extend from 25km to 50km above the planet’s surface. Beneath these clouds the atmosphere is clear down to the surface.

Venus’ atmosphere is made up of 96.5% carbon dioxide and 3% nitrogen. The remaining 0.5% is made up of traces of a variety of substances including the sulphuric acid in the clouds that make Venus so reflective.

This huge predominance of carbon dioxide may seem surprising but in fact the Earth’s atmosphere would have much the same amount were it not for the fact the bulk of the carbon dioxide is locked up in the Earth’s rocks.

Globally the upper atmosphere completes one rotation around the planet in about 4 days, much shorter than the planet’s own rotation of 243 days. This results in a wind speed of around 100m/s (roughly 360km/h) Descending through the atmosphere the wind speeds drop off and by the time the surface is reached they are at little more than a walking pace, often less than 2m/s.

Venus’ atmosphere is extraordinarily dense with a mass 93 times that of the Earth. As a result the surface pressure is over ninety times that of the Earth’s atmosphere, equivalent to that encountered nearly a kilometre beneath the surface of our oceans.

Being closer to the Sun, Venus receives about 1.9 times the amount of radiation that the Earth does. However, because of the reflective clouds and absorption in the atmosphere only around 2.5% of the incident radiation reaches the planet’s surface. As a result, at the surface, the illumination is about the same as that on Earth on a heavily overcast day. Because the atmosphere is so dense it averages out the temperature on the planet and there is little difference between the night and day sides.

The dense atmosphere with its large amount of carbon dioxide has caused a runaway greenhouse effect to occur on Venus. This had led to temperatures on the surface averaging around 460oC, hot enough to melt lead!

Surface
Figure 5. Map of Venus based on Pioneer Venus Orbiter observations. The different colours represent different heights of surface features. The yellow area along the equator is Aphrodite Terra. The large area upper left of centre is Ishtar Terra with Maxwell Montes, the highest area on the planet shown in red. Image, NASA Ames Research Centre, USGS and MIT.

Because of the all encompassing clouds nothing was known about the surface of Venus for a long time. It was not until the 1960’s and 1970’s that radar studies from Earth gave the first clues. Since then, radar and altimetry studies from orbiting satellites along with results from a small number of landers have greatly increased our knowledge.

There are two significant highland areas, Ishtar Terra, the size of Australia and Aphrodite Terra which is about the size of Africa (Figure 5). The former of these contains the massif Maxwell Montes a peak of which, at a height of around 6.4km, is the tallest mountain on the planet.

Geologically the surface of Venus is dominated by volcanic landforms. There are extensive plains of solidified lava covering about 75% of the planet. There are also a volcanoes numbered in their thousands, more than on any other planet in the solar system. It is usually considered that Venus’ volcanoes are long dead but there are indications of some possible activity on the planet. There have been large swings in the amount of sulphur dioxide in the atmosphere which could indicate emissions from active volcanoes. Also a small number of transient hot spots have been detected corresponding in position to volcanic features, the excess heat perhaps resulting from eruptions.

Images of the surface taken from spacecraft that have landed there reveal a bleak, rocky terrain.

One surprise was the comparatively small number of impact craters. Mercury, Mars and the Moon are all heavily cratered as would be the Earth were it not for the active resurfacing processes taking place on its surface. Venus has somewhat more than one thousand craters on its surface, more than the Earth but still a relatively small number. The implication is that there is some process resurfacing the planet over a comparatively short time scale which is erasing craters. As a result, the average age of the surface is in the order of 750 million years, much less than the age of the planet which is measured in billions of years.

Life on Venus?
In 2020, researchers at the University of Cardiff announced the discovery of traces of the gas phosphine high in Venus’ atmosphere. On Earth, this gas occurs naturally as the result of biological activity and it was suggested that the discovery of phosphine on Venus was due to the presence of primitive microorganisms on that planet. If confirmed, this would be the first detection of life beyond the Earth.

At the time of writing (early 2021) the claims have been challenged and there have been some issues with the data so it is fair to say the jury is still out.

There are three possibilities:

  1. The identification is mistaken and there is no phosphine on Venus.
  2. The identification is correct but the phosphine is produced by some not currently understood non-biological process.
  3. There is indeed primitive life in the atmosphere of Venus!

Only time will tell.

Conclusion
Figure 6. An amateur observation of Venus made on September 21st 2020 with a 203mm telescope. Image courtesy Paul Abel.
If you’d like to see Venus for yourself the regular BAA Sky Notes will indicate when and where in the sky it is visible.

Venus can be an interesting target in a telescope and there is an active Mercury and Venus section details of which are here.

Figure 6 shows an example of what can be seen and is a drawing made by a skilled observer with an average sized amateur telescope.

For a gallery of amateur observations of Venus made by members of the British Astronomical Association please click here.











Credits

Figure 1: NASA, Public domain via Wikimedia commons.
Figure 3: Andrew Paterson.
Figure 4: NASA/JPL-Caltech, Public domain via Wikimedia Commons.
Figure 5: NASA Ames, USGS and MIT, Public domain via Wikimedia Commons.
Figure 6: Paul Abel.

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