Venus: Earth’s Sister Planet

Venus has always been regarded as Earth’s Sister Planet. After all, it can be the closest planet to us and it is nearly the same size as Earth. But how similar is it really to Earth?

ESA’s Venus Express mission returned this image of sulphuric acid clouds over the southern hemisphere of Venus (Image credit: ESA)

Astronomers get asked this question very often:  “I saw this really bright light in the sky last night, it was just hanging there so it wasn’t a ‘plane. What could it have been?”

Well, this one is usually easy, probably what the enquirer saw was the planet Venus. This is the second closest planet to the Sun and is covered in highly reflective clouds, so it is very bright, in fact it is the brightest astronomical object in the sky after the Sun and Moon. It can occasionally be seen in daylight. This brightness is why it is sometimes reported as a UFO!

Venus is named for the classical goddess of love and beauty, a deity, who judging from Renaissance-era artwork available via Google, appeared to spend her days frolicking in the surf, rather like a Graeco-Roman Pamela Anderson. Venus was long thought of as Earth’s ‘sister planet’. Sadly, beneath her clouds she is a very ugly sister indeed!

As the orbit of Venus lies inside that of our own planet, Venus is always in the same part of the sky as the Sun. We see it best after sunset (or before dawn, depending on where the planet is on its orbit). Sometimes the planet is the ‘Evening star’, visible after sunset, at other times it is the ‘Morning star’, only visible before sunrise, and for a long time people thought it was two separate objects. Galileo was first to see that Venus shows phases just as our Moon does and he cleverly used this fact (along with the planet’s proximity to the Sun) to demonstrate that Venus orbited the Sun and not the Earth.

Venus rising from the waves? Our sister planet is seen reflected in the Pacific Ocean. (Image Credit: Mila Zinkova via Wikipedia)

The planet itself is almost the same size as Earth, and has about 80% of Earth’s mass. It has no moons and orbits about 110 million km from the Sun (Earth is about 150 million km from the Sun). One day on Venus is 243 Earth days long and its year lasts 225 Earth days. Yes, these are the right way round, the day is longer than the year! There is still more weirdness to come: since Venus rotates from east to west, observers on Venus would see the Sun rise in the west and set in the east

As its surface is hidden by clouds, Venus was regarded as a mysterious planet. Since nineteenth century astronomers could not see what was there, they could only speculate about that was lurking beneath the clouds. Perhaps surprisingly, usually they let their imaginations run riot. Their reasoning, such as it was, went like this:

•Venus is closer to the Sun than Earth so it must be hotter than Earth (so far so good);

•Venus is covered in clouds, maybe they are composed of water vapour (a reasonable guess);

•If the clouds are water vapour, the planet’s surface must be wet- presumably there are huge oceans and any land must be covered in lush jungles and steamy swamps (just about possible);

•The oceans and swamps of Venus are probably inhabited by ferocious space-dinosaurs! (A bit of a leap here.)

Right up to the 1950s, popular astronomy books carried artist’s impressions of torrid Venusian landscapes and seascapes populated with scaly behemoths and leviathans which could have lumbered straight from a Ray Harryhausen movie. The same books would also have paintings of the canals and vegetation on Mars: the Solar System was a more exciting place in those days (if you think this was silly, just check your recently published books for artist’s impressions of weird sea-creatures in the hypothetical oceans under the ice of Europa). At the beginning of the Space Age, this view of Venus as a ‘Lost World’ was already fading as new technologies such as radio astronomy were suggesting that the planet’s surface was probably very hot indeed. The last hope for a planet full of exotic life was literally crushed in the 1960s when the first spacecraft visited Venus.

Venera 14 photographed this desolate landscape in 1982. (Image credit: Don P. Mitchell)

The Soviet Union pioneered missions to Venus, beginning with the unsuccessful Venera 1 in 1961. It was followed by Venera 3 which was intended to land on the surface, and according to some sources was designed to be able to float just in case there really were oceans on Venus.  Unfortunately it crashed there in 1966 without returning any data. (Just imagine how different the next half century of spaceflight would have been if the Venera really had plopped into an ocean of water or a tropical swamp!)  In 1967 another Venera, the fifth in the series, was much more successful, returning all kinds of useful information as it descended on a parachute. Among its important revelations was that the planet’s temperature was about 500°C, and that the atmosphere was much denser than expected and consisted of more than 90% carbon dioxide. So dense was the atmosphere that its pressure squashed the probe flat before it even reached the surface. Later, tougher Veneras reached the surface and returned pictures of bleak and rocky landscapes with not a space monster to be seen. Fuzzy, distorted and marred by streaks of missing data, these images made little impression at the time despite the fact that they represented a major achievement in Solar System exploration. Recently, the raw data from the probes (stored on reels of tape for decades) has been reprocessed using modern technology by computer scientist Don P Mitchell. Thanks to his work we now have crisp snapshots of the hellish desolation that is Venus. Later missions from Russia and the US have added to our knowledge of our sister planet. The latest arrival is the European Venus Express spacecraft currently exploring the planet.

Here is a question which is trickier than you might think: which planet has the hottest surface? The answer is not Mercury (maximum surface temperature 430°C), closest planet to the Sun, but Venus (average 470°). The very dense atmosphere of mainly carbon dioxide has promoted a ‘runaway’ greenhouse effect. Energy from the Sun can pass through the planet’s atmosphere but a very large portion of this energy is trapped there (having no atmosphere this cannot happen on Mercury). As you may have heard, we have something similar here on Earth, and if studying Venus helps us learn how to prevent global warming, that justifies every penny spent on space exploration (although the greenhouse gases of Venus probably came from volcanoes rather than powerstations).

As a result of the greenhouse on Venus, the planet’s surface is overwhelmingly hostile: the temperature is higher than that of molten lead and the atmospheric pressure is 90 times that on Earth’s surface. All the space probes which have landed on the surface stopped working after only a few hours as they were simultaneously baked and crushed! About 40-60 km above the surface lies the unbroken blanket of thick clouds which make the planet so obvious in our skies. These consist mainly of sulphur dioxide and droplets of sulphuric acid. This uninviting composition has led many pundits to claim that visitors to the planet’s surface would be corroded to death by downpours of acid rain. This is very unlikely if you compare the surface temperatures with the boiling point of sulphuric acid…but even without this horror the Venusian environment is quite hideous enough. It is hard to imagine people ever visiting the surface in person.

A volcano on Venus is revealed in this Magellan radar image with added colour based on Venera lander data. (Image credit: NASA/JPL)

The surface of Venus was pretty thoroughly mapped by radar carried on an American probe called Magellan in the early 1990s. The terrain revealed was flat plains, with two continent-sized highlands, sprinkled with a mixture of familiar and novel terrain features. There were valleys, mountains, volcanoes and meteor craters (not many of these, though, the dense atmosphere is an effective meteor shield). There are odd volcanic structures, unlike anything on Earth, arachnoids are spidery fractures in the surface, while farra are flat pancakes of cooled lava. Most of its ‘geographical’ features are named after famous females from history and myth.

Oddly the surface is much younger than the planet itself (in case anyone asks, we can date the surface by the number of meteor craters- we know how often the planet should be hit by space debris). All the planets in the Solar System are about 4.5 billion years old, but Venus was resurfaced presumably by molten rock welling up from underground some 500 million years ago. In the distant past, Venus was probably cooler, and some scientists have suggested it was once cool enough for there to have been seas. Some daring scientists have proposed there could be still be microscopic organisms floating in the upper, more temperate, layers of atmosphere but these are probably as hypothetical as the Venusian dinosaurs were. However, the most Earth-like conditions anywhere else in the Solar System are probably found about 50 km over the baking surface of Venus (could we ever inhabit this region?)

Venus is a spectacular sight, even with the naked eye. Try looking for it in the west after sunset. Our Sister Planet’s beauty is only skin deep but she is always worth a look!