The Planet Venus - A Greensleeves Page

Venus is the second planet of our Solar System, lying as it does between Mercury and our own planet Earth. It is a planet which has a special place in our night sky, because it sparkles so brightly in the late evening or the early morning. As a result of this brilliance, mythology and a lot of speculation has surrounded this world for almost as long as human beings have existed. Today, of course, we know the truth behind this planet, and the truth is that it is one of the most extraordinary of all the worlds in our Solar System. Judged by a human perspective, it is also one of the most horrific places in the known universe.

The first photo is on this page is an image of the cloudy atmosphere of Venus, taken by the Pioneer misssion of 1978-9. Although beautiful, such pictures cannot reveal too much about the planet which lies beneath the clouds, but unfortunately, little else had ever been available. The clouds are much too thick to be penetrated by ordinary Earth-based or space-based cameras and telescopes, and the few craft which have successfully landed on the surface have not been able to send back many images before failing. It was left to the more recent development of radar-based images to reveal true details of the Venusian surface.

All other photos on this page are developed from radar images. Some are black and white; others have simulated colour based on photos from the Venera 13 and 14 missions. It should be pointed out that most of these images cover a vast scale (several hundred kilometres) and are taken from altitude. The vertical scale of the photos (height) has been exaggerated to enhance smaller surface features.

This page is the first of two pages about Venus.

Page 1) - A generalised overview of our current state of knowledge.

Page 2) - A more comprehensive study of the geology and climatology.

Venus is unmistakeable in the night sky. Excluding the Sun and the Moon, it is quite simply, the most brilliant object to be seen, blazing out several times brighter than the next brightest planet, Jupiter, and up to 15 times brighter than the most luminous star, Sirius (precise figures are not helpful because the visibility of Venus varies so much according to its constantly changing distance from the Earth, and its orientation in relation to the Sun, which illuminates the surface with starshine).

So bright is Venus at its peak that it can often be seen in daytime. But Venus offers only a fleeting show in the night sky; because the planet lies between the Earth's orbit and the Sun, it never strays more than 47° from the Sun in our sky, and therefore tends to be visible around dusk (in the west) and dawn (in the east), The planet may linger for a few hours low on the horizon during the hours of darkness, but will not be visible in the middle of the night.

Like Mercury, Venus can often be seen as a crescent when viewed with a telescope. This is because just like Mercury (and our own Moon) we see only the part of the planet which is lit by the Sun's rays. The full face is only visible to us when it is on the far side of the Sun (but at this time the planet is much further away from us, the globe is much smaller, and Venus is correspondingly dimmer.

It was in 1962 that Venus became the first planet to be visited by an Earth spacecraft when America's Mariner 2 flew within 34,0000 km (21,000 miles) of the surface and took photos. It was the start of a period in which many attempts were made to uncover the secrets of Venus, But it was also the start of a period of many failures and disappointments.

Prior to Mariner, the Soviet Union had actually already embarked on a series of missions under the name Venera, beginning with the launch on 12th February 1961 of Venera 1. Sadly, communication was lost with Venera 1 after just one week. A similar fate befell Venera 2 in 1965. Venera 3 did a litle better - it crash-landed on the surface of Venus on 1st March 1963, but it again failed to returrn any data. It was, however, the first spacecraft ever to land on another planet. Subsequent missions Venera 4, 5, and 6, revealed some information about Venus's temperature and its atmospheric conditions, but again equipment failure quickly ended the flow of data from all craft. Venera 7 became the first space probe to successfully transmit data from another planet when it touched down on Venus on 15th December 1970; transmissions from this probe also failed after 23 minutes. Venera 8 fared better - it lasted for 50 minutes on the surface. In 1975, the next two missions, Venera 9 and 10, managed to take a few photos and survived for just 53 minutes and 65 minutes respectively before malfunctioning.

As well as these missions, American Mariner probes were also sent on fly-bys to Venus. Venus, indeed, was attracting more attention than any other planet in the 1960s and early 1970s, but though many of these craft did contribute to our knowledge of the planet, the amount of information was seriously limited both by the difficulties of imaging through the extremely opaque atmosphere, and by the short lifespan which each of these craft enjoyed after entering the Venusian atmosphere and landing on the surface.

The reasons for this catalogue of equipment failure will be explained later on this page.

DISTANCE FROM SUN - This varies from 107 million kilometres and 109 million kilometres (about 67 million miles). Venus has the most circular orbit of any planet.

DIAMETER - 12,103 kilometres (7,520 miles). Venus is slightly smaller than Earth, but the two planets are quite similar in physical dimensions. Venus is the sixth largest, of the eight planets which are currently recognised. The surface area is 90% as great as the Earth's.

GRAVITY - Gravity is similar to Earth.

LENGTH OF YEAR - 225 Earth days.

LENGTH OF SIDERIAL DAY (Period of rotation) - 243 Earth days.

LENGTH OF SOLAR DAY (Sunrise to Sunrise) - 117 Earth days.

The revolution of Venus around the Sun takes 225 Earth days. Whilst revolving round the Sun, Venus rotates on its axis very slowly indeed. It takes 243 Earth days to complete just one 360° rotation - the longest rotation of any planet in our Solar System. (The next slowest is Mercury which takes 59 Earth days). However, even though the 'siderial' day (a full 360° rotation) is 243 days long, the 'Solar' day (the period of time from sunrise to sunrise), is actually much shorter at 117 Earth days.

This descrepancy between two definitions of day length which are essentially identical to each other here on Earth, comes about partly because of Venus's slow rotation, but also because Venus uniquely rotates in a retrograde direction; that is to say that when viewed from above the north pole, Venus spins clockwise - all other planets including Earth spin anti-clockwise. A second consequence of this reverse rotation is that on Venus the Sun rises in the west and sets in the East!

(The reason for this peculiarity of motion is uncertain, but it may be presumed that at some stage in its history Venus was acted upon by another sizeable body, causing it to reverse its direction of spin).

One other oddity about Venus's progress around the Sun make this planet 's orbit stand out as one of the most unusual. The orbit is the most nearly circular of all planets varying by less than 1% in its distance from the Sun (typically planetary orbits are quite elliptical).

The majority of the surface of Venus consists of extensive, low lying plains such as Atalanta Planitia, Lavinia Planitia and Guinevere Planitia (in keeping with the theme of the God Venus, many surface features on the planet have feminine names). These plains are disrupted and interrupted by several large mountain ranges and a multitude of impressive volcanoes. Two mountain ranges deserve special attention. Aphrodite Terra, which is in the southern equatorial region, is as large as many continents on Earth. Ishtar Terra, which is in the northern hemisphere, is smaller in area, but higher; many tall mountains surround the central high plateau of Lakshmi Planum, and these mountains include Maxwell Montes, the highest on Venus, which rises 11.2 kilometres (7 miles) above the average elevation of the Venusian surface (sea level is clearly a meaningless term on planets other than Earth!). One characteristic of the Venusian mountains is their highly fractured and ridged surface, which is probably the result of repeated upwellings of lava expanding and compressing the crustal surface.

Venus is a planet of volcanoes. Tens of thousands of volcanoes dot the surface. Many of these are small, but several hundred are massive, more than 400 kilometres (250 miles) in diameter. Volcanoes create lava flows, and on Venus huge lava flows spread out over the plains; some of these are very extensive and one is nearly 7,000 kilometres (4,300 miles) long - the longest flow in the Solar System. Other more curious features of probable volcanic origin have also been found on the planetary surface. There are giant calderas 100 kilometres (60 miles) across, and circular structures with radial or concentric fractures, variously called novae, or coronae, or arachnoids, depending on the form the fractures take. Also there are strange round features called farra, or pancake domes, most of which are 20-50 kilometres (12-30 miles) in diameter. The precise origin of the pancake domes is unknown, but again they are believed to be volcanic, and probably involve an upwelling of magma from within the planet.

As with most of the rocky planets and moons of the Solar System, Venus's surface is pockmarked with craters, but these are fewer in number than on many other worlds. This suggests that many craters on Venus have been eroded away, or more probably covered by extensive resurfacing of the planet's crust.

This then, is the surface of Venus today - a barren world of volcanoes and lava floodplains, mountain ranges and curious rounded structures all scarred with a multitude of ridges and fractures. There is no liquid water on the surface of Venus today, as will be discussed under the section on the Venusian atmosphere.

Venus has an internal geology which may be broadly similar to Earth's, though the details are unknown and must be surmised from gravitational and mass calculations, and magnetic field considerations. As on Earth, there is an iron core which is probably at least 3,000 kilometres (1,900 miles) in diameter, but which may or may not be solid. Above this lies a molten mantle, and a basalt crust, which is probably more than 20 kilometres (12 miles) thick.

This similarity does not extend to plate tectonics. Unlike the Earth where large plates of surface crust 'float' upon the mantle beneath, and seizmic activity and vulcanicity tends to be localised along the borders of these plates, on Venus, activity seems to have occured across the planet, relieving stress via a multitude of volcanoes and crustal fractures. Whether such eruptions still occur on a regular basis, is a matter of some conjecture.

There is also very little similarity in the magnetic fields of the two planets, as Earth has a considerable magnetic field, whilst Venus has virtually none.

85% of the Venusian surface is volcanic in origin, and one consequence of volcanic activity is that the surface is young in comparison with most other rocky worlds of the inner Solar System such as Mercury, Mars and the Moon. None of the surface seems older than 800 million years, and there there appears to have been a particularly extensive period of vulcanicity between 300 and 500 million years ago. Almost all current surface features probably date from this time.

(More comprehensive information regarding the make-up and development of Venusian geology, and also the Venusian atmosphere, should be published within one month. Please check back here in due course for a link)

The atmosphere of Venus is one of the most intriguing in the Solar System. Principally this is a carbon dioxide atmosphere, and the pressure generated by this intensely thick gas at ground level is 92 times greater than that on planet Earth. Carbon Dioxide makes up 96.5% of the atmosphere. Almost all of the remainder is nitrogen. Trace amounts of sulphur dioxide, water vapour and inert gases such as argon, helium and neon also exist. There also exist in the upper atmosphere, vast clouds of corrosive sulphuric acid, in which lightning has been detected by the recent Venus Express mission.

Winds in the upper atmosphere may exceed 350 kph (220 mph), but the wind speed at ground level is believed to be quite slow (one of the few mild aspects of Venus's atmosphere).

It seems likely that several billion years ago, Venus's atmosphere was rather more similar to Earth's, and surface water may have been abundant. However the evaporation of this water under conditions of intense heat, led to greenhouse gases reaching a critical level, whereby sunlight passed through the atmosphere, but the accompanying heat was not able to escape back through the atmosphere into space. As a result of all this, the surface temperature of Venus rose to even higher levels, and is now known to be about 480°C (900° F). This is hot enough to melt lead, and is actually higher than the temperature on the side of Mercury which faces the Sun, despite the fact that Mercury is nearly twice as close to the Sun and receives nearly four times as much radiation.

After reading these few sentences about Venus's atmosphere, it should now have become clear just why so many of those early Venera missions failed so very soon after entering the environs of Venus's atmosphere and surface; crushing pressure and ultra high temperatures mean that it takes a very specially designed craft to survive such a place for even a few hours, let alone to transmit photos or much data back to Earth.

The irony of all this is that Venus was compared by the ancients to the Goddess of Beauty and Love. It was long considered as Earth's twin - our nearest neighbour (apart from the Moon), a planet of similar size, with similar density and gravity, relatively few craters, and even relatively similar chemical compositions (geologically). In quite recent times in the early 20th century, Venus was thought to be a promising site for extraterrestrial life.

However the truth proved to be very different. John Gray, author of the best seller, 'Men are from Mars, Women are from Venus' presumably didn't mean his title to be taken too literally. If he did, it actually isn't very complimentary to women, because whereas Mars is relatively benign (as planets go), Venus must be just about the most nightmarish place in the Solar System. Of course nowhere other than Earth is hospitable to our species today, but few places are quite so obnoxious in quite so many different ways as Venus. Venus is Hell.

Venus is the only planet which rotates on its axis in an east-west direction, and it has the longest day of any planet in the Solar System.

Venus has the most circular (least oval or elliptical) of all planetary orbits.

Venus and Mercury are the only planets without attendant moons.

Venus has more volcanoes than any other body in the Solar System and it has the greatest lava flows in the Solar System.

Venus has the densest atmosphere of any planet in the Solar System.

Despite the fact that it is nearly twice as far away from the Sun as Mercury, Venus is the hottest of all planets and moons in the Solar System.

Venus is the only planet which has electrical storms generated by clouds of sulphuric acid rather than water vapour

The conditions prevalent on Venus have made it a difficult planet to investigate, despite its proximity to Earth. It remains a great challenge for mankind to land probes on this planet and uncover all of its secrets. It will be a challenge worth taking up, because Venus's similarity to Earth in so many ways, and yet its great difference from the Earth in so many other ways, suggest that lessons learned about what 'went wrong' with Venus will inform us as to what 'went right' with Earth, and make clearer the delicacy of the balance which spiralled out of control on Venus, but which today still keeps our planet so hospitable and green.