Venus

By Lesa Moore

Venus has been described since ancient times as both the "morning star" (Phospherus) and the "evening star" (Hesperus) because of its prominent appearance near the Sun. Venus is always bright because it is highly reflective, and it comes closer to Earth than any other planet.

Once called Earth's "sister planet" it is now known that Venus is the closest thing to Hell that you could ever imagine. It is a similar size to Earth and is a rocky body, but in all other respects it is nothing like our comfortable home planet.

With an atmosphere of almost pure carbon dioxide, atmospheric pressure about 90 times that of Earth's, a global temperature of 470 degrees C (higher than Mercury's day side) and clouds of sulphuric acid, you would be simultaneously asphyxiated, crushed, roasted and dissolved. This inhospitable place has only a meagre supply of water - less than 30 parts per million (which probably comes from occasional comet impacts). There is no ocean, and no precipitation (it does not rain sulphuric acid).

When observing Venus with a telescope, you will notice that it appears bland and bright. What you see is not the surface of the planet, but the top of a uniform haze of concentrated acid droplets some 60km deep, which extends to 90km above the surface. Its rotation is unusual in that is takes 243 days to rotate, longer than its 225 day year. Thus any spot on its surface has 122 days of sunlight and 121 days of darkness. Its contra-rotation means the Sun rises in the west and sets in the east.

Venus presents a diverse and puzzling geology. Some of its features are:

The longest channel in the Solar System (an old lava flow 7,000 km in length - longer than the River Nile)
Few impact craters, signifying a relatively young surface
Mountains thought to be "active" volcanoes, supported by plumes of upwelling material under the crust
Volcanoes typically of 400km diam, 1.5 km high
Evidence of explosive vulcanism in the past
Wind erosion patterns
Faults and fractures
Volcanic "pancake" domes up to 7.5 km diam
Volcanic calderas (craters in the top of a volcanic mountain)
Multiple impact craters (which would result from a split impactor like Comet Shoemaker Levy 9)
A double-ring impact basin named Cleopatra similar to some found on the Moon
Flooded impact craters
Intensely deformed terrain
Smooth halos formed by the bow shock of the atmosphere as projectiles plunge into the planet
Possible sedimentary deposits.

The youth of the surface is a puzzle which remains to be solved. The age of the surface is only about 500 million years. This is about twice as old as Earth's average surface age, but much younger than the Moon's. There doesn't seem to be any continuous resurfacing such as that found on Jupiter's moon, Io. There may have been a catastrophic resurfacing which occurred as a one-off event. Or Venus may undergo some cyclic process where heat in the core builds up, breaks through into lava flows, and then returns to a dormant state. Whatever processes are at work, the planet has been relatively quiet for the last 500m years. There are no plate tectonics working here. (Plate tectonics is the mechanism which causes "continental drift" on Earth and pushes up our highest mountain ranges.) Venus has a thick, one-plate shell, with a surface crust of basalt. The planet is not cool enough for rigid plates to form. It is thought that the crust is somewhat pliable. It gives and bends rather than breaking up into separate plates, because of its high internal and surface heat. Certain features apparent on the surface of Venus remind me of wrinkles on the cooling skin of marmalade.

Studies of Venus's surface have been undertaken by various methods designed to overcome the fact that we cannot see through the clouds which shroud it so effectively....

Mariner and Pioneer

The unmanned spacecraft, Mariner 2, flew by in 1962, confirming Venus's lack of a magnetic field, and its exact rotation period and direction. In 1978, two Pioneer craft were sent - an orbiter and a multi-probe spacecraft. The orbiter performed a long-duration radar mapping mission, whilst one of the probes lasted just over an hour on the surface. In March 1982, the Soviet Union's Venera 13 and 14 spacecraft transmitted the only color photos ever taken of the Venusian surface.

Earth-Based Observations

Pioneering work by the team at the Anglo Australian Observatory led by the late Dr David Allen produced the first infrared images of Venus's cloud layers in 1983 with an instrument called IRIS (Infrared Imaging Spectrometer). It was known that the overheated surface of the night side of Venus radiates in infrared, but this "light" only escapes at specific wavelengths which the carbon dioxide atmosphere doesn't absorb. Observations using IRIS with the AAT were made during the few weeks around inferior conjunction (which occurs once every 19 months). These observations revealed detailed cloud patterns circulating around the globe in only 6.5 days (this means they travel at 250 kph). More recent observations with the same technique have revealed bright variable patches of infrared radiation, which appear and disappear over a period of several hours (the probable explanation of the "ashen light" on the dark face reported by visual observers for many years). These variable airglow patches appear to be somewhat similar to aurorae on Earth, but arise through a different mechanism. Circulation of the atmosphere high above the clouds rapidly carries along oxygen atoms, created by the breakdown of carbon dioxide on the sunlit side of the planet. They are then dumped on the night side where the atmosphere heads downwards. As the atoms descend, they combine to form oxygen molecules, releasing a burst of infrared radiation and highlighting the position of the down flowing gas. Infrared observations have also been able to indicate the positions of cooler highland areas.

Galileo Fly-By

Galileo passed Venus on the Feb '90, producing filtered images which distinguished subtle contrasts in the cloud patterns. Its NIMS instrument (Near Infrared Mapping Spectrometer) also revealed cool spots which coincide with known mountains (as mapped by Pioneer and Magellan).

Magellan

Magellan was NASA's first interplanetary mission in 11 years. Launched in May '89 from the payload bay of the space shuttle Atlantis, Magellan arrived at Venus in August 1990. Its task was to orbit the planet every 3 hours & 9 minutes for one Venusian day, making 1852 orbits and covering 90% of the planet with its SAR, or synthetic aperture radar (this is imaging radar which looks sideways along the surface rather than straight down). The primary mission was set for shutdown in September 1993, but was eventually extended to allow 4 complete mapping cycles. Three cycles with the radar mapping gave topographical data on 99% of the surface. The final cycle included an aerobraking manoeuvre to lower its orbit and bring it closer to the poles. It then measured variations in the gravity field and searched for density anomalies. Other results of the mission included collecting electrical and magnetic data, and studying the effects of solar particle emission. Magellan finally plunged into Venus's atmosphere and burned up in October 1994.

Space Telescope

Hubble observations in 1995 have shown that no recent large-scale volcanic eruptions have occurred. Hubble can trace abundances of sulphur dioxide which would be produced in eruptions, but the sulphur dioxide content of the atmosphere has been declining ever since the Pioneer probe took the first readings in the 1970's.

Little Bits...

Pioneer Venus was originally designed to last one Venus day (243 Earth days), but lasted from 1978 until 1992, entering the atmosphere and burning up after 14 years of exploration. In addition to successful completion of its radar-mapping mission, its discoveries included evidence for the prior existence of liquid water on Venus's surface, and lightning in the atmosphere.

Probes sent to the surface of Venus only survive a short time before being crushed by the atmosphere and the corrosive effect of the acid vapour means that camera lenses need to be made from diamond.

Because Venus is closer to the Sun than the Earth, we are able to observe its phases. However, a paradox surrounds Venus's phases. If you were to observe Venus at either of its greatest elongations, which you can do quite easily with a 10cm telescope, you would notice the phase approaching dichotomy - the disk half lit, as seen from Earth. It should be a simple task to predict the date of dichotomy, but it always seems to be about four days away from the prediction, and no one knows why.

Dr David Allen (30.7.1946 - 26.7.1994) joined the Anglo Australian Observatory in 1975. He was a driving force behind the design and development of specialised infrared sensing instruments, including IRIS, built specifically for the Anglo Australian Telescope. In 1983, Allen became their first and only permanent staff astronomer, a position he continued to fill until his untimely death in 1994, the result of a brain tumor. He'd also served as president of the Astronomical Society of Australia from 1991 to 1993, and won the Eureka Prize in 1993 for his work in popularising science.

A Tale from Venus

Saturn may have rings, but did you know that Venus has a tail? First discovered in the late 1970's by Pioneer Venus Orbiter, the tail of charged particles was discovered 70,000 km from the planet. Recent findings from the SOHO solar probe reveal that the tail extends almost to Earth's orbit - some 45 million kilometres (see diagram).

The Earth does not have a similar tail because of its magnetic field which protects our atmosphere from the solar wind. In the case of Venus, the solar wind strips particles from the upper atmosphere and scatters them in a similar way to the formation of a comet tail.

By Lesa Moore, Senior Guide, Koolang Observatory (1999), and reproduced by permission.

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