Venus

Posted on Sep 24, 2011 under science | Comments are off

As viewed from Earth, Venus is the brightest celestial object in the sky except for the Sun and Moon. Like the Moon, Venus can he seen to go through a series of phases as it orbits the Sun and is … viewed from Earth. The Greek poet Homer even went so far as to call it the most beautiful star in the sky, while the Romans named it Venus after their goddess of beauty. The second planet from the Sun, Venus is a near twin of the Earth in terms of size, with a diameter 95 percent that of our own planet. Like Mercury, and unlike every other planet in the Solar System, Venus has no moon. Transits?in which the planet passes directly between the Earth and the Sun?arc characterized by an effect similar to an eclipse, although Venus appears as a mere tiny black dot creeping across the face of the Sun. Transits are rare. occurring in pairs eight years apart?and then not at all, for well over a century. The last pair of transits, for example. occurred in 1874-1882, and the next will occur in 2004-2012.

Any attempt to see the surface features of Venus were frustrated by the fact that the entire surface is covered by a thick cloud layer, a fact not known to early astronomers. Giovanni Cassini (1625-1712) produced the first ‘map in 1667, but as cloud patterns changed he could no longer find the features he had drawn. Johann Hieronymus Schroeter (1745-1816) was also fooled and reported having seen mountains on the surface. Schroeter. however, was the first to observe a very real phenomena, that of the ‘ashen light’ seen in the Venusian atmosphere on the dark side ofthe planet. This faint light was at one time thought to be the city lights of Venusian civilization, but is now attributed to lightning which occurs during the planet’s frequent electrical storms.
By the early twentieth century, it had been determined that the Venusian surface was obscured by clouds, and various theories evolved regarding the actual nature of the surface beneath those clouds. The nineteenth-century idea that the planet was covered by lush jungles was dismissed in favor of the two schools of thought that suggested either a vast desert or a vast ocean of water.

It had been established that the surface would be extremely hot because carbon dioxide in the thick atmosphere would prevent solar heat from escaping the surface, thus producing what is referred to as a ‘greenhouse effect.’
The first successful expedition to the vicinity of Venus came in December 1962 when the American unmanned spacecraft Mariner 2 traveled to within 21,600 miles
of the planet. The flight of Mariner 2 was a major milestone in unlocking the secrets of the mysterious planet. Among its achievements were confirmation that Venus has no detectable magnetic field, confirmation of the planet’s exact rotational period-245 Earth days?and confirmation that it rotates from east to west, rather than the opposite as previously supposed.

Mariner 2 also provided a more accurate reading of the planet’s surface temperature, which at 900 degrees Fahrenheit is too hot for the existence of an ocean, because water could exist there only as steam. Water vapor is in fact present in the atmosphere, and some astronomers have theorized that at an early stage in the evolution of Venus, oceans ‘nay have in fact existed on the surface.

In 1978 the United States undertook the Pioneer Venus project as a follow-on to several earlier Mariner probes. The project consisted of an Orbiter spacecraft and a Multiprobe spacecraft. The former under-
took the detailed radar mapping of the Venusian surface that made possible the maps on these pages, and which gave us much of the information we now have about the planet’s terrain. The Multiprobe was actually five probes designed to return data about the Venusian atmosphere as they plunged toward the surface. One of the Pioneer Venus Multiprobes continued to return data from the surface for just over an hour after impact.
The Soviet Union, meanwhile, prepared a series of spacecraft to conduct soft landings on the Venusian surface, which returned the only photographs ever taken of the Venusian surface. The Soviet Venera 9 and Venera 10 spacecraft each returned a single black and white image In 1975, and the Venera 13 and Venera 14 spacecraft returned color photos in March 1982.

While the Soviet Venera spacecraft provided the first photographs of specific points on the Venusian surface, the American Pioneer Venus Orbiter provided our first clear look at the overall global surface features of Venus.
Using a radar altimeter. Pioneer Venus was able to obtain the data necessary to produce a topographical map of 90 per? cent of the planet’s surface, from 73 degrees north latitude to 63 degrees south latitude.

On 4 May 1989. the United States used the Space Shuttle Orbiter Atlantis to launch the four-ton Magellan radar-mapping spacecraft on a mission to Venus. Arriving at the cloud-shrouded planet on 17 August 1990, Magellan initially conducted 1852 mapping swaths around the planet, a process which continued for 243 Earth days, or one Venusian day. The resolution of the data returned by Magellan was vastly superior to that achieved by Pioneer Venus a decade earlier. Indeed, the quality of the imagery was so good that the data it transmitted looked like actual photography of a cloudless planet!

Satellite data from Pioneer Venus in 19-8-1979 was thought to show that the surface was generally smoother than those of the other three terrestrial planets, but in 1990. Magellan, which was capable of ‘seeing’ in more detail, revealed a more rugged terrain. However. Magellan confirmed Pioneer’s findings that Venus has much less variation in altitude than is seen on Earth. Foe instance, 60 percent of the Venusian surface is within 1600 feet of the planet’s mean radius of 3752 miles. It has been suggested that this is due to the deeper lowlands having been filled with sand and other wind-blown material. Because there are no seas on Venus, the mean radius is used as a reference point in the same way that sea level is used on Earth.

Most of the surface of Venus is characterized as rolling uplands, rising to an altitude of roughly 3000 feet, while 20 percent of the surface is identified as lowlands and 10 percent as mountainous. The two largest upland regions, or continental masses, are Aphrodite Term (roughly the we of Africa), near the equator in the Southern hemisphere, and Ishtar Terra I roughly the size of Australia), in the northern hemisphere near the North Pole. These rap features constitute the Venusian ‘continents and are named respectively for the ancient Greek and ancient Babylonian god-desses of love.

The highest points on the mapped surface of Venus are in the Maxwell Mountains Maxwell Montes) in Ishtar Terra. High enough to have been identified by Earth-based radar prior to the Pioneer Venus project the Maxwell Mountains. which mayactually he a single mountain, rise to more than 35,000 feet above mean radius, or roughly 20 percent higher than Mount Everest rises above Earth’s sea level. If viewed from the surface they would be an impressive sight, rising nearly 27,000 feet above Lakshmi Planvin. the surrounding plateau which is roughly the same elevation as the Tibetan plateau on Earth.

Data obtained from Pioneer Venus indicates that the Maxwell Mountains may be the rim of an ancient volcano whose caldera had a diameter of roughly 60 miles. The lava flows, however, have long since been worn away by wind erosion, and the slopes of the Maxwell Mountains are strewn with rocks and debris. Another important upland region is Beta Regio with its great shield volcanos, Rhea Mons and Theia Mons, which are larger than the great shield volcanos of Hawaii on Earth. The mountainous Beta Regio is still in the process of formation and probably contains active volcanos. As such, it is the newest major surface feature on Venus.
The lowest point on the Venusian surface is actually a canyon, Diana Chasma, located within central Aphrodite Terra. At just 9500 feet below mean radius, Diana Chasma is much shallower than the corresponding lowest point on Earth, the Marianas Trench. The largest and lowest lowland region on Venus is the Atalanta Plain (Atalanta Planitia) located northeast of Aphrodite Terra and due east of Ishtar Terra. It is roughly the same size as the Earth’s North Atlantic Ocean, although it is shallower by comparison.

The atmosphere of Venus has long been known to consist primarily of carbon dioxide, and the instruments of Pioneer Venus and Venera have pinpointed the proportion of carbon dioxide at 96 percent. Nitrogen constitutes more than three percent of the Venusian atmosphere. and there arc also traces of neon and several isotopes of argon.
There is some water vapor present in the Venusian cloud eover, where it has a density of 200 ppm?ten times the density of water vapor in the clear air near the surface. In the clouds the water vapor combines chemically with traces of sulphur dioxide to produce droplets of sulfuric? acid, which give the Venusian cloud cover its distinctive yellowish color.

The Venusian cloud cover is complete and unbroken. The cloud layer is roughly 15 miles thick, with its base about 30 miles above the surface of the planet, relatively higher than the thinner cloud cover on Earth. The air at the surface is probably quite clear and the air relatively still. The clouds, however, are pushed by winds with speeds up to 200 mph and circulate around the entire planet once every four Earth days, in contrast to the rotation period of Venusian ‘day’ or 243 Earth days. Electric storms are common within the clouds and lightning has been detected by both American and Soviet spacecraft.

The most notable visible feature in the Venusian atmosphere. and one that misled so many would-be mapmakers in earlier days, is the Y Feature, whose tail sometimes stretches around the planet. The feature is actually the prevailing winds in the northern and southern hemispheres as they diverge at the equator. This pattern is constantly changing, and sometimes it is seen as a reversed C. It always, however, retains an approximate north-south symmetry.
On the surface of Venus, atmospheric pressure is roughly 100 times that of the Earth. A yellowish glow like that of a smoggy sunset on Earth is all pervasive. The daytime surface temperature of 900 degrees Fahrenheit is a global constant because the carbon dioxide and sulfuric acid atmosphere and cloud cover function as an insulating blanket, trapping the heat and producing convection currents that redistribute it across the entire surface area.