Venus

Venus and Earth are similar in size, mass, density, composition, and gravity. There, however, the similarities end. Venus is covered by a thick, rapidly spinning atmosphere, creating a scorched world with temperatures hot enough to melt lead and surface pressure 90 times that of Earth (similar to the bottom of a swimming pool 1-1/2 miles deep). Because of its proximity to Earth and the way its clouds reflect sunlight, Venus appears to be the brightest planet in the sky.

The image below is from the same Mariner 10 probe that visited Mercury.  It shows the clouds on Venus, the image below it is based on radar data from the more recent Magellan mission.

We cannot normally see through Venus' thick atmosphere, but NASA's Magellan mission during the early 1990s used radar to image 98 percent of the surface, and the Galileo spacecraft used infrared mapping to view both the surface and mid-level cloud structure as it passed by Venus on the way to Jupiter. In 2010, infrared surface images by the European Space Agency's Venus Express provided evidence for recent volcanism within the past several hundred thousand years. Indeed, Venus may be volcanically active today.

Venus' atmosphere consists mainly of carbon dioxide, with clouds of sulfuric acid droplets. Only trace amounts of water have been detected in the atmosphere. The thick atmosphere traps the sun's heat, resulting in surface temperatures higher than 470 degrees Celsius (880 degrees Fahrenheit). The few probes that have landed on Venus have not survived longer than 2 hours in the intense heat. Sulfur compounds are abundant in Venus' clouds; the corrosive chemistry and dense, moving atmosphere cause significant surface weathering and erosion.

Atmospheric lightning bursts, long suspected by scientists, were confirmed in 2007 by the European Venus Express orbiter. On Earth, Jupiter, and Saturn, lightning is associated with water clouds, but on Venus, it is associated with sulfuric acid clouds.

Our only direct close-up view of the surface of Venus comes from a series of missions carried out by the Soviet Union.  The Venera (Russian for Venus) missions made multiple successful soft landings on Venus and returned images of a bleak, inhospitable surface.  Due to the extreme surface conditions, the instruments only worked for between 23 min to approximately 2 hrs.  The image at the top of this page is from Venera 13 and the image below was transmitted by Venera 10.

Our understanding of the larger features on Venus come from radar mapping the surface. Most of that information was gathered by the Magellan mission. Between 15 September 1990 to  13 September 1992 Magellan mapped 98% of the surface of Venus.  The picture that emerged was that the surface of Venus looked very different from the surface of the Earth. The remarkable image below is based on all available radar data.  It is worthwhile to read the figure caption in the block quote before examining this image in detail.
 

The hemispheric view of Venus, as revealed by more than a decade of radar investigations culminating in the 1990-1994 Magellan mission, is centered at 180 degrees east longitude. The Magellan spacecraft imaged more than 98 percent of Venus at a resolution of about 100 m; the effective resolution of this image is about 3 km.
A mosaic of the Magellan images (most with illumination from the west) forms the image base. Gaps in the Magellan coverage were filled with images from the Earth-based Arecibo radar in a region centered roughly on zero degree latitude and longitude, and with a neutral tone elsewhere (primarily near the south pole). The composite image was processed to improve contrast and to emphasize small features, and was color-coded to represent elevation.
Gaps in the elevation data from the Magellan radar altimeter were filled with altimetry from the Venera spacecraft and the U.S. Pioneer Venus missions. An orthographic projection was used, simulating a distant view of one hemisphere of the planet.

Here is a link to a full resolution version of that image, along with some more in-depth explanation. After we have examined surface processes on Earth in module 3, we will return to Venus to try and understand why it does not look similar to the surface of the Earth.  But for now, we will just leave this information.

Craters smaller than 1.5 to 2 kilometers (0.9 to 1.2 miles) across do not exist on Venus, because small meteors burn up in the dense atmosphere before they can reach the surface. It is thought that Venus was completely resurfaced by volcanic activity 300 to 500 million years ago. More than 1,000 volcanoes or volcanic centers larger than 20 kilometers (12 miles) in diameter dot the surface. Volcanic flows have produced long, sinuous channels extending for hundreds of kilometers. Venus has two large highland areas - Ishtar Terra, about the size of Australia, in the north polar region; and Aphrodite Terra, about the size of South America, straddling the equator and extending for almost 10,000 kilometers (6,000 miles). Maxwell Montes, the highest mountain on Venus and comparable to Mount Everest on Earth, is at the eastern edge of Ishtar Terra.

Venus has an iron core that is approximately 3,000 kilometers (1,900 miles) in radius. Venus has no global magnetic field - though its core iron content is similar to that of Earth, Venus rotates too slowly to generate the type of magnetic field that Earth has.