The "Cloudy" Planet:
Venus, the jewel of the sky, was once know by ancient astronomers as the morning star and evening star due to its low elongation with respect to the Sun. Venus, which is named after the Roman goddess of love and beauty, is veiled by thick swirling cloud cover. Due to its closeness to the Sun, Venus was once thought to be two planets, Hesperus (evening star) and Phosphorus (morning star).
Venus is often called the twin planet to Earth because 1) it has a similar radius/size, 2) is has a similar mass, 3) it has a similar density and 4) it has an atmosphere. However, the environment of Venus is very different from the Earth, its ground temperature is over 800 degrees F and its atmosphere is carbon dioxide and sulfuric acid.
Notable features to Venus are its:
The above image of Venus is a mosaic of three images acquired by the Mariner 10 spacecraft on February 5, 1974. It shows a heavy atmosphere covered with thick clouds that prevents optical observation of the surface of Venus.
radar mapped terrain
This hemispheric view of Venus is the result of more than a decade of radar investigations culminating in the 1990-1994 Magellan mission. The effective resolution of this image is about 3 kilometers. It was processed to improve contrast and to emphasize small features, and was color-coded to represent elevation.
Radar imaging shows that a Venusian day is 243 Earth days and is longer than its year of 225 days. Oddly, Venus rotates from east to west. To an observer on Venus, the Sun would rise in the west and set in the east.
The atmosphere of Venus is composed of 97% CO2, 2% N2 and less than 1% of O2, H2O and CH4 (methane). There is also a substantial amount of sulfuric acid in the lower atmosphere.
Understanding the pressure, density and temperature of an atmosphere means understanding the Ideal Gas Law. The Ideal Gas Law states that the pressure, density and temperature of a gas are all related by a simple formula. For example, if you increase the pressure of a gas, its temperature also goes up or its density goes down. If you lower the density, the pressure goes down or the temperature goes up. The formula to express this relation is:
where ρ is the density (in gm/cc), T is the temperature (in Kelvins), P is the pressure (in bars), N is Avagadro's number (a constant) and k is Boltzmann's constant. Since there are two constants and three variables, it is often useful to express the ideal gas law as the ratio of the values. So, for example, the pressure, density and temperature of two planets' atmospheres (planet 1 and planet 2) are expressed as:
The surface temperature of Venus is around 890 degrees F, the hottest average temperature in the Solar System. This is due to the rich amount of CO2 which leads to a runaway greenhouse effect.
There are two types of greenhouse effects, normal and enhanced. Normal greenhouse is a natural activity that all the Terrestrial planets with atmospheres undergo (Venus, Earth and Mars). Both types are based on the property that some types of gases will absorb short wavelength light (visual) and remit it as long wavelength IR (heat).
Enhanced greenhouse effect is when the natural greenhouse gases are added by by human activity. Important components are CFC's and N2O from fossil fuels. Is this real? Is there any evidence that human civilization has changed the biosphere? The following plot indicates that we are nearing an all-time record in temperature and CO2 abundance based on ice core samples (note the upward trend starts 20,000 years ago when humans began to modify the environment).
The enhanced greenhouse effect is a complicated political issue. There are numerous popular misconceptions. For example, it is a misconception that the destruction of the Amazon rainforest contributes to the enhanced greenhouse effect. It does not, rainforest are mostly greenhouse gases producers. It is the total biomass on the planet that is important, and agriculture has filled in much of the holes that cities and population have taken.
What are the results of an enhanced greenhouse effect?
- global warming (3 to 5 C rise by 2050)
- increased cloud cover
- increased hurricane strengths
- rise in sea level by 1 to 2m over next 50 years
- increased growing season at northern latitudes
Venusian surface features:
Due to an optically thick atmosphere, the surface features on Venus are known only through radar mapping. Venus' surface is relatively young geologically speaking. It appears to have been completely resurfaced 300 to 500 million years ago. It's not clear how and why this occurred. The Venusian topography consists of vast plains covered by lava flows and mountain or highland regions deformed by geological activity. Some of the prominent features revealed by the Magellan mission are:
1) Craters - impact craters, Venus is scarred by numerous impact craters distributed randomly over its surface. Small craters less that 2 kilometers are almost non-existent due to the heavy Venusian atmosphere. The exception occurs when large meteorites shatter just before impact, creating crater clusters. Note the lava flows around the rims.
2) Volcanoes - Volcanic features are common on Venus with at least 85% of the Venusian surface is covered with volcanic rock. Hugh lava flows, extending for hundreds of kilometers, have flooded the lowlands creating vast plains. More than 100,000 small shield volcanoes dot the surface along with hundreds of large volcanoes. Flows from volcanoes have produced long sinuous channels extending for hundreds of kilometers, with one extending nearly 7,000 kilometers.
3) Fault lines - In images of the Alpha Regio, bright terrain is shown to be a series of troughs, ridges, and faults that are oriented in many directions. The lengths of these features generally range from 10 kilometers to 50 kilometers.
4) Arachnoids - As the name suggests, arachnoids are oval features with concentric rings and a complex network of fractures extending outward. The arachnoids range in size from approximately 50 kilometers to 230 kilometers in diameter. They might have resulted from an upwelling of magma from the interior of the planet which pushed up the surface to form "cracks".
Although there is no direct evidence for tectonic activity (active volcanoes, plumes, vents, etc.), three of the above features suggest weak geological activity in the last 107 years.
Due to the intense heat and pressure, a mission to the surface of Venus was challenging. The Venera missions were a series of unmanned Soviet planetary probes that were sent to Venus. Venera 5 and 6 (1969) made soft landings on Venus, but ceased transmitting data before reaching the surface because of the extreme heat and pressure. Venera 9 and 10 (1975) sent back the first closeup photographs of the planet's surface; these images showed that certain parts of Venus were covered with sizable sharp-edged rocks and others with fine-grain dust.
In general, the Venusian soil is rocky material intermixed with gravel. The rocks appear to be igneous in nature and smooth from erosion.
The principal Venus' characteristcs are:
- Thick atmosphere and "runaway greenhouse effect" causes 730K surface temperatures.
- Atmosphere consists mostly of CO2 (carbon dioxide), clouds of sulfuric acid—no water vapor or oxygen.
- Huge volcanoes.
- Clouds are highly reflective (high albedo)—Venus is one of the brightest objects in our night sky.
- Rotates very slowly (243 Earth days) and it rotates in a retrograde manner (opposite of the Earth & Sun).
- Not in a synchronous orbit—one Venus year = 225 Earth days.
- No moon.
- Almost identical in size to the Earth.
- A number of missions to Venus have been sent by the USA and USSR. In 1994, Magellan mapped the surface of Venus.
Interestingly, only the Soviet Union ever landed any craft on the surface of Venus, with one accidental exception. An atmospheric probe from the US Pioneer Venus mission that was supposed to drop through the atmosphere until it was crushed or baked to death survived descent all the way to the surface, but it wasn’t designed to be a lander, only an atmospheric probe. That it survived all the way to the ground surprised everyone, but it did not last long past reaching the surface. The Soviet Union had an effective monopoly on Venus landers, and all photos of the planet’s surface come from those craft.
On March 5, 1982, the Soviet Union’s Venera 14 Lander settled onto Venus’ surface. The image of the surface here was taken by that lander. Veneras 13 and 14 were basically sealed pressure vessels that descended through the atmosphere of Venus all the way to the planet’s surface.
The lander had quartz windows through which cameras were able to observe the surface (glass would have melted). The craft was heavily insulated, but there was really no way to keep all the heat out, and it was expected to die after about half an hour on the surface. Venera 14 exceeded expectations and lasted 57 minutes. It performed spectroscopic analysis of the ground, and was supposed to measure the soil compressibility, but the lens cap over its quartz window landed right where the ground compressibility probe extended. The craft landed in a large basalt field (there is reason to believe that this is a typical surface for Venus).
Currently exists only one active mission on Venus. The Venus Express. It will study its complex dynamics and chemistry, and the interactions between the atmosphere and the surface, which will give clues about surface’s characteristics. It will also study the interactions between the atmosphere and the interplanetary environment (solar wind) to better understand the evolution of the planet.
There is a special astronomical phenomenon related to Venus. It is called Transit of Venus. The last one was on 5 and 6 June 2012, and was the last Venus transit of the 21st century. The previous one took place on 8 June 2004, and the next will be on December 10–11, 2117, and in December 2125.