Credit NASA (click on image for a larger view)
Mercury mosaic by Mariner 10 (1974)
Credit NASA (click on image for a larger view)
Mercury mosaic by Mariner 10 (1974)
Actually, very little is known about Mercury. This small planet is slightly larger than our moon, is heavily cratered, has no atmosphere, and orbits so close to the sun (.39 AU) that we really can't ever view it well from the Earth. The first spacecraft to visit this barren, sun baked planet was Mariner 10 in 1974. We have much more information about Mercury since Messenger reached orbit in 2011.
It only takes Mercury 88 days to orbit the sun (sidereal period) and rotates on its axis once every 59 days, .... which makes a day on Mercury last 176 earth days. During that "day", the temperature ranges from -300 degrees Fahrenheit (at night) to 800 degrees Fahrenheit during the day. No life biological is possible under these conditions.
Mercury has a rather high density, indicating that is has a large metallic core .... likely composed of iron. Mariner 10 measured the presence of a magnetic field on Mercury. This was totally unexpected because this would indicate that the interior is still molten (and acting like a dynamo). Radar images from the earth (and confirmed by Messenger) show that there is something shiny and reflective at the poles of Mercury. Could this be water (and other volatiles) trapped in deep, cold impact basins (which receive no sunlight)? If so, how did it get there? Astronomers are curious because they believe impacts by comets could provide the answer. If that were the case, perhaps a lot of the water on the earth was delivered by comets as well.
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Venus was always considered "Earth's twin" by earlier astronomers . They are both about the same distance from the sun (.72 AU and 1.0 AU), and both have the same diameter. Perhaps the surface of each planet was similar? What a surprise we got when we actually went to visit Venus. The Russian Venera probes landed on the surface only to find a modern version of Dante's Inferno. No liquid water, perpetual cloud cover, no breathable air, scorching temperatures, crushing surface pressure, and sulfuric acid rain. Earth's evil twin maybe.
Venus has a very dense atmosphere composed mainly (96%) of carbon dioxide (CO2), which is a greenhouse gas. This means that the solar infrared radiation enters the atmosphere, but does not allow heat to escape easily. As a result of this ultra-greenhouse, the surface temperature reaches 890 degrees Fahrenheit - which is extremely hot! In addition, the dense atmosphere produces a surface pressure about 90 times the surface pressure at the surface of the earth.
How did planets so similar in size end up with such different atmospheres? The answer is water! On earth, carbon dioxide enters the atmosphere from volcanoes (and comets), but is removed when rain takes in the CO2 and washes it to the oceans. The deep oceans of the earth represents a huge reservoir of carbon (in the form of a bicarbonate in solution) taken from the atmosphere. Marine plants and animals are able to use some of this carbon to produce a natural defense (coral, shells, clams, etc). These mollusks pile up as oceanic sediments which eventually form rocks like limestone. Some of this carbon is recycled back into the atmosphere as out-gassing from volcanoes. The carbon cycle on earth acts as a thermostat, keeping the surface temperatures within a limited range. If the earth's surface temperature increases, nature provides a fix. The increased temperatures allow more efficient absorption and weathering of rocks by rainwater, ... taking more carbon dioxide out of the atmosphere and putting it into the ocean. Less CO2 in the air makes the temperature drop. Venus is unable to do this, so it becomes a furnace. Could it happen to the Earth? Click here if you want to read some predictions.
Venus has almost no water now! Probes can see hydrogen and oxygen escaping to space (the parts of water). This is no surprise. The lack of magnetic field allows the solar wind to "erode" water to its component parts and then blasts it to space. However, there is indirect evidence that Venus did have liquid water in the distant past. The evidence comes from a high concentration of deuterium (heavy hydrogen) found in the Venetian atmosphere. The thought is .... if Venus did have a lot of liquid water in the distant past, and it eventually all boiled away into the atmosphere. The heavier variety of hydrogen would find it harder to escape and build up in concentration.
Since Venus has the same size and density as the earth, it probably has a similar interior. However, Venus does not have a magnetic field. This suggests that either Venus has no liquid outer core (as the earth has) or it is rotating too slow to act as a dynamo. The rotation rate of Venus is only 243 days (in the opposite direction).
The Magellan spacecraft was able to penetrate the thick clouds of Venus (using radar) which presented planetary geologists with more surprises. One of the standing questions they had was - "Does Venus show any evidence of plate tectonics?". Magellan showed that the crust of Venus is not sectioned into plates! How can this be?
Pancake domes on Venus (Credit NASA)
On earth, the majority of the heat within the earth escapes via plate interactions, but not always. The Hawaiian islands are a volcanic feature (shield volcano) formed far from any plate boundary. Geologists suggest that there are "hot spots" in the mantle where the interior heat is excessively high. In order to release this heat (from radioactivity), the mantle material simply unwells, and punches a hole in the crust. Other examples are found in Yellowstone National Park and Iceland. Apparently Venus releases its interior heat this way on a global scale. The vast majority of the Venetian surface shows evidence of shield volcanoes (and other types known as pancake domes and Coronae). Accompanying these flat "pancake domes" are vast regions of lava flows. Venus has two uplifted plateaus (similar to continents on the earth) which apparently formed when an entire region was pushed upwards from material in the mantle. The rocks in the crust must be extremely rigid in order to maintain vertical profile seen on Venus (despite that fact that the rocks are very hot). However, none of these volcanic features appear to be currently active .... maybe. Since 2006, the European Space Agency's Venus Express spacecraft has shown significant spikes in sulfur dioxide which could be explained by volcanic outgassing.
The surface of Venus gives us another surprise - impact craters.
An impact crater on Venus
There are a little over 100 confirmed impact sites found on the earth, but over 1000 found on the surface of Venus. They seem to be evenly distributed and suggest that the surface has not changed for the last 500 million years. This is odd because geologists are quite certain that Venus has to release interior heat (via radioactivity) just as the earth does. Apparently Venus does it in spurts and all at once (and the earth does it on an ongoing basis). It is much like a pressure cooker, when the temperature builds to a critical point, then releases the steam. If so, when will the surface of Venus "blow"? Exactly what triggers the eruptions is a matter of debate. Below are some possible ideas:
The crust of Venus is rigid and thick. This makes it difficult for the interior heat to escape and it builds up. Once the mantle temperature passes a critical point, volcanic eruptions take place on a global scale, giving Venus a new surface. Once a volcano erupts on Venus, it could trigger more eruptions elsewhere as the increase in CO2 heats the surface and weakens the crust. Eventually the activity stops and the crust again acts like a lid, keeping the building heat inside the planet until the cycle repeats
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The atmosphere plays a critical role when the volcanoes occur. Perhaps, Venus goes through huge temperature swings. When it is hot (hotter than it is now), crustal rocks are weakened and this triggers the eruptions.
Life on Venus?
In September 2020, things got really interesting when astronomers discovered a molecule called phosphine in the atmosphere of Venus. Maybe the surface of Venus is too hot to support life, but for decades astronomers have suggested that the upper atmosphere may be capable of supporting bacterial life. That idea got a big boost with the discovery of phosphine in the atmosphere of Venus. Phosphine is a by-product of anaerobic (oxygen free) bacterial life. I'm sure this discovery will produce a future mission to investigate this exciting possibility.
ŠJim Mihal 2004, 2014, 2020- all rights reserved
