Venus

Venus is the second planet out from the sun. Until the space-age, Venus was considered to be a “sister-planet” to earth. It is about the same size, but was covered with clouds so we could not see its surface. many thought that, under those clouds, a planet something like earth existed. Science-fiction stories of strange creatures living in a perpetual warm fog were common.

Venus1 Venus
Venus as seen from a telescope
Venus as seen from a spacecraft

 

Venus is the closest of the planets to Earth in size: our diameter is a little less than 8000 miles, and Venus' is 7550. That makes the gravitational pull of each planet about the same. But there the similarity stops.

The farther out from the sun a planet is, the longer its year is. So Venus' year is longer than Mercury's and shorter than Earth's. A Venusian year is 224 Earth days long.

Because of the clouds that cover the entire planet, we had no way of knowing Venus' rotation rate -- how long a Venusian day is. The American Pioneer Probe used radar and infra-red instruments to look through the clouds at the planet's surface. When this was done it was discovered that Venus, like Mercury, has a very slow rate of rotation. One day on Venus, from sunrise to sunrise lasts for 117 Earth days. However, its sidereal rate is much longer, at 243 Earth days. Due to its slow rotation, Venus has no magnetic field. If someone were to be on the surface of Venus (which you will see is impossible) with a compass, there would be no magnetic north for the compass to point to.

On Earth, just as on Mercury, the sun rises in the east and sets in the west. That is because both planets are rotating from the west to the east. Venus is not. It is rotating in the opposite direction so the sun would appear to rise in the west and set in the east. This is called retrograde motion. It is for this reason that astronomers have concluded that something tipped the planet upside down. If it were exactly upside down, its "tilt" would be 180 degrees. It is only 3 degrees off, with a tilt of 177 degrees.

axis tilts

The first space crafts which landed on Venus were the Russian Venera probes in 1975 and 1982. However they melted within 90 minutes of landing. This gave us a hint that Venus was not at all like Earth. The American Pioneer probe of 1978, orbited the planet and used radar to look through the clouds at the surface. Later the and Magellan craft (1990) was able to look through the clouds using both radar and infra-red instruments.

Venus cloud/radar

Venus’ cloud tops (left) next to a color-enhanced radar map deep below the clouds' surface (right)

So, what is Venus really like? As you approach it, the top of the cloud layer is impressive. There are strong winds up to 250 miles per hour (370 kilometers per hour).

Venus winds

The cloud layer itself moves up and down by over a mile over a period of four days. Venus appears so bright in our sky because its clouds reflect 80% of the sunlight that reaches it. If its heat were from the sun, then that would mean the surface should be very cold. But remember, the Venera probes melted! As it turns out, Venus itself emits, or gives off, more heat than it receives from the sun. The surface of Venus reaches temperatures of up to 860 degrees F. It is hotter than Mercury. This surface temperature does explain the up and down movement of the clouds -- they are like the lid of a kettle that is boiling.

Approaching Venus, the cloud layer appears yellow to yellow-orange. First there is a hazy layer and then this gives way to a much thicker cloud layer. Both these top layers are composed of sulfuric acid drops. Sulfuric acid corrodes everything -- it eats its way through metal rather quickly. Sulfuric acid is also the reason for the yellow color of the clouds. The probe was dropping down at a fast enough rate to survive those layers. Below the sulfuric acid layer is a clear layer, composed of 96.5% carbon dioxide and 3.5% nitrogen.

Venus atmosphere

The above chart shows how the temperature and pressure increase dramatically at the surface. The troposphere is the layer composed of primarily carbon dioxide.

The compositions of these two layers of Venus' atmosphere were very good indications that Venus had volcanoes. Further, and conclusive, evidence was a type of water found in the atmosphere in small amounts. Water as we are aware of it is H2O, or two normal hydrogen atoms attached to an oxygen atom. The water in Venus' atmosphere is not quite like that. Instead of normal hydrogen atoms which only have one proton with one electron circling it, the Venusian atmosphere has "heavy" hydrogen atoms that each have a neutron in the nucleus as well. That form of hydrogen is called deuterium. So when we symbolize it, we write D2O. This, then, is called "heavy water." When water vapor has a high D2O content, it can only come from planetary interiors. So here was the strongest evidence of volcanic activity on Venus. At some stage, a huge amount of water must have been expelled from Venus' interior.

What happened to the water on Venus?

We know that Venus had a lot of water initially present. This can be deduced from the concentration of deuterium in the atmosphere. Hydrogen and deuterium both combine with oxygen to form water. The sources of water from the earth’s mantle (between the crust and the core) have a specific ratio of deuterium to hydrogen. This is called the D/H ratio. As water comes to the surface from the interior by volcanic or other processes some of it evaporates and goes into the atmosphere. The D/H ratio for Earth’s atmosphere is well-known, and its link with the amount of water in our oceans is established.

On Venus, the D/H ratio in the atmosphere is 120 times greater than on earth. Even when allowance is made for a higher evaporation rate, the D/H ratio is over 10 times what is expected. In an article in Nature for the 3 June 1993, D. H. Grinspoon has stated that it could be construed as coming from a lost primordial ocean or from continual volcanic outgassing from the interior.

The question therefore is, what has happened to the literally oceans of water that supplied the observed quantity of deuterium?

It has already been seen that there was a volcanic resurfacing event which covered the majority of the surface with molten rock or magma. This magma has solidified and now behaves just like basalt does on earth. This resurfacing event has been placed in the interval between 700 million atomic years and 500 million atomic years ago. In other words, this was well after the atomic date when the planet was formed. If the ocean was outgassed from the interior originally, or was originally part of the surface (which is possible on the plasma model discussed in a later lesson), then, when the molten rock was forced out from the interior to cover the planet, the ocean would be evaporated, and the D/H ratio in the atmosphere could then be accounted for.

 

After plummeting through the cloud layers, the Venera probe approached the surface. The measurements it sent back to Earth indicated that the atmosphere is so dense that the air pressure on the surface of Venus is 90 times the air pressure on the surface of Earth. That amount of pressure is what you get if you are 2700 feet down in the ocean here on Earth. On Venus, that pressure would mean that even a slight breeze would feel like hammer blows.

The Venera spacecraft encountered continuous lightning flashing from one mile up to about 20 miles up. Lightning flashes were counted at 25 per second.

When a Pioneer probe travelled through Venus' atmosphere, it was on the night side of the planet. At the upper levels the probe recorded a glow discharge in the ultraviolet range coming from the upper, hazy layer of sulfuric acid drops. This glow was the result of electrical discharges. It then recorded a much stronger glow from the surface of Venus. On Earth, this sort of glow, sometimes seen in marshes and even seen on ships' masts and aircraft, is called St. Elmo's fire. It is the result of electrical discharges between an object and the atmosphere.

At eight miles above Venus' surface both of the Venera and the Pioneer probes went haywire -- each experienced a power surge. Why? Because with that much electrical activity going on between the surface and the atmosphere, plasma covered the probes, producing massive static discharges. (You can get this same static reaction in a very much smaller way by rubbing a balloon against your hair and then watching your hair stand up, trying to get to the balloon so they can get the electrons back where they belong).

The Venus Double Polar Vortex

Two enormous atmospheric vortices, with very complex shapes and behaviour, rotate vertically over the poles of Venus, recycling the atmosphere downwards. The polar region or the 'black hole' seen in the images is where the polar dipole dominates. The polar dipole is the name given to a giant double-vortex, each of which is about 2000 km across, similar to the eye of a hurricane. The double-vortex has been seen at both the north and south poles, rotating in opposite directions (clockwise at the north pole and counter-clockwise at the south pole). Observations with Venus Express show that the vortex at the south pole also changes its shape rapidly, from one orbit to the next.

Here is the north pole of Venus

double vortex

 

Here is the south pole

south pole

 

Surface Features

The only actual photographs we have of Venus' surface come from the Russian Venera probes:

Venera photo1

Views of the surface of Venus taken by the Venera probe, whose electronics melted after a few minutes in the intense heat. The foreground object (the white 'half moon') is the camera cover. The rocky plates and debris around the craft are probably fractured lava.

The clues from the atmosphere that there were volcanoes on Venus proved correct. There are thousands of volcanoes on Venus. 1600 of them are considered major. The volcanoes are of different kinds: shield volcanoes, both large and small, and some unusual volcanoes.

Venus volcanoes

These volcanoes indicate a lot of activity in the interior of Venus. The heat from that activity is not just coming up through the volcanoes, but through the general surface of the ground as well. That is why that, although Venus' clouds radiate so much heat back from the sun, the planet itself is still so hot. It is the hottest planet in the solar system with a uniform surface temperature of 860 degrees F. (That's about 200 degrees hotter than Mercury.)

The other thing we see on the surface of Venus is a series of impact craters. But whereas little Mercury has over 45,000 known large impact craters (larger than a mile in diameter), Venus only has about a thousand, and they are uniformly distributed across the planet. That is because the volcanoes have re-layered the surface and so we only see the craters from impacts that have happened after any resurfacing. In addition, any incoming debris from space must penetrate through the thick atmosphere, and that means most of them would experience some breaking up or reduction in size. So we do see fewer craters for this reason as well. We also see clusters of small craters where a larger incoming body broke up and hit in pieces.

Venus impact craters

The largest crater on Venus is Meade. It is about 174 miles across.

 

Venus Meade

This radar view from the Magellan spacecraft shows several distinct features. The flat radar-bright central floor (A) has several cracks showing as even brighter lines. The crater has both inner (C) and outer (D) rings. Beyond, areas of ejecta (E) show as rough patches, whose slopes facing the radar making them appear brighter than the surrounding plain (G). These outside regions are covered with fine dust and debris which appears dark in the image.

The smallest craters are about a mile across. There are no smaller craters because any smaller object would be entirely burnt up by the atomosphere. It is impossible to estimate how big any of the impacting objects were because so much depends on the speed and the original size of the object, as well as its composition. For instance, comet nuclei will shatter and vaporize much more easily than asteroids. A more complete explanation of cratering can be found in the section on Earth's Moon.

Venus crater cluster

The above radar image shows a crater cluster, probably the result of one large object breaking up in the atmosphere.

Venus' surface has been disrupted by both its own interior heating and impacts -- resulting in vertical movements which caused rift valleys and faults. On Earth most of our quake activity is horizontal because we are on tectonic plates which are sliding slowly toward or away from each other. There are no tectonic plates on Venus, so there is no horizontal movement when the surface is disrupted. There are, however, massive uplifted areas that we refer to as 'continents.' The impacts which caused the most damage were the earliest ones. They may have caused the large mountains, but we don't know that for sure. We do know that the mountains were there before the resurfacing occurred. The resurfacing was a rsult of magma coming up through fissures and cracks in the surface, as well as through volcanic action. It is after both of those times that the craters we see today were formed.

Venus topography

The red, yellow and green areas are raised areas of the crust. The blue is NOT water, but lower areas, making up the lava plains of Venus where the molten rock, or lava, was poured out. The colors are artificial.

Other than that, here is the problem with trying to map Venus....

Venus-radar

Everything on Venus above 13,000 feet is very bright in radar. This may be from one of several causes: the surface may be rough above that height or it is due to St. Elmo's fire (or both). That plasma discharge shows bright on radar because it is strongly reflective of radar waves. Another option is that these areas mark metal deposits, which would also be highly reflective of radar waves. The white feature at the center top is Maxwell Montes, a large volcano. It is 6.8 miles high. It is the highest point on the planet's surface.

Venus Interior

Like Mercury, Venus has a solid nickel and iron core surrounded by a liquid nickel and iron layer. Over this is the mantle, of unknown composition, topped by the crust. Above that is the atmosphere of first carbon dioxide and then the thick clouds of sulfuric acid droplets.

Venus interior

 

Because Venus rotates so slowly, and does not have a magnetic field, that means there are no magnetic north and south poles. That means there are no polar cusps where the ionosphere dips into the planet. As a result, Venus' plasma sphere looks like this:

Venus ionosphere

However, what we have seen from earliest times was something that looked like hair streaming from Venus. It was called "the hairy planet." This was long ago when the plasma spheres of all planets were in glow mode. The plasma tail would have been quite visible from earth then. However, as time went by, and the the plasma spheres lost some of their charge, going into dark mode, scientists mocked the idea of Venus having 'hair.' Then, as spacecraft went into orbit around Earth and towards Venus, they found that "stringy things" from Venus' ionosphere came out as far as Earth's orbit. These things are part of Venus' plasma tail, and validate the ancient name given to Venus regarding it having 'hair.' This may have been one reason why Venus was considered a female goddess. We could not see Mercury's plasma tail as it was often pointing away from us and, when pointing toward us (when Mercury was between us and the sun), would only reach out to about Venus' orbit. The tails of the other planets would always be pointing away from us. Mercury, Mars, Jupiter and Saturn, the only planets we are sure could be seen with the naked eye, were all considered male gods.

 

A Brief History of Venus.

  1. The Late Heavy Bombardment (LHB) occurred from what is now the Kuiper Belt. Remember this formed craters on Mercury (as well as elsewhere in the Solar System.) The impact of these large objects on Venus probably resulted in the upwelling of granitic material and the uplift which gave rise to the “continental areas” on Venus.
  2. Due to the high concentration of radioactive material in the core, the planet, which originally started off in a cool condition, heated up. This drove the majority of water from the interior rocks towards the surface where it eventually burst out and formed possible original oceans.
  3. The initial breakup of the planet which occupied what is now the asteroid belt, around 750 to 600 million atomic years ago and sent debris down on the inner planets including Venus. By this time, the interior of Venus had heated to high temperatures and the originally cool rocks were now becoming molten. When rock changes from solid to liquid, its volume increases by 10% or more. This placed pressure on the crust of Venus. When the impacts of the first debris from the asteroid belt occurred, this triggered a fracturing of the entire Venusian crust, and molten rock from the interior poured out through the fissures, faults and fractures. This molten rock flooded the entire planet except for the upper parts of the high continental areas. In so doing, any original oceans were vaporized. The immense amount of carbon dioxide and sulfur dioxide given off by the molten rock reacted with the water and formed the carbon dioxide atmosphere that Venus has today as well as the sulfur and sulfuric acid clouds. . At the same time, all the impact craters from this event, and the LHB were obliterated by the magma.
  4. The secondary break-up occurred in the asteroid belt around 250 – 260 million atomic years ago. This event left the larger craters on the now solid volcanic surface of Venus. Volcanoes would still be active.
  5. The breakup of the moon of the original asteroid planet occurred about 65-70 million atomic years ago. This left some large and many of the smaller craters on Venus.
  6. As the interior of the planet still continued to heat because of radioactive decay, volcanoes remained active and the atmosphere trapped the heat in and became very hot, with its 4 day cycle of pulsing up and down like the lid of a boiling kettle.
  7. This is basically what we see of Venus today.

NOTE: The notation of "atomic years" is made because time measured by atomic processes is not the same as time measured by how many times we go around the sun. The difference is explained in Dating Methods.

 

 

 

.