Astronomy for Students

by Barry and Helen Setterfield

 

Lesson 4 -- Venus

Level 1

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. Here is what Venus looks like through a telescope (For the older kids, please notice the circles at the poles. This is going to be important later).

Venus

 

Then astronomy developed powerful instruments and spacecraft and the whole picture changed. The Russians were the first to send space-probes to Venus. They sent a few brief pictures of the surface. What they found was an atmosphere with a high carbon dioxide content, intense heat, and enormous atmosphere pressure. Here on earth, at sea level, the atmosphere is pushing down on us at 14 pounds for every square inch. But we have bodies that are made to resist that. If you dive into the deep end of a pool though, you will feel extra pressure on your ears. When you travel over a mountain pass, or climb a mountain, your ears will hurt then, too, because there will be a few pound less pressure on you for every square inch. Just a few pounds makes a very big difference to our bodies.

The average surface temperature on Venus is about 860 degrees F (212 is boiling water),or about 460 degrees C. The atmospheric pressure on Venus is about 90 earth atmospheres. This pressure is about the same as being at a depth of 2,700 feet in our oceans. You cannot go that deep without protection -- you would be crushed. Here are two images from the Russian probes, Venera 9 and 13 respectively, which landed on Venus on October 20th 1975, and 3 March 1982. The craft were probably melted by the heat and crushed by the great atmospheric pressure. Venera 13 only lasted 127 minutes before the electronics melted.

Venera9

 

The object on the ground in the middle is the camera cover in the picture below:

Venus surface

 

Venus is just a little smaller than Earth with a diameter of 7550 miles or 12,100 km. Earth is 7970 miles in diameter (easy to remember as about 8,000 miles) or  12,750 km. Its equator is inclined, or tilted, to the orbit at 177.4 degrees.That means Venus is almost upside down! Not quite, but almost.

Our earth rotates on its axis so that the sun rises in the east and sets in the west. Most of the other planets do the same thing. But two of them go the other way. Venus is one of the 'backwards' planets. There, the sun rises in the west and sets in the east! This is called "retrograde rotation." But it rotates on its axis so slowly that one day on Venus takes over 116 days on Earth.

Because Venus is closer to the sun than we are, it circles the sun faster, so its years are shorter. A year on Venus is only 224.7 of our days.

Here on Earth, when we fly in a large jet, it usually cruises at about 30,000 feet or maybe a little more. That's about 5.5 to 6 miles up. We can see the rain clouds underneath us then, although the wispy cirrus clouds are still above us. Sometimes our clouds can be so low we can see them winding around mountain tops. Our clouds are made of water vapor.

Now let's compare Venus. First, it's clouds are poisonous. They are made of drops of sulfuric acid. Sulfuric acid is a very strong acid which can eat its way through metal, plastic, and, definitely, a living body. The clouds are yellow to yellow-orange. There is sulfur dust in large amounts in the upper cloud layers. In the lower cloud layers there are large amounts of sulfur dioxide and hydrogen sulfide (the smell of rotten eggs). The clouds themselves are composed of droplets of concentrated sulfuric acid. The clear atmosphere is 96.5% carbon dioxide with about 3.4% nitrogen. The rest is water vapor. The carbon-dioxide and sulfur compounds have all come from the volcanic activity that can be seen on its surface. These cloud and haze layers impart a yellow color to everything on the surface. The Russian Venera craft registered this as seen in these color photos. The object on the ground near the upper right corner is the camera cover.

Venus surface color

 

These clouds on Venus are not close to the surface, though. If you go up 30 full miles, the air begins to get a little hazy with sulfuric acid droplets. That haze continues for fifteen miles. Then you get to the acid clouds themselves, and they are 13 miles thick. Above the clouds is another 15 miles of haze. From the surface of Venus up to the haze is quite clear, but you don't want to breathe it. There's no oxygen. It's mostly carbon dioxide -- 30 miles thick. The chart below shows you the distance in kilometers, but you can still see how it is there:

Venus atmosphere

Venus receives almost two times as much radiation from the sun as the earth does, so we expect it to be hotter. But the clouds on Venus reflect 80% of that. In fact it can be shown that with that amount energy from the sun being reflected, the temperature on the surface should be low enough to freeze water. Instead it is hot enough to melt lead! Here is a puzzle!

The Pioneer Venus probe took measurements that showed that the atmosphere of Venus was transmitting twice as much energy upwards from below as it was receiving from the Sun. In other words the heat energy was coming from below the clouds not from above. That was one part of the puzzle.

As we get closer to the surface things get more strange.

The Russian fleet of Venera spacecraft all encountered continuous lightning activity as they came down through the atmosphere. The lightning was persistent from a height of 32 km down to 2 km in altitude above the surface. The Veneras were registering 25 lightning bolts per second.

The surface of Venus is also very strange. From the photographs above, we can see that it is very rocky. But what was a big surprise is that Venus is loaded with volcanoes. Look at this map of them:

Venus volcanoes

We got that map from this page, which has a lot more pictures of volcanoes and lava flows on Venus taken from the Magellan spacecraft. Scroll down a bit for pictures of the surface. The blue color does NOT mean water. They add colors to the photographs to see the different levels, or heights, of the landforms.

So while Venus is about the same size as Earth, no one could live there. It is a very, very strange planet!

 

Level 2

Venus presents another puzzle. On Earth, we get winds because the tilt of the Earth's axis causes different parts of the Earth to heat up differently at different times. So as the cold and warm air cells shift, they create winds. Venus has tremendous winds, but the air is uniformly hot and so there are no temperature diiferences to explain the incredible speed of the winds measured there. Furthermore, because the planet rotates very slowly, the winds are not caused by the energy from rapid rotation.

Take a look at the speeds of these winds (a kilometer is about 5/8 of a mile, so you can get a rough estimate in miles per hour by dividing the km/hr in half):

Venus winds

The Venutian atmosphere is so thick that winds of this speed would be like getting hit with thousands of hammers.

The picture above is taken from a very good article about the winds on Venus. It is interesting to note that there appear to be a four or five day cycles to some of the winds and that is another puzzle for the scientists.

In 1972 it was found that the atmosphere moved up and down by about a mile over a 4 day period. This 4-day cycle was confirmed by the Magellan spacecraft. In other words, the atmosphere of Venus was behaving like the lid on a kettle that was bouncing when the kettle was boiling. The heat, then, can only be coming up from the interior of Venus. This heat is probably coming from radioactive decay.

Two of the Pioneer probes sent to Venus came down in the night hemisphere. At a height of 16 km they both noticed a glow from the surface of the planet. It turned out to be St. Elmo’s fire – a glow discharge in a strongly electrical environment.

At a height of 12.5 km above the surface every probe went haywire. All the sensors underwent a power surge. The best explanation is that the probes became covered with a plasma of charged particles in a static discharge. This effect also occurs in the upper atmosphere where there is a glow discharge is in the ultra violet. Observers of Venus from Earth could see this glow coming from the dark part of the planet when it was in its crescent phase. It was called the “Ashen Light.”

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

Venus double vortex

 

Here is the south pole

Venus south pole

 

These double-lobed vortex structures may give us a hint as to why the Venus atmosphere is whirling like it is. The atmosphere has a 4 day rotation period at the equator and a two day rotation period at the poles. The additional clue we need is given by the strike-rate for lightning (25 times a second), the glowing arcing surface and the plasma phenomena encountered by probes from a height of 12.5 km down. There is an intense electrical environment associated with the planet. In such an environment Birkeland currents as seen in plasma physics will play a big part. Birkeland currents usually come in pairs and twine around each other like this:

Birkeland currents

 

We will be dealing a lot more with this and explaining some of what is puzzling the scientists when we start the section on plasma interactions (after the planets).

[Note: because it was impossible to separate the following material into two levels without repeating much of it, a number of those who are comfortable with level 2 will probably also be able to deal with most of the material in level 3.]

 

Level 3

The Surface of Venus
The surface of Venus has no tectonic plates like the earth has, but it does have uplifted continental type areas. There is nothing equivalent to mid-ocean ridges. Tectonic plates require horizontal movement, but vertical movements may have occurred to form the continental or uplifted areas.

The first image is a general map of Venus laid out flat from 70 degrees North to 70 degrees South. On this map the landing places of the Russian Veneras is marked. The blue areas are the lowest and the red areas are the highest. The two main “continents” or high standing areas are Ishtar Terra and Aphrodite Terra. “Terra” means “land”; “Planitia” means “plain” or flat land; “Regio” means “region”; “Montes” means “mountains” and “Mons” means “mountain”. These three maps which follow all came from data collected by the Magellan Mission. The blue areas are rolling or undulating plains of volcanic material (probably basalt) .

Venus landing sites

 

The second is the hemisphere of the planet centered on the high region Aphrodite Terra. This “continent” is given a generally pinkish color on this map. See if you can trace the same outlines of this “continent” on the flat map above along with the curving features of Artemis Chasma (chasm). Note that the pink areas on this hemispherical map correspond to areas that are yellow and green on the flat map. Again the blue areas are rolling volcanic plains.

Venus Hemisphere

 

This third map is also a hemisphere map, just as we might see it from space. This hemisphere is centered on Venus’s North pole and has a small part of the flat map included in it. The flat map only went to 70 degrees North.  Since the pole is at 90 degrees much of this map was not seen on the flat map. The main pink area on this map is Ishtar Terra with the whitish feature being Maxwell Montes.

 

Venus north pole

 

Everything above 13,000 feet is very bright in radar. This may be from one of 2 causes. (1) it may be very rough. (2) It is due to St Elmos fire. This plasma discharge is bright in radar because it is strongly reflective of radar waves. This explanation fits what is known about the electrical peculiarities of Venus. Here is a map taken by radar.

Venus radar

This Radar map shows the white feature of Maxwell Montes at the top center. Note the other high white areas and those that are off-white of slightly pinkish. These are the areas which are either rough or electrically active. In view of the fact that all of them have a filamentary form to them (rather like some forms of lightning), the evidence suggests that we are dealing with an electrical discharge.

“Impact” Craters on Venus (defined here as all craters not of volcanic origin):

Unlike the Moon, Mercury (or Mars) which contain hundreds of thousands of “impact” craters, the surface of Venus has comparatively few. There is a total of about 1000 altogether. They range in size from 1.5 km (about 1 mile) up to 280 km (174 miles). Their distribution across the surface of Venus is extremely uniform. That can be seen from this map where every circle is an impact crater:

Venus craters

 

The number of craters with diameters less than about 35 km falls off rapidly with decreasing diameter. The reason is that small meteors burn up or break up in the dense atmosphere. Small craters tend to come in multiples, or are overlapping, presumably because the infalling object broke up into several pieces in the dense atmosphere. A typical cluster of three small craters from the Eistla region is here:

Venus crater cluster

 

An excellent group of Magellan photos of the Venus surface are on the web, including the above shot.

In contrast to the smaller meteors, the large meteors would get through basically intact and ought to leave their mark on the surface. But the rarity of these meteorite craters on Venus compared with the rest of the Solar System suggests that the surface of Venus is relatively young. So, too, does the fact that most of these craters look clean, sharp and unmodified. Geologists suggest an age for the surface of 300 – 500 million atomic years old. Their interpretation is that there must have been a major event where molten rock was outpoured from the interior that wiped out the earlier impact scars. This molten rock was extruded through the immense system of cracks, faults, fissures and fractures that connect the surface to the interior of the planet. The outpouring process ended with the formation of the various sorts of volcanoes on the surface of Venus that we see today.

The largest impact crater on Venus is Mead which is 174 miles (280 km) across. Here is what it looks like.

Mead Crater

The multi-ringed Mead Crater  (named after anthropologist Margaret Mead) is the largest impact crater on Venus, measuring 280 km in diameter. It is located north of Aphrodite Terra at latitude 12.50 N, longitude 57.20 E.

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 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. (This image is available here)

Continent Formation: The images of objects on the surface of Venus (except for the shots taken by the 8 Russian Venera probes) are all photographs taken by radar from the Magellan or infrared from Venus Express probes in orbit around the planet. Because the Venus Express probe used infra red imaging, it also picked up differences in rock types and their chemical composition. This has shown that the highland plateau rocks in Ishtar Terra and Aphrodite Terra and other similar areas. The Russian probes touched down away from the highlands and all landed on basalt-like rocks. The new data show that the Phoebe and Alpha Regio plateaus are lighter in color and look old compared with the rest of the planet. On earth, such light colored rocks are usually granites which are related to continent formation

On Wednesday 15 July 2009 a news report said that “The new data is consistent with suspicions that the highland plateaus of Venus are ancient continents, once surrounded by ocean that might have evaporated away into space…” “This is not proof, but it is consistent” said Nils Muller of the University  Munster and DLR in Berlin who headed the mapping program.

The suspicion arose because we know that Venus had a lot of water initially present. This can be deduced from the concentration of deuterium in the Venusian atmosphere. Deuterium is a special form of hydrogen where the usual proton and electron has had a neutron added to the nucleus. It looks like this:

deuterium

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. This, then, leads us to consider in brief the history of Venus.

A Brief History of Venus.

The planet was formed. How all the planets were formed will be dealt with in a later lesson.

  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 the 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 fissured, faults and fractures. This molten rock flooded the entire planet except for the upper parts of the high continental areas. In so doing, the 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 and the sulfur and sulfuric acid clouds and haze that also exists. 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.

 

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