Venus is the third brightest regular object in the sky after the sun and the moon and can often be seen at noon with the unaided eye. Venus is an inferior planet and so exhibits the phases of the moon through a synodic period. These phases (especially the gibbous) were observed by galileo in 1610 and lent strong evidence to the Copernican heliocentric hypothesis.

Venus and Earth are sister planets. Venus is a bit smaller and the uncompressed density a little lower (smaller iron/nickel core). But Venus is closer to the sun and this evidently makes all the difference. The solar insolation is greater by a factor of 1/a2=1.9 (which is not at all offset by the planets greater albedo Venus/Earth=0.65/0.37=1.8).

The visual appearance of Venus is quite disappointing except for the phases. The disk is quite featureless due to a 10km layer of sulfuric acid droplets and sulfur dust at an altitude of 50km. Some features can be made out in the ultraviolet and Mariner 10 found cloud patterns that indicated an upper atmosphere rotation period of about four days. The first visible features were returned by the Russian Venera 7 in 1970 which landed and broadcasted pictures for 20 minutes before it succumbed to the 730K surface temperature. Previous Venera craft were crushed by the dense 90 bar atmosphere before reaching the surface. The data indicates a flat layered basaltic surface. A fractured lava crust?

The surface has been revealed in detail by Radar. The 300m Arecibo telescope can map the surface with a resolution of a few km comparable to telescopic observation of the moon.

But this technique is plagued by a weird coincidence wherein Venus shows the same face to the earth when best placed for observation. From earth, Venus turns exactly four times every synodic year! The rotation of Venus was determined by radar to rotate from east to west -retrograde rotation- with a period of 243.08d (d means Earth day), about 19d longer than its sidereal period of 224.7d. So the length of a solar day on Venus (where the sun rises in the west) is 116.67d. The synodic period of Venus is 583.92d during which Venus rotates 583.92/116.67=5.0049 times or almost exactly four times as seen from earth.

So earth radar has mapped under half of Venus. Space vehicles, especially the Magellan side scanning radar mapper (1990), have given us a better map of the surface of Venus than we have of the earth at a resolution of 100m (altitude as well as surface). The resulting picture of the planet is a remarkably flat planet covered by rolling hills and volcanic lava flow plains covering 80% of the planet. There are two major "continents" or highlands, Aphrodite Terra about the size of South America a bit to the south of the equator and Ishtar Terra, about the size of Australia in the North. Aphrodite Terra is covered by a vast network of faults and fractures. Ishtar Terra is a high plateau ringed by towering mountains, one of which is the highest point on the surface, Maxwell Montes, rising some 11km.

Less than a thousand impact craters have been found and there are no small craters. One of the largest is Mead crater named in honor of Margaret Mead. Evidently the atmosphere only allows larger bodies to reach the surface. The low crater count suggests the surface is young and must be refreshed often. Possibly by volcanic flooding. It has been suggested that Venus has a relatively thick mantle, a very good insulator, that periodically melts the crust as internal heat builds up and nature finds a way to dispel the heat I hope this does not happen on earth...

The Venerian atmosphere is an extremely thick blanket of 96% carbon dioxide (CO2), 3% nitrogen (N2), and traces of water (H2O), O2, Ne, SO2, Ar. The surface pressure is 90 atmospheres (bar) and at the surface the weather is clear and calm. Rising from the surface we find at an altitude of 30-50km the pressure falls to about a bar , the temperature falls to room temperature and the atmosphere becomes ever hazier due to sulfur dust and sulfuric acid aerosols, eventually merging into a lower cloud layer.

In 1985 the VEGA spacecraft deployed two French-Russian balloons which floated around for some two days at an altitude of 53km where the pressure was half a bar and the temperature 305K (32ºC) not too bad if you don't mind a bit of acid rain and catty your own oxygen.

The clouds gradually thin from 50-70km altitude. The sulphurous compounds should precipitate so must be continuously replenished presumably by volcanic activity. Indeed, compounds similar to those emitted by terrestrial volcanoes are detected in the atmosphere and the concentrations change over decades, lending further evidence to volcanism. Finally, the orbiters have detected static suggesting lightning, another indicator of active volcanism.

At the top of the clouds winds blow from east to west at about 360kph blowing the tops of the cloud cover around the planet in four days giving the appearance of a four day retrograde period.

Venus reflects two thirds of the solar energy that falls on it and is black in the infrared so it is a good radiator when T<1000K. So the cloud tops at least should be quite cold. They are. Of the energy that penetrates into the cloud layer a significant amount is absorbed, heating the clouds, and only a few percent of the incident energy reaches the ground. So the surface is dimly lit and since red light penetrates the clouds easiest the light is rather red. This is the reason for the color in the pictures sent back via the Venera landers. With white light the surface would be shades of grey. The feeble light warms the surface which radiates back into the atmosphere where it is promptly absorbed and returned. The thick CO2 atmosphere is opaque to infrared radiation and acts like a thick blanket. Since the heat transfer is very slow the temperature gradient must be very steep so that in equilibrium the surface must be very hot. It is. "Blanket effect" would be a better term than "greenhouse" but we use the latter term because everyone knows what we mean. Of course a greenhouse works by inhibiting convection. A greenhouse always has windows at the top and bottom so the temperature can be convectively controlled.

Atmospheric gasses are thought to be the result of outgassing. Venus and the earth both have active volcanoes that pump CO2 into the atmosphere. The CO2 is removed in the earth's case by absorption in the ocean followed by reactions that turn it into beach sand and limestone. But the absorption and outgassing are temperature dependent in a complicated nonlinear fashion (words we use when we don't really know what's going on) so if the temperature increases the outgassing increases (H2O evaporates and is a greenhouse gas) and the ocean absorption decreases, the blanketing further raises the temperature and… A very bad thing happens if the situation gets to the point that the planet's water trap (the altitude at which water freezes and cannot easily go higher) rises to an altitude where the sun's ultraviolet radiation can dissociate the H2O into H2 and O. The H2 will be lost in space and the O2 will eventually combine in the surface rocks so the water is destroyed forever. This process is not reversible so the planet is doomed. Just like Venus.