Two Different Stories
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.
This image of Venus is a mosaic of three images acquired by the Mariner 10 spacecraft on February 5, 1974. It shows the thick cloud coverage that prevents optical observation of the surface of Venus.
This hemispheric view of Venus, as revealed by 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.
The surface temperature of Venus is around 890 degrees F, the hottest average temperature in the Solar System. This is due to a runaway greenhouse effect. The atmosphere of Venus is composed of 97% CO2, 2% N2 and less than 1% of O2, H2O and CH4 (methane). Since CO2 is a major greenhouse gas, the radiation from the Sun is trapped in the atmosphere of Venus producing an extremely high surface temperature.
Due to an optically thick atmosphere, the surface features on Venus are known only through radar mapping. Such mapping by the Magellan mission revealed:
1) Craters - impact craters, but note the lava flows around the rims.
2) Volcanoes - hotspot volcanoes normally associated with tectonic activity
3) Fault lines - This images show the Alpha Regio. The bright terrain is 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 circular to ovoid 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 10**7 years.
On March 3, 1982 the Venera 13 lander touched down on the Venusian surface. It was the first Venera mission to include a color TV camera. This image is the left half of the Venera 13 photo.
In general, the Venusian soil is rocky material intermixed with gravel. The rocks appear to be igneous in nature and smooth from erosion.
In its global properties Venus is very similiar to the Earth with the following important differences:
The Magellan Mission to Venus represents a nearly complete radar image map of Venus at a resolution of about 300 meters.
The Soviet Union's Venera 8 and 9 missions (1975) represent the only landings on the Venusian surface. As this surface has a temperature of about 450 Farenheit and is constanly raining sulfuric acid, these spacecraft did not function long. The return images show the base of the spacecraft and a bunch of rocks.
Quick Synposis of Venusian Geology:
Up till the early 1920's we thought Mars looked like this or this. Note the ``canals'', which originally described as ``channels'' in Italian but then badly translated to ``canals'' which implied they were built by intelligent beings.
This is Mars from the Hubble Space Telescope
This image is a mosaic of the Valles Marineris hemisphere of Mars. It is a view similar to that which one would see from a spacecraft. The center of the scene shows the entire Valles Marineris canyon system, more than 3,000 kilometers long and up to 8 kilometers deep. Many huge ancient river channels begin from the chaotic terrain and north-central canyons and run north. Many of the channels flowed into a basin called Acidalia Planitia, which is the dark area in the extreme north of this picture. The three Tharsis volcanoes (dark red spots), each about 25 kilometers high, are visible to the west. Very ancient terrain covered by many impact craters lies to the south of Valles Marineris.
Although Valles Marineris originated as a tectonic structure, it has been modified by other processes. This image shows a close-up view of a landslide on the south wall of Valles Marineris. This landslide partially removed the rim of the crater that is on the plateau adjacent to Valles Marineris. Note the texture of the landslide deposit where it flowed across the floor of Valles Marineris. Several distinct layers can be seen in the walls of the trough.
This image of the head of Ravi Vallis shows a 300-kilometer long portion of a channel. Like many other channels that empty into the northern plains of Mars, Ravi Vallis originates in a region of collapsed and disrupted ("chaotic") terrain within the planet's older, cratered highlands. Structures in these channels indicate that they were carved by liquid water moving at high flow rates. The abrupt beginning of the channel, with no apparent tributaries, suggests that the water was released under great pressure from beneath a confining layer of frozen ground. As this water was released and flowed away, the overlying surface collapsed, producing the disruption and subsidence shown here.
The water that carved the channels to the north and east of the Valles Marineris canyon system had tremendous erosive power. One consequence of this erosion was the formation of streamlined islands where the water encountered obstacles along its path. This image shows two streamlined islands that formed as the water was diverted by two 8-10 kilometer diameter craters lying near the mouth of Ares Vallis in Chryse Planitia. The water flowed from south to north (bottom to top of the image). The height of the scarp surrounding the upper island is about 400 meters, while the scarp surrounding the southern island is about 600 meters high.
This image shows the south polar cap of Mars as it appears near its minimum size of about 400 kilometers. It consists mainly of frozen carbon dioxide. This carbon dioxide cap never melts completely. The ice appears reddish due to dust that has been incorporated into the cap.
Before 1800's it was known that Mars had surface features and seasons (the polar caps changed size). Two small moons discovered, Phobos and Deimos (Fear and Panic). Not regular moons like our Moon but rather irregular shaped objects that means they are captured asteroids.
1) craters - impact craters with heavy erosion due to atmosphere
2) featureless terrain - large regions devoid of faults or craters (not maria since no young craters are found). Investigation by Viking Orbiter showed them to be deserts with dunes.
3) chaotic terrain - highlands and broken hills, probably old tectonic regions.
4) Polar caps - change in size with seasons, but since the temperature is less than 0 degrees C at all times, the ice is mostly CO2 ice with an H2O ice core. Viking Lander confirmed that the atmospheric pressure goes up in the summer as the CO2 ice melts.
5) volcanoes - such as Olympus Mons and Olympus Mons from the side - The Tharsis and Elysium are rich in old cone volcanoes, averaging over 500 km across and 25 km high. These are ``hotspot'' volcanoes like the Hawaii Islands. Extreme size due to the fact that there is no tectonic plate motion on Mars.
6) canyons - long, fractured regions. Most canyons on Earth are caused by either a) wind erosion, b) H2O flow or c) lava flow. But, the atmosphere of Mars is too thin for wind erosion, H2O is all ice, all volcanoes are inactive. Canyons must be tectonic features leftover from early epochs.
Water on Mars:
There is a great deal of evidence of the existence of liquid water on Mars, at least in the far past. A secondary atmosphere for all terrestrial worlds is rich in CO2, H2O and SO2. On Earth, the temperature is just right for H2O to rain out and form oceans. On Venus, the temperature is too hot and H2O stays as a vapor to be destroyed by photodisintegration. On Mars, it is too cold for liquid water. All the H2O is locked up in permafrost under the soil and subsurface ice reservoirs. Notice that most of the water flow features are near the base of old volcanoes or impact craters. These early events heated the subsurface ice to produce a short-lived flow of liquid H2O.
Mars is another example of a secondary atmosphere from outgassing (therefore, we know that Mars had an early epoch of tectonic activity). However, unlike the Earth or Venus, the atmosphere is very thin, about 1% the mass of Earth's atmosphere. Its composition is 95% CO2, 3% N2, 2% Ar and less than 1% O2. A high noble gas content implies that Mar's atmosphere was much thicker in the past (noble gases do not react with other elements and are heavy enough to stay within the gravitational field of Mars). The climate on Mars is very desert-like due to its thin atmosphere. There is too little mass in the atmosphere to hold in heat so the warmest daytime temperatures are around 50 degrees F, but the nighttime temperatures are -170 degrees F. Other weather features are massive dust storms and occasional CO2 fog in the canyons.
The images from Viking Lander 1 and Viking Lander 2 indicate a surface and soil that is mostly old impact debris with sand-blasted gravel. The soil is rich in Si and Fe, Fe oxide (rust) given Mars its red color. The low gravity of Mars has produced a crust and soil that is not as chemically differentiated as the Earth's crust. The frost in winter is mostly CO2 ice.
This has runned rampant for many years. A few highlights:
Searching for Life in the Martian Top Soil:
The Viking Landers primary mission was to test for microbial life in the Martian soil. They landed at latitudes of 23 and 48 degrees. There were three identical experiments on board each lander designed to see if anything in the soil did the following:
The Three Experiments and the results are the following:
An excellent summary of the Viking Lander Experiments can be found in a article in Scientific American in 1977 by Horowitz. A brief summary is given below:
The major flaw associated with the Viking Lander experiments is that they could only access the topsoil. As this is the product of wind blown dust, it is possible it has become sterile by this process. Hence, it is desireable to return to Mars and sample deep into the soil, down near the bedrock, to test for the presence of organic matter.
There is ample scientific motivation to return to Mars with a manned mission. Here's predicting it will happen before the year 2025.