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.

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Atmosphere:

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.

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Surface features:

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.

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Soil:

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.

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Venus:

In its global properties Venus is very similiar to the Earth with the following important differences:

• It still retains its original atmosphere of Carbon dioxide at a pressure which 90 times that of the Earth's Atmosphhere
• It essentially is non-rotating

The thick atmosphere has prevented any visual observations of its surface geology. Recent developments in radar imaging systems have allowed this thick atmosphere to be penetrated. Differences in radar timing at different locations represent different elevations. In this way radar imaging can build up a toplogical map of Venus.

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:

• There are no regions on Venus with saturated impact sites although there are some regions of fairly high crater density
• The surface is clearly young and active geology may be still be occurring. Some estimates are that lava flows data as recently as 300-500 million years ago but this is mostly conjecture.
• There are large scale uplifted regions above lower lying regions. The hemisphereic topographic maps strongly support the idea that Venus has been involved in some kind of continent building activity.
• There are features that look like large shield Volcanoes and giant calderas.
• Radar Bright features associated with large craters may be reflective of an explosive volcanic event that has locally shattered the crust.
• Overall, Venus is a suprisingly active surface.

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MARS:

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.

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Surface features:

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.

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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.

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Atmosphere:

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.

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Soil:

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.

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Quick Summary:

• Mars is relatively small, having only about 10% the mass of the Earth
• Mars has an atmosphere made up mostly of Carbon Dioxide but its pressure is only 1% of the Earth's atmospheric pressure. Thus Mars has 10,000 times less Carbon Dioxide in its atmosphere than Venus.
• The strangest thing about Mars atmospheric content is the high degree of residual Argon
• Mars is tilted on its axis by 25 degrees, very similar to the Earth and hence Mars has seasonal variations
• These seasonal variations cause frozen carbon dioxide to sublime from the Polar Caps thus giving Mars a seasonal atmospheric pressure

Speculation about Life on Mars:

This has runned rampant for many years. A few highlights:

• War of the Worlds by HG Wells in the late 19th century (this is actually an entertaining social commentary instead of a book about Martians invading the Earth even though Hollywood disagrees)
• Percival Lowell (late 1920s): A semi-deranged astronomer who thought he saw canals which connected the polar cap regions to the equatorial regions. He also saw changing patterns on Mars which he attributed to seasonal vegetation growth. He made up an elaborate story that Agrarian Martians built a network of aqeducts to bring water from the polar regions to irrigate equatorial crops. The New York Times believed him.
• Orson Wells: On Halloween night 1939 he gave a radio broadcast announcing that the Martians were landing in New Jersey. People believed him (must be how Rush Limbaugh got his start) - totally amazing. See the movie "Buckaroo Banzai" for more details on Cherry Hill New Jersey and John Bigbooty.

Spacecraft Encounters with Mars:

• Mariner 4 crash landed on the surface in July 1965. Limited images revealed a cratered surface
• Mariner 6 and 7 in 1969 took additional images (before crashing) which indicated the surface was not uniformly cratered like the lunar surface. Hence, Mars must have a younger surface
• Mariner 9 in 1971 was the first orbiter mission. Approaching mars revealed a surface that appeared suprisingly smooth and uniform with a few black dots. At the time of the approach, Mars was engulfed in one of its periodic global dust storms and the black dots were the tops of huge volcanoes sticking above the dust storm. These dust storms account for the shifting large scale patterns on Mars that Percival Lowell thought were agriculturual fields. Mariner 9 returned the first evidence that periodic massive floods (of water) occur on Mars.
• Viking Landers and Orbiters in 1976 took extensive images of Mars at fairly high resolution (like a few cm in the case of the Landers). These pictures are quite striking and should be inspected here

Surface Geological Evolution on Mars:

• No age dataing is available so everything is still a guess.
• Crater erosion is occuring as over time the craters are filled in with wind-blown dust
• Some evidence for primitive plate tectonics:
  • Valley Marineris: Looks like a huge rift valley on Mars as if crustal separation was occurring
  • Tharsis Volcanic Region: Region of 3--4 large shield volcanoes. Very reminiscent of the Hawaiian Islands as being on a moving plate over a stationary hot spot (magma source) deep in the interior of the earth. The large size of these Volcanoes is probably related to the low surface gravity on Mars (40\% of the Earth's)
• Fluvial Erosion (running water) from two sources. Note that the current atmospheric pressure on Mars is insufficient for water to exist as a liquid. However, during volcanic episodes the atmospheric pressure (mostly provided by carbon dioxide) is sufficient for liquid water to exist for a short time.
  • Much of the Martian topsoil is supported by permafrost. Periodic melting would cause this support to collapse and on Mars there does appear to be large scale collapse features (jumbled up terrain if you want a technical term)
  • Many fluvial features are associated with large impact craters. This suggest that the heat dissipated during the impace melts much of the permafrost causing a large scale flood. This flooding events are similar to what occurred in the massive flood 15,000 years ago that shaped much of Eastern Washington

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:

• Ate
• Breathed
• Deposited Waste

The Three Experiments and the results are the following:

• Gas Exchange Experiment: Some soil was put in a small chamber on board the Viking spacecraft to which water was added. There was a change in the atmospheric composition in the chamber but this could be entirely explained via chemical reactions with peroxides (a peroxide molecule has 2 hydrogens and 2 oxygens and is highly chemically reacting).
• The labelled Release Experiment: This was designed to see if anything in the Martian soil would "eat" some nutrients that were mixed in. The results can again be explained entire via chemical reactions
• The Pryolictic Release Experiment: This was designed to detect photosynthesis. When illuminated with light, photosynthesis should deposit carbon in the soil in the chamber. There was a small increase in the amount of carbon fixed in the soil which can not be entirely explained through chemistry.

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:

• No Martians were observed standing in front of the Camera and waving shouting "Hey, we are on TV!".
• Overall the results are ambiguous but most are consistent with a purely chemical reaction
• The soil of Mars is highly chemically reactive with a high oxidizing capacity (which of course is why the soil is red)
• A photochemically active surface is evident. However, due to low temperature and absence of water this surface is maintained in a state far from chemical equilibrium. Hence, the Viking lander experiments chemically woke-up some dormant top soil.
• The mass spectrometer aboard each lander carefully analyzed the topsoil of Mars and found no organic matter down to a level of 10 parts per billion.

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.