Venus' Atmosphere: Composition, Climate and Weather Venus' Atmosphere: Composition, Climate and Weather By Nola Taylor Redd, Space.com Contributor | November 16, 2012 06:23pm ET MORE Venus has the distinction of being the hottest planet in the solar system, and the fault lies solely with its atmosphere. What is it about the air on Venus that keeps the planet cooking? False-colour image of cloud features seen on Venus by the Venus Monitoring Camera (VMC) on Venus Express. Credit: ESA/MPS/DLR/IDA Atmospheric makeup The atmosphere of Venus is made up almost completely of carbon dioxide. Nitrogen exists in small doses, as do clouds of sulfuric acid. The air of Venus is so dense that the small traces of nitrogen are four times the amount found on Earth, although nitrogen makes up more than three-fourths of the terrestrial atmosphere. This composition causes a runaway greenhouse effect that heats the planet even hotter than the surface of Mercury, although Venus lies farther from the sun . When the rocky core of Venus formed , it captured much of the gas gravitationally.

In addition to warming the planet, the heavy clouds shield it, preventing visible observations of the surface and protecting it from bombardment by all but the largest meteorites. Although Venus and Earth are similar in size , someone standing on the ground on Venus would experience air about 90 times heavier than Earth's atmosphere; pressures are similar to diving 3,000 feet beneath the ocean. Ironically, the most Earth-like atmosphere in the solar system occurs 30 to 40 miles (50 to 60 kilometers) above the surface of Venus. Both oxygen and hydrogen rise above the heavier gas layer covering the ground, and the pressures are similar to our planet. Carbon dioxide: 96 percent The Atmospheres of the Terrestrial Planets How did the terrestrial planets get their atmospheres? Two distinctly different processes exist for the formation and development of the atmosphere of a terrestrial planet. The first is capture of a primitive, or primary, atmosphere. The second is the outgassing of a secondary atmosphere. Primitive atmospheres: If these atmospheres ever existed, they would have been the first (or primary) atmospheres for the terrestrial planets. These atmospheres would have been captured from the gas-rich solar nebula as the protoplanet grew.

With increasing size, the protoplanet's gravity may have become a sink for the concentration of solar nebula gas. Such a gas would have been dominantly H, H2, He, Ar, Ne, and perhaps small amounts of H2O, CH4, and NH3. The lightest of these would have easily escaped (or never been captured by the protoplanet), leaving an atmosphere dominated by the heavier inert gases and hydrogen rich compounds. Such a hydrogen rich atmosphere is called a reducing atmosphere. Did the terrestrial planets ever have primitive atmospheres of any significance? For many years, astronomers assumed that Earth's earliest atmosphere was a dense, primitive atmosphere. Hence, the earliest Miller-Urey experiments (making amino acids in a test tube filled with methane, ammonia and water) imitating the supposed primeval soup were done in reducing environments. We can test this hypothesis and it is found wanting. How? Neon. Neon is heavy and so none can escape from Earth's atmosphere. (Do the calculation and test this claim!) It is inert so it does not bond with rocks or have any other way of escaping into the Earth from the atmosphere.

It is not produced via radioactive decay, so whatever neon is presently in our atmosphere is a measure of the upper limit for the total amount of neon left over from a primitive atmosphere (note that some neon could have entered the atmosphere via outgassing over Earth's history). Given the known ratio of neon to hydrogen and helium and other gases in the sun, we can estimate the total mass of the Earth's primitive atmosphere. The result is that the primitive atmosphere could have been no more than 0.9% of Earth's present day atmosphere. Therefore, the Earth never had a primitive atmosphere of any significance. How do secondary atmospheres form? Gases are vented (outgassed) through volcanic eruptions. The terrestrial planets must have been warmer and more volcanically active when they were younger. These gases will accumulate at the surface of the planet and in the atmosphere. Some can bond to rocks and cycle in and out of the atmosphere; others will dissolve in water, reaching equilibria between atmospheric and oceanic abundances. Some will remain almost wholly in the atmosphere. What would an outgassed atmosphere look like? The following gives observed and theoretical estimates of gases added to Earth's atmosphere (as percent composition by weight).

The data is from the William Hartmann's book Moons & Planets (p. 320). carbonic acid reacts with silicate rocks (or minerals dissolved in sea water) to form carbonate rocks enstatite (MgSiO3) + carbonic acid (H2CO3) -> <- magnesite (MgCO3) + quartz (SiO2) + water (H2O) [Note: one could think of this reaction as simply silicate rocks + carbon dioxide yields carbonate rocks and quartz; however this ignores the critical catalytic role played by water. Without the water, this process doesn't go forward.] magnesite (and many other carbonates) is not soluble in water so it precipitates and forms rock on the ocean bottom; most CO2 on Earth is tied up in carbonate rocks and oceans; when these rocks are subducted, the heat drives the reactions backwards and the CO2 vents from volcanoes. high temperature drives reaction to the left (Venus) water drives reaction to the right (Earth) on Venus, absence of water keeps CO2 gaseous what about Mars? Is CO2 locked into rocks? If so, would the presence of carbonate rocks be evidence for ancient Martian oceans?

And finally, let's compare the nitrogen contents: What have we learned from comparing these three terrestrial atmospheres? Venus and Earth have the same carbon dioxide budgets. Venus has no water; more than one part in ten thousand of the Earth is water. Comets are the most likely source of both water and carbon dioxide. If you capture a comet, you get both. If we assume that Venus and Earth formed in the same part of the solar system from the same general process of accretion formed from similar materials obtained their volatiles from the same reservoirs of volatile-rich material (outgassing of cometary material collected during early stages of planet formation or late veneer bombardment from comets) And we find that Venus and Earth have the same total content of carbon dioxide similar amounts of nitrogen (to within a factor of 3) Then where is all the water on Venus?

Terrestrial Planetary Atmospheres Greenhouse gases (CO2, H2, O3, CH4) absorb IR as it tries to escape Which solar system objects have atmospheres Moon, Mercury - no atmosphere to speak of Earth - yes, atmosphere of mostly N2 and some O2 Venus - yes, thick atmosphere of mostly CO2 Mars - yes, tenuous atmosphere of mostly CO2 Other objects? Titan, Triton, Io What do these objects have in common? Volcanoes - source of atmospheric gases Larger objects - with enough gravity to hold gases Planetary Atmospheric Evolution In discussing the properties of the atmospheres of the different terrestrial planets, we mentioned that the source of planetary atmospheres is mainly volcanism where gases from the interior of the planet are brought up to the surface. The main gases produced by volcanoes are H2O, H2S, CO2, N2, and CO. Here is the cartoon again that shows the atmospheric sources . Figure 10.20 shows evaporation and bombardment as additional sources of atmosphere.

These were more important than volcanic sources in the early stages of planetary evolution (and remain important in the outer solar system) but are less important than volcanism for the bulk of the geological history for the terrestrial planets. What happens to these gases when they enter the atmosphere? Volcanoes produce mostly water, carbon dioxide, nitrogen, carbon monoxide - plus some other gases such as sulfur dioxide and hydrogen sulphide (both very smelly!) If the planet is cold enough the water condenses to form an ocean Carbon dioxide dissolves into the water - if there is life in the ocean, the carbon dioxide is converted to calcium carbonate - rock - by shellfish and coral, etc. Carbon monoxide and water can react in the early planet's atmosphere to make hydrogen and carbon dioxide. The hydrogen, a light gas, can escape more easily than other gases. Thus, more carbon dioxide is left behind. If the planet does not have water, then the atmosphere collects carbon dioxide. Plants on Earth convert carbon dioxide to O2 - the gas that humans (and other mammals) breath in the Earth's atmosphere.

(13) (a) On Earth, H2O condensed to form oceans, CO2 dissolved into the ocean (and became carbonate rocks), CO reacted with water to make more CO2--which of the main volcanic gases does this leave to make an atmosphere? (b) In which ways has the presence of life on Earth affected the Earth's atmosphere? Why are Earth and Venus so Different? Earth and Venus both have volcanoes, both have the same amount of gravity and Venus is really not that much closer to the Sun than the Earth. Yet Venus as a thick, poisonous atmosphere and a surface temperature equivalent to a "high broil" in your oven. The critical difference is the presence of liquid oceans on the Earth: (14) (a) Comparing the evolution of the atmospheres of Venus and Earth, the critical step is the formation of an ocean--why is the Earth's ocean so important? (b) Water not only does not condense on Venus, it is broken up into its constituent elements. What are these elements? (c) What happens to the hydrogen? (d) What does this mean for the possibility of ever having an ocean on Venus, even if the temperature could somehow become cooler?

(e) This is sometimes called the "runaway greenhouse effect" - what does "runaway" mean in this case? Earth's Climate Change The Evidence for Global Warming Studying the differences between the atmospheres of Earth, Venus and Mars is not just an academic exercise. Comparing atmospheres tells us how atmospheres work and warns us that we may be changing Earth's atmosphere at our peril. Or maybe not. Below are two theories of climate change. They have radically different implications for predicting the effects of human activity on the Earth's climate and on the possibility of life existing elsewhere. First, let us look at the evidence for climate change on Earth. There is a great deal of talk in the public media about "global warming" - what is the evidence? Look at these graphs of global climate change . The top plot is for long term variations - over hundreds of thousands of years. The bottom one is for the past 150 years. They both have temperature as the left axis and carbon dioxide concentration on the right axis. How these data obtained? Well, for the modern data we can just average measurements of temperature taken all over the world.

The carbon monoxide measurements are made at a few special atmospheric observatories (such as the one run by the National Center for Atmospheric Research on Mauna Loa, Hawaii). The long-term data come from ice cores from the Greenland icesheets. The ratio of oxygen isotopes is believed to be an accurate indicator of temperature, so by measuring the oxygen isotope and CO2 concentrations with depth down a core we can get an estimate of the climate at the time that the layer of ice was deposited. The temperature scales are in degrees Celsius as the change from average values in the 1970s. The CO2 concentration scale is in parts of CO2 per million parts of air ("parts" just means volume - so that 300 parts per million of CO2 means that for every million cubic meters of air, there is 300 cubic meters of CO2). The important things to notice, however, are the changes in these quantities with time. Look at how the red CO2 concentration lines match the black temperature lines. This just says that it is warmer when the CO2 concentration is higher. (15) (a) How is a warmer temperature consistent with more CO2 in the atmosphere?

(b) About 140 thousand years ago there was a large increase in temperature and CO2 concentration. By how many degrees did the temperature increase? How much did the CO2 concentration increase? (c) These long-term variations are believed to be caused by wobbles in the Earth's orbit and the tilt of the Earth's spin axis. The times of colder temperatures are often called "ice ages" since there were extensive glaciers that spread south from artic regions into the mid-latitudes. From the plot you can see that there are roughly 2 timesscales - one between deep, freezing temperatures (or between very hot spells - this is perhaps easier to measure) and the other, much shorter variations. What are the two timescales for these variations? (d) Now, look at the lower plot of modern variations. Note that the temperature scale has now changed to a range of -0.8 C to +0.4 C - about a factor of 10 smaller range that the upper plot. How much is the variation in average, global temperature from year to year? (e) How much is the variation in temperature, smoothing out the year-to-year variations, over the past 150 years? Are YOU convinced that there is global warming?

(f) The red plot is really as much a plot of human activity as a plot of CO2 concentration. What are the ways in which human activity contributes to CO2 production? (g) What is the prognosis for the future? The top plot shows that the Earth has suffered much larger ranges in CO2 and temperature in the distant past. What would be the impact on present flora and fauna - including humans - of similar ice ages / global warmings? Climate Models So far we have explored the effects of distance of a planet from the Sun, albedo and greenhouse gases on a planet's surface temperature. We are able to do a reasonable job of modeling the temperature of the planets - at least to an accuracy of a few degrees. However, the Earth's environment is very sensitive to small changes in temperature. A few degrees can have drastic effects on vegetation and melting of the ice caps (which would flood low-lying areas of land), for example. This means that the effects of changing the composition of the atmosphere need to be calculated with sophisticated models.

Scientists at the National Center for Atmospheric Research (up on the mesa above Boulder) have developed models of the atmosphere that include many more effects - multiple layers, clouds, geographical variations across the globe, coupling to the oceans, etc.; in their Global Climate Models (GCMs). For an example, go to the Genesis site. Clicking on a picture will give you a full-page version. (16) (a) What are the NCAR scientists predicting for the increase in the amount of carbon dioxide over the next (i) 20 years? (c) food production? Theories of Planetary Climate Change These models are just for the Earth. The political issues of global warming are very immediate for us on Earth. In a more philosophical way, we can look at the Earth as the only (seriously) habitable planet in the solar system and wonder what the implications may be. Here are 2 ideas about planetary climates: CLIMATE CHANGE - THEORY #1 Basic Idea = the Earth is just in the right place. A little farther away from the Sun and you get a "runaway icebox" (Mars) and a little closer to the Sun and you get a "runaway greenhouse" (Venus).

Estimated habitable zone is small (0.95-1.05 AU) Few habitable planets in the Galaxy Earth's atmosphere is precariously balanced - we could tip it either way, catastrophically, if conditions changed. CLIMATE CHANGE - THEORY #2 Basic Idea = Earth's biosphere is self-stabilizing (Gaia hypothesis). The biosphere on the Earth adapts to maintain its existence - it reacts to changes in climate to maintain a habitable environment. Large range of distances from the Sun habitable Many habitable planets No need to worry about human impact on the environment - the rest of the biosphere will adapt to keep Earth habitable. (18) Which theory do you think is closer to the truth? What Happened on Mars? While Venus is much more similar to Earth, in size and location, Mars is more likely to have had life. Yet, presently there is no evidence of life existing on Mars right now. An important factor is Mars' tenuous atmosphere. As we discovered when discussing erosion in session 12, there is plenty of evidence of there having been liquid water on the surface of Mars in the past. What happened to Mars' atmosphere? What happened to the water?

Mars is a little farther from the Sun, making it a little colder Mars is smaller: Atmosphere of Venus - Universe Today Universe Today by Jerry Coffey Earth and Venus share many physical characteristics. Atmosphere is not one of them. The atmosphere of Venus is so harsh that it is the main reason that no one has ever been able to make optical observations of anything other than the planet’s upper atmosphere. Visualization of the planet is made impossible be the high amounts of sulfuric acid in the atmosphere. Clouds in the upper atmosphere are full of sulfuric acid droplets. Sulfur is highly reflective of visible light, thus preventing observation much deeper than the upper limits of the clouds themselves. The next challenge that the atmosphere of Venus presents is its composition. It is made up of 96% carbon dioxide. Oxygen can not be detected even as a trace element. At the surface the atmospheric pressure is 92 times that of Earth. If you were able to find a way to survive the intense pressure and had enough oxygen, you would be standing on a surface that is hot enough to melt lead. The temperature across Venus, from pole to pole, is a steady 462°C as a result of the atmosphere’s greenhouse qualities.

In the hottest parts of the hottest deserts here on Earth, the temperatures never top 50°C. The sweltering surface of the planet is covered by volcanoes, impact craters, and old lava flows. There are 1,000 volcanic craters and crater remnants that are over 20 km in diameter. The surface has never been impacted by a small meteorite. The atmosphere burns up anything small. It could take an asteroid in excess of 50 km to survive long enough to hit the surface and then it would have lost close to half of its mass in the atmosphere. The entire planet was resurfaced by volcanic flows between 300 and 500 million years ago. Several spacecraft have entered the atmosphere of Venus. The first successful probe was the Soviet mission Venera 3, but its instruments failed before entry. Venera 4successfully deployed a number of science experiments. They showed a surface temperature of almost 500°C and an atmosphere that was 90 to 95% carbon dioxide. The atmosphere was much denser than anticipated and its slower descent caused its batteries to run down before reaching the surface.

NASA’s Mariner 5 reached the planet one day after Venera 4, but only flew by. The data from the two missions were compared and were an early step in international space exploration cooperation. The Soviet space agency also launched Venera’s 5 and 6. They returned a great deal of information, but were crushed by atmospheric pressure within 20 km of the surface. There have been over a dozen missions to study the surface and atmosphere of Venus since the Venera program. Currently, the Venus Express mission is in orbit. Bepicolumbo and the Venus in-Situ Explorer are planned. Each mission returns new data, it is hard to imagine what mysteries may be unraveled with a surface explorer. We have written many articles about Venus for Universe Today. Here are some interesting facts about Venus , and here are some pictures of planet Venus . If you’d like more information on Venus, check out Hubblesite’s News Releases about Venus , and here’s a link to NASA’s Solar System Exploration Guide on Venus . We’ve also recorded an entire episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus .

References: Planetary Atmospheres Lecture 29 -- Planetary Atmospheres The are some striking differences between the planetary atmospheres present in the solar system. The earth has an atmosphere that is about 3/4 nitrogen, 1/4 oxygen, and trace amounts of carbon dioxide and other gases. In contrast, our next-door neighbor, Venus, has a thick carbon-dioxide atmosphere with no oxygen, but with sulfuric acid clouds. Mars also has an atmosphere that is primarily carbon-dioxide, but on Mars, the atmosphere is extremely thin, and prone to high winds. The giant outer planets Jupiter, Saturn, Uranus, and Neptune have extremely thick atmospheres of hydrogen, helium, methane, and ammonia. Titan, the moon of Saturn also has a methane-ammonia atmosphere. For reference, Titan has roughly the mass as the planet Mercury (which doesn't have an atmosphere at all). We know that it is possible for planets to change their atmosphere over their existence: photographs of the Moon and Mars show dried river beds. Liquid water cannot exist on either of these bodies today, but at some point in the past, the conditions for liquid water must have existed. Why all this diversity?

The presence of hydrogen and hydrogen-rich molecules in the atmospheres of the outer planets is easy to rationalize. Most of the universe is hydrogen; the only reason hydrogen (and helium) are rare in the inner solar system is that these light gases were blown away in the early stages of the solar system via radiation pressure and the solar wind. Planets like Jupiter kept their hydrogen, and, since the hydrogen atom likes to combine with other atoms, you get things like methane (carbon with 4 hydrogens), and ammmonia (nitrogen with 3 hydrogens). Note that Jupiter is very much like the Sun in many respects. Of course, the Sun is more massive, so it has been able to fuse hydrogen in its core. Consequently, its surface is hotter, and most of its molecules have been torn apart by the heat. One interesting feature of the atmosphere of Jupiter is its turbulence. Because Jupiter is rotating so rapidly, the gas around the material moves at extremely high speed.

This causes wind patterns that are somewhat similar to the trade winds on earth, only much stronger; there is a band of rapidly moving air near the equator, followed by a calmer region, followed by another region of rapid atmospheric circulation, followed by another calm region near the planet's poles. In addition to these overall patterns, there are great storm systems on Jupiter (such as the Red Spot) that exist of for hundreds of years. (Galileo saw the Red Spot back in the 1600's.) Although the precise nature (and reason) for the longevity of the storms is not well known, at some level, such features are easy to understand. The cloud-tops of Jupiter are rather cold, since the planet is far from the Sun. On the other hand, the mass of Jupiter is large, so the inside of the planet is under great pressure. High pressure means high temperature, so down deep the in atmosphere, Jupiter is very hot. Just like on earth, when warm air hits cold air, you get storms. Now let's contrast Titan with the planet Mercury. Both are roughly the same mass, but while Titan has an atmosphere, Mercury doesn't. Why? The answer lies in the planet's temperature.

Mercury is close to the Sun, and is extremely hot. This high temperature means that the molecules and atoms in the planet's atmosphere are moving extremely rapidly. Occasionally, one of these atoms will reach a velocity such that Mercury's limited gravity will not be able to hold it. The atom will then be lost into space. The temperature on Titan, of course, is much lower, so the motions of the molecules are much less. Escape velocity is rarely attained, hence Titan can hold onto its atmosphere. For this same reason, Mars doesn't have much of an atmosphere -- its mass, though larger than Mercury's, is still rather low. This same mechanism explains why there are no light gases (such as hydrogen) in the atmospheres of the Earth, Venus, Mars, and Titan. At a given temperature, heavy molecules move more slowly than light molecules. (Both have the same amount of energy, but just as a slowly-moving bowling ball carries the same amount of energy than a fast moving misquito, slowly moving heavy molecules carry the same energy as fast moving light molecules.) Now recall that the hydrogen molecule is made up of two hydrogen atoms, each of which has one proton.

(Protons and neutrons weigh about 2000 times more than electrons, so we can ignore the electrons.) So the weight of a hydrogen molecule is 2. Helium (which doesn't form any molecules) has two protons and two neutrons, so its weight is 4. Methane is made up of one carbon (6 protons and 6 neutrons) and 4 hydrogens, so its weight is 16. Ammonia is nitrogen (7 protons plus 7 neutrons) with 3 hydrogens, so its weight is 17. Nitrogen gas is made up of two nitrogen atoms bonded together: its weight is 28. Similarly, oxygen gas has two oxygen atoms (each with 8 protons and 8 neutrons), so its weight is 32. Finally, carbon-dioxide is formed from 1 carbon (weight 12) and two oxygens (16 each), so its total weight is 44. Molecules such as hydrogen and helium are extremely light, so they move very rapidly. Consequently, they often reach velocities that are too fast for the weak gravity of the terrestrial planets. These molecules escape into space.

Hydrogenic molecules such as methane and ammonia are heavier, so their velocities are significantly less. These are trapped by the gravitational pull of Titan and the inner planets. That's why Titan has a methane and ammonia atmosphere. Nitrogen and carbon-dioxide atmospheres are common in the inner part of the solar system. The reason for this is that ultraviolet photons from the Sun cause methane and ammonia to dissociate; when this happens, the results are nitrogen gas, carbon-dioxide, and hydrogen. The hydrogen is light, so it escapes into space, but the nitrogen and carbon-dioxide can be retained. Mars has only enough gravity to retain a little bit of carbon dioxide, while Venus can retain virtually all of its CO_2. Note that Mars is cold enough so that carbon-dioxide can freeze into solid form. (This is called ``dry ice.'') The Martian ice-caps are mostly dry ice with some water ice mixed in (most likely trapped below the dry ice). Since Mars' axis is tipped 23 degrees from the ecliptic plane (like the earth), Mars undergoes seasons, and each ``summer'', some of this ice (dry and water) evaporates into the atmosphere.

Thus, parts of the Martian atmosphere are continually freezing out, and boiling in. This is what drives the Martian winds. An intruiging mystery is why the Earth has a nitrogen-oxygen atmosphere, while Venus has a thick carbon-dioxide atmosphere. The answer is related to Venus' surface conditions, which are extremely hot --- Venus is, in fact, the hottest location in the Solar System. This difference is striking, especially when one considers that both planets have roughly the same mass, were formed roughly out of the same material, and are roughly the same distance from the Sun. In fact, the atmospheres of Venus and Earth evolved along very different paths because of a slight difference in the beginning. Venus started out slightly closer to the Sun than the Earth. As a result, Venus' surface temperature started out a bit higher. This made the difference. There are two interesting properties to carbon-dioxide. The first is that carbon-dioxide readily dissolves in liquid water. When you mix CO_2 with water (H_2O), the initial result is carbonic acid, (H_2CO_3). Carbonic acid then reacts with minerals to form rocks.

For instance, when carbonic acid encounters calcium, it eats through it to form caclium-carbonate, CaCO_4 (otherwise known as limestone); when carbonic acid hits silicon, it forms silicon-dioxide (sand, quartz, etc.) So the net result is that, as long as there is water, carbon-dioxide can be taken out of the air, and deposited into rocks. The second important property of carbon-dioxide is that it is a gas that is transparent to optical light, but opaque (i.e., it absorbs) infrared light. (That's one of the reasons that infrared telescopes are on top of high mountains, in airplanes, or even in space.) Carbon-dioxide acts like the glass sides of a greenhouse -- it allows the energy of optical light in, but none of the heat to get out. Now, consider the early history of Venus. Since Venus was a bit closer to the Sun than the Earth, it started out with a slightly higher temperature; this caused most of the water to be in gaseous form (water vapor) rather than liquid. As the planet evolved, methane and ammonia were outgassed, and then dissociated by the Sun's UV light into carbon-dioxide.

However, unlike the earth, there was little water around to dissolve the CO_2. Thus, the carbon-dioxide remained in the atmosphere to trap the Sun's heat. This, of course, warmed the planet even more, and evaporated whatever liquid water there was lying around. The greenhouse effect was increased, and, as more and more methane and ammonia were outgassed, the effects got even worse. In other words, a runaway greenhouse effect was the result. In contrast, the Earth's liquid water enabled most of carbon-dioxide to condense out of the air and into rocks. With the carbon-dioxide gone, and the hydrogen and helium molecules lost to space, the remaining atmosphere on earth consisted mostly of nitrogen and some methane and ammonia. Eventually, however, photosynthetic plant life developed. These green plants worked to further remove carbon-dioxide from the air, and replace it with oxygen. Over the years, the precentage of oxygen in the air has gradually increased due to plants (it was only 5 to 10% in the days of the dinosaurs), while carbon-dioxide has decreased.

(Without the plants, oxygen would disappear, as it would quickly react with the soil and get tied up in solids.) Along with normal oxygen gas (which consists of 2 atoms of oxygen), some molecules consisting of 3 atoms of oxygen (ozone) are also formed. Ozone is unstable and quickly reacts with other molecules, but, what does exist in the air does a very good job at absorbing ultraviolet photons. Without ozone, this high energy light would penetrate alot further into the earth's atmosphere and destroy animal (and plant) cells. ch24 - Geology 1000 with Duvall at Brevard Community College - StudyBlue Which one of the following statements is believed to be true of comets? A)They may gain slightly in mass with each orbit around the Sun. B)Their bright, glowing appearance in the night sky is due to frictional heating in Earth's atmosphere. C)They are composed mostly of dense rock particles. D)They have highly elliptical orbits around the Sun. D)They have highly elliptical orbits around the Sun A (an) ________ probably caused the Tunguska event of 1908.

A)supersonic impact of an asteroidal-size body in central Siberia, Russia B)explosive disintegration of a meteorite or small comet high in the atmosphere over central Siberia, Russia C)impact of a small, low-velocity, iron meteorite in northern Arizona D)explosive disintegration of a meteorite or small comet in the atmosphere over northern Arizona B)explosive disintegration of a meteorite or small comet high in the atmosphere over central Siberia, Russia Which one of the following is most likely to be associated both with impact craters and with volcanic calderas that originate by collapse? A)outwardly radiating, fragmental, bright, surface streaks B)sharp rims that rise well above the surrounding area far away from the edge of the depression C)landslides and other mass wasting movements along the walls of the depression D)sharp, central peaks that rise above the floor of the depression C)landslides and other mass wasting movements along the walls of the depression ________ was struck by a string of small asteroids and cometary fragments in the highly publicized Shoemaker-Levy event in 1994.

A)Saturn D)Saturn and Uranus B)Jupiter and Mars Rayed craters on the Moon, such as Copernicus, formedduring an intense, early period of bombardment prior to the formation of thelunar maria. false The lunar maria are thought to be huge, dust-filled, impactcraters. false Jupiter is the largest and most massive planet in the solarsystem. true The Cassini gap refers to the planetless belt of asteroidsbetween Mars and Earth. false Of the terrestrial planets, Mercury exhibits the greatestlateral variations in surface temperatures. true Rays in the rings of Saturn are bright, radial streaks thatconverge inward toward the planet's surface. true Lunar regolith breccia contains crystalline rock fragments and glassy fragments. true Meteorites disintegrate and burn up as shooting stars; meteoroids survive an impact event and a trip through Earth's atmosphere. false Comets are thought to be composed of dust and tiny, icy particles encased in small, solidified, metallic fragments. false An extreme example of the greenhouse effect is thought to be associated with an abundance of carbon dioxide in the Martian atmosphere. false The Martian polar caps are thought to be mainly frozenmethane and ammonia.

false Large impact craters and large, domal features (probablyrepresenting viscous lava flows) have been imaged on the surface of Venus. true Under similar conditions of temperature and pressure, solidcarbon dioxide (dry ice) evaporates at a much faster rate than water ice. true Eruptive plumes and lava flows rich in sulfur have beenphotographed on Jupiter's moon, Io. true The Jovian planets are more massive but less dense than the terrestrial planets. true Climate Change: Vital Signs of the Planet: Causes Vital Signs of the Planet Questions (FAQ) A blanket around the Earth A layer of greenhouse gases – primarily water vapor, and including much smaller amounts of carbon dioxide, methane and nitrous oxide – acts as a thermal blanket for the Earth, absorbing heat and warming the surface to a life-supporting average of 59 degrees Fahrenheit (15 degrees Celsius). Most climate scientists agree the main cause of the current global warming trend is human expansion of the "greenhouse effect" 1 — warming that results when the atmosphere traps heat radiating from Earth toward space. Certain gases in the atmosphere block heat from escaping.

Long-lived gases that remain semi-permanently in the atmosphere and do not respond physically or chemically to changes in temperature are described as "forcing" climate change. Gases, such as water vapor, which respond physically or chemically to changes in temperature are seen as "feedbacks." Gases that contribute to the greenhouse effect include: Water vapor. The most abundant greenhouse gas, but importantly, it acts as a feedback to the climate. Water vapor increases as the Earth's atmosphere warms, but so does the possibility of clouds and precipitation, making these some of the most important feedback mechanisms to the greenhouse effect. Carbon dioxide (CO2). A minor but very important component of the atmosphere, carbon dioxide is released through natural processes such as respiration and volcano eruptions and through human activities such as deforestation, land use changes, and burning fossil fuels. Humans have increased atmospheric CO2 concentration by more than a third since the Industrial Revolution began. This is the most important long-lived "forcing" of climate change. Methane. A hydrocarbon gas produced both through natural sources and human activities, including the decomposition of wastes in landfills, agriculture, and especially rice cultivation, as well as ruminant digestion and manure management associated with domestic livestock.

On a molecule-for-molecule basis, methane is a far more active greenhouse gas than carbon dioxide, but also one which is much less abundant in the atmosphere. Nitrous oxide. A powerful greenhouse gas produced by soil cultivation practices, especially the use of commercial and organic fertilizers, fossil fuel combustion, nitric acid production, and biomass burning. Chlorofluorocarbons (CFCs). Synthetic compounds entirely of industrial origin used in a number of applications, but now largely regulated in production and release to the atmosphere by international agreement for their ability to contribute to destruction of the ozone layer. They are also greenhouse gases. Not enough greenhouse effect: The planet Mars has a very thin atmosphere, nearly all carbon dioxide. Because of the low atmospheric pressure, and with little to no methane or water vapor to reinforce the weak greenhouse effect, Mars has a largely frozen surface that shows no evidence of life. Too much greenhouse effect: The atmosphere of Venus, like Mars, is nearly all carbon dioxide. But Venus has about 300 times as much carbon dioxide in its atmosphere as Earth and Mars do, producing a runaway greenhouse effect and a surface temperature hot enough to melt lead. On Earth, human activities are changing the natural greenhouse.

Over the last century the burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO2). This happens because the coal or oil burning process combines carbon with oxygen in the air to make CO2. To a lesser extent, the clearing of land for agriculture, industry, and other human activities has increased concentrations of greenhouse gases. The consequences of changing the natural atmospheric greenhouse are difficult to predict, but certain effects seem likely: On average, Earth will become warmer. Some regions may welcome warmer temperatures, but others may not. Warmer conditions will probably lead to more evaporation and precipitation overall, but individual regions will vary, some becoming wetter and others dryer. A stronger greenhouse effect will warm the oceans and partially melt glaciers and other ice, increasing sea level. Ocean water also will expand if it warms, contributing further to sea level rise. Meanwhile, some crops and other plants may respond favorably to increased atmospheric CO2, growing more vigorously and using water more efficiently. At the same time, higher temperatures and shifting climate patterns may change the areas where crops grow best and affect the makeup of natural plant communities.

The role of human activity In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change, a group of 1,300 independent scientific experts from countries all over the world under the auspices of the United Nations, concluded there's a more than 95 percent probability that human activities over the past 50 years have warmed our planet. The industrial activities that our modern civilization depends upon have raised atmospheric carbon dioxide levels from 280 parts per million to 400 parts per million in the last 150 years. The panel also concluded there's a better than 95 percent probability that human-produced greenhouse gases such as carbon dioxide, methane and nitrous oxide have caused much of the observed increase in Earth's temperatures over the past 50 years. The panel's full Summary for Policymakers report is online at . Solar irradiance It's reasonable to assume that changes in the sun's energy output would cause the climate to change, since the sun is the fundamental source of energy that drives our climate system. Indeed, studies show that solar variability has played a role in past climate changes.

For example, a decrease in solar activity is thought to have triggered the Little Ice Age between approximately 1650 and 1850, when Greenland was largely cut off by ice from 1410 to the 1720s and glaciers advanced in the Alps. But several lines of evidence show that current global warming cannot be explained by changes in energy from the sun: Since 1750, the average amount of energy coming from the sun either remained constant or increased slightly. If the warming were caused by a more active sun, then scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere, and a warming at the surface and in the lower parts of the atmosphere. That's because greenhouse gases are trapping heat in the lower atmosphere. Climate models that include solar irradiance changes can’t reproduce the observed temperature trend over the past century or more without including a rise in greenhouse gases. References

Question: Which gas is the principal gas in the atmosphere of Venus?
Answer: Carbon dioxide