Friday 8 April 2011

************ Old exam on Solar System **************

Look at to prepare for exam 3 on 22 April 2011. NOTE THAT THE ANSWER FOR Question # 32 IS NOT CORRECT The correct answer is:

8 o'clock IN THE MORNING WEDNESDAY 11 MAY 2011

old solar system exam

***********Results from exam 2 (stars) *******************

Exam 2 key

Exam 2 histogram of scores

Exam 2 scores vs exam 1 scores

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********** A Tale of Two Planets *************

It was the best of planets, it was the worst of planets... (With apologies to Mr. Dickens).

Venus ,Earth, Mars atmosphere Basic numbers about atmospheres. The most important are the numbers showing the composition "principal gases" and surface density. Basically these numbers can be summarized as follow: Venus and Mars both have atmospheres of CO2. Venus has a very thick dense atmospher, almost 100 times density of air in this room, while Mars has a very low denisty atmosphere, about 200 times less dense than the air in this room. Earth has an atmosphere that is primarily nitrogen and oxygen, with some CO2 and H2O.

The greenhouse effect A greenhouse (upper picture) can keep its interior much warmer than its surroundings (if its sunny!). Visible wavelengths of sunlight carry energy through the glass (the glass is transparent to visible light) some of which is absorbed by the interior, which heats up and radiates infrared radiation. However, the glass is far less transparent to infrared radiation, so the heat it trapped, warming the interior.

A planets atmosphere can act as a greenhouse, if it contains molecules that are relatively transparent to visible light and relatively opaque to infrared. Many molecules with 3 or more atoms are greenhouse gases. In particular, carbon dioxide (CO2) is a greenhouse gas.

The Earth is warmed by the greenhouse effect due to CO2 and H2O in the atmopshere. (The oxygen and nitrogen in the atmosphere are diatomic - two atom- molecules- and are NOT greenhouse gases.) If there were no greenhouse effect, the Earth would be 30 or so degrees colder than present, so the Earth would be a very different place. So the greenhouse effect at present is probably a good thing as it keeps the Earth at the temperature at which life flourishes.

Average temperature of Earth over last century The average temperature over the Earth has been rising. Most atmospheric scientists attribute this warming to CO2 released by humans burning fossil fuels- global warming.

Thus, there may be too much greenhouse warming, caused by human activity, than is good for life on Earth. If the temperature continues to rise for a century or two, the Earth will be a very different place than it is now.

Global Carbon Emission This shows the amount of carbon dumped into Earths atmosphere by burning fossil fuels (and making cement). Over the past 100 years or so, humans have dumped about 200 to 250 gigatons of carbon into the atmosphere.

The coal-powered I-phone Electricity seems like such a clean power source. Ain't no smoke coming out of the power outlet in my house! (well, as long as you don't cross the wires!). But, at least in the US, almost 70% of the generation of electricity is powered by burning fossil fuels- coal, natural gas and (just a little) petroleum. Coal is by far the worst fossil fuel from an environmental viewpoint. It is mostly carbon, and so burning coal leads to the most CO2 per unit of heat generated compared to say natural gas. Coal also has nasty impurities- sulfur, mercury, even some thorium and uranium (radioactive elements). When coal burns, some of these impurities go into- you guessed it- the air that we all breath.

CO2 content of atmosphere- 50 years By burning fossil fuels, humans have increased the amount of CO2 in the atmosphere. Careful observations of CO2 (here plotted as ppmv - parts per million by volume) have been made from Mauna Loa HI (a place not much affected by "local" sources) for almost half a century. The observations show that the CO2 in the atmosphere has increased by about 1/4 over just the last half century.

CO2 content of atmosphere- last 400,000 years We can use airbubbles in Antartic ice to measure the properties of the Earths atmosphere in the past. Previous to the industrial revolution, the CO2 varied from about 200 to 280 ppmv. Over the past century, it has gone to about 380 ppmv, due to burning fossil fuel.

Ocean acidity increasing Some of the increased CO2 we are dumping into the atmosphere finds its way into the ocean, where it makes the ocean water acidic. This will have consequences for sealife. This is the first page of an article on this topic from the April 2011 NATIONAL GEOGRAPHIC magazine. You can read the article here .

Recent Sea Level Rise The sea level has been rising during the last century. Experts agree that some significant portion of this has been due to warming of the ocean. This causes water to expand and hence the level to rise. Melting of ice (on land) also plays some part in the sea level rise.

So, maybe its time to sell your beach house in Florida and remember to WALK UPHILL as the oceans rise!

Earths carbon inventory The amount of carbon in various reservoirs on Earth. The atmospheric carbon is of 2000, and includes a contribution of about 1/3 from recent human activity (about 200-250 GT out of 720 GT). Note that by far the largest carbon reservoir is carbonate rocks, such as limestone (CaCO3). This is found primarly on the ocean floor, and the carbon in these rocks is sometimes recycled into the atmosphere by volcanoes.

CO2- rock cycle Exchange of CO2 between atmosphere and carbonate rock reserviors may help moderate temperature fluctuations. If the CO2 gets "high", the Earth warms, there is more water evaporation and rain, which causes more rock weathering, which puts more CO2 into ocean, which decreases atmospheric CO2, which cools Earth.

Presumably, this general mechanism may help rid the atmosphere of the fossil fuel CO2, but the timescale to do so is FAR beyond human timescales (and the process of absorbing extra CO2 will alter ocean chemistry - ocean acidification- with at present unknown ramifications).

Atmospheres of Venus and Earth So why are the surface conditions of Venus and Earth so utterly different? The planets have very similar sizes and surface gravity. We know little of the history of the atmosphere of Venus, so there is no definitive answer to the question posed. However, the answer is undoubtedly related to carbon, water, and life. Earth may have started out with a thick blanket of CO2 as did Venus. In fact, since the Sun was less luminous in the distant past, the Earth should have been frozen solid billions of years ago, but there is evidence for liquid water over a large fraction of Earth's history. One obvious way out of this "faint Sun paradox" is that the Earth had much more CO2 (and hence a bigger greenhouse effect) in the past. On Earth, liquid water (and eventually life) managed to "scrub" most of the CO2 out of the air, turning into carbonate rocks, kerogens, and fossil fuel (see earlier slide on "Earth carbon inventory"). Perhaps there was never much liquid water on Venus, or if there was there was never life to help scrub the CO2 out of the atmosphere.

The rocks found on Venus (from Russian lander data) are somewhat similar to terrestrial basalts. Basalts are mostly silica (SiO2), with various amounts of things like FeO , Al2O3, CaO amd MgO- but no carbon.

So, perhaps the "simple" answer to the question is that the Earth managed to get rid of most of its original thick blanket of CO2, but Venus has been cursed to keep its thick CO2 cloak.

Chu remarks on climate change The content of the Earth's atmosphere, how and why it is changing, and the effects of such changes has become very political. Some people still deny there is any problem, other say there is a problem but there is nothing we can do, others want to do something to at least try to address the problem. This is from an interview with Steven Chu in Newsweek 11 April 2011. Chu is a Nobel prize winning physicist and current Secretary of Energy in the US.