5 October 2011

****************** Venus **********************

Venera color Most of what we know about atmosphere and details of surface of Venus comes from Russian spacecraft. The Russians landed a number of spacecraft on Venus, and also floated instrumented balloons in the atmosphere.

The two key scientific discoveries from the surface landings are: (1) surface temperature of Venus is far hotter than was expected (2) the atmosphere is almost pure CO2 and is much denser than the atmosphere of the Earth.

These two discoveries are related, of course. Venus is so hot because CO2 is a good greenhouse gas and this has caused the high surface temperature.

Venus global topography This shows "elevation" of Venus on a global scale. Note the two large "continents"- Aphrodite Terra (roughly size of Africa) and Ishtar Terra (roughly size of Australia). About 80% of surface of Venus is within a kilometer of the average altitude.

Magellan in orbit around Venus Almost everything we know about the large-scale surface features on Venus comes from the US Magellan mission. This spacecraft orbited Venus from about 1990 to 1994, and used synthetic aperture radar to make a map of almost the entire surface of Venus at about 60 meter resolution.

*********** The next half-dozen images are all from Magellan and illustrate some of the geological features on Venus. Now, the surface area of Venus is almost *3 times* the land area of the Earth, so this is only a tiny, tiny slice of the vast Magellan dataset. The official Magellan web site is at www2.jpl.nasa.gov/magellan , where you can find links to vast quantities of data from Magellan.***********

Three impact craters on Venus Magellan "image" of 3 impact craters. For scale, the rim of the one in the upper left (Danilova) is about 50 km in diameter. All geological features on Venus are named after (Earth) women. (Wonder what the Venusian women think of that?!) Danilova was a Russian born 20th century prima ballerina.

Large shield volcanos on Venus and Mars These large shield volcanoes are roughly analogous to the Big Island of Hawaii, but on a larger scale.

Arachnoid An arachnoid is a large structure of unknown origin, found only on the surface of the planet Venus. Arachnoids get their name from their resemblance to spider webs. They appear as concentric ovals surrounded by a complex network of fractures, and can span 200 kilometers. Over thirty arachnoids have been identified on Venus, so far. The arachnoid might be a strange relative to the volcano, but possibly different arachnoids are formed by different processes.

Pancake dome volcano A pancake dome is an unusual type of volcano found on the planet Venus. Pancake domes have a broad, flat profile similar to shield volcanos and are thought to form from one large, slow eruption of viscous silica-rich lava.

Fotla Corona On Venus, coronae are large (typically several hundred kilometres across), crown-like, volcanic features. It is believed that coronae are formed when plumes of rising hot material in the mantle push the crust upwards into a dome shape, which then collapses in the centre as the molten magma cools and leaks out at the sides, leaving a crown-like structure: the corona.

Lava carved channels? Hope the Venusians had titanium inner tubes to go tubing when these puppies were flowing!

Impact craters on Venus- global view Experts have identified over 900 impact craters on Venus. There are very few "small" craters, say as big as the Arizona Meteor Crater. This is understandable, as the very dense atmosphere of Venus breaks up much bigger rocks than does Earth's atmosphere. However, rocks that make bigger craters (say 1 km rocks that make 10 km craters) are NOT significantly affected even by the thick atmosphere of Venus. There are several important properties of the Venusian impact craters: (1) there are only ~900 or so of them (2) they are randomly distributed over the surface and (3) they look surprisingly "fresh"- few have, for example, lava flows in them. 900 may sound like a lot of craters, but if Venus had a "dead" surface, like Moon or Mecury, we would expect many times as many impact craters to be visible. These facts have lead to a hypothesis called the "global resurfacing of Venus". The idea is that about 0.5 to 1 billion years ago, pretty much the entire surface of Venus melted (wiping out any older craters), then rather quickly cooled and solidified (so that new craters were not filled in with lava). Venus does have numerous old volcanoes, but does not appear to have plate tectonics, like the Earth. Perhaps the "resurfacing event" was Venus's way of getting rid of excess internal heat??? (Planets the size of Venus and Earth generate internal heat from decay of radioactive elements. The Earth gets rid of some internal heat by constantly bringing hot lava to the surface in divergent plate boundaries- see discussion below). We really need seismometers on Venus to start to map out its internal structure, but such devices would have to operate in the extreme heat on the surface.

****************** Earth and Moon ************************

(1) Activity (earthquakes, volcanoes) on Earth (2) Tectonic plates The Earths outer shell is divided into a dozen or so "plates" that move around. The plate boundaries are outlined by earthquakes and volcanoes.

Earthquakes on Earth Same idea as previous, but with lots more real data! This shows position of over *1/3 of a Million different earthquakes** over 35 years.

(1) Plate boundaries (2) Plate boundaries - spreading ocean floor to subduction zone (3) Subduction zone, ocean trench and orogeny - west coast of South America (4) San Andreas transform (slip fault) boundary There are 3 kinds of plate boundaries- converging, diverging and transform. At diverging boundaries (in oceans) "new" sea floor is made, which is then "lost" at converging boundary (subduction zone). By the way, "orogeny" has nothing to do with erogenous or orgy- orogeny means the process of mountain formation.

Age of sea floor This color- coded map of the ages of sea floor rock shows the spectacular success of the idea of sea floor spreading- you can clearly see the long linear "sources" of the sea floor rocks as the youngest rock (coded as red- make sure you look at the color-age bar in lower left).

(1) Continental drift (2) Pangea breakup animation The continents are lighter rock (mostly granite- about 2700 kg/m**3) that "float" on the denser sea floor (basaltic rock- about 3300 kg/m**3). The continents move about slowly.

Fossil map evidence of continental breakup Maps of fossils of ancient creatures show that the ranges of some creatures, presumably once contiguous land, are now split onto two continents- just what you would expect from continental breakup and drift.

Earthquakes in Fiji Sea and plate subduction By plotting position and depth of earthquakes in the Fiji Sea, we can "see" a subducting plate. This drawing shows the relationship between ocean trench, volcano and subducting plate.

Earth impact sites As we discussed earlier, we have identified fewer than 200 impact structures on the Earth- far fewer than the other inner planets. The primary reason for this is, of course, that the Earth is much more geologically active than the other planets. Many Terran craters presumably were formed in the oceans, and have been destroyed by seafloor spreading. On land, small to medium sized craters are quickly erased by geological processes- remember the image of the Roter Kamm crater in Namibia being filled with sand. Most very large, old craters have been filled in, but can sometimes be found by studying buried rocks. Certainly more impact craters remain to be found on Terra. It seems ironic that it is much easier to see the impact craters on the Moon or Mercury than the ones on our own planet!

Near side of Moon Our old friend (actually, the astronomers enemy, due to it lighting up the sky!) the Moon. The surface is covered by some 30,000 impact craters larger than 1 km in diameter, as well as "billions and billions" of smaller ones (well, no one has counted them all!). As Shoemaker said in the DVD "The Moon is a slate that no one has been erasing". The large dark areas are called maria. These are really "super craters". These very large impacts broke through the lunar crust and the craters were flooded by lava. As discussed earlier, these maria may have been created in the "Late Heavy Bombardment" period about 3.9 Ga ago.

Far side of Moon The far side of the Moon has lots of impact craters, but only a few small maria. The reason is shown in the following slide.

Schematic Moon structure The Moon has a much thicker crust on the far side than on the near side. (We know something about the Moon's interior because of seismometers left on the Moon by Apollo astronuts.) The large impacts that punched through the crust to create the maria on the near side seem not to have been able to penetrate the thicker crust on the far side.

Giant Impact Theory of Moon formation. How did our Moon form? Over the past century or so, there have been several theories: (1) formation somewhere else and then capture by Earth, (2) fission from the forming Earth and (3) separate formation alongside the Earth. In the 1970s, using chemical and isotopic data from Apollo moon rocks, a new theory was proposed: the Giant Impact theory. In the GIT, the Moon formed as the result of a collision between the forming Earth and a Mars sized body (often called Theia) in the early solar system. This theory comes closest to explaining what we know of the Earth-Moon system: angular momentum, orbit of Moon, differences in chemical and isotopic compostion between the Earth and Moon.

Several groups have done detailed computer simulations of the GI. One short "movie" derived from one such simulation can be found here .

Tidal bulge. The tidal bulge is dragged around by rotation of Earth, so that it "leads" the Moon slightly. This has all sorts of interesting results, including lengthening of the Earths day and lengthening of the orbital period of the Moon around the Earth.

This happens because of conservation of angular momentum (AM). The Earth-Moon system AM consists of the rotational (spin) AM of Earth and Moon and the orbital AM of Earth and Moon around their center of mass. For purposes of AM, we can consider the Earth-Moon system an isolated system (no external torques)- thus the total AM of the Earth-Moon system is conserved. The near tidal bulge pulls the Moon just a little bit along its direction of motion (see right hand part of figure). The far bulge tends to cancel this, but it can't quite do so because it is farther from the Moon than the near bulge. This results in the Earth produces a torque on the Moon- an off-center force. Now, from physics you should know that a torque on a body changes the bodies AM. In the Earth-Moon system, tidal friction (of tides with coastline) cause the Earth to spin more slowly as time goes on, and thus to lose spin AM. The AM the Earth loses goes to increase the orbital AM of the Moon (through the physical mechanism of the not-quite balanced tidal bulges pulling on Moon!). As the Moon gains orbital AM, it slowly spirals away from Earth and the length of the "month" increases. The change in Earth-Moon distance is about 4 centimeters/year. We can actually measure the increase in Earth-Moon distance by timing laser pulses sent from the Earth that are reflected from corner reflectors the Apollo astronauts left on the Moon. Mind-boggling!!!

Corner reflector A corner reflector (here in 2dim- real ones are in 3dim of course) returns light rays in a direction *parallel* to the incident direction, unlike a flat mirror, where the angle of incidence equals the angle of reflection.

Lunar corner reflector array Apollo astronauts and Russian robotic lunar landers have left several corner refectors on the Lunar surface. By shooting laser beams at these, and measuring the time to get an "echo" precise measurements of the Earth- Moon distance have been made for the past 40 years.

Detail of lunar corner reflector Looking like a thing for the astronauts to store their empty soda bottles in, each "hole" is actually a glass corner reflector like the one I passed around in class. These reflect laser beams sent through a telescope from Earth directly back to that telescope.

***************** Atmospheres of Inner Planets *************

Venus ,Earth, Mars atmosphere Basic numbers about atmospheres. Other very useful numbers: mass of Earths atmosphere= 5.2E18 kg; mass of Earths oceans: 1.4E21 kg. (Mass of oceans about 270 times mass of atmosphere)

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 atmosphere, almost 100 times pressure of air in this room, while Mars has a very low density 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.

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.

CO2 content of atmosphere By burning fossil fuels, humans have increased the amount of CO2 in the atmopshere. 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 atmsophere has increased by about 1/3 over just the last half century.

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!

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.

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.

Faint Sun "Paradox" Computer models of the Suns evolution predict that the Sun is slowly becoming more powerful. This means that the Sun was fainter in the distant past, and would not have kept the Earth warm enough for liquid water (all other things being equal). As we have fossil and mineralogical evidence for liquid water for most of past 3.5 Gyr, this realization was at first called the "faint sun paradox". But the obvious way out is that the Earths atmosphere was far different in the past. It is probable that the early atmosphere had MUCH more CO2 than todays atmosphere.

Possible CO2 vs time This diagram shows much more CO2 in the past than now. (How close to reality this is is not clear) This is one obvious way out of the "faint sun paradox"- the Earth had a much greater greenhouse effect in the past, and that allowed the less powerful Sun to produce a surface temperature that allowed liquid water over a large fraction of the Earths history.

The "Deep glaciations" mark times of possible "Snowball Earth Events" (you gotta have a catchy phrase for your discovery or hypothesis!). In an SEE, most of Earth (including oceans- at least surfaces) were presumably frozen. SEEs are hard to "recover" from, as the frozen Earth has a high albedo, causing even lower temperatures- also, the cold air can't hold much water vapor which acts like a greenhouse gas. SO, it sounds like a SEE is a one-way ticket to a frozen hell. Perhaps the humble CO2 (and CH4 = methane) molecules- spewed from volcanoes- could help break the SEE.

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).

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

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

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 (see slide on CO2 vs time). 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.

As we search the Milky Way for planets around other stars (exoplanets) you should always keep in mind the amazing story that is told by the comparison of Venus and the Earth. In mass, size and surface gravity, Venus is VERY "Earth like". However, the surface conditions on Venus are very UN-"Earth-like". ALWAYS remember this as you read (and I am sure you will) reports of astronomers finding "Earth-like" planets.