******************** Mercury *******************
Mercury and Earth to scale
Mercury from Mariner flyby spacecraft Mercury, the closest planet to the Sun, has no atmosphere. It is close to the Sun and hence very hot, so that any atoms and molecules that were near the planet would move very fast. This, coupled with the relativly low gravity, meant that any atmosphere that had been around Mercury would have simply flown off into space long ago. The surface of Mercury is covered with ancient impact craters, created long ago when there was lots more debris flying around the solar system. Mercury, like our Moon, appears to be geologically dead, so these ancient craters has not been changed much at all over billions of years.
Mercury has been the least studied inner planet so far, with only a few brief flybys by spacecraft way back in the 1960s. However, a new spacecraft, called Messenger, is just now starting to orbit Mercury. New pictures and data will be forthcoming in the next few months.
********************** Venus surface *********************
Venus and Earth (middle 2 planets) are very similar in size
(1) Russian lander on Venus (2) Venera color The Russian Venera landers are the only spacecraft ever to send back data from the surface of Venus. They landed on a flat plain of volcanic rock. The spacecraft reported back that the temperature on the surface was a toasty 900 Farenheit! (Thats not a typo- 900= nine hundred)
Venus has a very thick atmosphere of CO2 (carbon dioxide). I will talk about the atmospheres of Venus, Earth and Mars later.
(1) Magellan leaving shuttle near Earth (2) Magellan orbiting Venus (artist conception) In the 1990s, the US spacecraft Magellan spent several years orbiting Venus. This spacecraft used radar to penetrate the clouds that hide the surface of Venus. The Mgellan radar created a planet-wide map of the geological features of the planet.
(1) Impact craters on Venus (2) Volcanoes on Venus These show just a tiny sample of the surface of Venus, as revealed by the Magellan spacecraft. The geology of Venus is dominated by volcanic structures. However, there is no evidence of plate tectonics as we find on Earth. From the number and distribution of impact craters, we think that Venus underwent a complete resurfacing about 0.5 billion years ago, with the crust completely renewed by liquid rock from Venus's mantle in a planet wide volcanic spasm. Why this might have happened is not known for certain. It may have something to do with how the internal heat from the planet escapes from Venus.
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 (30 mile) in diameter. All geological features on Venus are named for women. Danilova was a Russian-American 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.
********************** Moon *************************
(1) Gibbous Moon (2) Full Moon Some pictures taken with a telescope of our Moon. Much of the Moon is carpeted with craters, but large darker areas, called "maria" have fewer craters. Features such as craters and mountains show up best near the day-night line, as the shadows help highlight the features. Thats why the full moon, with no day-night line visible, looks "flat"- there are no shadows to highlight features.
The maria are thought to be "super craters". These were impacts of bodies maybe a 100 miles or more in size. The impacts were so powerful that they broke through the lunar crust, flooding the large impact craters with lava which hardened into a relatively smooth surface.
The Moon has no essentially atmosphere, because its low gravity can't hold on to one- like Mercury, any atmosphere that might have been around the Moon was stripped away by the Sun's heat.
Typical moderate sized lunar crater The larger crater imaged here would easily hold the OKC metro area, from Norman to Edmond. There are many thousands of craters greater than a mile across on the moon, with too-many-to count smaller ones as well.
Sputnik The story of how (and more importantly *why*) we went to the Moon is a long and fascinating story of Cold War geopolitics. But much of it started with the Russians launching the first artificial Earth-orbiting satellite,called Sputnik, in 1957, commemorated in this Russian postage stamp. The announcement of this event, deeply shrouded in secrecy until it was successful, as was the way Russians did things, deeply shocked the US to its very core. How, Americans asked, could the "backward" Russians have beaten us into space? One real fear was that the Russians apparently now had a means to quickly rain down atomic bombs on the US and we had no way to stop them or to retaliate with our own satellite weapons. The Russians went on to a series of other space "firsts", most importantly sending the first human to orbit the Earth. On 12 April 1961, Yuri Gagarin became the first human to orbit the Earth.
In the early 1960s, president Kennedy decided we would go to the moon. Certainly much of the motivation was to show the world that the US had superior technical capability than the Russians. A discussion of all the reasons for the moon trip would be far beyond what we have time for here. (Going to do science was pretty low on the list! Kennedy said something like "Just get the damn guys to the Moon- I don't give a damn what they do when they get there")
The Apollo Program, like the Manhattan Project which had developed the A-bomb in WWII, was a spectacular engineering tour de force, done in what seems like an impossibly short period of time. These projects show that amazing things are possible, given determined leadership and sufficient resources.
(1) Apollo lunar exploration (2) Genesis Rock There were 6 Apollo landings on the Moon between 1969 and 1972. A total of 12 men walked on the Moon. Further missions had been planned, but Congress cut funding after the novelty wore off.
One of the real treasure troves from Apollo were hundreds of rock samples gathered by the astronauts. These samples were analyzed to find their detailed chemistry and ages from radioactive dating techniques. All Moon rocks are far older than most any rock on the Earth. This particular rock, dubbed the Genesis Rock, was the oldest, at 4.5 billion years old. This rock is thought to be part of the original crust of the Moon- the first rocks that solidified after the Moon formed. (On the Earth, rocks have been repeatedly melted and re-solidified because of volcanic action. The "age"of the rock found by radioactive dating is the time since the rock last solidified.)
Large Impact Theory of Moon formation. The best current theory on the formation of the Moon is that it formed as the result of a collision between the forming Earth and a Mars sized body in the early solar system. This is often called the "Large Impact Theory" (or the "Giant Impact Theory"). 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.
The Big Splat, or How Our Moon Came to Be This is the cover of a popular book about the large-impact theory of Moon formation. Its one of my favorite book titles! (Subtitle= Scariest Day in the Earth's History!) The image is, of course, from an artist's imagination. Parts of it aren't perhaps too accurate- for one thing, this event probably happened before Earth had its nice blue oceans. But the image displays the idea- SPLAT!
The idea that the Moon formed in this way was one of the results of the detailed study of the chemistry of the Apollo moon rock samples. For hunderds of years, scientists had debated how the moon formed- did it form someplace else and then get captured by Earth? did it form alongside the Earth? was it once part of the Earth that was "spun off" by the early, rapidly rotating Earth?? All these theories were found to have near-fatal flaws that rendered them almost impossible to accept as reality. The Large Impact theory at first sounds VERY unlikely- how likely was it that there had been a Mars-sized body that hit the Earth? But the early solar system was a MUCH more crowded place than the current solar system. Many bodies show evidence for very large impacts. At present, the Large Impact theory is our best idea on how the Moon came to be.