More cool images from outer space

I recently wrote about how 2 of Saturn’s lesser-known moons were recently shown to look astonishingly like flying saucers. (“Saturn’s flying saucers” from January 11, 2008.) Well, it looks like NASA is at it again. They’ve just released pictures of the planet Mercury, showing scientists a side of the planet never before seen by human eye.

Mercury is the smallest planet in our solar system (now that Pluto is no longer considered a planet, of course). It’s about one-third the size of Earth, with a diameter of around 3000 miles. (That’s roughly the distance from Los Angeles to Maine.) Being closest to the sun, Mercury has a very rapid orbit – it circles the sun once every 88 earth days. It doesn’t have a normal orbit, though, it has an elliptical one. The distance from the sun to the surface of Mercury varies from 28 million to 43 million miles. While there was much speculation some years ago as to why Mercury has such an eccentric orbit (including the theory that there was another planet even closer to the sun that was tugging on it), it now appears that this elliptical orbit is explained in Einstein’s General Theory of Relativity. Despite the fact that it orbits the sun very rapidly, it rotates on its axis relatively slowly. Mercury only rotates 3 times for every 2 orbits that it makes. (This would make for some very odd sunrises and sunsets on the surface!)

A lot of interesting comparisons can be made between Mercury and Earth. Both planets have similar densities. Also, both planets have similar cores made of iron. However, Mercury’s iron core is much larger than ours - it seems to make up around 42% of the volume of the planet, and is at least partly liquid. Like Earth, the core of Mercury is surrounded by a rocky mantle and crust; however, Mercury’s mantle and crust is much smaller than our own, coming in at approximately 370 miles thick. This crust is highly cratered, and actually looks a lot like the surface of our moon. Both Mercury and Earth also have magnetic fields (of the 4 rocky planets, they are the only 2 to have one.) Mercury might have even had an atmosphere, except for the fact that it is so close to the sun - its atmosphere is constantly getting blasted away by solar wind. Being so close to the sun has other consequences, too. Temperatures at the equator have an 1100 degree Fahrenheit difference between day and night – and despite the possibility of extreme heat, there is evidence of the existence of ice at the poles.

Mercury is visible from the surface of Earth, but just barely. It’s hard to see because it stays so close to the sun; this makes it hard to see except at twilight and dawn. In fact, about the only information we had until this year about the surface of Mercury came from NASA’s Mariner 10 spacecraft, which mapped about 40% of the planet’s surface in 3 fly-bys in the 1970s. But here’s the odd thing about the Mariner 10 mission. The combination of Mercury’s elliptical orbit and its slow rotation means that the same side of Mercury is always illuminated by sunlight when it is closest to the sun. So that’s the only side that the Mariner 10 was able to see in its pictures! That means that there was an entire half of the planet we’d never seen.

So, on August 3, 2004, NASA launched the MESSENGER spacecraft to do a detailed mapping of Mercury’s surface. (MESSENGER stands for Mercury Surface, Space Environment, Geochemistry and Ranging.) MESSENGER will make 3 flybys of the planet over the next 3 years before it settles in to a stable orbit around it. During those passes, the spacecraft will pass within 124 miles of the surface, allowing for some highly detailed images. And the first set of pictures, released in January, did not disappoint.

I think my favorite image is the giant spider crater. It’s a massive crater with faint lines radiating out from the center of it, kind of like a squished spider. Scientists think that this may be the remnants of a volcano. There are also other neat images, though. For example, there is one taken near Mercury’s terminator – the line between the side of the planet lit by sunlight and the side in darkness. This image shows a vast plain of craters, which are enhanced because of the long shadows that exist at the terminator. And another shows a double ringed crater – it actually looks kind of like a doughnut on the surface. Scientists hope that these images will allow them to better understand the physical properties of Mercury, as well as its geological history. (For example, whether there has been volcanic activity on the planet.)

I highly encourage everyone to check out the website of the MESSENGER mission. You can see all of the images I’ve described (this is where I got the spider crater picture), as well as others that will be released as the spacecraft does more flybys. (The next one is scheduled for October of this year.)
http://messenger.jhuapl.edu/index.php