Earth and Luna
Published on Mar 12, 2008 at 7:02 pm.
2 Comments.
Filed under Earth, moon.
NASA’s Mars Reconnaissance Orbiter (MRO) left Earth August 12, 2005. It arrived at Mars about two years ago, March 10, 2006, joining a flotilla of other spacecraft studying the red planet. Last October, the MRO turned its HiRISE camera towards home, capturing the image above. The HiRISE camera (High Resolution Imaging Science Experiment) is a telescopic camera capable of 1m (or better) resolution on Mars. That sort of resolution is something that until recently was only available in military reconnaissance satellites looking down at Earth. Now, even civilian imaging satellites have that resolution at Earth. HiRISE operates in visible and near infrared light.
The image above shows Earth and Luna, our Moon. It’s a pretty picture. But, more than that, it shows the large size of Earth’s moon compared with Earth. Most planets’ moons are very small compared with the planet. Luna’s diameter is 0.27 that of Earth. Mercury and Venus have no moons. Mars’ largest moon, Phobos, is a paltry 0.0032 of Mars’ diameter! Mars’ two moons, Phobos and Deimos, though are believed to possibly be asteroids that got caught in Mars’ gravity. Jupiter has more respectable sized moons, with its largest, Ganymede, being even larger than the entire planet Mercury. Still, Jupiter is huge. Because of that Ganymede is still only 0.037 of Jupiter’s diameter! Titan, Saturn’s largest moon, is also larger than Mercury, but it manages to be only 0.043 of Saturn’s diameter. Uranus’ largest moon, Titania, is 0.031 of Uranus’ diameter. Triton, Neptune’s largest moon, is 0.055 of Neptune’s diameter.
So, you see that something is amiss with Earth’s moon. With the exception of Mars, who’s moons are not native to that planet, none of the inner, rocky planets have moons other than Earth. And, all other planets with moons have moons with diameters only a few percent of the planet’s diameter. The largest of the bunch, relative to its planet is Triton, and there is reason to believe that even Triton may be a captured body around Neptune (it orbits the wrong way). But, Neptune’s next largest moon, Proteus, is only 0.0085 of Neptune’s diameter. So, what is wrong with Earth’s moon? Why is it close to 27% of Earth’s diameter, when all other planetary moons are so small?
The answer is believe to rest in the origin of Earth’s moon. Unlike the other moons of the solar system, which likely either formed around their planets or are captured bodies, Earth’s moon is believed to have been formed in the earliest days of the Solar System by a humongous collision of a very large body with Earth. A wonderful, and very readable, book about what astronomers consider the most likely scenario for the formation of the Moon is Dana MacKenzie’s book The Big Splat. I know the name doesn’t sound serious, but the book is good. And, it explains the whole thing in a way that is easy to understand for a layman.
That is what comes to my mind when I see the image above. I think of just how unusual the Moon is for Earth. But, that also has had a major impact on our world. A number of years ago, Neil Comins wrote a book entitled What if the Moon Didn’t Exist that looks at some ideas of just how the Moon has influenced Earth. That is another interesting book, and it, too, is written at a popular science level and is very readable.
-Astroprof
Image courtesy NASA, JPL
(Note: Pluto’s moon Charon is just over 50% of Pluto’s diameter, but Pluto is not really a planet. In fact it is not even fair to call Charon Pluto’s moon, since Charon does not really orbit Pluto. Rather, both bodies orbit around the center of mass point between them. Pluto and Charon are more of a binary whatever-they-are. When Pluto we used to think of Pluto as a planet, we quickly realized that it and Charon were really a binary planet. Now, Pluto is, more properly, classified a Dwarf planet, and Pluto and Charon are together a binary dwarf planet. The formation and history of Pluto and Charon are quite different from that of the eight major planets, so its moon size does not bear to this discussion.)
Paleoprof on March 12, 2008 at 7:54 pm: 1
Great post I’m going to use the picture in my Earth Science class. At the beginning of the third paragraph second sentence don’t you mean Mars instead of Mercury?
Astroprof on March 12, 2008 at 10:58 pm: 2
Thanks, Paleoprof. I changed the typo.