Messenger from Space

A space craft called Messenger was launched back in August of 2004, en route to the smallest of the planets, Mercury. Tiny and dense, closest to the sun, and with a very ancient surface, Mercury should be a planet of great interest, but has been largely ignored for most of the era of solar system exploration.

Only one other spacecraft has visited Mercury. From 1974-1975, Mariner 10 flew by and sent back the pictures of the heavily cratered planet. The data showed a hot little world, burning and freezing alternately through its day and night. The surprising thing learned by Mariner about the closest planet to the sun is that there are spots that are colder and darker than many places in the solar system. One of the reasons we’re so curious about Mercury is the fact that it may harbor ice in a deep, dark polar crater. Ice on Mercury? How could this be?

All planets spin as they orbit the sun, and usually those spins have some harmonic resonance, thanks to gravity. This means that Mercury’s spin has slowed to the point that for every two times it orbits the sun, it rotates three times. A day on Mercury lasts about 176 earth days, baking the landscape for 88 days at a time from sunrise to sunset. The polar regions are the exception; they see little sun due to Mercury’s axis being nearly upright. It is deep craters in these polar regions that may harbor ice from a long past comet collisions or outgassing from Mercury itself.

Mercury’s curious spin has also led to the discovery that it may still have a molten core. Raw eggs tend to wobble more than hard boiled ones when they spin. That’s a good way to tell which ones have been cooked. Using that same principal, scientists have used radar to learn that Mercury wobbles more than a hard-boiled planet should wobble. This is surprising since tiny Mercury has had time to cool enough to solidify in the past 5 billion years. Its core is surprisingly large, as well. It makes up about 75% of the diameter of the planet. Such a large, dense core can’t be explained by compression, as it can in the cases of earth and Venus, so astronomers are curious to find out how Mercury accumulated such a great proportion of the solar system’s heaviest elements.

The Messenger spacecraft should be able to shed some light on these questions. The Mariner data was limited and up to now we have only had pictures of one side of Mercury. New pictures of before unseen parts of Mercury are streaming in, and soon this little world will tell us more about how the solar system formed.

There are many mysteries about Mercury, some of which Messenger will try to answer. But in the meantime, we can try to spy the tiny planet in the fierce glow of the sun. The evening sky of late January still has elusive Mercury visible very low in the west after sunset. In the morning, Venus and Jupiter will be together, shining brightly in the February pre-dawn sky. On February 4th, the waning crescent moon will join the party in the eastern morning sky.