This week NASA released a photo of Earth and moon in space taken by the Mercury Messenger spacecraft from a distance of 114 million miles.Â This is how our “double planet” would appear to someone on Venus with a pair of binoculars. There’s nothing like the perspective afforded by a pair of distant, robotic eyes to stir our thoughts and fire our imaginations about where we fit in the scheme of things. This view reminds of looking at double stars through the telescope. The similarity is no coincidence — the moon is so large in relation to our planet, the Earth-moon system is often considered a “double planet”.
NASA’s reason to take the provocative photo was not connected to a higher purpose on this occasion. It was a delightful byproduct of a search for “vulcanoids”, small rocky asteroids that may be orbiting between Mercury and the sun. What, you say? NASA looking for Spock’s planet? Well, not really. No vulcanoids have been discovered yet, so they’re still hypothetical. The glare of the sun prevents easy hunting from Earth, so NASA directed the spacecraft, which is much closer and has a better view, to seek out this potential new class of solar system bodies. May it live long and provide prosperous imagery.
There was more NASA news this week from the Lunar Reconnaissance Orbiter Mission (LRO) of the discovery of lobate scarps on moon. These cliffs on the lunar crust point to a moon that’s been shrinking, and not four billion years ago, but as recently as a couple hundred million years ago. The moon won’t be crunching itself into a tennis ball anytime soon since the total shrinkage is only on the order of 300 feet. Compared to its 2,160 mile diameter thatâ€™s a tiny fraction, but the consequences are evident on its surface. Even more intriguing, we’re waking up to a moon thatâ€™s not as dead as once thought. Since the Apollo missions, weâ€™ve learned the moon has water ice embedded at many sites across its surface; now it also appears to have been geologically active in recent times.
After its formation some 4 billion years ago, the moon was mostly molten rock. As it cooled, the outer crustal rock solidified first. Later, magma beneath the crust also began to cool and solidify. As the molten materials went from a liquid to solid state, their density increased. If you take a certain volume of rock and increase its density, the amount of space or volume it occupies drops. As the moon cooled and grew denser, its internal rock slowly cooled and compressed itself into a smaller ball. The outer crust responded in the only way it could — it broke into faults and slid over itself in an attempt to form a tight fit over the ever-shrinking mantle and core. We see the results of this adjustment in the numerous lobate (rounded or lobed) scarps or cliffs hither and yon across the moon. And we know some are recent because they partially cover small, relatively fresh craters.