The Sciences

Watermelon planet

Bad AstronomyBy Phil PlaitOct 7, 2008 1:33 PM


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Holy Haleakala. Look at those rays! They go all the way across the planet! This is Mercury as seen by MESSENGER, which flew by the planet for a second time yesterday (out of three passes on its way to orbiting the planet in March 2011). This overview was taken when the probe was 27,000 km (17,000 miles) from Mercury, 90 minutes after closest encounter. What you're seeing here is pretty much the opposite side of the planet as was seen last January at MESSENGER's first pass, so most of this is territory never seen before in this detail (in this case, at about 5km/pixel). The bright streaks or stripes are called rays. They're material ejected from what appears to be a young crater first seen during the initial flyby. When an object impacts the surface of a planet, material can spray out in long rays; check out an image of the full Moon to see similar rays radiating out from the crater Tycho. Amazingly, the rays were known before MESSENGER; radar signals bounced off Mercury from Earth indicated the rays were there. The ray material lying on the surface reflects radar differently than rock, and that was detected even from Earth. But this is the first time they've been seen. The bright crater in the middle of Mercury is Kuiper, known since the Mariner 10 days of the 1970s. But not like this! It has a ray system as well. In fact, if you look closely, you'll see that almost all the bright small craters can be seen to have rays, too. Over time, the solar wind and meteorite impacts erase ray systems, so they come from young craters, and young craters tend to have brighter floors. Now check out this close-up:

That is not the Moon! This is a shot taken just about 9 minutes after closest approach, when MESSENGER was about 1800 km (1100 miles) from the surface. You can almost pick out the order in which events happened here, with fresher craters overlaying older ones. The big sucker at the top is called Polygnotus, and has an inner ring, common in larger impact events. Polygnotus is actually more of a basin than a crater; the floor is smooth, indicating it got filled with lava after impact. Another double-walled basin can be see on the left, called Boethius. You can see a cliff running up-and-down near the center of Boethius; that is a scarp that must have formed after the impact event. You can tell because of the short, bright feature that is cut in half near the top of the scarp (halfway from the center of the basin to the rim). Reading a planet's surface may seem a little bit like phrenology or palm reading. In a sense, it is; you look at the overview as well as the details laid out in front of you, and try to tease out information from the jumble. But unlike those pseudosciences, planetary surface geology tells you about the way the world really is. And it's a different world! We can read Mercury like, well, like a map. We can look at the features and peel away the history of this planet, from massive impacts to the subsequent shrinkage of the crust, causing cracks to penetrate all over the surface. Huge cliffs, deep chasms, steep crater walls... all brutally baking under an unforgiving Sun much closer than it is from the Earth. And this is almost all new. Our previous maps of Mercury were amazing for their time, have no doubt, but we are a restless bunch, and love to fling ever-better technology at the places we've only seen fuzzily in the past. Now our vision is sharper... but this is only a glimpse. MESSENGER will take a third swing at Mercury next year on September 29, and then, 18 months later, will settle down for a long stay, orbiting the planet and returning thousands of images such as these. There's much we still don't understand about the smallest planet, even with these fantastic images to help us. In fact, they are only whetting our appetite, but the feast will start soon enough. Image credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

MESSENGER's Mercury. Click to embiggen.

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