With this week's full Moon, the dazzling crater Tycho will be in full regalia. Time to catch some rays!
It's Tycho time! The Moon waxes to full on March 1st and its most famous crater, Tycho, is favorably tipped into view for the entire week. Tycho is one of the few lunar craters you can actually see with the naked eye. If you stare squarely at it, you'll see a tiny, unresolved bright spot about a quarter of the way in from the southern limb.
Tycho stands out because it's still fresh with an estimated age of 108 million years, far younger than the 3.9 billion years for many large lunar craters. We know its date of birth because the Apollo 17 astronauts bagged a sample from one of the rays and returned it to Earth for study in 1972.
An 8–10 kilometer-wide asteroid excavated Tycho during Earth's Cretaceous Period, when dinosaurs still clomped through fields and forests. The 86-kilometer-diameter crater sits at the epicenter of a web of brilliant and bountiful rays created by massive rocks hurled outward from the impact. They arced above the airless lunar landscape before crashing down and digging out innumerable secondary craters. Those impacts hurled additional rocks and soil downrange, extending and expanding the rays.
Freshly excavated lunar material is bright, but it weathers dark over time. Tycho's rays are bright because not enough time has passed for cosmic rays, solar storms, and micrometeorites to darken and sandpaper them into dust. If you could walk along a ray today, you'd see heaps of light-toned crushed rock alternating with craters dug by catapulted ejecta.
Tycho reveals much in a small telescope. Besides its crisply defined rim — another sign of the crater's youth — the central peak is immediately obvious. The main peak stands two kilometers above the crater floor and looks like a bright point at full Moon. A much smaller secondary peak pokes out right next to it to the northeast. With a magnification of 214× on my 10-inch scope it appears as a tiny mound. Both are formed from rock that rebounded upward after the crust "relaxed" in the aftermath of the impact.
Like many larger craters, Tycho has step-like terraced walls that formed as the sides of the crater gradually gave way and slumped downward under the force of gravity. Both the terraces and peaks are best seen within a couple days of sunrise and sunset at the crater, i.e., when it's on or near the lunar terminator. Try on the morning of March 9–10 (sunset) or on the evening of March 24–25 (sunrise). Slanted sunlight at these times creates the shadows that highlight fine details otherwise lost under the high Sun of full Moon.
If you're lucky, you might catch sight of the peak intercepting the first or last rays of sunlight with the crater bowl steeped in black shadow below — a most arresting lunar spectacle. On rare occasions, I've seen the darkness filling the bowl leavened to a deep gray by sunlight reflecting off the crater's rim.
Tycho's floor is 4.8 km deep and appears relatively smooth east of the central peak and rougher west of it. It's composed of melted rock (called impact melt) created by the tremendous heat generated during the impact. The view through a telescope only hints at what's there. To fully appreciate the floor's bizarre humpback mounds of melted rock, wormy troughs, and crisscrossing cracks, visit the LROC-Quickmap site, an interactive, zoomable tool that takes you so close to the Moon you can almost smell it. High resolution images from LRO were stitched together to create the map. Center the view on Tycho, zoom in with your mouse, and explore.
Glassy impact melt extends beyond the crater wall to create the unique dark "collar" around Tycho best visible at full Moon. The collar sets the crater off from a larger nimbus of exceptionally bright rays that overlap and weave in intricate ways. You can almost lose your way here, so take your time. A neutral density filter will help to tone down the light and make it easier on the eye.
Look closely at the craters within and just beyond the dark collar several days before or after full Moon; their floors and walls display narrow, linear gouges that trace the blast tracks of impact debris across the region. What a sight it must have been in the day. Lunar erosion scratches away details relatively slowly, allowing us to appreciate the magnitude of the bombardment to this day.
Tycho's rays form a web of white spokes that extend up to 2,000 km across the lunar nearside, even as far as Mare Serenitatis, where the Apollo 17 astronauts collected their crucial sample. One possible ray bisects the mare and reaches even further, but it's still unclear if it belongs to Tycho or the crater Bessel, which it overlaps. The rays aren't equally distributed but form a butterfly pattern with most extending to the lunar east, south, and northwest. Very few reach west, indicating that the projectile approached at a low angle from the west to create the off-center ballistic pattern.
A short, bright ray to the south-southwest of Tycho doesn't fit the pattern: instead of pointing back to the crater, it's nearly tangent to it. I've tried hard to imagine what might have happened in the chaos of impact to cause a stream of boulders to turn askew and land in such a non-radial fashion, but I'm still at a loss. Was it redirected after colliding with another ejecta stream? Unrelated to Tycho?
A remarkable pair of rays to the crater's northwest (upper left) run parallel, like the ruts cut by an the old wagon trains across the prairie. These features and others will be in full view this week during full Moon. But wait, there's more. With two full Moons this month (the next falls on March 31st), we'll get to see it all over again!