RISING MOON
I Old lava, fresh craters
CONSIDER DOING a little sidewalk astronomy this month to show off Luna to those who’ve never seen it before. Whether it’s July’s first or last weekend, or while waiting for the fireworks to start, newcomers will gasp at the most impressive feature near Full Moon: Tycho and its magnificent ray system. The long lines were formed from the splash of a high-energy impact that excavated the lunar regolith and spread it outward.
Take note of the dark ring of melt products immediately surrounding the 53-mile-wide crater. Then let your eye naturally follow its longest ray to the northeast, where it splits the Sea of Serenity in near perfection.
The bright circular crater at the south edge of Serenity is the 15-milewide Menelaus. Its sharp edges and light-hued apron of debris are characteristics of relative youth, just like the smaller Bessel farther up the long stream. It seems as if Bessel disrupted the spray as it was spreading out, creating a sheltered spot on its north side, but it’s tough to be sure since other rays are broken without an immediate crater nearby.
Now look carefully at the surrounding lava. The darkest shade is the oldest, estimated at
3.8 billion years, while the lighter layer with a different composition that lies just to the north clocks in at closer to 3 billion. Can you see more white flecks (tiny impact craters) in the darker, older zone?
Astronomers have discovered that rays and aprons darken over time, due to the ceaseless solar wind and its energetic particles that microscopically sandblast the surface.