The Moon’s bright crater rays are streaks of material thrown outward when an asteroid or comet struck the surface. Some rays stand out because they contain relatively fresh, reflective debris; others contrast with the darker ground underneath. Tycho, in the Moon’s southern highlands, is one of the easiest examples to spot around full Moon.
What makes Tycho’s rays look like a white splash?
Imagine a pebble striking a tray of flour and sending powder in several directions. A high-speed impact on the Moon is vastly more energetic: it excavates rock, breaks surrounding layers and throws fragments across the surface. Some debris forms long, thin-looking streaks or clusters of secondary impact craters. Scientists call this material ejecta.
Tycho is roughly 85 kilometers (53 miles) wide, but its rays extend much farther. It stands out not because it is the Moon’s biggest crater, but because its impact happened about 110 million years ago—young on a lunar geological timescale. Its bright material has not been darkened as thoroughly by long exposure to space.
See the rays in a real spacecraft image
The Galileo spacecraft photographed the near side of the Moon in December 1992. Look toward the lower part of the lunar disk: Tycho is the bright, starburst-like feature with rays crossing large areas of the surface. NASA notes that this image combines several spectral filters, including wavelengths beyond visible light, so its colors are enhanced rather than a natural-eye color view.
Why do rays fade over time?
The Moon has almost no atmosphere to weather rocks with rain or wind. But its surface is constantly exposed to micrometeorite impacts and charged particles from space. These processes—often grouped under space weathering—can alter the reflectance of lunar soil over very long periods. Fresh and older ejecta gradually become harder to distinguish from surrounding ground.
Do all lunar craters have white rays?
Not all have rays you can easily see today. Newly formed impact craters can display obvious ejecta patterns, but older rays may darken and blend into their surroundings. Brightness can depend on the composition of the underlying terrain as well as the freshness of exposed rock. A ray on dark volcanic plains may be especially noticeable.
How to find Tycho from your backyard
- Wait until the Moon is near full, when the bright ray system is especially obvious.
- Look toward the bottom half of the Moon’s familiar near-side disk (the southern highlands in standard north-up images).
- Start with the naked eye, then try binoculars or a small telescope for more detail. Never point binoculars or a telescope toward the Sun.
- Compare how the rays look at different Moon phases.
Frequently asked questions
Are Tycho’s rays cracks?
No. The striking streaks are primarily impact-related surface deposits and secondary crater patterns, not cracks splitting the Moon open.
Did Tycho’s impact happen while dinosaurs lived on Earth?
Yes. An age of roughly 110 million years places it in the Cretaceous Period, long before the impact associated with the end-Cretaceous mass extinction.
Why are rays easier to see near full Moon?
When the Sun shines almost straight onto the lunar face we see, broad differences in surface brightness stand out. Near the crescent phase, long shadows make mountains and crater walls more visually prominent instead.
For more on lunar geology, see NASA’s explanation of how impact craters form, and read why astronauts’ footprints remain sharp on the Moon.
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