Sunbeams and moonbeams. Those are the familiar terms for crepuscular rays, alternating shafts of shadow and sunlight that stream through clouds to form an ethereal crown of light around the sun or moon. Crepuscular comes from the Latin word for twilight, since the phenomenon is most eye-catching during early morning and evening hours.
On Feb. 2, 2023, NASA's Curiosity rover captured a Martian version of crepuscular rays from the slope of Mt. Sharp in Gale Crater. As the sun sank over the horizon its filtered rays illuminated a bank of clouds to create a dappled pattern of light and shadow much like what we see from Earth. Imagine how this familiar sight might tug at the heart of a future astronaut after a year or more away from home.
Curiosity captured the scene during the rover’s most recent twilight cloud survey, and it's the clearest picture yet of Martian sunbeams. Clouds might seem unlikely on a dry, cold planet like Mars where the atmosphere holds only about 0.03% water vapor (Earth's contains 2%). But the bitter cold air and low atmospheric pressure keeps the atmosphere close to the saturation point, making it conducive to cloud formation. Clouds are especially common in the polar regions during the fall and winter seasons as well as at high elevations hugging the slopes of the planet's ancient volcanoes.
Most Martian clouds hover no more than 37 miles (60 km) above the ground and are composed of water-ice crystals. Those in these recent images appear to lie at a higher altitude, where it’s colder yet and are mostly likely composed of dry ice crystals — frozen carbon dioxide.
Crepuscular rays form when sunlight streams past a bank of broken clouds. Shafts of sunshine reach our eyes through holes or gaps between and within the clouds. Shadows are cast where a cloud's bulk blocks the sun. Together they produce alternating beams and shadows that appear to radiate from the sun or moon. In truth, all the rays are parallel to one another and only appear to converge the same way parallel railroad tracks "meet" in the distance at the vanishing point . A convincing — and inspiring — optical illusion!
ADVERTISEMENT
Watching clouds on Mars helps scientists understand how and when they form. They also provide information on the composition of the atmosphere, its temperatures and winds. Curiosity also photographed a feather-shaped cloud on Jan. 27 aglow with subtle colors caused by diffraction. Really tiny things like minute water droplets or ice crystals in clouds, especially if they're all of similar size, scatter light waves and spread them into their component colors, creating a wash colorful bands reminiscent of an abalone shell. You'll see the same effect on bubbles and light reflected from a thin oil film on pavement.
“Where we see iridescence, it means a cloud’s particle sizes are identical to their neighbors in each part of the cloud,” said Mark Lemmon, an atmospheric scientist with the Space Science Institute in Boulder, Colorado. “By looking at color transitions, we’re seeing particle size changing across the cloud. That tells us about the way the cloud is evolving and how its particles are changing size over time.”
Light can also be bent or refracted. When sunlight passes through raindrops they act like prisms, bending and spreading the light into a rainbow. Larger ice crystals in cirrus-type clouds also refract light to create familiar "ring around the sun" halos.
Full Worm Moon Wriggles into View
We're up for a full moon this week! On Tuesday, March 7, the Full Worm Moon will rise in the eastern sky in Virgo around sunset. For Duluth, moonrise occurs at 6:19 p.m. To find your local time visit timeanddate.com/sun .
Later in the evening, when the moon has climbed up the sky, you'll notice it doesn't get quite as high as the January moon. That's because the sun is moving north (higher) in the sky as we approach spring (March 20, 4:24 p.m. CDT). Since the full moon shines directly opposite the sun, it slides south and sinks lower in tandem.
Looking for patterns in the sky comes natural to us. Likewise seeing shapes and creatures in the face of the full moon. Maybe you've seen the man-in-the-moon, but have you seen the squirrel or rabbit? Give it a try!
The Trust Project
