Glow up (3/23/23)

Good afternoon, and happy Thursday. Hope you’re having a wonderful week. Let’s get right into it.

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Reading the Signals

Jupiter and Saturn have glowing moons. 

Okay, not really. What they do have, though, are icy satellites that reflect so much light and create such strange results in radar observations that they’ve been stumping observers for decades. These satellites create bright radar signatures even when they should be in darkness, and scatter light differently than rocky moons or smaller icy bodies like comets.

In a study published today in Nature Astronomy, researchers at the Southwest Research Institute and NASA’s Jet Propulsion Laboratory posed an explanation for these strange radar observations—a phenomenon called the coherent backscatter opposition effect.

Let’s break that down: In astronomy, opposition occurs when an observer comes directly between a light source and an observed object. For instance, if we’re standing on the Earth and using the Sun as a light source, then an object at opposition would be on the exact opposite side of the Earth from the Sun.

Objects at opposition appear brighter than they otherwise would. Two explanations are generally used to explain this:

1. Shadow hiding. Since you view an object at opposition from the same angle as the light source hits it, you wouldn’t see shadows cast by roughness on the surface, and the absence of shadow makes the surface look brighter.

2. Coherent backscatter. This effect is much more difficult to measure. Sometimes, when light hits a surface at a narrow angle, the surface conditions are just right to reflect that light right back in the direction it came from (or “coherently”). This lining-up of the light beams makes objects appear brighter to an observer.

Back to the icy moons…For icy, reflective surfaces like the moons analyzed in this study, the coherent backscatter effect at opposition is particularly strong. The researchers used radar data from the Arecibo Observatory (RIP) to rule out shadow hiding as the primary reason for the outsized brightness of the moons at opposition, and determined that the coherent backscatter opposition effect is the only explanation that makes sense for every moon observed.

“I think that tells us that the surfaces of these objects and their subsurfaces down to many meters are very tortured,” said Jason Hofgartner, lead author on the study, in a press release. “They’re not very uniform. Icy rocks dominate the landscape, perhaps looking somewhat like the chaotic mess after a landslide. That would explain why the light is bouncing in so many different directions, giving us these unusual polarization signatures.”

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Other News from the Cosmos

• JWST data reveals swirling clouds of sand-like silicate particles in the atmosphere of exoplanet VHS 1256 b.

• Small stars could host big planets—at least, larger gas giants than previously identified, according to new data from the TESS (Transiting Exoplanet Survey Satellite) mission.

• ispace’s HAKUTO-R lunar lander has entered lunar orbit ahead of its planned touchdown in late April.

• “Space skin” developed by Airbus and the University of Surrey and made of carbon composites shows promise for shielding space assets from radiation.

• Black holes and gluons—the particles that hold nuclei together—share similar organizational characteristics, a DoE study found.

• Samples from the asteroid Ryugu contained uracil, a nucleic acid found in RNA.

• Fissures on the seafloors of Europa and Enceladus, ocean moons of Jupiter and Saturn, may be too scarce to support life.

Rachael's Recs

👽 No alien spaceship: The…uh…object ‘Oumuamua drew the world’s attention six years ago as it passed through our solar system. The small, pancake-shaped rock moved like a comet made of rock and ice, but had no comet’s tail to speak of. Theories on the object’s identity abounded, ranging from asteroid to alien spaceship. Now, a study in Nature has determined a more reasonable explanation for ‘Oumuamua—but not everyone agrees. The Wall Street Journal explored the mystery.

🌭 Into the multiverse: It’s hard enough trying to understand our own universe. But what if there are others out there? For the Great Mysteries of Physics podcast with The Conversation, host Miriam Frankel talked with several cosmologists to understand the multiverse theory and everything (everywhere all at once) its supporters and detractors have to say.

The View from Space