The Puzzling Radar Signatures of Icy Satellites Around Jupiter and Saturn

Category Space

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A collaborative investigation led by researchers from the Southwest Research Institute and NASA Jet Propulsion Laboratory provides new insights on the puzzling radar signatures of icy satellites orbiting Jupiter and Saturn. The researchers argue that the coherent backscatter opposition effect, combined with an icy surface's enhanced opposition effect, can provide an explanation for the anomaly in the satellites' radar properties.

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A collaborative investigation led by Dr. Jason Hofgartner, Senior Research Scientist at the Southwest Research Institute, sheds light on the puzzling radar signatures of icy satellites orbiting Jupiter and Saturn. The distinct radar attributes of these celestial bodies, which deviate considerably from those of rocky planets and the majority of Earth’s ice, have long confounded researchers.

"Six different models have been published in an attempt to explain the radar signatures of the icy moons that orbit Jupiter and Saturn," said Hofgartner, first author of the study, which was published in Nature Astronomy. "The way these objects scatter radar is drastically different than that of the rocky worlds, such as Mars and Earth, as well as smaller bodies such as asteroids and comets." .

The Arecibo Observatory was thought to be the single most powerful radar system in the world.

The objects are also extremely bright, even in areas where they should be darker.

"When we look up at Earth’s moon it looks like a circular disk, even though we know it’s a sphere. Planets and other moons similarly look like disks through telescopes," Hofgartner said. "While making radar observations, the center of the disk is very bright and the edges much darker. The change from center to edge is very different for these icy satellites than for rocky worlds." .

Light waves propagate from the transmitter to the icy satellite and back to the receiver

In collaboration with Dr. Kevin Hand of NASA’s Jet Propulsion Laboratory, Hofgartner argues that the extraordinary radar properties of these satellites, such as their reflectiveness and polarization (the orientation of light waves as they propagate through space) is very likely to be explained by the coherent backscatter opposition effect (CBOE).

"When you’re at opposition, the Sun is positioned directly behind you on the line between you and an object, the surface appears much brighter than it would otherwise," Hofgartner said. "This is known as the opposition effect. In the case of radar, a transmitter stands in for the Sun and a receiver for your eyes." .

The opposition effect enhances the brightness of the surface of an icy satellite when the sun is behind the observer

An icy surface, Hofgartner explained, has an even stronger opposition effect than normal. For every scattering path of light bouncing through the ice, at opposition there is a path in the exact opposite direction. Because the two paths have precisely the same length, they combine coherently, resulting in further brightening.

In the 1990s, studies were published stating that the CBOE was one explanation for the anomalous radar signatures of icy satellites, but other explanations could explain the data equally well. Hofgartner and Hand improved the polarization description of the CBOE model and also showed that their modified CBOE model is the only published model that can explain all of the icy satellite radar properties.

The CBOE model is the only model that can account for all of the polarizing radar features of icy moons

"I think that tells us that the surfaces of these objects and their subsurfaces down to many meters are very tortured," Hofgartner said. "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." .

The radar observations Hofgartner and Hand used were from the Arecibo Observatory, which was one of only two telescopes on Earth capable of successfully collected this data. Unfortunately, Arecibo’s radar transmission system has been retired in 2020 as a result of severe damage.

The distinct radar signatures of icy satellites deviate considerably from those of rocky planets and Earth's ice

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