The icy satellites appear extremely bright, even in areas where they should be darker. Astronomers say that while planets and other moons look like disks through telescopes, the change from the center to the edge of these icy satellites' disks is very different than for rocky worlds.
Unraveling the extraordinary radar properties of Jupiter and Saturn’s satellites
A recent study co-authored by Dr. Jason Hofgartner, Senior Research Scientist at Southwest Research Institute, delves into the unusual radar signatures of icy satellites orbiting Jupiter and Saturn. These signatures, which vary significantly from those of rocky planets and most ice on Earth, have long puzzled scientists.
“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, the study’s first author. The research was published this month 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.”
Exceptional brightness and distinctive radar properties
The icy satellites appear extremely bright, even in areas where they should be darker. Hofgartner explained that while planets and other moons look like disks through telescopes, the change from the center to the edge of these icy satellites’ disks is very different than for rocky worlds. Collaborating with Dr. Kevin Hand of NASA’s Jet Propulsion Laboratory, Hofgartner suggests that the coherent backscatter opposition effect (CBOE) is the most likely explanation for the extraordinary radar properties of these satellites, such as their reflectiveness and polarization.
Coherent Backscatter Opposition Effect (CBOE) sheds light on unusual signatures
Hofgartner explained that an icy surface exhibits a stronger opposition effect than usual, resulting in further brightening. Studies from the 1990s proposed that CBOE could be one explanation for the anomalous radar signatures of icy satellites. Hofgartner and Hand have since improved the polarization description of the CBOE model, demonstrating that their modified CBOE model is the only published model capable of explaining all of the icy satellite radar properties.
The researchers suggest that the surfaces and subsurfaces of these icy satellites, extending down to several meters, are likely quite uneven, possibly resembling the disorderly aftermath of a landslide. This could account for the light bouncing in various directions, leading to unusual polarization signatures.
Looking to the Future: Further Observations and Studies
Hofgartner and Hand used radar observations from the now-decommissioned Arecibo Observatory, which was one of only two telescopes conducting radar observations of icy satellites. The researchers have planned to conduct further observations whenever feasible and have the intention of examining supplementary archival data that could provide more insights into icy satellites and the CBOE. Additionally, they aim to carry out radar investigations on the ice located at the poles of Mercury, the Moon, and Mars.
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