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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Explanation for unusual radar signatures of icy satellites in the outer   
   solar system    
    Researchers present answers for long-debated mysterious radar properties   
   of moons of Jupiter and Saturn    
      
     Date:   
         March 23, 2023   
     Source:   
         Southwest Research Institute   
     Summary:   
         A study explains the unusual radar signatures of icy satellites   
         orbiting Jupiter and Saturn. Their radar signatures, which differ   
         significantly from those of rocky worlds and most ice on Earth,   
         have long been a vexing question for the scientific community.   
      
      
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   FULL STORY   
   ==========================================================================   
   A study co-authored by Southwest Research Institute Senior Research   
   Scientist Dr. Jason Hofgartner explains the unusual radar signatures   
   of icy satellites orbiting Jupiter and Saturn. Their radar signatures,   
   which differ significantly from those of rocky worlds and most ice on   
   Earth, have long been a vexing question for the scientific community.   
      
      
   ==========================================================================   
   "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 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."  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."  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."  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.   
      
   "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 making radar observations of icy satellites until   
   it was severely damaged by the collapse of its support structure, antenna   
   and dome assembly and subsequently decommissioned. The researchers hope   
   to make follow-up observations when possible and plan to study additional   
   archival data that may shed even more light on icy satellites and the   
   CBOE, as well as radar studies of ice at the poles of Mercury, the Moon,   
   and Mars.   
      
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   ==========================================================================   
   Story Source: Materials provided by Southwest_Research_Institute. Note:   
   Content may be edited for style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Jason D. Hofgartner, Kevin P. Hand. A continuum of icy satellites'   
      radar   
         properties explained by the coherent backscatter effect. Nature   
         Astronomy, 2023; DOI: 10.1038/s41550-023-01920-2   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/03/230323135452.htm   
      
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