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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