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EI2GYB > ASTRO 01.09.21 09:41l 138 Lines 7138 Bytes #999 (0) @ WW
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Subj: Highest-resolution measurements of asteroid surface
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Sent: 210901/0836Z @:EI2GYB.DGL.IRL.EURO #:13970 BPQ6.0.22
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Highest-resolution measurements of asteroid surface temperatures ever
obtained from Earth
A close examination of the millimeter-wavelength emissions from the
asteroid Psyche, which NASA intends to visit in 2026, has produced
the first temperature map of the object, providing new insight into
its surface properties.
The findings are a step toward resolving the mystery of the origin of
this unusual object, which has been thought by some to be a chunk of
the core of an ill-fated protoplanet.
A close examination of the millimeter-wavelength emissions from the
asteroid Psyche, which NASA intends to visit in 2026, has produced
the first temperature map of the object, providing new insight into
its surface properties.
The findings, described in a paper published in Planetary Science
Journal (PSJ) on August 5, are a step toward resolving the mystery of
the origin of this unusual object, which has been thought by some to
be a chunk of the core of an ill-fated protoplanet.
Psyche orbits the sun in the asteroid belt, a donut-shaped region of
space between Earth and Jupiter that contains more than a million rocky
bodies that range in size from 10 meters to 946 kilometers in diameter.
With a diameter of more than 200 km, Psyche is the largest of the
M-Type asteroids, an enigmatic class of asteroids that are thought to
be metal rich and therefore potentially may be fragments of the cores
of proto-planets that broke up as the solar system formed.
"The early solar system was a violent place, as planetary bodies
coalesced and then collided with one another while settling into
orbits around the sun," says Caltech's Katherine de Kleer,
assistant professor of planetary science and astronomy and lead
author of the PSJarticle.
"We think that fragments of the cores, mantles, and crusts of these
objects remain today in the form of asteroids.
If that's true, it gives us our only real opportunity to directly
study the cores of planet-like objects."
Studying such relatively tiny objects that are so far away from
Earth (Psyche drifts at a distance that ranges between 179.5 and 329
million km from Earth) poses a significant challenge to planetary s
cientists, which is why NASA plans to send a probe to Psyche to examine
it up close. Typically, thermal observations from Earth -- which
measure the light emitted by an object itself rather than light
from the sun reflected off of that object -- are in infrared
wavelengths and can produce only 1-pixel images of asteroids.
That one pixel does, however, reveal a lot of information; for example,
it can be used to study the asteroid's thermal inertia, or how fast it
heats up in sunlight and cools down in darkness.
"Low thermal inertia is typically associated with layers of dust,
while high thermal inertia may indicate rocks on the surface," says
Caltech's Saverio Cambioni, postdoctoral scholar in planetary science
and co-author of the PSJ article. "However, discerning one type of
landscape from the other is difficult."
Data from viewing each surface location at many times of day
provide much more detail, leading to an interpretation that is
subject to less ambiguity, and which provide a more reliable prediction
of landscape type prior to a spacecraft's arrival.
De Kleer and Cambioni, together with co-author Michael Shepard of
Bloomsburg University in Pennsylvania, took advantage of the Atacama
Large Millimeter/submillimeter Array (ALMA) in Chile, which became fully
operational in 2013, to obtain such data. The array of 66 radio
telescopes enabled the team to map the thermal emissions from
Psyche's entire surface at a resolution of 30 km (where each pixel is 30 km
by 30 km) and generate an image of the asteroid composed of about 50 pixels.
This was possible because ALMA observed Psyche at millimeter
wavelengths, which are longer (ranging from 1 to 10 millimeters) than
the infrared wavelengths (typically between 5 and 30 microns).
The use of longer wavelengths allowed the researchers to combine the
data collected from the 66 telescopes to create a much larger effective t
elescope; the larger a telescope, the higher the resolution of the
images it produces.
The study confirmed that Psyche's thermal inertia is high compared to
that of a typical asteroid, indicating that Psyche has an unusually
dense or conductive surface.
When de Kleer, Cambioni, and Shepard analyzed the data, they also
found that Psyche's thermal emission -- the amount of heat it radiates --
is just 60 percent of what would be expected from a typical surface
with that thermal inertia.
Because surface emission is affected by the presence of metal on
the surface, their finding indicates that Psyche's surface is no
less than 30 percent metal.
An analysis of the polarization of the emission helped the researchers
to roughly determine what form that metal takes.
A smooth solid surface emits well-organized polarized light; the
light emitted by Psyche, however, was scattered, suggesting that
rocks on the surface are peppered with metallic grains.
"We've known for many years that objects in this class are not, in
fact, solid metal, but what they are and how they formed is still an
enigma," de Kleer says. The findings reinforce alternative proposals
for Psyche's surface composition, including that Psyche could be a
primitive asteroid that formed closer to the sun than it is today
instead of a core of a fragmented protoplanet.
The techniques described in this study provide a new perspective on
asteroid surface compositions.
The team is now expanding its scope to apply these techniques to
other large objects in the asteroid belt.
The study was enabled by a related project by the team led by
Michael Shepard at Bloomsburg University that utilized de Kleer's
data in combination with data from other telescopes, including
Arecibo Observatory in Puerto Rico, to pin down the size, shape, a
nd orientation of Psyche. That in turn allowed the researchers to
determine which pixels that had been captured actually represented
the asteroid's surface.
Shepard's team was scheduled to observe Psyche again at the end of
2020, but damage from cable failures shut the telescope down before
the observations could be made.
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