[meteorite-list] NASA's WISE Colors in Unknowns on Jupiter Asteroids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 15 Oct 2012 12:44:55 -0700 (PDT)
Message-ID: <201210151944.q9FJit38004773_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?release=2012-322

NASA's WISE Colors in Unknowns on Jupiter Asteroids
Jet Propulsion Laboratory
October 15, 2012

Scientists using data from NASA's Wide-field Infrared Survey Explorer,
or WISE, have uncovered new clues in the ongoing mystery of the Jovian
Trojans -- asteroids that orbit the sun on the same path as Jupiter.
Like racehorses, the asteroids travel in packs, with one group leading
the way in front of the gas giant, and a second group trailing behind.

The observations are the first to get a detailed look at the Trojans'
colors: both the leading and trailing packs are made up of predominantly
dark, reddish rocks with a matte, non-reflecting surface. What's more,
the data verify the previous suspicion that the leading pack of Trojans
outnumbers the trailing bunch.

The new results offer clues in the puzzle of the asteroids' origins.
Where did the Trojans come from? What are they made of? WISE has shown
that the two packs of rocks are strikingly similar and do not harbor any
"out-of-towners," or interlopers, from other parts of the solar system.
The Trojans do not resemble the asteroids from the main belt between
Mars and Jupiter, nor the Kuiper belt family of objects from the icier,
outer regions near Pluto.

"Jupiter and Saturn are in calm, stable orbits today, but in their past,
they rumbled around and disrupted any asteroids that were in orbit with
these planets," said Tommy Grav, a WISE scientist from the Planetary
Science Institute in Tucson, Ariz. "Later, Jupiter re-captured the
Trojan asteroids, but we don't know where they came from. Our results
suggest they may have been captured locally. If so, that's exciting
because it means these asteroids could be made of primordial material
from this particular part of the solar system, something we don't know
much about." Grav is a member of the NEOWISE team, the asteroid-hunting
portion of the WISE mission.

The first Trojan was discovered on Feb. 22, 1906, by German astronomer
Max Wolf, who found the celestial object leading ahead of Jupiter.
Christened "Achilles" by the astronomer, the roughly 220-mile-wide
(350-kilometer-wide) chunk of space rock was the first of many asteroids
detected to be traveling in front of the gas giant. Later, asteroids
were also found trailing behind Jupiter. The asteroids were collectively
named Trojans after a legend, in which Greek soldiers hid inside in a
giant horse statue to launch a surprise attack on the Trojan people of
the city of Troy.

"The two asteroid camps even have their own 'spy,'" said Grav. "After
having discovered a handful of Trojans, astronomers decided to name the
asteroid in the leading camp after the Greek heroes and the ones in the
trailing after the heroes of Troy. But each of the camps already had an
'enemy' in their midst, with asteroid 'Hector' in the Greek camp and
'Patroclus' in the Trojan camp."

Other planets were later found to have Trojan asteroids riding along
with them too, such as Mars, Neptune and even Earth, where WISE recently
found the first known Earth Trojan:
http://www.jpl.nasa.gov/news/news.php?release=2011-230 .

Before WISE, the main uncertainty defining the population of Jupiter
Trojans was just how many individual chunks were in these clouds of
space rock and ice leading Jupiter, and how many were trailing. It is
believed that there are as many objects in these two swarms leading and
trailing Jupiter as there are in the entirety of the main asteroid belt
between Mars and Jupiter.

To put this and other theories to bed requires a well-coordinated,
well-executed observational campaign. But there were many things in the
way of accurate observations -- chiefly, Jupiter itself. The orientation
of these Jovian asteroid clouds in the sky in the last few decades has
been an impediment to observations. One cloud is predominantly in
Earth's northern sky, while the other is in the southern, forcing
ground-based optical surveys to use at least two different telescopes.
The surveys generated results, but it was unclear whether a particular
result was caused by the problems of having to observe the two clouds
with different instruments, and at different times of the year.

Enter WISE, which roared into orbit on Dec. 14, 2009. The spacecraft's
16-inch (40-centimeter) telescope and infrared cameras scoured the
entire sky looking for the glow of celestial heat sources. From January
2010 to February 2011, about 7,500 images were taken every day. The
NEOWISE project used the data to catalogue more than 158,000 asteroids
and comets throughout the solar system.

"By obtaining accurate diameter and surface reflectivity measurements on
1,750 Jupiter Trojans, we increased by an order of magnitude what we
knew about these two gatherings of asteroids," said Grav. "With this
information, we were able to more accurately than ever confirm there are
indeed almost 40 percent more objects in the leading cloud."

Trying to understand the surface or interior of a Jovian Trojan is also
difficult. The WISE suite of infrared detectors was sensitive to the
thermal glow of the objects, unlike visible-light telescopes. This means
WISE can provide better estimates of their surface reflectivity, or
albedo, in addition to more details about their visible and infrared
colors (in astronomy "colors" can refer to types of light beyond the
visible spectrum).

"Seeing asteroids with WISE's many wavelengths is like the scene in 'The
Wizard of Oz,' where Dorothy goes from her black-and-white world into
the Technicolor land of Oz," said Amy Mainzer, the principal
investigator of the NEOWISE project at NASA's Jet Propulsion Laboratory
in Pasadena, Calif. "Because we can see farther into the infrared
portion of the light spectrum, we can see more details of the asteroids'
colors, or, in essence, more shades or hues."

The NEOWISE team has analyzed the colors of 400 Trojan asteroids so far,
allowing many of these asteroids to be properly sorted according to
asteroid classification schemes for the first time.

"We didn't see any ultra-red asteroids, typical of the main belt and
Kuiper belt populations," said Grav. "Instead, we find a largely uniform
population of what we call D-type asteroids, which are dark burgundy in
color, with the rest being C- and P-type, which are more grey-bluish in
color. More research is needed, but it's possible we are looking at the
some of the oldest material known in the solar system."

Scientists have proposed a future space mission to the Jupiter Trojans
that will gather the data needed to determine their age and origins.

The results were presented today at the 44th annual meeting of the
Division for Planetary Sciences of the American Astronomical Society in
Reno, Nev. Two studies detailing this research are accepted for
publication in the Astrophysical Journal.

JPL manages, and operated, WISE for NASA's Science Mission Directorate.
The spacecraft was put into hibernation mode in 2011, after it scanned
the entire sky twice, completing its main objectives. Edward Wright is
the principal investigator and is at UCLA. The mission was selected
competitively under NASA's Explorers Program managed by the agency's
Goddard Space Flight Center in Greenbelt, Md. The science instrument was
built by the Space Dynamics Laboratory in Logan, Utah. The spacecraft
was built by Ball Aerospace & Technologies Corp. in Boulder, Colo.
Science operations and data processing take place at the Infrared
Processing and Analysis Center at the California Institute of Technology
in Pasadena. Caltech manages JPL for NASA. More information is online at
http://www.nasa.gov/wise , http://wise.astro.ucla.edu and
http://jpl.nasa.gov/wise .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
Whitney.clavin at jpl.nasa.gov

2012-322
Received on Mon 15 Oct 2012 03:44:55 PM PDT