[meteorite-list] Dawn's Early Light - Newsletter of the Dawn Mission To Ceres and Vesta

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:35 2004
Message-ID: <200308250519.WAA01127_at_zagami.jpl.nasa.gov>

http://www-ssc.igpp.ucla.edu/dawn/newsletter/html/20030822/

Dawn's Early Light
Volume 2, Issue 2
August 2003

Dawn Status
Christopher T. Russell
Dawn Principal Investigator, UCLA

The Dawn mission is presently in the formulation phase, preparing to
demonstrate at its Preliminary Design Review (PDR) that it is ready to
proceed with the implementation phase. As part of the preparation for PDR,
design reviews of four of the five scientific instruments have been
completed, (Visible and Infrared Mapping Spectrometer - VIR; Gamma Ray and
Neutron Detector - GRaND; magnetometer, and laser altimeter). All
instruments passed their reviews with flying colors. The review of the fifth
instrument, the framing camera, will take place in September. Much of the
work on the camera will now take place at the Max Planck Institut fur
Aeronomie (MPAe) in Katlenburg-Lindau. In conjunction with the new division
of labor, Dawn welcomes two new members from MPAe to its science team, Uwe
Keller, who will lead the camera development and Uli Christensen, MPAe
Director, who will assist with the science planning and analysis.

This summer we also completed testing of the solar cells that will power the
Dawn spacecraft and its ion propulsion system. Mission success depends
greatly on the efficiency of these cells. When the spacecraft journeys away
from the Sun, the illumination drops and the arrays cool. The cooler arrays
are more efficient, compensating somewhat for the drop in illumination, but
the combined effects have not previously been well characterized. Since most
outer solar system missions have used radioactive thermal generators, the
data on this low intensity, low temperature effect (LILT) is sparse.
Fortunately, our LILT testing confirmed the efficiencies that were assumed
when Dawn was proposed to NASA.

The project opted to add a fifth solar panel to each wing to increase Dawn's
power margin at Ceres (at which time the spacecraft could reach 2.9 AU). The
project has also been asked to increase its financial reserve, necessitating
other mission trades and descopes. Since the mission is well along in its
design phase, there are limited options to do this. Our optimized solution
to meet this challenge is to launch on a standard Delta 2925, rather than a
heavy, using Mars for a gravity assist. This mission plan also provides much
needed calibration data for the instruments, and an early test of the
science operations and data analysis systems. There will be some delay in
arrival at Vesta but the very interesting southern polar region will still
be illuminated. After acquiring its Vesta observations, the spacecraft
departs for Ceres. This plan requires some rephasing of the budget profile
and has not yet been approved by NASA HQ, but we are hoping and expecting it
will, and that Dawn will move into the implementation phase late in the
calendar year.

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Ceres Evolution and Current State: A Summary

Tom McCord[1] and Christophe Sotin[2]

[1] Dawn Co-investigator, Univ. of Hawaii, Honolulu & Planetary Science
Institute, Winthrop, WA
[2] Laboratory de Planetologie et Geodynamique, Nantes, France

Ceres orbits the sun and is large enough to have experienced many of the
processes normally associated with planetary evolution. Therefore, it should
be called a planet. Ceres probably survived from the earliest stages of
solar system formation, when its sibling objects probably became the major
building blocks of the Earth and the other terrestrial inner planets. Thus,
Ceres is an extremely important object for understanding the early stages of
the solar system as well as basic planetary processes.

Ceres apparently retains considerable volatile material. The latest gross
properties indicate that Ceres has a density of about 2100 kilograms per
cubic meter, suggesting that the body's composition may be half water. Its
density is similar to that of Ganymede (1940 kg/m3) and Callisto (1860
kg/m3).

Observational evidence also points to a wet Ceres. Its reflectance spectrum
contains a 3-µm absorption interpreted to be due to OH and perhaps 2O in
aqueously altered material such as clays and hydrated salts similar to CI
and CM, i.e. primitive, carbonaceous chondrite (CC) meteorites. A 3.1-µm
absorption also exists that suggests water ice or, alternatively,
HN4-bearing minerals such as saponite reported in aqueous alteration
products in CV and CI carbonaceous chondrites. Further, International
Ultraviolet Explorer (IUE) spectroscopic observations of a 3080A emission at
the northern limb of Ceres suggests the OH molecule, indicating the
production of H2O from Ceres is in the range of 1024 to 1025 sec-1, which is
a flux that could be sustained over a long period from a subsurface ice
layer.

This inference suggests that Ceres may harbor active chemistry that produced
evolved materials, considering that it was heated, is still wet and likely
started with primitive materials rich in silicates, organics, water and
perhaps other volatile materials. Ceres, located between Mars and the
Galilean satellites, is the perfect place to study the evolution of volatile
rich objects at the interface between our hometown terrestrial planets and
colder but volatile-richer outer solar system objects.

We modeled the thermal evolution of Ceres to find out what Dawn might find
when it orbits this small planet. We built on the vast literature on the
state and evolution of solar nebula material and on meteorites and the
earlier thermal modeling work by Hap McSween (a Dawn team member), who
explored how the aqueous alteration seen in CC meteorites could occur in the
parent bodies early in their histories. In an article submitted to J.
Geophys. Res. we describe the probable active thermal evolution of Ceres and
its favorable environment for fertile wet chemistry, and the probability
that Dawn will find very interesting materials and landforms as evidence of
these processes.

Even with a low-energy model - cold accretions, no effect from 26Al or other
short-lived radionuclides - and using only energy from long-lived
radionuclides with abundance derived from study of carbonaceous chondrite
meteorites and a uniform mixture of 75% silicates and 25% water ice, we find
that the water ice in Ceres must have quickly melted and continued to
circulate, transporting heat by convection and preserving a nearly
isothermal mantel approximately 100 km thick. A crust does not melt because
it is too conductive. The circulating warm water would alter the silicates,
leading to carbonaceous chondrite-like compositions. As heat is lost by
conduction through the unmelted crust, water begins to freeze out at the
base of the crust [Figure 1]. When the crust reaches a thickness of about 28
km, solid-state convection in the ice-rich crust would become continuous,
transporting more heat as well as altered materials to near the surface.
Ceres' water layer eventually freezes after about 2 billion years, forming a
layered density structure. Additional possible sources of heat, including
short-lived radionuclides and exothermal mineralization, enhance the melting
of water ice, extend the lifetime of the liquid water mantel, and alter the
temperature profiles with depth and time, but they do not produce total
melting of the crust or of the silicate core. The frozen crust lid would
contain most of the volatiles and altered materials, except that which
reached the surface by convection, volcanism and impacts, and preserve
evidence of the differ-entiation and the chemistry, perhaps including
hydrates, hydroxolates and evolved organics. In addition, melting and
freezing plus mineralization would, over time, create topographic features.
Thus, present day compositional units and topography on Ceres' and its
internal structure, should be of considerable help in constraining Ceres'
history and thereby the evolution of the protoplanets in general.

Some specific and intriguing possibilities come to mind. First, NH4-bearing
compounds have been suggested to explain the 3.1-µm absorption feature of
the surface material. Such materials might have been incorporated at
formation. Significant presence would lower the melting point of the liquid
water sufficiently to extend the lifetime of the liquid layer even to the
present. Second, clathrates could have formed. Considerable carbonaceous
material was likely present at formation. Interior thermal processing and
mixing with the freezing water could create methane and CO2 clathrates. If
these materials were brought near the surface, they would volatilize and
could produce explosive release of gases, bringing considerable quantities
of altered materials to the surface, and creating surface expressions that
Dawn might detect.

Third, Ceres would have shrunk as water formed from ice. Then, as the water
froze, Ceres would have expanded, and present day features should be those
associated with tension, such as cracks, faults and dropped blocks.
Topographic features on the surface could be formed that will be visible to
Dawn. Finally, there is the possible evolution of the carbon compounds in
addition to formation of methane and CO2. Could more complex compounds have
formed?

Ceres appears to be a complicated object in some ways similar to the outer
Galilean satellites. With so much water present and the energy to distribute
it in liquid form, Ceres probably experienced complex chemistry at least in
its interior, perhaps including organic materials. It is further likely
that expressions of these processes and materials made it to the surface, at
least in places. Dawn may well discover and analyze these. Vesta, the other
object on Dawn's itinerary, is much denser, dryer and has extensively
melted. Pallas, being denser, also seems to have evolved further and lost
more water or it may have had less to start. A major unanswered question is
why such different objects as Ceres, Vesta and Pallas, all protoplanets,
could have evolved so differently yet so close together in the Solar System.
Dawn may give us some answers by visiting these two very different objects.

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A GRaND New Instrument
Bill Feldman and Tom Prettyman
Los Alamos National Laboratory (LANL)

The Los Alamos National Laboratory (LANL) is responsible for the Gamma Ray
and Neutron Detector (GRaND) for the Dawn mission. The GRaND was designed to
provide an accurate and robust measurement system that fits within the cost
and resource constraints of the mission, with ample margins and sufficient
heritage to maximize the science return during the nine-year mission to the
asteroid belt. Features of the GRaND instrument include provisions to
suppress and subtract spacecraft background and the ability to fully resolve
gamma ray peaks for most elements. The latter is provided by new technology,
CdZnTe, which will make its debut in planetary science on the Dawn mission.

Using a combination of gamma ray and neutron spectroscopy, the GRaND will
measure the abundance of major elements, including O, Si, Ti, Al, Fe, Ca,
and Mg. Knowledge of the composition of all major rock-forming elements is
needed to provide context for meteoritic data and to constrain models of
planetary structure and evolution. The gamma ray spectrometer will also
determine the abundance of radioactive elements, including K, U, and Th. The
ratio of the volatile element K to the refractory element U provides a
measure of the depletion of volatile elements in the source material from
which the asteroid was accreted. The neutron spectrometer along with the
gamma ray spectrometer will measure the abundance of H and its stratigraphy.
H can be in the form of water or hydrated minerals. Ceres, for example, in
addition to having a wet, clay veneer at mid-latitudes, may have polar caps
that consist of water ice.

A diagram of the GRaND is shown in Fig. 1. The instrument contains a 4x4
array of CdZnTe semiconductor radiation detectors which will serve as the
primary gamma ray spectrometer. The CdZnTe array is mounted on top of a BGO
crystal that acts as an anticoincidence shield to suppress gamma rays that
come from the spacecraft. The BGO sensor is also used to acquire gamma ray
spectra and will serve to augment the detection efficiency of the CdZnTe
array (in coincidence mode) and as a backup, stand-alone, spectrometer if
needed. The CdZnTe array has sufficient pulse height resolution and counting
efficiency to map all major elements and radioactive elements with improved
accuracy compared to the BGO-based spectrometer flown on Lunar Prospector.

Surrounding the BGO crystal and CdZnTe array are four boron-loaded plastic
scintillators (labeled BC454 in the figure). Note that structural materials
in Fig. 1 hide the zenith facing scintillator. The nadir (asteroid) facing
and zenith facing elements are primarily sensitive to fast and epithermal
neutrons because they are shielded by 6Li-loaded materials that absorb
thermal neutrons. The two side facing elements will respond to neutrons of
all energies and are primarily intended for cosmic ray suppression. The
plastic scintillators are arranged so that the flux of neutrons originating
from the spacecraft can be suppressed, separately measured, and subtracted
from the neutrons that originate in the asteroid. Light produced in the
scintillating components is measured by five photomultiplier tubes (shown in
Fig. 1), one for the BGO crystal and four for the BC454 scintillators.
Thermal neutrons from the asteroids will be detected by the 6Li-loaded glass
(GS20), which is optically coupled to the nadir facing plastic scintillator,
forming a phoswhich. Signals from the plastic and glass will be separated
and analyzed electronically using a time-domain filter.

The GRaND instrument on Dawn derives its heritage from the 2001 Mars Odyssey
neutron spectrometer and the Lunar Prospector gamma ray and neutron
spectrometers. Data acquired by the GRaND will be comparable in quality to
these previous missions. The neutron spectrometer will map the near surface
abundance of water-equivalent hydrogen on Ceres and Vesta (as illustrated in
Fig. 2 for Mars). The gamma ray spectrometer, in some cases combined with
the neutron spectrometer, will map the abundance of major- and radioactive
elements (as illustrated in Fig. 3 for the Moon). The GRaND instrument on
Dawn exploits the rich heritage of past successful investigations to answer
key questions concerning the divergent evolution of Vesta and Ceres. Its new
technology CdZnTe detectors also pave the way for a new generation of more
capable and compact instruments.
Received on Mon 25 Aug 2003 01:19:03 AM PDT