| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains the New Horizons Pluto Encounter geology and
geophysics science theme team derived mosaics, topographic and bond
albedo maps for Pluto and Charon.
Global Monochrome Mosaics of Pluto and Charon
=============================================
Detailed, high-quality global mosaics of Pluto and Pluto's largest moon
Charon, were assembled from nearly all of the highest-resolution images
obtained by the Long-Range Reconnaissance Imager (LORRI) and the
Multispectral Visible Imaging Camera (MVIC) on New Horizons.
The mosaics are the most detailed and comprehensive global view yet of
the surfaces of Pluto and Charon using New Horizons data. Standing out
on Charon is an enormous trough at least 350 kilometers long, and
reaching 14 kilometers deep - more than seven times as deep as the Grand
Canyon. The mosaics are available in Equirectangular projection at an
equatorial pixel scale of 300 meters per pixel.
Mosaic Processing Parameters
============================
The Pluto map was produced using a radius of 1188.3 kilometers,
and the Charon map was produced using a radius of 606 kilometers.
Geometric correction was necessary because the reconstructed SPICE
kernels still showed a slight mismatch between the SPICE position and
the actual location of the object. Correction was performed by bringing
images into the ISIS system [KESZTHELYIETAL2014] and associating
them with their reconstructed SPICE information. All LORRI images were
inspected and image to image control points were assigned with the ISIS
'qnet' program. Then the ISIS bundle adjustment program 'jigsaw' was
used on all of the LORRI images to adjust only the instrument pointing
parameter (but not the spacecraft position), and letting jigsaw solve
for the local radii of the given control points. The process generated a
new SPICE C kernel that describes the updated spacecraft pointing at the
time of each observation. A controlled LORRI mosaic was then created
from this control solution.
A similar approach was used with the MVIC images, but in this case qnet
was used while setting the LORRI mosaic as a 'Ground source' such that
the locations of features in the LORRI mosaic were treated as known
control points. The resultant control network that contained MVIC to
MVIC control points, as well as MVIC to LORRI mosaic ground control
points was then given two runs through jigsaw. The first run only
adjusted the spacecraft position. The second run was allowed to solve
for camera angles and their angular velocities as well as update
spacecraft position. This produced a pointing (CK) and spacecraft
location (SPK) solution for each MVIC image that allowed reprojection of
the individual MVIC color bands together to allow for registered color
mosaics.
Photometric correction was performed by using the equations in
[PETERSONETAL2007] in order to make an approximation from instrument DN
to I/F values. The 'photomet' program was also used with a lunar Lambert
photometric function to correct for the changing observation angles due
to planetary curvature within a scene, normalized to the approach phase
angle of fifteen degrees. See [SCHENKETAL2018] for additional details
on the photometric correction.
The 8-bit values of the mosaics are in units of relative brightness,
which approximate I/F but are not.
More information about mosaic creation can be found in [SCHENKETAL2018]
and [SCHENKETAL2017].
Topographic Maps (Digital Terrain Models - DTMs)
================================================
New Horizons 2015 flyby of the Pluto system has resulted in high-
resolution topographic maps of Pluto [SCHENKETAL2018] and
Charon [SCHENKETAL2017], the most distant objects so mapped. A variety
of individual DTMs over about 30% of each object were produced at
300-800 m/pixel ground scales and with stereo height accuracies from
100 to 1500 m.
To facilitate geologic investigation of these two bodies [STERNETAL2015]
[MOOREETAL2016], imaging strategies were designed to enhance
cartographic and topographic mapping products for Pluto and Charon.
Cartographic control was complicated by the the high-speed encounter and
imaging resolution was variable across both bodies. Selection of tie
points between the approach and encounter hemispheres required selection
of points at resolutions from 1 to 20 km/pixel. Nonetheless, redundant
imaging enhanced bundle adjustments and resulted in stable cartographic
solutions and global map products.
Topographic data for Pluto and Charon come from several sources. Bundle
adjustments allow for determination of local radii; stereo images allow
for direct DTM production; and limb observations reveal local relief
along linear traces. Stereo mapping was strictly limited to the
encounter hemispheres due to the rapidly decaying resolution around the
backside of each sphere. Parallax in the approach images was simply
insufficient to resolve topography on these bodies at these distances.
Shape-from-shading compliments the stereo with pixel-scale slope
measurement over areas of low Sun.
Stereo measurements based on the LORRI framing camera are stable and
provide stereo height accuracies as good as 100 m and post-spacings of
1 km.
Stereo measurements based on MVIC line-scanner images are equally as
good but are complicated by the method of image acquisition, resulting
in DTM rumpling in the direction of scan in the highest resolution
images. Limb profiles were also possible over narrow restricted parts of
the surface, and these extend topographic information to unseen areas.
More information about terrain model creation can be found in
[SCHENKETAL2017] and [SCHENKETAL2018].
The values are elevations in kilometers from the reference radius of
Pluto: 1188.3 km. So an elevation value in the NH_Pluto_DTM.img of 1
would be a radius of 1,189,300 m.
For Charon, the values are elevations in kilometers from the reference
radius of Charon: 606 km. So an elevation value in the
NH_Charon_DTM.img of 1 would be a radius of 607,000 m.
Bond Albedo Maps of Pluto and Charon
====================================
The exploration of the Pluto-Charon system by the New Horizons
spacecraft represents the first opportunity to understand the
distribution of albedo and other photometric properties of the surfaces
of objects in the Solar System's 'Third Zone' of distant ice-rich
bodies. Images of the entire illuminated surface of Pluto and Charon
obtained by the Long Range Reconnaissance Imager (LORRI) camera provide
a global map of Pluto that reveals surface albedo variegations larger
than any other Solar System world except for Saturn's moon Iapetus.
Normal reflectances on Pluto range from 0.12-1.0, and the low-albedo
areas of Pluto are darker than any region of Charon. Charon exhibits a
much blander surface with normal reflectances ranging from 0.20-0.53.
LORRI Observations used for the Bolometric Bond Albedo Maps
===========================================================
The full list of the LORRI images used in this derivation, along with
their integration times and their associated geometric information, can
be found in [BURATTIETAL2017]. These images represent the best spatial
resolution obtained for each geographical location within the week prior
to closest approach. For most of the data, Pluto and Charon appear on
the same image (It wasn't until three days before closest approach that
the binary pair exceeded the LORRI Field-of-View.) Pipeline calibration
procedures were employed to flatfield each image, remove blemishes, and
transform data numbers (DNs) into radiometric units using the flight
calibration current as of late February 2016. These procedures are
documented with the LORRI calibrated datasets.
Global Maps of Normal Reflectance
=================================
Since geologic analysis of images requires the knowledge of intrinsic
values of the albedo, changes due solely to viewing geometry must be
modeled and removed from the data. The images used in this study were
obtained at small solar phase angles (although still larger than any
observed from Earth); thus the corrections for solar phase angle effects
are not large. Photometric changes on a surface are due to two primary
factors: changes in the viewing geometry as the incident, emission, and
solar phase angle change, and the physical character of the surface.
This latter factor includes the anisotropy of scatterings in the
surface, which is expressed by the single particle phase function; the
compaction state of the surface, which leads to the well-known
opposition surge attributed to the rapid disappearance of mutual shadows
among regolith particles as the surface becomes fully illuminated to an
observer, and to coherent backscatter [HAPKE1981] [IRVINE1966] and to
macroscopic roughness, which both alters the local incident and emission
angles and removes radiation due to shadowing [HAPKE1981] [BURATTI1984].
Radiative transfer models have been developed that fully describe the
specific intensity returned from a planetary surface [HAPKE1981]
[BURATTI1984] [SHKURATOVETAL2005]. Empirical photometric models have
been developed that are more appropriate for the data set in hand:
observations at small solar phase angles (~10-15 degrees) leading up
the flyby.
Two widely used models are those of [MINNAERT1961], which is
essentially a first-order Fourier fit that describes the distribution of
intensity on a planetary surface, and a lunar-Lambert model that is the
superposition of a lunar, or Lommel-Seeliger law, describing singly
scattered radiation, and a Lambert law describing multiple scattered
photons [SQUYRESETAL1981].
Bond Albedo Map Construction
============================
A preliminary map of the Bolometric Bond albedo at LORRI wavelengths
can be constructed with a rudimentary phase curve and our normal albedo
maps. LORRI Images of Pluto and Charon for which the full disk is
included in the image exist for a small range of solar phase angles.
The images at large solar phase angles are contaminated by scattered
light or atmospheric contributions in the case of Pluto. In future
studies, synthetic integral values of Pluto's and Charon's solar phase
curves will be constructed from disk-resolved observations.
For these preliminary Bond albedo maps, we make use of the fact that
phase integrals of objects that scatter like Pluto and Charon have been
derived, and we use these values for this study. For Pluto we adopt the
phase integral of Triton of 1.16 derived from Voyager images obtained in
the green filter, which at 0.55 um is the closest in wavelength to LORRI
[HILLIERETAL1990]. For Charon, we use the lunar phase integral at 0.63
um of 0.60 [LANEETAL1973]. For this preliminary study, the assumption of
a lunar-like phase curve for Charon is reasonable and supported in
[BURATTIETAL2017].
The LORRI images in this study have been scaled to geometric albedos
determined from ground based observations. For Pluto, the value is 0.62
+/- 0.02 near the time of the New Horizons encounter for the R-filter
Table Mountain Observatory, which is centered at 0.62 um [BUIEETAL2010]
near the LORRI pivot wavelength of 0.607, while for Charon, it can be
computed from the New Horizons radius of 606 km [STERNETAL2015] combined
with the ground-based opposition magnitude of 17.10 [BUIEETAL2010],
transformed to the R-filter using the spectrum of Charon [FINKETAL1988]
[SAWYERETAL1987]. This method yields a geometric albedo at LORRI
wavelengths of 0.41 +/- 0.01. These maps were multiplied by
the phase integrals for Triton (in the case of Pluto) and the Moon (for
the case of Charon).
The preliminary Bond albedo of Pluto is 0.72 +/- 0.07 and that of Charon
is 0.25 +/- 0.03. The Bond albedo is the geometric albedo (p) times the
phase integral (q). The best determination of the geometric albedo is
from the ground, as it is based on observations at small solar phase
angles. [BURATTIETAL2015] gives the visible geometric albedo for Pluto
as 0.56 +/- 0.03, if an opposition surge is included. From the paper
based on New Horizons data [BURATTIETAL2017], we have a phase integral
for Pluto of 1.16 (this number has been confirmed by additional
unpublished data) for a Bond albedo of 0.65, in the visible.
For these maps, the 8-bit Data Numbers (DN, from LORRI data) can be
converted to albedo values using the equation albedo = DN/255.
The DNs are the 8-bit integers in the .img file. The data is read in
with readPDS with this DN/255 scaling factor already applied so that
the data goes from 0 to 1. The accuracy of the data is based on the
original 8-bit integers, not the scaled values, and is dimensionless.
Please note that the data along the edges may not be completely accurate
due to photometric processing artifacts. In addition, these maps were
built based on an early control solution of the spacecraft and the maps
may not line up exactly with the other DTM and mosaic maps included in
this dataset.
Version
=======
This is VERSION 1.0 of this data set.
Processing
==========
The data in this data set were created by a software data
processing pipeline on the Science Operations Center (SOC) at
the Southwest Research Institute (SwRI), Department of Space Operations.
This SOC pipeline assembled data as FITS files from raw telemetry
packets sent down by the spacecraft and populated the data labels
with housekeeping and engineering values, and computed geometry
parameters using SPICE kernels. The pipeline did not resample
the data. This data was then used by the Science Theme Teams to
generate derived data products as provided in this dataset.
Contact Information
===================
For any questions regarding the data format of the archive,
contact
New Horizons Principal Investigator:
Alan Stern, Southwest Research Institute
S. Alan Stern
Southwest Research Institute
Department of Space Studies
1050 Walnut Street, Suite 400
Boulder, CO 80302
USA
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