| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Near Infrared Mapping Spectrometer (NIMS) on the Galileo
spacecraft took unique data of Comet Shoemaker-Levy/9's impact with
Jupiter. A preliminary analysis of this data is presented in this
submission to the Planetary Data System (PDS). It consists of nine
small tables with detached labels and documentation.
Background
==========
The Galileo spacecraft, enroute to Jupiter, was situated 240 million
kilometers from Jupiter with a spacecraft-Jupiter-Sun phase angle of
51 degrees during the collision of Comet Shoemaker-Levy/9 with the
planet. This geometry allowed a direct view of the impacts, which
occurred on the nightside of Jupiter, not viewable from the Earth,
and provided an opportunity to investigate the early temporal
evolution of the impact events. Much of the radiation occurs in the
infrared region, and time-resolved infrared spectral observations,
obtained over a broad wavelength range, are ideal for studying these
phenomena.
The Galileo Near Infrared Mapping Spectrometer (NIMS) instrument
(CARLSONETAL1992, see REF.CAT) observed the C, F, G, and R events,
simultaneously with the Photopolarimeter (PPR) and Ultraviolet
Spectrometer (UVS) instruments. Only data for the G and R events
were telemetered to Earth.
In order to ensure successful observations of the impacts, given
uncertainties in the absolute spacecraft pointing, a 'checkerboard'
scan pattern was used, covering Jupiter and the immediate vicinity.
One dimension of scanning was provided by the NIMS mapping
capability, giving a 10 mrad column of 20 pixels. Each pixel is
acquired in 1/63 sec and is 0.5 mrad by 0.5 mrad in size (Jupiter's
diameter as seen from Galileo was 0.6 mrad). The spacecraft scan
platform provided the second dimension, scanning back and forth by 3
mrad at 0.92 mrad/sec and a period of 10 2/3 sec. Jupiter was in
the field-of-view for only a fraction of each scan, giving a net
time resolution of 5 1/3 seconds.
The instrument was operated in the 'Fixed Map' mode in which, for
each spatial pixel, 17 spectral bands are simultaneously monitored.
The wavelength setting was chosen to include continuum bands, where
the atmospheric gases are transparent, and bands with differing
absorption strengths so as to perform vertical sounding of the
fireball in the atmosphere. It also included a band for possible
H3+ emissions. For short wavelengths, the intense reflected
sunlight signal precludes ready identification of fireball emission
while Jovian thermal emission obscures the fireball signature in the
5 micron region. Between these limits, in the 1.8 to 4.4 micron
region, the reflected sunlight signal is weak and little atmospheric
thermal emission occurs. Consequently, we employ this region for
our preliminary analysis. The corresponding wavelengths and
atmospheric absorption properties are listed in the table below.
The spectral resolution for each wavelength channel is 0.025
microns.
Wavelengths and Jovian Atmospheric Absorption Properties
Det. No. Wavelength Wavenumber Absorber, Emitter
(microns) (cm-1)
======== ========== ========== ==========================
6 1.84 5430 Continuum
7 2.12 4710 Molecular hydrogen,
pressure induced
8 2.40 4160 Methane (stratosphere)
9 2.69 3720 Continuum
10 2.97 3370 Ammonia (troposphere)
11 3.25 3075 Methane (stratosphere)
12 3.53 2830 Methane (stratosphere),
H3+
13 3.82 2620 Methane (stratosphere)
14 4.10 2440 Continuum
15 4.38 2280 Phosphine (troposphere)
Some of the above has been abstracted from CARLSONETAL1995A.
Another useful reference is CARLSONETAL1995B. In addition, a
comprehensive paper on the G fireball is being submitted to Icarus,
and an analysis of the G and R splash spectra is currently
underway.
Tables in the Dataset
---------------------
The data provided here for the G and R events are of three types:
calibration data, raw and averaged data numbers, and processed data,
giving source intensities in physical units.
CAL_DATA.TAB: A file of calibration data and related information
useful for interpretation.
JREF_DNS.TAB: Reference spectra for the undisturbed full disc
Jupiter, in data numbers (DNs).
JREF_GAM.TAB: Reference spectra for Jupiter's morning hemisphere,
just prior to the G fireball event, in DNs.
JREF_RAM.TAB: Reference spectra for Jupiter's morning hemisphere,
just prior to the R fireball event, in DNs.
G_DATA.TAB: Raw DNs versus time for pixels containing the G impact
site.
R_DATA.TAB: Raw DNs versus time for pixels containing the R impact
site.
SI_G_1.TAB: Source intensities versus time for the G event using
chi squared minimization for the fireball period,
which finds the fraction (eta) of reflected light to
subtract to obtain the best fit of a blackbody
spectrum. A regression fit of eta to detector 1 was
developed from the fireball period and applied to the
pre- and post-fireball periods.
SI_G_2.TAB: Same as above, but the regression algorithm found in
the chi squared minimization was used for all data,
including the fireball period. This is to test the
sensitivity of the results to the analysis procedure.
SI_R_2.TAB: Same as above, but for the R event.
Details of the structure and contents of these tables may be found
in the detached label accompanying each table and in the
AAREADME.TXT file.
Processing
==========
The raw NIMS data received from the spacecraft were collected on
EDRs by the Image Processing System (MIPS) at the Jet Propulsion
Laboratory (JPL) and are archived on PDS CD-ROM volume GO_1004. The
tables in the submitted dataset were generated by Dr. Robert W.
Carlson of JPL, Principal Investigator of the NIMS experiment on the
Galileo Orbiter. The detached PDS labels were prepared by Bob
Mehlman of the NIMS team and UCLA, who also edited the documentation
supplied by Dr. Carlson into PDS format.
Data structure
==============
The table files follow PDS structure and labelling conventions. A
detached PDS label accompanies each file, and describes its
structure and contents using ASCII keyword=value statements.
Ancillary Data
==============
Calibration files derived from ground and flight calibration of the
NIMS instrument were used in generating the CAL_DATA.TAB file in
this dataset, but are not included with it. These calibration files
are continually being improved. The most recent calibration files
are available with the Integrated Software for Imaging Spectrometers
(ISIS) system.
Software
========
The nine tables in this dataset are small and in ASCII character
format. They are therefore printable and accessible with ordinary
editing software. They are also easily ingested by most database
systems.
Media/Format
============
This dataset is an electronic submission to PDS but may be included
on a future CD-ROM volume of NIMS data from the Jupiter Tour.
Formats are based on standards for table objects established by
PDS.
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