Mars Science Laboratory
(MSL)
Software
Interface Specification
SAM
Reduced Data Record (RDR)
Version 2.0
JPL D-38123
MSL SIS-SCI020-MSL
DATE: August 13,
2013
Prepared by:
Heather Franz, UMBC/GSFC
Copyright 2013. All rights
reserved.
Jet Propulsion Laboratory
California Institute of
Technology
Pasadena,
California
Signatures
Prepared
by:
Author/Custodian:
Heather
Franz, UMBC/GSFC Date
Approval:
Instrument Scientist:
Kimberly
Lichtenberg, JPL Date
OPGS CDE:
Maher
Hanna, JPL Date
SAM
Accommodation
Engineer Alicia Allbaugh, JPL Date
Concurrence:
PDS Geosciences Node:
Raymond
Arvidson, PDS Date
Principal Investigator:
Paul
Mahaffy, GSFC Date
CHANGE LOG
|
DATE
|
SECTIONS CHANGED
|
REASON FOR CHANGE
|
REVISION
|
|
4/3/12
|
1.2,
1.3, Table 2-8, 2.4.8, 3.2.2, 4.1, 4.2, Appx A
|
Minor
edits
|
S.
Slavney / PDS
|
|
4/3/12
|
Table
2.5
|
Suggested
data set ID revisions
|
S.
Slavney / PDS
|
|
4/3/12
|
2.3.2
|
Comment
about adding calibration and processing information
|
S.
Slavney / PDS
|
|
4/3/12
|
Appx A
|
Comment
about completing definitions for SAM-specific keywords to be added to the MSL
Data Dictionary
|
S.
Slavney / PDS
|
|
4/3/12
|
2.3.4,
Appx C
|
File
names must be case-insensitive
|
S.
Slavney / PDS
|
|
5/9/12
|
Appx
A, Appx B, Appx C
|
Updated
SAM-specific keywords in Appx A and sample label files in Appx B; added
missing values to Appx C
|
H.
Franz/UMBC/
GSFC
|
|
2/13/13
|
2.2; Appx
B; Appx C
|
Eliminated
L1C product from table 2-5 and Appx B; changed COMPXXX to ATMCOMP and EGACOMP
in the list of valid L2 file descriptors.
|
H.
Franz/UMBC/
GSFC
|
|
4/22/13
|
2.1,
2.3; 3.1; Appx B; Appx C
|
Minor
wording changes; eliminated Appx B and references to it within the text; renamed
Appx C to Appx B and modified references to Appx C accordingly.
|
H.
Franz/UMBC/
GSFC
|
|
8/13/2013
|
Tables
2.3, 2.8
|
Added
variants of noble gas and nitrogen enrichment experiment.
|
H.
Franz/UMBC/
GSFC
|
CONTENTS
CHANGE LOG............................................................................................................................................................................ iii
CONTENTS.................................................................................................................................................................................. iv
ACRONYMS AND ABBREVIATIONS.................................................................................................................................... v
1. INTRODUCTION...................................................................................................................................................................... 1
1.1 Purpose and Scope..................................................................................................................................................... 1
1.2 Contents.......................................................................................................................................................................... 1
1.3 Applicable Documents and Constraints.................................................................................................. 1
1.4 Relationships with Other Interfaces....................................................................................................... 2
2. Data Product Characteristics and Environment................................................................................ 3
2.1 Instrument Overview............................................................................................................................................ 3
2.2 Data Product Overview..................................................................................................................................... 10
2.3 Data Processing....................................................................................................................................................... 11
2.3.1 Data Processing Level............................................................................................................................................. 11
2.3.2 Data Product Generation....................................................................................................................................... 12
2.3.3 Data Flow.................................................................................................................................................................. 12
2.3.4 Labeling and Identification................................................................................................................................... 13
2.4 Standards Used in Generating Data Products.................................................................................. 17
2.4.5 PDS Standards.......................................................................................................................................................... 17
2.4.6 Time Standards......................................................................................................................................................... 17
2.4.7 Coordinate Systems................................................................................................................................................. 18
2.4.8 Data Storage Conventions..................................................................................................................................... 18
2.5 Data Validation....................................................................................................................................................... 19
3. Detailed Data Product Specifications....................................................................................................... 19
3.1 Label and Header Descriptions.................................................................................................................... 19
3.1.1 PDS Label.................................................................................................................................................................. 19
3.1.2 PDS TABLE Object................................................................................................................................................... 20
3.1.3 PDS SPREADSHEET Object.................................................................................................................................. 20
3.1.4 PDS TEXT Object..................................................................................................................................................... 20
3.1.5 PDS DOCUMENT Object........................................................................................................................................ 20
4. Applicable Software................................................................................................................................................ 20
4.1 Utility Programs.................................................................................................................................................... 20
4.2 Applicable PDS Software Tools.................................................................................................................... 21
4.3 Software Distribution and Update Procedures............................................................................... 21
5. References......................................................................................................................................................................... 21
Appendix A: PDS Keyword Definitions................................................................................................................ 22
Appendix B: SAM data descriptors for RDR product file names.................................................. 29
ACRONYMS AND ABBREVIATIONS
|
ASCII
|
American
Standard Code for Information Interchange
|
|
CODMAC
|
Committee
on Data Management and Computation
|
|
EDR
|
Experiment
Data Record
|
|
GC
|
Gas
chromatograph
|
|
GDS
|
Ground
Data System
|
|
IOT
|
Instrument
Operations Team
|
|
JPL
|
Jet
Propulsion Laboratory
|
|
MIPL
|
Multi-mission
image Processing Laboratory
|
|
MPCS
|
Mission
Data Processing and Control System
|
|
MSL
|
Mars
Science Laboratory
|
|
NASA
|
National
Aeronautics and Space Administration
|
|
OCM
|
Organic
check material
|
|
ODS
|
MSL’s
Operations Data Store
|
|
OPGS
|
Operations
Products Generation Sub-system
|
|
PDS
|
Planetary
Data System
|
|
QMS
|
Quadrupole
mass spectrometer
|
|
RDR
|
Reduced
Data Record
|
|
RTO
|
Real
Time Operations (MSL terminology)
|
|
SAM
|
Sample
Analysis at Mars
|
|
SIS
|
Software
Interface Specification
|
|
TBD
|
To
Be Determined
|
|
TCD
|
Thermal
Conductivity Detector
|
|
TLS
|
Tunable
laser spectrometer
|
The purpose of this Software
Interface Specification (SIS) is to provide suppliers and consumers of the Mars
Science Laboratory’s (MSL) Sample Analysis at Mars (SAM) instrument with a
detailed description of SAM’s Reduced Data Records (RDR).
The users for whom this SIS is intended are the scientists
who will analyze the data, including those associated with the MSL Project, SAM
instrument engineers, and other users in the general planetary science
community.
This SIS contains a very high level description of how SAM
works/operates. It will also describe, at high level, how the SAM data product
is acquired by the instrument, and how it is processed, formatted, labeled, and
uniquely identified on the ground.
The document discusses standards used in generating the
product and software tools that may be used to access the information. The data
product structure and organization are described in sufficient detail to enable
a user to read the product. Finally, an example of the product’s PDS label is
provided.
This Data Product SIS is responsive to the following MSL
documents:
1. Mars
Exploration Program Data Management Plan, R. E. Arvidson and S. Slavney, Rev. 4,
June 15, 2011.
2. Mars
Science Laboratory Project Archive Generation, Validation and Transfer Plan,
Joy Crisp, JPL D-35281, November 13, 2006.
3. Planetary
Data System Standards Reference, February 27, 2009, Version 3.8, JPL D-7669
Part 2.
4. Planetary
Archive Preparation Guide, April 1, 2010, Version 1.4, JPL D-31224, http://pds.nasa.gov/tools/archiving.shtml.
5. SAM Functional Description Document, September 24, 2010, Revision A, JPL
D-34225, MSL 375-1235.
6. SAM Science
Team and PDS Geosciences Node ICD, Version 3.0, April 5, 2011.
7. SAM
RDR Archive Volume Software Interface Specification, May 9, 2012, Version 1.0,
JPL D-75417, MSL SIS-SCI030-MSL.
This SIS is meant to be consistent with the contract
negotiated between the MSL Project and the Instrument Principal Investigator
(PI) in which reduced data records (RDR) and documentation (SIS) are explicitly
defined as deliverable products.
Changes to this SAM SIS document will affect the following
products, software, and/or documents.
Table 1-1.
Product and Software Interfaces to this SIS
|
Name
|
Type
P-product
S-software
D-document
|
Owner
|
|
Other SAM
Programs/Products/Documents
|
P/S/D
|
SAM Science Team
|
This section is included for convenience of PDS data users, but a
comprehensive description of the Sample Analysis at Mars (SAM) Suite
Investigation in the MSL Analytical Laboratory may be found in Mahaffy et al.
(2012). SAM is designed to address the present and past habitability of Mars by
exploring molecular and elemental chemistry relevant to life. SAM has the
capability to analyze volatiles thermally released from rocks and soils as well
as gases from the Martian atmosphere. A primary focus of the suite is the
detection and identification of organic compounds. However, SAM will also study
the chemical and isotopic state of elements besides carbon that are relevant to
life or Mars’ geological and environmental history. SAM's science goals are summarized
in Table 2-1.
Table
2-1. SAM Science Goals
|
Science and Measurement Goal
|
Habitability Question
|
|
(1)
Survey carbon compound sources and evaluate their possible mechanisms of
formation and destruction.
(2)
Search for organic compounds of biotic and prebiotic importance, including
methane.
|
What
does the inventory or lack of carbon compounds near the surface of Mars tell
us about its potential habitability?
|
|
|
(3)
Reveal the chemical and isotopic state of elements (i.e., N, H, O, S, and
others) that are important for life as we know it.
(4)
Determine atmospheric composition, including trace species that are evidence
of interactions between the atmosphere and soil.
|
What
are the chemical and isotopic states of the lighter elements in the solids
and in the atmosphere of Mars and what do they tell us about its potential
habitability?
|
|
|
(5)
Better constrain models of atmospheric and climatic evolution through
measurements of noble gas and light element isotopes.
|
Were
past habitability conditions different from today’s?
|
|
|
|
|
|
|
SAM is a suite of three instruments: a Quadrupole Mass
Spectrometer (QMS), a Gas Chromatograph (GC), and a Tunable Laser Spectrometer
(TLS). The QMS and the GC can operate together in a GCMS mode for separation
(GC) and definitive identification (QMS) of organic compounds. The TLS obtains
precise isotope ratios for C and O in carbon dioxide, H and O in water, and also
measures trace levels of methane and its carbon isotope ratio.
The three SAM instruments are supported by a sample manipulation
system (SMS) and a Chemical Separation and Processing Laboratory (CSPL) that
includes high-conductance and micro-valves, gas manifolds with heaters and
temperature monitors, chemical and mechanical pumps, carrier gas reservoirs and
regulators, pressure monitors, pyrolysis ovens, and chemical scrubbers and
getters. The Martian atmosphere is sampled by CSPL valve and pump manipulations
that introduce an appropriate amount of gas through an inlet tube to the SAM
instruments. Solid phase samples are analyzed by transporting finely sieved
materials to one of 74 SMS sample cups that can then be inserted into a SAM
oven and thermally processed to release volatiles. The SAM mechanical
configuration and a top-level schematic of its sample flow configuration are
illustrated in Figures 2-1 and 2-2. SAM instrument components are summarized in
Table 2-2.
Figure
2-1. The illustration of the mechanical configuration of SAM shows the
three instruments and several elements of the Chemical Separation and
Processing Laboratory.
Figure
2-2. The path of solid and gas samples delivered by MSL subsystems to
the SAM instruments is shown. Arrows designate the direction of gas and solid
transport.
Table 2-2. SAM Instrument
Characteristics
|
Quadrupole
Mass Spectrometer
|
|
Tunable
Laser Spectrometer
|
|
Summary: QMS analyzes
the atmosphere, gases thermally evolved from solid phase samples to sub ppb
sensitivity. QMS is the primary detector for the GC and can operate in static
or dynamic mode.
|
Summary: Two-channel
Herriott cell design spectrometer that provides high sensitivity, unambiguous
detection of targeted species (CH4, H2O, and CO2)
and selected isotope ratios. One channel is at a wavelength of 3.27 mm for CH4, and the
second is at 2.78 mm for CO2
and H2O.
|
|
Mass
Range
|
2-535
Dalton (Da)
|
Sensitivity
|
Methane:
2 ppb direct
Water:
2 ppm direct
CSPL
enrichment improves by ~100x.
|
|
Detector
dynamic range
|
>
1010 with pulse counting and Faraday Cup
|
Isotopes
|
13C/12C
in methane and CO2
18O/17O/
16O in CO2 and water
D/H
in methane and water
|
|
Crosstalk
|
<
106 adjacent unit mass channels (below 150 Da)
|
Isotope
precision
|
Typically
< 10 ‰
|
|
|
|
Gas
Chromatograph
|
|
Summary: GC separates
complex mixtures of organic compounds into molecular components for QMS and
GC stand-alone analysis. Helium carrier gas is utilized.
|
|
Injection
|
Injection
from traps into the SAM manifold or from 3 GC injection traps incorporated
into the GC subsystem.
|
|
6
GC Columns
|
GC1:
w/o trap; w/o TCD; MXT20 (C5-C15 organics)
GC2:
w/o trap; TCD; MXT5 (> C15 organics)
GC3:
trap; TCD; Carbobond (permanent gases)
GC4:
trap; TCD; ChirasilDex (chiral compound separation)
GC5:
trap; TCD; MXT CLP (C5-C15 organics)
GC6:
trap; TCD; MXTQ (C1-C4 organics, N/S compounds)
|
|
Detection
Limit
|
10-11
mole
|
|
|
|
|
|
|
SAM
is designed to deliver nine data set types that are acquired via the experiment
sequences described in Table 2-3. These experiment sequences may utilize
different elements of the SAM suite. In addition to these science sequences,
the SAM vacuum elements will be cleaned as necessary during the course of the
mission by in situ bakeout.
Pyrolysis
is the primary method for detection of organic molecules and examination of
mineral content. This approach samples the gas thermally evolved from a small
aliquot of sample delivered from the MSL sample acquisition/sample preparation and
handling unit (SA/SPaH) to one of 59 quartz cups of the SAM sample manipulation
system. Each quartz cup can accommodate up to 0.5 cm3 of sample and
the incremental volume of sample delivered from the SA/SPaH is approximately
0.05 cm3. This allows delivery of a specified volume of sample to a cup
and possible reuse of cups in an extended mission by deposition of fresh sample
on the devolatilized residue in a cup. For direct analysis by the QMS or TLS,
the sample in the quartz cup is heated from ambient to ~1000 °C
with a programmable temperature ramp, nominally 35 °C/min.
As gases are released, they are swept through the gas manifold by a helium
carrier gas for detection by the spectrometers. Typically, water of hydration
is released from samples early in the temperature ramp. Moderately volatile
organics are released in the 300 °C to 600 °C
range
Table 2-3. SAM Experiment
Sequences
|
Solid Sample Analysis Sequences
|
|
S-PYR
Pyrolysis
with GCMS
(seq.
#1)
|
Measurement: Chemical and
isotopic analysis of gases evolved from samples as a function of temperature
(EGA) and GCMS analysis of organics thermally released from samples.
Experiment Sequence: Quartz cell
cleaned in pyrolysis oven; Sample delivered to cooled cup; Sample heated from
ambient to ~1000 °C in helium gas
stream while evolved gases are monitored by QMS and TLS; GCMS analysis
initiated using gases trapped during gas processing in previous step with
detection by QMS and GC thermal conductivity detectors.
Notes: 59 quartz cups
allow this sequence to be repeated many times over the course of the landed
mission. Cups can typically be reused several times.
|
|
S-DER
Derivatization
(seq. #2)
|
Measurement: Analysis of
chemically derivatized polar compounds such as amino acids and carboxylic
acids that would not otherwise be detected by GCMS.
Experiment Sequence: Foil on metal
cup in SMS punctured; Sample delivered to one-cup solvent
extraction/derivatization cell; Sample cup moved to oven for thermal
processing; Venting of solvent; Thermal injection of derivatized compounds
into SAM GC traps; GCMS.
Notes: 9
derivatization cups in SMS.
|
|
S-CMB
Combustion
(seq. #8)
|
Measurement: Analysis of
carbon isotopes in carbon dioxide produced by combustion in oxygen.
Experiment Sequence: Quartz cell
cleaned in pyrolysis oven; Sample delivered to cooled cup; Sample combusted
using oxygen gas; Carbon dioxide produced analyzed by TLS for 13C/12C
isotopic composition.
|
|
Atmospheric Analysis Sequences
|
|
A-DIR
Direct
Atmospheric
Measurement
(seq.
#4)
|
Measurement: QMS and TLS
analysis of atmospheric chemical and isotopic composition and seasonal and
diurnal variations in trace species abundance.
Experiment Sequence: Atmospheric
gas directed into SAM gas manifold; Analysis by QMS and TLS scans.
Notes: Duration and
number of day time operation may be limited by thermal and energy
considerations.
|
|
A-ENR
Atmospheric Enrichment
(seq. #5)
|
Measurement: QMS, TLS, and
GCMS analysis of atmospheric trace species.
Experiment Sequence: Atmospheric
gas directed over SAM gas traps for enrichment of trace species; Gases
released from traps and analyzed by GCMS for trace species.
Notes: Direct TLS and
QMS measurement also possible during this sequence.
|
|
A-MET
Methane
Enrichment
(seq. #6)
|
Measurement: TLS methane
analysis with SAM CSPL methane enrichment.
Experiment Sequence: Atmospheric
gas directed over SAM gas scrubbers and cold traps; Methane released into TLS
for isotopic and abundance determination.
Notes: Enrichment
sequence may be repeated as necessary for improved methane detection
sensitivity.
|
|
A-NGE
Noble Gas
Enrichment
(seq. #7)
|
Measurement: Noble gas and
nitrogen analysis with SAM CSPL noble gas enrichment.
Experiment Sequence: Atmospheric
gas directed over SAM gas scrubbers; Noble gases and nitrogen analyzed in SAM
QMS operated in static mode.
Notes: Enrichment
sequence may be repeated as necessary for improved noble gas and nitrogen detection
sensitivity and precision in isotope measurements.
|
|
A-DNG
Dynamic
Noble Gas
Enrichment
(seq. #7a)
|
Measurement: Noble gas and
nitrogen analysis with SAM CSPL noble gas enrichment.
Experiment Sequence: Atmospheric
gas directed over SAM gas scrubbers; Noble gases and nitrogen analyzed in SAM
QMS operated in dynamic mode.
Notes: Variant of
A-NG. Enrichment sequence may be repeated as necessary for improved noble gas
and nitrogen detection sensitivity and precision in isotope measurements.
|
|
A-SNG
Semi-static
Noble Gas
Enrichment
(seq. #7b)
|
Measurement: Noble gas and
nitrogen analysis with SAM CSPL noble gas enrichment.
Experiment Sequence: Atmospheric
gas directed over SAM gas scrubbers; Noble gases and nitrogen analyzed in SAM
QMS operated in semi-static mode, in which high-conductance valve is
partially closed.
Notes: Variant of
A-NG. Enrichment sequence may be repeated as necessary for improved noble gas
and nitrogen detection sensitivity and precision in isotope measurements.
|
|
Calibration Sequences
|
|
CAL-GAS
In situ Gas
Calibration
(seq. #9)
|
Measurement: Calibration
gas sampled by QMS, TLS, and GCMS to check instrument performance and changes
with time.
Experiment Sequence: Gas from SAM
calibration gas cell released into manifold; QMS and TLS scans; Fluorocarbons
trapped on SAM trap; GCMS analysis of fluorocarbons.
Notes: Calibration
sequence repeated on approximately a monthly basis.
|
|
CAL-SOL
Solid Sample
in situ
Calibration
(seq. #3)
|
Measurement: Identical to
S-PYR sequence but using internal calibration standard consisting of
carbonate and/or fluorinated carbon compounds.
Experiment Sequence: Metal cup
containing sample opened by SMS foil puncture operation; Sample heated from
ambient to ~850 °C in helium gas
stream while evolved gases are monitored by QMS and TLS; GCMS analysis
initiated to detect fluorocarbons thermally released from cup and trapped
during gas processing in previous step; GCMS analysis.
Notes: 6 solid sample
calibration cups will be used during the landed mission to check instrument
performance.
|
due
to thermal desorption and the breakdown of macromolecular organic matter. At
higher temperatures, gases evolve due to the further pyrolysis of refractory
organics and the breakdown of minerals. For example, carbonates and sulfates
thermally dissociate to release CO2 and SO2,
respectively, at temperatures above 500 °C. The temperature at which
minerals degrade is often diagnostic of the mineral type.
In
addition to direct analysis of the evolved gases, SAM has the option to direct
the gas flow over a high-surface-area adsorbent to trap organic molecules, thus
separating organic from inorganic volatiles for subsequent analysis by GCMS.
Passing the trapped and released organic volatiles through one of six GC
columns effectively separates different molecules, allowing for individual
detection by the QMS. As a consequence of pyrolysis, the complex refractory
organic molecules embedded in a mineral matrix or thermally unstable species
may break down during thermal processing to produce lower molecular weight or
more stable pyrolysis products. Thus, information on the parent organic
molecules must be inferred from the patterns of stable products evolved with
temperature. For example, pyrolysis of microbial material typically evolved
amines, but the more fragile amino acids are destroyed (Glavin et al., 2006).
The detection limit of the SAM GCMS is ~10-14 to 10-13
mole, depending on the compound and instrument background. The mass range of
the QMS is 2-535 Da to sample a wide range of organic compounds.
The
second tool used by SAM to understand the state of carbon in Martian rocks and
fines is combustion. Reduced carbon in samples delivered to the SAM sample cups
may be oxidized by stepped combustion using isotopically pure 16O2,
with the 13C/12C ratio of the CO2 product
determined by QMS and TLS analysis. The 13C/12C ratio in
organic matter is used as a biomarker on Earth because organisms prefer the lighter
12C isotope and typically incorporate 2-4% more 12C into
their cells than is present in the CO2 carbon source. The utility of
these measurements will depend on the ability of SAM to reveal the isotopic
composition of the most important reservoirs of inorganic carbon for comparison
with organic carbon. Two such reservoirs are the atmosphere itself and carbonates
that may be found in rocks or sedimentary materials sampled by MSL. The
combined evolved gas and atmospheric sampling should enable this comparison.
In
addition to the dry pyrolysis experiments, nine SAM sample cups are dedicated
to simple single-step solvent extraction and/or chemical derivatization processes.
Resource constraints on MSL preclude a more ambitious fluid extraction analysis
approach. Depending on the chemistries encountered in Martian rocks and soils,
this technique may enable analysis of several classes of molecules that could
be of biotic or prebiotic relevance, including amino acids, amines, carboxylic
acids, and nucelobases. Without derivatization, these compounds would not elute
from the GC columns under SAM protocols.
Seven
of the SAM wet-chemistry cups use dimethylformamide (DMF) to extract organics
from the powdered sample delivered to the cup, while the derivatization agent
is a silylation reaction using
N,N-Methyl-tert-butyl(dimethylsilyl)trifluoroacetamide (MTBSTFA). The selected
volumes of derivatization agent and solvent were mixed together during SAM
integration and then hard-sealed into a metal cup. The top cup consists of an
electron-beam welded foil that can be punctured on the surface of Mars using
the vertical motion of the sample cup into a foil puncture station. The cup is
then placed under the SAM sample inlet tube so that these fluids can be mixed
with the Martian powdered sample. The cup is next delivered to the pyrolysis
oven, where the desired reaction temperature (~80 °C)
is set. The hard seal into the pyrolysis chamber ensures that vapor does not
escape during the derivatization reaction. After several tens of minutes, much
of the solvent is evaporated to the atmosphere through a microvalve and a
heated vent, and the chemical products of the derivatization reaction are
flash-heated into the injection trap of the gas chromatograph.
The
SAM team has exposed the selected solvents and derivatization agents to more
radiation than is expected over the course of the mission to ensure that the
radiation-induced chemistry is negligible over the course of the nominal MSL
mission. The volume of the SAM wet cells enables a substantial excess of
derivatization agent to be supplied, to mitigate the impact of side reactions
that could compromise the detection of amino acids and carboxylic acids. An
internal standard (a fluorinated amino acid compound in a separately punctured
dry chamber) will allow evaluation of whether undesired side reactions have
fully or partially prevented reaction with organics of interest. See Buch et
al. (2006) for further details on the SAM “one-pot” derivatization protocol.
The
remaining two SAM wet-chemistry cups are dedicated to thermochemolysis
experiments with 25% tetramethyl-ammonium hydroxide (TMAH) in methanol.
Thermochemolysis involves concurrent mild thermal bond cleavage and selective
base-catalyzed cleavage (hydrolysis) of ester, amide, and ether bonds in
high-molecular-weight, complex organics. The free lower molecular-weight
carboxylic acid, alcohol, and amine hydrolysis products are subsequently
methylated to form highly volatile products amenable to GCMS analysis. During
tests of the SAM protocol with Mars analog materials, TMAH thermochemolysis was
successful in yielding molecules from samples with challenging mineral matrices
or chemistries, including those with oxides, hydroxides, salts, and water.
Carboxylates, organic acids, and PAHs are readily detectable with
thermochemolysis, although halogenated, alkylated, and other complex free
molecules often produce multiple products with this technique. See Eigenbrode
et al. (2011) for more details about the SAM thermochemolysis experiment.
An
Organic Check Material (OCM) has been developed for SAM as a sample standard
for verification of organic cleanliness and characterization of potential
sample alteration by the sample acquisition and portioning process of the MSL
rover. OCM samples will be acquired using the same procedures for drilling,
portioning, and delivery that are used to study Martian samples during surface
operations. Because the SAM suite is highly sensitive to organic molecules, the
mission can better verify the cleanliness of sample acquisition hardware if a
known material can be processed through SAM and compared with results obtained
from Martian samples. The OCM is comprised of amorphous SiO2 glass
doped with fluorinated hydrocarbons (3-fluorophenanthrene and 1-fluoronaphthalene)
to ensure that the lack of other detectable molecules is indeed due to their
absence, rather than failure of the sample delivery process.
The
MSL rover carries five OCM samples, each one a cylindrical brick individually
encapsulated in a hermetically sealed can. The five cans are accommodated on the
front of the rover and the OCM sample will be acquired by puncturing the top of
a can and drilling a brick in place, just as a natural Martian rock is to be
sampled. Since the OCM is an expendable resource, the decision to sample an OCM
brick will be made by consensus of the MSL project’s science operations working
group. Possible scenarios for use could include analyzing the OCM immediately
after observing any organics in a Martian sample. Perhaps an OCM sample may be
acquired if no organics are observed to test whether reduced carbon phases are being
destroyed by the drilling, portioning, and delivery process. An OCM sample
could be characterized very early in the mission to determine if the EDL processes
have contaminated the sampling hardware. See Conrad et al. (2011) for more
information about the SAM OCM.
SAM
will bring a mixture of several calibration gases to Mars to be used at regular
intervals during the mission. The calibration gas cell includes N2,
CO2, Ar, and a Xe mix heavily spiked with 129Xe to
clearly distinguish it from terrestrial or Martian xenon. This cell includes
the same fluorocarbon compounds that are incorporated into the OCM bricks, to
evaluate the performance of the GCMS before launch and over the duration of the
mission.
The
SAM Tunable Laser Spectrometer (TLS) offers a direct, non-invasive technique to
produce sub ppb sensitivities for gas detection and isotope ratio measurements
for samples of small mass and volume, without the mass interferences seen by
the QMS. The TLS is a high resolution (0.0005 cm-1) IR spectrometer
that includes two semiconductor laser sources: a NIR tunable laser at 2.78 mm
from Nanoplus, Germany, that operates at room temperature with a single stage
thermoelectric cooler (TEC) and accesses lines of CO2, water, and
their isotopic forms; and a JPL-built interband cascade IC laser at 3.27 mm
that operates at 245 K with a two-stage TEC for methane and its isotopic forms 13CH4
and 12CH3D. Detectors are high-sensitivity HgCdZnTe
photovoltaics with immersion linses provided by Vigo Systems S.A. of Poland.
The TLS optical head has 3 chambers, the smallest containing the science detectors
and their field lenses at the far end. At the near end is a foreoptics chamber
that houses the lasers and their collimators, detectors, reference gas cells,
beamsplitters that sit outside the main chamber, a multipass gas cell ~5 cm
internal diameter and ~20 cm long. The cell volume is ~405 cm3. This
“Herriott” cell contains two internal spherical mirrors with drilled holes for
laser entrance and exit, and is fitted with 3 high vacuum valves that connect
to SAM. The cell is fitted with two heaters for fringe washing, its own
pressure gauge, and temperature sensors. The TLS Herriott cell is designed to
provide 81 passes for methane measurements and 43 passes for carbon dioxide and
water measurements.
Spectral
regions for the TLS were carefully selected for high sensitivity in detection
but insensitivity to temperature in line pairs chosen for isotope ratio
measurements. TLS employs second harmonic (2f) detection techniques that
enhance its sensitivity since the 2f signal is zero-based and the phase sensitive
detection regime is moved to higher frequencies (~ 10 kHz) where 1/f noise is
lower.
Each
channel of the TLS is run by its own microcontroller (8051 CPU) that returns
housekeeping data to the SAM CDH computer (including temperatures and cell pressure,
fore-optics pressure) and six spectra: 3 from the measurement Herriott cell
(direct absorption, 2f low-gain, 2f high-gain) and 3 from the reference channel
that contains calibration gases within tiny optical reference cells housed in
the fore-optics and sampled by the same laser simultaneously using a
beam-splitter. TLS was calibrated during thermal-vacuum testing using
NIST-traceable standard calibration gases over a wide range of operating
temperatures and pressures. In addition, TLS carries a reference gas cell for
each channel with an isotopic standard (checked with IRMS) for in-flight
reference to improve isotope ratio precision and accuracy.
Table
2-4 gives the expected measurement capabilities of the SAM TLS with a 15-min
integration time, based on actual results from thermal-vacuum testing of the
flight instrument. For additional details about the TLS, see Webster and
Mahaffy (2011).
Table 2-4. Expected TLS
Measurement Capabilities
|
TLS Channel
|
Wavelength
|
Scan Name
|
Expected
Capability in 15-min integration time
|
|
1 – IC laser
|
3.3 mm
|
Methane
|
CH4
to 0.3 ppbv (pptv in SAM)
d13C to 2‰
|
|
2 – Near-IR
|
2.785 mm
|
Carbon dioxide
|
CO2
to 0.2 ppmv
H2O
to 0.2 ppmv
d13C to 2‰
d18O to 3‰
d17O to 5‰
|
|
Water
|
H2O
to 0.1 ppmv
dD to 2‰
d18O to 3‰
d17O to 5‰
|
Each RDR will have unique PDS detached label files providing
all the meta and ancillary data for that RDR. The SAM data files for NASA
processing levels 0 through 1B, consisting of variable-width comma-separated-value
(CSV) tables and fixed-width CSV tables, will have file extensions of “.CSV”
and “.TAB,” respectively. The SAM level 2 product will consist of a report file
in PDF format that describes the findings of the SAM science team. All
associated label files will have file extensions of “.LBL”. The data or report
file(s) together with the associated label file(s) are collectively referred to
as an RDR. Each RDR product will contain data for a single SAM experiment
sequence.
Table 2-5 summarizes SAM standard Reduced Data Record (RDR)
data products.
Table 2-5. SAM
Standard RDR Data Products
|
Product Name
|
NASA Level
|
Description
|
PDS Data Set
ID
|
|
SAM L0
|
0
|
Unpacking of telemetry into data numbers as raw ADC
values or counts and verification of data integrity
|
MSL-M-SAM-2-RDR-L0-V1.0
|
|
SAM L1A
|
1A
|
Conversion of raw ADC values or counts to science
units
|
MSL-M-SAM-3-RDR-L1A-V1.0
|
|
SAM L1B
|
1B
|
Application of corrections to data, e.g., detector
dead time, TCD temperature, noise removal, corrections for saturation and
instrument response function. Instrument-specific data may include the
following types of information:
QMS: time, m/z, signal; oven temperature for EGA
runs; GC column temperature for GCMS runs
GC: TCD signal vs. retention time; pressure;
temperature; column used
TLS: direct and harmonic spectra
|
MSL-M-SAM-4-RDR-L1B-V1.0
|
|
SAM L2
|
2
|
Results of data interpretation completed by the SAM
science team. Instrument-specific data may include the following types of
information:
QMS: gas composition; isotope ratios; gas
composition vs. sample temperature for EGA runs
GC/GCMS: species; relative abundance; identification
of pyrolysis products and derivatized compounds
TLS: abundance and isotope ratios
|
MSL-M-SAM-5-RDR-L2-V1.0
|
All SAM RDR products are considered reduced data products
as defined by both NASA and “Committee on Data Management and Computation (CODMAC).
See Table 2-6 for more details.
SAM RDR data products will be generated by the SAM team
led by principal investigator Mahaffy at NASA GSFC, co-investigator Cabane at
the University of Paris, and co-investigator Webster at JPL. SAM instrument
calibration is described in detail in Mahaffy et al. (2012).
Table 2-6.
Processing Levels for Science Data Sets
|
NASA
|
CODMAC
|
Description
|
|
|
Packet data
|
Raw - Level 1
|
Telemetry
data stream as received at the ground station, with science and engineering
data embedded.
|
|
Level-0
|
Edited - Level 2
|
Instrument
science data (e.g., raw voltages, counts) at full resolution, time ordered,
with duplicates and transmission errors removed.
|
|
Level 1A
|
Calibrated - Level 3
|
Level
0 data that have been located in space and may have been transformed (e.g.,
calibrated, rearranged) in a reversible manner and packaged with needed
ancillary and auxiliary data (e.g., radiances with the calibration equations
applied).
|
|
Level 1B
|
Resampled - Level 4
|
Irreversibly
transformed (e.g., resampled, remapped, calibrated) values of the instrument
measurements (e.g., radiances, magnetic field strength).
|
|
Level 1C
|
Derived - Level 5
|
Level
1A or 1B data that have been resampled and mapped onto uniform space-time
grids. The data are calibrated (i.e., radiometrically corrected) and may have
additional corrections applied (e.g., terrain correction).
|
|
Level 2
|
Derived - Level 5
|
Geophysical
parameters, generally derived from Level 1 data, and located in space and
time commensurate with instrument location, pointing, and sampling.
|
|
Level 3
|
Derived - Level 5
|
Geophysical
parameters mapped onto uniform space-time grids.
|
|
|
|
|
|
|
|
Raw data will be delivered to the SAM team as Experiment
Data Record (EDR) products generated by the Multi-mission Image Processing
Laboratory (MIPL) at JPL under the Operations Products Generation Subsystem
(OPGS), using the telemetry processing software MSLEdrGen. Products are created
by MIPL during operations from: a) MPCS data products, b) SPICE kernels, and c)
a meta-data database. They are created on the MSL Operations Data Store (ODS)
and then deposited into MIPL’s File Exchange Interface (FEI) for electronic
distribution to remote sites/users via a secure subscription protocol. The EDR
product is considered NASA packet-level data. SAM EDR data products will be
stored at the PDS for safekeeping, but will not be formally archived in PDS.
The SAM team will convert raw data to a more manageable format before delivery
to the PDS for archiving as the lowest-level RDR product, which is considered
to be NASA level 0 data. Higher-level RDR products will represent the results
of further data manipulation and interpretation.
A file-naming scheme has been devised for SAM RDR data
products, based on the convention used for other MSL products. The file-naming
scheme adheres to the Level II 36.3 filename convention to be compliant with
PDS standards. It also contains a minimal level of meta-data that retains
uniqueness and is searchable.
All MSL data products can be uniquely identified by
incorporating key information into the product filename, including the
instrument identifier and a version number. Additional fields within the file
names have been modified for SAM RDR products to be more suited to the type of
data produced by SAM, which differs from that of other MSL instruments.
Each SAM RDR has a detached PDS label associated with the
SAM data file. The file-naming scheme for the data products is formed by:
Table 2-7. File naming template
|
1-2
|
3-7
|
8
|
9-12
|
13-15
|
16-17
|
18
|
19-22
|
23
|
24-26
|
27
|
28-34
|
35
|
36
|
37
|
38-40
|
|
instr (2 char)
|
SAM exp ID
(5 alphanumeric)
|
venue/source (1 char)
|
sol (4 alphanumeric)
|
prod (3 char)
|
level (2 alphanumeric)
|
_
|
exp seq (4 char)
|
_
|
type (3 char)
|
_
|
descr (7 alphanumeric)
|
_
|
ver (1 alphanumeric)
|
·
|
ext (3 char)
|
Table 2-8. Detailed description of product name
components
|
Name
|
Description
|
|
instr
|
(2 characters) MSL science
instrument identifier
Valid value: SAM – “SM”
|
|
SAM exp ID
|
(5 alphanumeric) SAM
experiment ID.
|
|
venue/
source
|
(1 character) Venue and source
(Flight Model vs Testbed) identifier
Valid values are:
|
|
Venue/source
|
Value
|
|
Flight model, pre-launch
calibration
|
“C”
|
|
Flight model, ATLO
|
“A”
|
|
Flight model, OPS
|
“F”
|
|
Testbed
|
“T”
|
|
sol
|
(4
alphanumeric) Sol, solar days since first full day on Mars, when data
transferred from SAM to rover. Landing day is Sol 0. For pre-launch tests,
this field is filled with ASCII lowercase x.
The valid values, in their
progression, are as follows (non-Hex):
Range 0000 thru 9999 - “0000”,
“0001”, … “9999”
Range 10000 thru 10999 - “A000”,
“A001”, … “A999”
Range 11000 thru 11999 - “B000”,
“B001”, … “B999”
·
·
Range 35000 thru 35999 - “Z000”,
“Z001”, … “Z999”
|
|
prod
|
(3
characters) Product identifier.
Valid
value: SAM RDR - “RDR”
|
|
level
|
(2
alphanumeric) NASA data processing level.
Valid
values:
level 0 – “0_”
level 1A – “1A”
level 1B – “1B”
level 1C – “1C”
level 2 – “2_”
|
|
_
|
ASCII
underscore.
|
|
exp seq
|
(4 characters) SAM experiment
sequence.
Valid
values:
|
|
SAM Experiment Sequence
|
Description
|
4-character value
|
|
#1
|
S-PYR, Solid sample pyrolysis
with GCMS
|
SPYR
|
|
#2
|
S-DER, Solid sample
derivatization
|
SDER
|
|
#3
|
CAL-SOL, Solid sample
calibration
|
CSOL
|
|
#4
|
A-DIR, Direct atmospheric
measurement
|
ADIR
|
|
#5
|
A-ENR, Atmospheric enrichment
|
AENR
|
|
#6
|
A-MET, Atmospheric methane
enrichment
|
AMET
|
|
#7
|
A-NGE, Atmospheric noble gas
enrichment
A-DNG, Dynamic noble gas
enrichment
A-SNG, Semi-static noble gas
enrichment
|
ANGE
ADNG
ASNG
|
|
#8
|
S-CMB, Solid sample combustion
|
SCMB
|
|
#9
|
CAL-GAS, Gas calibration
|
CGAS
|
|
_
|
ASCII
underscore.
|
|
type
|
(3
char)
Valid
values:
|
|
“MSG”
|
Message
log
|
|
“HK_”
|
Housekeeping
data, levels 0 and 1A only
|
|
“QMS”
|
QMS
data
|
|
“GC_”
|
GC
data
|
|
“TLS”
|
TLS
data
|
|
_
|
ASCII
underscore.
|
|
descr
|
(7
alphanumeric) Type-dependent; description of file contents. If unused, filled
with ASCII X.
Valid
values are listed in Appendix B.
|
|
_
|
ASCII
underscore.
|
|
ver
|
(1
alphanumeric) Version identifier, independent of the Special Processing
field.
The
valid values, in their progression, are as follows (non-Hex):
Range 1
through 10 - “1”, “2”, … “9”,
“0”
Range 11 through 36 -
“A”, “B”, … “Z”
Range
37 and higher - “_”
(underscore)
The
Version number increments by one whenever an otherwise-identical filename
would be produced. In general, the highest-numbered Version represents the
“best” version of that product.
|
|
ext
|
(3 characters) Product type
extension.
Valid
values for nominal operational data products:
Non-imaging instrument data
– "CSV", "TAB", "TXT"
Detached PDS labels for SAM
– “LBL”
Image files – “TIF”
|
|
|
|
|
|
|
|
|
|
|
|
instr
SAM Exp ID
venue / source
sol
prod
level
exp seq
type
descr
ver
ext
|
=
=
=
=
=
=
=
=
=
=
=
|
“SM”
“30008”
“F”
“0157”
“RDR”
“1B”
“SPYR”
“QMS”
”MASSXXX”
”1”
“TAB”
|
=
=
=
=
=
=
=
=
=
=
=
|
SAM
SAM Experiment
ID 30008
Flight model,
ops
Sol 157
RDR
NASA data
processing level 1B
Solid sample
pyrolysis with GCMS
QMS data
Mass spectral
data
Version 1
ASCII data
file
|
|
|
|
|
|
MSL instr
SAM exp ID
venue / source
sol
prod
level
exp seq
type
descr
ver
ext
|
=
=
=
=
=
=
=
=
=
=
=
|
“SM”
“21157”
“F”
“0089”
“RDR”
“0_”
“ADIR”
“HK_”
“XXXXXXX”
“1”
“LBL”
|
=
=
=
=
=
=
=
=
=
=
=
|
SAM
SAM Experiment
ID 21157
Flight model,
ops
Sol 89
RDR
NASA data
processing level 0
Direct
atmospheric measurement
Housekeeping
data
Unused
Version 1
PDS label
|
SAM data products comply
with the PDS standards for file formats and labels, as specified in the PDS Data
System Standards Reference, February 27, 2009, Version 3.8, JPL D-7669, Part 2.
The following time standards and conventions are used
throughout this document, as well as the MSL project for planning activities
and identification of events.
Table 2-9. Time standards
|
Time Format
|
Definition
|
|
SCET
|
Spacecraft event time. This is the time when an event
occurred on-board the spacecraft, in UTC. It is usually derived from SCLK.
|
|
SCLK
|
Spacecraft Clock. This is an on-board 64-bit counter, in
units of nano-seconds and increments once every 100 milliseconds. Time zero
corresponds to noon on 1-Jan-2000.
|
|
ERT
|
Earth Received Time. This is the time when the first bit
of the packet containing the current data was received at the Deep Space
(DSN) station. Recorded in UTC format.
|
|
Local Solar
Time
|
Local Solar Time (LST). This is
the local solar time defined by the local solar days (sols) from the landing
date using a 24 “hour” clock within the current local solar day (HR:MN:SC).
Since the Mars day is 24h
37m 22s long, each unit of LST is slightly longer than the corresponding
Earth unit. LST
is computed using positions of the Sun and the landing site from SPICE
kernels. If a landing date is unknown to the program (e.g. for calibration
data acquired on Earth) then no sol number will be provided on output
LST examples:
SOL
12 12:00:01
SOL
132 01:22:32.498
SOL
29
|
|
RCT
|
Record Creation Time. This is
the time when the first telemetry packet, containing a given data set, was
created on the ground. Recorded in UTC format.
|
|
True Local Solar Time
|
This is related to LST, which
is also known as the mean solar time. It is the time of day based on the
position of the Sun, rather than the measure of time based on midnight to
midnight “day”. TLST is used in all MIPL/OPGS generated products.
|
|
SOL
|
Solar Day Number, also known as
PLANET DAY NUMBER in PDS label. This is the number of complete solar days on
Mars since landing. The landing day therefore is SOL zero.
|
The following coordinate systems
are used within the project to refer to the position of the rover and its
instruments.
Table 2-10. Coordinate systems
|
Coordinate System
|
Origin
|
Orientation
|
|
Local Level
|
Same as payload frame, and it
moves with the rover
|
+X North
+Z down along gravity
vector
+Y East
|
|
Payload Frame
|
At the shoulder of the Robotic
Arm. Attached and moves with the rover
|
+X along rover –X ( point
out into the work space)
+Z down along rover
(vertical
axis)
+Y along rover -Y
|
|
Site Frame
|
Same as payload frame when
first defined and never moves relative to Mars. Possible to define multiple
site frames in case the rover moves/slips.
|
Same as local level
|
SAM low-level RDR products containing housekeeping data will
be stored as PDS SPREADSHEET objects, which comprise collections of logically
uniform rows containing ASCII values stored in variable-width fields separated
by field delimiters. Each row within a SPREADSHEET has the same number of
fields, in the same field order, and each field contains the same logical
content. Files containing SPREADSHEET objects have the extension CSV.
SAM products containing QMS science data will be stored as
ASCII TABLE objects. They will have fixed-length records, although the size of
the records in each file may differ depending on the context of the data. Files
containing ASCII TABLE objects have the extension TAB.
Label keywords will provide the necessary information to
determine the organization of each RDR.
SAM RDRs, as with all other MSL RDRs, are subject to PDS
peer review and verification.
Validation of the SAM RDRs, during production, will be
performed according to specifications in the MSL Archive Plan. The SAM Team
will validate the science content of the data products. The PDS Geosciences
node will validate the archive volumes for compliance with PDS standards and
conformance with the design specified in this SIS.
Each of the data products described in Table 2-5 will be
saved as a different RDR, as defined in the following sections. Each RDR
product will consist of a collection of files that contain data for the
different channels or the different SAM instrument components. The SAM RDR file
naming convention will uniquely identify each data product component. See Section
2.3.4 for a detailed description of the SAM RDR file naming convention. The
structure of the SAM RDR consists of detached ASCII PDS labels and associated
data files, as shown in Figure 3-1.
Figure 3-1. The
SAM RDR consists of at least two files.
The PDS label, with file extension “.LBL,” contains
keywords for product identification and table object definitions. The label
also contains descriptive information needed to interpret or process the data
objects in the file. Detached labels are paired with science data of the same
file name, but with different file extensions depending on the data format.
PDS labels are written in Object Description Language
(ODL). PDS label statements have the form of "keyword = value". Each
label statement is terminated with a carriage return character (ASCII 13) and a
line feed character (ASCII 10) sequence to allow the label to be read by many
operating systems. Pointer statements with the following format are used to indicate
the location of data objects in the file:
^Object = location
Where the carat character (^, also called a pointer) is
followed by the name of the specific data object. The location is the starting
record number for the data object within the file.
Each PDS label will include all keywords defined for SAM.
If a keyword does not have a value, a value of “N/A” will be given. Per PDS
rules, “N/A” is used when a keyword exists, but it does not apply to a
particular data product, and “UNKNOWN” is used when the value of an applicable
keyword cannot be determined at the time the PDS label is generated.
The TABLE object is a uniform collection of rows
containing ASCII or binary values stored in columns. Each field is defined as a
fixed-width COLUMN object; the value of the COLUMNS keyword is the total number
of COLUMN objects defined in the label. All TABLE objects must have fixed-width
records.
PDS SPREADSHEET objects are collections of logically
uniform rows containing ASCII values stored in variable-width fields separated
by field delimiters. Each row within a SPREADSHEET has the same number of
fields, in the same field order, and each field contains the same logical
content. By definition, the SPREADSHEET object is used only to describe ASCII
data objects. The delimiter used for SAM data files is the comma, thus SAM
low-level RDR products containing housekeeping data are stored with the file
extension “.CSV.”
PDS TEXT objects describe files that contain plain text.
Use of the carriage-return/line-feed sequence (<CR><LF>) is
required for cross-platform support. The SAM archives may contain text files
describing the contents of other files in the archive volume or providing
information about experiments that would be helpful in data interpretation by
PDS users.
PDS DOCUMENT objects provide information in support of
archived data products. They may consist of one or more files in a single
format, such as images to assist PDS users in understanding interpreted SAM
science data in the level 2 product.
There are no SAM-specific utility programs provided with
these data products. All products are ASCII text, whether in TABLE,
SPREADSHEET, or TEXT objects, and are therefore viewable in any text editor.
TABLE and SPREADSHEET products are also suitable for use in spreadsheet and
data base management programs.
No special PDS software tools are needed to view SAM RDR
data products.
There are neither software-distribution nor update
procedures associated with these data products.
Buch, A.,
Glavin, D.P., Sternberg, R., Szopa, C., Rodier, C., Navarro-Gonzalez, R.,
Raulin, F., Cabane, M., and Mahaffy, P.R.(2006) Planet. Space Sci. 54,
1592-1599.
Conrad, P.G.,
Eigenbrode, J., Mogensen, C.T., Von der Heydt, M.O., Glavin, D.P., Mahaffy,
P.R., and Johnson, J.A. (2011) LPI, #2076.
Eigenbrode, J.,
Glavin, D., Dworkin, J., Conrad, P., and Mahaffy, P. (2011) LPI, #1460.
Glavin, D.P.,
Cleaves, H.J., Buch, A., Schubert, M., Aubrey, A., Bada, J.L., and Mahaffy,
P.R. (2006) Planet. Space Sci. 54, 1584-1591.
Mahaffy, P.R.
(2008) Space Sci. Rev. 135, 255-268.
Mahaffy, P.R.,
Trainer, M.G., Eigenbrode, J.L., Franz, H.B., Stern, J.C., Harpold, D.N.,
Conrad, P.G., Raaen, E., Lyness, E., and the SAM Team. (2011) LPI, #1556.
Mahaffy, P. R., et al. (2012) Space Sci. Rev. DOI:
10.1007/s11214-012-9879-z.
Webster, C.R.
and Mahaffy, P.R. (2011) Planet. Space Sci. 59, 271-283.