Data Set Overview
=================

The Labeled Release (LR) instrument on each Viking Lander operated
throughout the Primary Mission and continued into the Extended
Mission.  The basic analysis cycle for the LR experiment was to
deliver a sample of martian soil to one of four test cells, seal the
cell and moisten the soil with an aqueous solution of carbon-14
labeled organic media.  A second nutrient injection was typically done
about 7-8 Mars days after the first injection.  The test cell was
continuously monitored for the release of radioactive labeled gas as
an indicator of heterotrophic metabolism.  Viking Lander 1 (VL1)
conducted four analysis cycles with two nutrient injections for cycles
1, 2, and 4 and three injections for cycle 3.  Viking Lander 2 (VL2)
conducted five analysis cycles.  The first four cycles had two
nutrient injections each, and the fifth cycle had one injection
[LEVIN&STRAAT1976B; LEVIN&STRAAT1977A; LEVIN&STRAAT1979A].

Data collected by the LR instruments included radioactivity counts
from evolved carbon-14 labeled gas and temperatures of the detectors
and the head-end assembly.  Radioactivity was counted for 16-minute
intervals during an analysis cycle, except for several hours around
the time of nutrient injection where intervals of 2- to 4-minutes were
used.  Detector and head-end temperatures were measured at 16-minute
intervals throughout the cycle.

The LR data were originally archived on microfilm housed at NSSDC.  LR
data were also available from a set of computer listings in the
personal archives of Co-Investigator Dr. Patricia Ann Straat.  The
data in this PDS archive were transcribed into digital files from
these two sources.  The Straat listings were the primary source used.

The LR experiment included an extensive test program that not only
tested the operating characteristics of the LR instrument, but also
analyzed many terrestrial samples for comparison with the Mars
results.  These test data are not currently part of the PDS Labeled
Release archive, but much of it has been published in the scientific
literature [e.g., LEVIN&STRAAT1981A; LEVIN&STRAAT1981B;
LEVIN&STRAAT1979B; LEVIN&STRAAT1977B; LEVIN&STRAAT1976B].


Parameters
==========

The LR data set contains three basic data measurements: radioactivity
counts, detector temperature, and head-end temperature.  Measurements
are tagged with the time they were made and are stored in a
time-ordered format.

RADIOACTIVITY COUNTS: Radioactivity in the LR data set are given as
counts/minute.  Counting was generally done over a 16-minute period
and then normalized by the length of the counting interval to
determine the average number of counts/minute within that interval.
Each LR instrument had two solid-state beta detectors and could be
commanded to use either one or both detectors.  In cases where only
one detector was used, radioactivity values were corrected to values
equivalent to counting with both detectors.  The radioactivity
recorded by the LR detectors included a background signal from the
Mars environment and from the Radioisotope Thermoelectric Generators
(RTGs) that powered the Viking Landers.  Background levels were
determined from radioactivity measurements made prior to the start of
an analysis cycle.  Radioactivity values with and without background
correction are provided for measurements made during the incubation
period of an analysis cycle.

TEMPERATURES: Both detector and head-end temperatures in the Labeled
Release data set are given in degrees Celsius.  Temperatures were
measured every 16-minutes throughout an analysis cycle.  Sample
temperatures are less than those recorded by the head-end temperature
sensor because the test cell heaters and temperature sensors are
located at the top of the test cell above the sample.  See
LEVIN&STRAAT1977A for information on correlating head-end and sample
temperatures.

TIME: The primary time tag for the LR data set is known as Mars
mission time.  It is defined as the number of Earth seconds past
midnight of Sol 0, where Sol 0 is the day of landing (P. A. Straat,
personal communication, 2000).  Mars mission time values in this
archive have been transcribed from the computer listings or microfilm.
The LR data files include several other time-related fields that were
computed from the transcribed Mars mission time values during archive
production (see Processing Section below).  The LR data files include
time fields for elapsed time from nutrient injection and elapsed time
from a specific Sol number.  These fields are included to allow
display of the data in forms similar to previously published graphs
[e.g., LEVIN&STRAAT1979A; LEVIN&STRAAT1977A; LEVIN&STRAAT1976B].
Other computed time fields are UTC and local lander time.  The Viking
project used the format of sol number, hour, minute, and second for
local lander time with times in a sol extending beyond 24 hours.  This
usage is different from the PDS standard for local time.  As a result,
local lander time is called native time in the LR data files.


Processing
==========

All radioactivity, temperature and Mars mission time data in this LR
data set were transcribed by hand into data files.  The primary data
source was computer listings supplied by P. A. Straat.  The NSSDC
microfilm was used to recover data for gaps in the computer listings.
Personnel at the PDS Geosciences Node transcribed the computer
listings, whereas personnel at NSSDC transcribed the microfilm.  All
data entered by hand was checked for accuracy.  However, typographical
errors may still exist in the data set.  See the CONFIDENCE_LEVEL_NOTE
for estimates of the data entry accuracy.

All ancillary time fields in the LR data set were computed from the
Mars mission time parameter.  The computation of UTC and local lander
time from Mars mission time require the UTC of midnight at Sol 0 and
the number of seconds in a martian day.  The intent of including these
time fields in the LR data set was to provide a means of comparison
with times in other Viking data sets.  As such, the methods used here
to compute UTC and local lander time attempted to reproduce, as
closely as possible, values consistent with values from the Viking
project.  A value of 88775.241 seconds/Sol was derived from analyzing
a series of local lander time and UTC values published in
CLARKETAL1977.  The UTC value for midnight of Sol 0 was estimated from
times in CLARKETAL1977 and from acquisition times of Viking Lander
images taken on Sol 0 for each lander [TUCKER1978].  The estimated UTC
values were 1976-07-19T19:39:54 and 1976-09-03T12:48:45 for VL1 and
VL2, respectively.  These values are slightly different from those
published in ALLISON1997 and ALLISON&MCEWEN2000.


Data
====

The LR data consists of three files for each analysis cycle.  Each
table has an associated detached PDS label.  The tables are:

(1) Detector temperatures
(2) Head-end temperatures
(3) Radioactivity counts

(1) Detector temperatures

This table includes fields of detector temperature, Mars mission time,
and several derived time fields (see PDS label).  Data are sorted in
increasing Mars mission time order.

(2) Head-end temperatures

This table includes fields of head-end temperature, Mars mission time,
and several derived time fields (see PDS label).  Data are sorted in
increasing Mars mission time order.

(3) Radioactivity counts

This table includes fields of raw and background-corrected
radioactivity counts, channel number, Mars mission time, and several
derived time fields (see PDS label).  The background-corrected field
was derived from the raw radioactivity field by subtracting a constant
background value determined from data collected prior to nutrient
injection.  Data collected in single channel mode were corrected to
dual channel mode (using a constant multiplier) before background
correction.  Data are sorted in increasing Mars mission time order.


Analysis Cycles
===============

A number of published papers describe the soil sampling and analysis
rationale for the LR experiment, along with the scientific results
[e.g., LEVIN&STRAAT1979A; LEVIN&STRAAT1977A; LEVIN&STRAAT1976B].  A
summary of the sampling and analysis strategy information from these
papers is given here.

Viking Lander 1 Cycle 1
-----------------------

The sample for VL1 cycle 1 was acquired from a smooth patch of
fine-grained material named Sandy Flats.  The sample came from within
4 cm of the surface.  VL1 cycle 1 was performed autonomously according
to preflight instructions.

  Sample Site             :  Sandy Flats
  Collection Temperature  :  -83 deg C
  Collection Sol          :  8 (fresh)
  Experiment Type         :  Active
  First Injection Sol     :  10
  Second Injection Sol    :  17
  Purge                   :  23

Viking Lander 1 Cycle 2
-----------------------

This sample was acquired on Sol 8 (see VL1 cycle 1) from Sandy Flats
and stored in the biology hopper for about 20 sols.  Before nutrient
incubation, this sample was heated to 160 deg C for about 3 hours to
sterilize the sample as a control for comparison to VL1 cycle 1
results.

  Sample Site             :  Sandy Flats
  Collection Temperature  :  -83 deg C
  Collection Sol          :  8 (stored in hopper for 20 sols)
  Experiment Type         :  160 deg C control
  First Injection Sol     :  29
  Second Injection Sol    :  35
  Purge                   :  37

Viking Lander 1 Cycle 3
-----------------------

The VL1 cycle 3 sample was also acquired from Sandy Flats close to the
site of the first VL1 sample (cycles 1 and 2).  The sample came from
within 4-5 cm of the surface.  This analysis was a long active
incubation (about 50 sols), where the sample received three nutrient
injections.

  Sample Site             :  Sandy Flats
  Collection Temperature  :  -21 deg C
  Collection Sol          :  36 (fresh)
  Experiment Type         :  Active, long incubation
  First Injection Sol     :  39
  Second Injection Sol    :  55
  Third Injection Sol     :  80
  Purge                   :  89

Viking Lander 1 Cycle 4
-----------------------

This sample was collected during the Extended Mission from the same
Sandy Flats area as the previous VL1 samples.  It was collected on Sol
91 and stored in the Biology hopper in the dark, open to the Mars
atmosphere, and at temperatures between 10 and 26 deg C until analysis
started on Sol 230.  The original intent for this cycle was to collect
a fresh sample, but concerns by the surface sampler team about
possible damage to the sampler arm changed the scenario.  The two
nutrient injections for this sample occurred about 3 hours apart.  It
was calculated that enough nutrient remained in the reservoir for the
two injections [LEVIN&STRAAT1979A].

  Sample Site             :  Sandy Flats
  Collection Temperature  :  -71 deg C
  Collection Sol          :  91 (stored)
  Experiment Type         :  Active
  First Injection Sol     :  232
  Second Injection Sol    :  232
  Purge                   :  N/A

Viking Lander 2 Cycle 1
-----------------------

This sample was collected from a crusty to cloddy material in an area
known as Beta.  The material came from within 3.5 cm of the surface.
VL2 cycle 1 was performed autonomously according to preflight
instructions.

  Sample Site             :  Beta
  Collection Temperature  :  -23 +/- 5 deg C
  Collection Sol          :  8 (fresh)
  Experiment Type         :  Active
  First Injection Sol     :  11
  Second Injection Sol    :  18
  Purge                   :  24

Viking Lander 2 Cycle 2
-----------------------

The VL2 cycle 2 sample was collected from the same area of Beta as for
VL2 cycle 1 sample.  It was a fresh sample collected on Sol 28.  The
sample was heated in the test cell at about 50 deg C for 3 hours
before the nutrient was injected.  The sterilization process was
intented as a control for comparison to cycle 1 results.

  Sample Site             :  Beta
  Collection Temperature  :  -23 +/- 5 deg C
  Collection Sol          :  28 (fresh)
  Experiment Type         :  51 deg C control
  First Injection Sol     :  34
  Second Injection Sol    :  38
  Purge                   :  47

Viking Lander 2 Cycle 3
-----------------------

This sample was acquired from under a rock called Notch Rock.  The
rock was pushed aside to expose the soil.  The pushing occurred about
1 hour after sunrise and the soil was exposed to low angle sunlight
for approximately 37 minutes before delivery to the biology sample
processor.  The intent of this sample was to analyze soil that was
protected from ultraviolet radiation for a long period of time.  This
analysis performed a long incubation of nearly 90 sols with two
nutrient injections.

Radioactivity data was collected in single channel mode for part of
the beginning of this cycle.  Data were corrected to dual channel mode
by multiplying by a factor of 1.95 prior to nutrient injection and a
factor of 2.06 after the first injection.

  Sample Site             :  Beneath Notch Rock
  Collection Temperature  :  -66 deg C
  Collection Sol          :  51 (fresh)
  Experiment Type         :  Active, long incubation
  First Injection Sol     :  53
  Second Injection Sol    :  60
  Purge                   :  140

Viking Lander 2 Cycle 4
-----------------------

This sample was collected from the same general area as the samples
for VL2 cycles 1 and 2 in the Beta location.  The material for this
sample mostly came from the top 5 cm of the surface.  The sample may
also contain material disturbed during the cycle 2 sample acquisition.
The sample was heated to 46 deg C for about 3 hours before nutrient
was injected.

  Sample Site             :  Beta
  Collection Temperature  :  -84 deg C
  Collection Sol          :  145 (fresh)
  Experiment Type         :  46 deg C, control
  First Injection Sol     :  147
  Second Injection Sol    :  161
  Purge                   :  171

Viking Lander 2 Cycle 5
-----------------------

The original intent for VL2 cycle 5 was to incubate a fresh sample at
sub-freezing temperatures to more closely simulate the actual Mars
surface conditions.  A previously used test cell was required for this
cycle because all four test cells had been used.  A surface sampler
anomaly during the sample acquisition attempt on Sol 195 prevented the
collection of a fresh sample.  Thus, a sample collected on Sol 145 was
used for VL2 cycle 5.  The sample was stored for 84 sols prior to
nutrient injection.  The 0.5 cc sample was placed on top of about 1.2
cc of unused sample dumped into the cell from previous acquisitions
plus about 0.5 cc of material from VL2 cycle 1.

The LR instrument froze prior to nutrient injection during a power
shutdown.  It is possible that nutrient delivery lines or valves may
have ruptured due to freezing.  In addition, only a small amount of
nutrient (0.64 cc) was calculated to remain in the reservoir.  Thus,
it is not clear whether a complete nutrient injection was delivered,
although as least a partial delivery is likely [LEVIN&STRAAT1979A].

  Sample Site             :  Beta
  Collection Temperature  :  -84 deg C
  Collection Sol          :  145 (stored)
  Experiment Type         :  Soil-on-soil in used cell
  First Injection Sol     :  229
  Purge                   :  N/A

Confidence Level Overview
=========================

The sources for the LR data were microfilm stored at NSSDC and
photocopies of Co-Investigator P. A. Straat's original computer
listings.  The P. A. Straat computer listings were used as the primary
data source with the microfilm data used to fill in gaps.  The
microfilm contained images of computer pages with variable quality.
The data transcribed from the P. A. Straat source were entered twice.
The two versions were compared and corrected to ensure that the two
versions exactly matched.  For the microfilm data source, information
was manually entered by one person and then checked by a different
individual.

For VL2 Cycle 3, there was some data that were available in both
sources.  A comparison of about 2000 radioactivity readings revealed a
0.25% error rate (5 values total) in NSSDC's keypunch and quality
control process.  The errors found included a single digit miskeyed,
two digits reversed, or fractional digits replaced with zeroes.  It is
possible that the error rate is similar for data values where the
microfilm is the only source.  The NSSDC microfilm remains available
from NSSDC for access by future users.


Data Coverage and Quality
=========================

Steps were taken to insure that this data set is complete and contains
as few errors as possible.  However, there are several data gaps.
Most data gaps are on the order of one or two sampling intervals, but
some are on the order of Sols.  It would appear that these data were
never taken or lost in transmission.

The data set contains a few spikes of spurious readings.  These are
usually obvious as a single data point that is very much larger than
the values prior to and after the spike.