Instrument Overview
  ===================
    Each Viking Lander had identical Labeled Release (LR) instruments
    that used two solid-state beta detectors (Geiger counters) to
    measure the release of radioactive gas from samples of martian
    soil that had been inoculated with an aqueous nutrient solution.
    The instrument contained additional sensors to measure headspace
    (i.e, the volume of test chamber above the sample) and detector
    temperatures.  Heaters were also included in the LR instrument
    for the detectors and test chambers.  A number of published
    papers describe the characteristics and performance of the LR
    instrument.  The scientific rationale and early design of the
    instrument are described in LEVIN1972 and a detailed description
    of the flight module is given in LEVIN&STRAAT1976A.  A summary of
    the information from these papers is given here as a high-level
    description of the LR instrument and its operation.  The Labeled
    Release instrument was part of a package of three biology
    experiments on the Viking Landers [KLEIN1974].
 
 
  Science Objectives
  ==================
    The major scientific objectives of the Viking Lander biology
    investigation were to detect the plausibility and/or presence of
    life on Mars.  In particular, the LR experiment was designed to
    detect microbial life in the martian soil.  The Labeled Release
    experiment tested for heterotrophic metabolism by monitoring the
    release of radioactive gases from a soil sample inoculated with
    carbon-14 labeled organic substrates.  The experiment also was
    designed to analyze control samples that were heat sterilized.
 
    Several assumptions were used in the design and operation of the
    Labeled Release experiment.  These assumptions included the
    following: A) life on Mars was carbon based; B) one or more of
    the nutrient compounds would be metabolized by possible microbial
    life; and C) one end product of metabolism would be a carbon
    based gas.
 
 
  LR Detailed Description
  =======================
    The Labeled Release instrument contained four incubation test
    cells mounted on a carousel.  Each test cell could be rotated and
    sealed beneath an assembly that contained a heater, plumbing
    terminals for nutrient delivery and gas removal, and a tube
    leading to the beta detectors.  Gas and nutrient moving through
    the plumbing system were controlled by eight miniaturized
    solenoid valves.  The headspace was connected to the two
    solid-state beta detectors through a tube bent at several spots.
    The connecting tube was bent to prevent radioactive particles
    from reaching the detectors from the test cell.  The instrument
    also had two temperature sensors.  One sensor measured the
    temperature of the detectors and the other measured the
    temperature of the headspace within the test cell.  For a
    schematic and photographs of the Labeled Release instrument see
    LEVIN&STRAAT1976A.
 
    The Labeled Release nutrient was stored in a sealed glass ampoule
    within a reservoir.  The reservoir, in turn, was connected to the
    test cells by the instrument plumbing system.  The ampoule
    containing the nutrient was broken by a mechanical striker driven
    by high pressure helium shortly after the spacecraft landed on
    the surface of Mars.  Low pressure helium was then bubbled
    through the nutrient in the reservoir for several hours in order
    to degas the nutrient.  At the start of an analysis cycle high
    pressure helium was used to route a portion of the nutrient into
    a sealed test cell.  Testing prior to launch indicated that 0.115
    cc +/- 8% of nutrient was delivered during each injection.  The
    pressure in the test cell headspace was kept above Mars ambient
    atmospheric pressure with helium from the plumbing system to
    prevent boiling of the nutrient.  The total pressure at the start
    of an analysis cycle was about 9200 Pa (92 millibar).  At the end
    of an analysis cycle, radioactive gas was purged from the test
    cell through the plumbing system.
 
    The LR instrument had two identical solid-state beta detectors
    (Geiger counters) to monitor the evolved radioactive gas from the
    soil sample.  There were two detectors as a contingency against
    failure of one detector.  The detectors continuously counted
    gaseous radioactivity as it evolved from the test cell.  The
    instrument could be commanded to count with either one or both
    detectors.  Detectors were referred to as the right and left
    channels.  The Viking Landers were powered by radioisotope
    thermoelectric generators (RTGs), which provided background
    radiation to the detectors.  As a result, this background signal
    had to be determined by monitoring the detectors for one to
    several Mars days before nutrient was injected into a test cell.
 
    The test cell heaters were used in a number of ways.  Samples
    could be heated in the test cell prior to nutrient injections to
    provide sterilized control samples.  During an analysis cycle the
    test cell was heated to prevent the temperature from falling
    below 9-10 degree C during the martian night.  The heater was
    also used at the end of an analysis cycle to dry the sample.
 
    The Labeled Release instrument shared common support services
    with the other biology instruments.  This included the sample
    delivery system [KLEIN1974].  Martian soil samples were collected
    by the Viking Lander surface sampler arm [MOOREETAL1987;
    CROUCH1977].  Soil from the sampler arm was dumped into a hopper
    on top of the lander.  The hopper contained a sieve that only
    allowed particles less than 2 mm in size to enter the instrument
    test cells.  The sieved samples entered a distribution assembly
    that automatically delivered measured volumes of soil to each
    biology experiment [KLEIN1974].  The Labeled Release instrument
    typically received 0.5 cc of soil.
 
    The Biology common support services and the Labeled Release
    instrument held enough nutrient and helium to conduct up to two
    injections on four soil samples.  Testing prior to launch showed
    that after proper drying of a soil sample and purging of gases
    from a test cell that the cell could be used a second time by
    adding more soil and nutrient.  One of the test cells on Viking
    Lander 2 was used a second time.
 
 
  Nutrient Description
  ====================
    Considerable effort went into selecting, preparing, and testing
    the nutrients for the Label Release experiment
    [LEVIN&STRAAT1976A].  Selection criteria were partially based on
    the assumption that life evolved similarly on Mars as on Earth.
    There were several criteria used in selecting compounds for the
    nutrient that included: A) Compounds that were likely to be
    produced on Mars; B) Compounds that primitive organisms were
    likely to use in metabolism based on terrestrial analogs; and C)
    Compounds that were unlikely to have nonbiological reactions with
    martian soil.  In addition, each compound in the nutrient was
    tested to show that it produced a rapid response in a variety of
    terrestrial soils and in cultures of several types of organisms.
    The stability of the materials was also considered given that the
    nutrient was stored for about 2 years from the time it was
    prepared until it was used on Mars.  The storage period included
    the nearly one year cruise phase of the spacecraft on its journey
    to Mars.  Nutrient material also had to undergo sterilization of
    the biology module (54 hr at 120 degree C) and the entire
    spacecraft (20 hr at 100 degree C).  It was expected that some
    decomposition (<1%) of the nutrient would occur due to the long
    storage time and sterilization.  Any radioactive carbon dioxide
    produced by the nutrient decomposition was removed by flushing
    the nutrient reservoir with helium before the first injection.  A
    complete list of nutrient compounds is given in
    LEVIN&STRAAT1976A.  The nutrient included sodium formate, calcium
    glycolate, glycine, D- and L-alanine, sodium D-lactate and sodium
    L-lactate.  The concentrations of each nutrient component were
    dilute in case any particular compound was toxic to possible
    martian organisms.  Each compound in the nutrient was labeled
    with a precise amount of radioactive carbon-14.
 
 
  Operation and Sampling Modes
  ============================
    The basic analysis cycle for the Labeled Release experiment was
    to have a sample of martian soil delivered to a test cell and to
    be moistened 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 headspace
    above the sample was monitored continuously for evolved
    radioactive gas as evidence for metabolism.  Viking Lander 1
    completed four cycles, whereas Viking Lander 2 completed five
    cycles for a total of nine cycles between the two landers.  VL2
    cycle 5 was performed in a previously used test cell using a
    stored portion of the cycle 4 soil sample.  Eight of the cycles
    received two injections of nutrient.  VL1 cycle 3 received three
    injections of nutrient.
 
    Three of the analysis cycles were control cycles where the soil
    sample was heated for several hours before nutrient injection.
    The control cycle for Viking Lander 1 was heated to 160 degree C.
    After receiving highly attenuated results from Viking Lander 1,
    Viking Lander 2 control cycles were modified to be 50 degree C.
 
    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.
    Background levels were determined by measuring the test cell for
    several Mars days prior to nutrient injection.  The headspace and
    detector temperatures were measured every 16 minutes throughout
    an analysis cycle.
 
    At the end of the cycle, the radioactive gas was removed by
    purging and the soil was dried by brief heating to prevent
    evaporation upon opening the cell.  A fresh test cell was then
    rotated beneath the head end assembly and a three-hour cleanup
    was accomplished by heating both the head assembly and detectors
    during continuous helium purging.  After cooling, trapped
    nutrient was vented from the system.
 
    The instrument was fairly automated with preprogrammed sequences.
    Commands could be sent from the ground to change the
    preprogrammed sequences to perform nutrient injections, to select
    active or control sequences, to select a fresh soil sample, and
    to initiate or terminate an analysis cycle.  Commands could be
    sent to conduct single or double channel counting.
 
 
  Principal Investigator
  ======================
    The principal investigator for the Viking Lander Labeled Release
    experiment was Dr.  Gilbert V.  Levin of Biospherics, Inc.  Much
    of the work on selecting the nutrient was directed by
    Co-Investigator Dr.  Patricia Ann Straat.