Labeled Release Instrument

The following description of the LR instrument and experiment as flown on Viking was largely summarized from Levin and Straat, 1976a.

The LR instrument was part of a package of three biology experiments on the Viking Landers. The scientific objective of the Viking Lander biology investigation was to seek the presence of microbial life in the soil on Mars (Klein et al., 1976a; Klein, 1974). The principal assumptions upon which all three Viking biology experiments are based are: (1) that microorganisms exist within the biotic system, (2) that metabolic rates are sufficient for detection during the elapsed time of the experiment, and (3) that the microorganisms are widely disseminated. The LR experiment tested for heterotrophic metabolism by monitoring for the release of radioactive gases from a soil sample inoculated with carbon-14 labeled organic substrates. In the event of a positive response, the experiment was designed to analyze a heat sterilized control sample of the same soil for comparison to the unheated sample (Levin and Straat, 1976a). Several assumptions were used in the design and operation of the LR experiment including: 1) life on Mars was carbon based; 2) one or more of the nutrient compounds would be metabolized by any microbial life present; and 3) one end product of metabolism would be a carbon based gas that would rise from the sample for ready detection (Levin and Straat, 1976a). Each sample analyzed is referred to as a cycle.

The LR instrument shared common support services with the other biology instruments. This included the sample delivery system (Klein, 1974). Martian soil samples were collected by the Viking Lander surface sampler arm (Moore et al., 1987). Soil from the sampler arm was dumped into a hopper on top of the lander. The hopper contained a sieve that only allowed only 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 (Klein, 1974). The LR instrument was designed to receive 0.5 cc of soil. The biology common support services and the LR instrument held enough nutrient and pressurized helium to conduct two nutrient injections on each of four soil samples. Testing prior to launch showed that after proper drying of a soil sample and purging of gases, a cell could be used a second time by adding more soil and nutrient (Levin and Straat, 1979b; Levin and Straat, 1976a).

Each Viking Lander had identical LR instruments that used two solid-state beta detectors to measure the release of radioactive gas from samples of martian soil 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. The LR instrument contained four incubation test cells mounted on a carousel. After receiving a soil sample, a 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 33.02-cm long, 0.267 cm inner-diameter tube bent at several spots to prevent radioactive particles or aerosol in the test cell from reaching the detectors. The total volume of this assembly was approximately 8.5 cc and gas equilibrium between the test cell and the detectors was established within thirty minutes. All materials that contacted soil or nutrient were tested for toxicity and chemical compatibility prior to selection (Levin and Straat, 1976a).

The LR solid-state beta detectors monitored 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 use either one or both detectors. Background radiation came from radioisotope thermoelectric generators (RTGs) that were part of the Viking Lander power system. As a result, a 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. During an analysis cycle the test cell was heated to prevent the temperature from falling below 9-10°C during the martian night. Samples could be heated in the test cell prior to nutrient injection to provide sterilized control samples. the heaters proved versatile in allowing samples to be heated temperatures other than the pre-programmed values. The heaters were also used at the end of an analysis cycle to dry the sample before opening the cell.

A schematic of the LR Module is shown below.

Click on image to enlarge (Levin and Straat, 1976a).

The LR 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 (930,790 Pa) shortly after the spacecraft landed on the surface of Mars. Lower pressure helium (124,106 Pa) was then bubbled through the nutrient in the reservoir for several hours via valves S/61, S/59, and S/47 in order to degas the nutrient, thereby removing any radioactive gas produced by degradation of the nutrient during the 11-month trip to Mars. At the start of an analysis cycle high pressure helium was used to route a portion of the nutrient into the space bound by valves S/59, S/61, S/44, and S/45. The meter volume of nutrient was then released through S/45 to the sealed test cell.  Testing prior to launch indicated that 0.115 cc ± 8% of nutrient was delivered during each injection. 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. Total pressure at the start of an analysis cycle was about 9200 Pa (92 millibar). Following injection, radioactive gas evolved was counted at 2- to 4-minute intervals for the first two hours and at 16-minute intervals thereafter. The experiment was conducted in predominantly moist conditions but provided a gradient from wet to dry in the event that Mars organisms may be intolerant of large quantities of liquid water. At the end of an analysis cycle, radioactive gas was purged from the test cell through the plumbing system. The soil sample was dried by brief heating prior to opening the cell to prevent explosive evaporation. A fresh test cell was then rotated beneath the head assembly and a three-hour cleanup was accomplished by heating both the headspace and detectors during continuous helium purging. After cooling, nutrient trapped in the metered volume between S/59, S/61, S/44, and S/45, was wasted through S/44. The system was then ready to accept a fresh soil sample.

In the event a positive result was obtained, the instrument was designed to heat soil from the same sample for three hours at 160°C in the test cell prior to the addition of nutrient as a control. After attaining highly attenuated results from the 160°C control cycle performed on Viking Lander 1, the Viking Lander 2 control cycles were modified to heat the sample to only 50°C. Although such a "cold sterilization" had never been performed on a flight instrument, the experiment was conducted in an attempt to distinguish further between biology and chemistry as the cause of the LR response. By activating only one of the two heaters temperatures of 46°C and 51°C were achieved and maintained for three hours for two VL2 control cycles (Levin and Straat, 1976b).

All consumables (helium, nutrient, electrical power) are sufficient to support four complete soil cycles, each with two injections of nutrient during the cycle. The experiment is conducted in ambient Martian atmosphere modified by the over-pressure of helium as discussed above. The LR instrument was fairly automated with preprogrammed sequences. However, commands could be sent from the ground to change the preprogrammed sequences to perform nutrient injections, to select active or control sequences, to modify the control heat treatments, to thermally purge an active sample, to select a fresh soil sample, and to initiate or terminate an analysis cycle. Commands also could be sent to conduct single or double channel counting.

The LR experiment also 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. This testing was accomplished with a "Test Standards Module" (TSM). The TSM instrument was similar to the flight instruments. It was configured with test cells, head assembly, detector assembly, and nutrient reservoir of the same dimensions and materials as the flight instruments. These components were contained under a bell jar with a simulated Martian atmosphere of carbon dioxide. These test data have been published in the scientific literature (Levin and Straat, 1981a; Levin and Straat, 1979b; Levin and Straat, 1977a; Levin and Straat, 1976a).