SOLID was designed for automatic in situ detection and identification of substances from liquid and crushed samples under the conditions of outer space. The system uses hundreds of carefully selected antibodies to detect lipids, proteins, polysaccharides, and nucleic acids. These are complex biological polymers that could only be synthesized by life forms, and are therefore strong indicators —biosignatures— of past or present life.
SOLID consists of two separate functional units: a Sample Preparation Unit (SPU) for extractions by ultrasonication, and a Sample Analysis Unit (SAU), for fluorescent immunoassays. The antibody microarrays are separated in hundreds of small compartments inside a biochip only a few square centimeters in size.
SOLID instrument is able to perform both "sandwich" and competitive immunoassays using hundreds of well characterized and highly specific  antibodies. The technique called "sandwich immunoassay" is a non-competitive immunoassay in which the analyte (compound of interest in the unknown sample) is captured by an immobilized antibody, then a labeled antibody is bound to the analyte to reveal its presence. In other words, the "sandwich" quantify antigens (i.e. biomolecules) between two layers of antibodies (i.e. capture and detection antibody). For the competitive assay technique, unlabeled analyte displaces bound labelled analyte, which is then detected or measured.
An optical system is set up so that a laser beam excites the fluorochrome label and a CCD detector captures an image of the microarray that can be measured.
The instrument is able to detect a broad range of molecular size compounds, from the amino acid size, peptides, proteins, to whole cells and spores, with sensitivities at 1–2 ppb (ng/mL) for biomolecules and 104 to 103 spores per milliliter. Some compartments in the microarray are reserved for samples of known nature and concentrations, that are used as controls for reference and comparison. SOLID instrument concept avoids the high-temperature treatments of other techniques that may destroy organic matter in the presence of Martian oxidants such as perchlorates.