AbstractStrong radar reflections have been previously mapped at the base of the Martian South Polar Layered Deposits. Here, we analyze laboratory measurements of dry and briny samples to determine the cause of this radar return. We find that liquid vein networks consisting of brines at the grain boundaries of ice crystals can greatly enhance the electrical conductivity, thereby causing strong radar reflections. A brine concentration of 2.7–6.0 vol% in ice is sufficient to match the electrical properties of the basal reflection as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When brine is mixed with sediments, the brine‐ice mixture in the pores must be 2–5 times more concentrated in salt, increasing the brine concentration to 6.3–29 vol%. Our best fit of the median observed MARSIS value suggests a salt‐bulk sample concentration of ∼6 wt%. Thus, salt enhancement mechanisms on the order of a magnitude greater than the Phoenix landing site are needed. To form brine, the basal reflector must reach a temperature greater than the eutectic temperature of calcium perchlorate of 197.3 ± 0.2 K, which may be possible if more complex thermal modeling is assumed. Colder metastable brines are possible, but stability over millions of years remains unclear. Conversely, gray hematite with a concentration of 33.2–59.0 vol% possess electrical properties that could cause the observed radar returns, but require concentrations 2–3 times larger than anywhere currently detected. We also argue that brines mixed with high‐surface‐area sediments, or dry red hematite, jarosite, and ilmenite cannot create the observed radar returns at low temperatures.