Mars was observed at 1.35 cm (22 GHz) with the Very Large Array (VLA) on February 19, 1995. We mapped the thermal emission from the martian surface in the Tharsis and Amazonis regions. Observed variations of microwave brightness temperature were compared with the results of a subsurface thermal model to derive the emissivity and dielectric constant of the soil. The results revealed a region with anomalously high values of emissivity (ε = 0.99 ± 0.01), which is approximately inversely proportional to the density of the material. We interpret this region as having a low density (ρ = 0.5 ± 0.3 gm cm-3) near the surface. It lies within the boundaries of Stealth - the echoless radar feature discovered by Muhleman's radar group in 1991 (Science253, 1508-1513). We suggest two hypotheses for the formation of Stealth. It may consist of ash flows from the nearby Tharsis volcanoes, or it might be an eolian deposit.