A purpose of the Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport (InSight) mission is to reveal the Martian interior structure with seismic data. In this work, ambient noise autocorrelation of the continuously recorded vertical-component seismic signals has extracted the Rayleigh waves that propagate around Mars for one cycle, R2. The Mars orbiting surface waves are observed at a lag time of ∼6,000 s in the stacked autocorrelation series filtered between 0.005 and 0.01 Hz. Synthetic seismograms from a set of radially concentric velocity models were computed to find the best-fitting one as the starting model for a Monte Carlo inversion. The starting model was randomly perturbed iteratively to increase the correlation coefficients and reduce the absolute time shifts between the synthetic and observed R2. An S-wave low-velocity layer in the inverted velocity model extends to ∼400 km depth, consistent with Marsquake observations, geophysical inversion, and high-pressure experiments.