To enable reliable in situ, or sample return, life detection missions, it is critical that Mars missions be free of any biological materials that originated from Earth and could contaminate samples collected for analysis. Therefore, it is important that likely cross-contamination mechanisms be thoroughly studied and understood.Three simple models have been developed to estimate the maximum soil contamination that could originate from a bio-contaminated lander. All three models estimate the ground contamination concentrations at various distances from the lander. The first model estimates the ground concentration if the microorganisms covered the soil 360° around the lander. The second model uses a steady state Gaussian plume to transport the microorganisms from the Lander. The third model determines the ground contamination level from an instantaneous Gaussian puff release, probably at the time of landing.Input to the models includes the total spacecraft (s/c) contamination level, the height of the lander, the size distribution of the particles and their microbial burden, the fraction of the total contamination that is removed, the wind speed, and the diffusivities of the plumes. The results are given for input data available from old studies performed at the Kennedy Space Center. More realistic data are now being obtained at JPL.