We report an experimental analysis of the surface structures induced on silicon by approximate to 900 fs laser pulses at 1055 nm, both in air and high vacuum conditions. The direct comparison between the results obtained in atmosphere and in high vacuum highlights the influence of ambient pressure in the formation of surface micro-grooves with above-wavelength period. The effect is ascribed to the presence of nanoparticles of the target material on the sample surface when the process is carried out in air. In the case of high vacuum, we have also observed the formation of a peculiar structured region, elliptically shaped, and with the major axis directed along the polarization of the laser beam. These results are particularly interesting to further clarify some of the mechanisms involved in the process of direct ultrashort laser surface structuring. (c) 2017 Elsevier B.V. All rights reserved.