The day side ionosphere of Mars is primarily the product of solar photoionization of the planetary neutral atmosphere and subsequent photochemical processes. Its major characteristics are the main layer M2 and the underlying smaller M1 layer. While M2 is in general produced by solar radiation (extreme ultraviolet), the M1 layer is primarily produced from solar X-ray photoionization and secondary impact ionization of photo-electrons. O2+ yields the highest ion density in the main peak altitude range while at higher altitudes O+ dominates. MaRS observations from 2004 to 2014 show the day side maximum of M2 at altitudes from 120 km to 140 km. Generally, the main peak can be found at lower altitudes when the position of the sun is high and the incidence angle of the solar radiation is steep. The M2 electron density in the MaRS observations varies from 4e10 to 13e10 electrons/m3, also a function of the solar radiation incidence angle and solar activity. Information on additional properties of the Mars ionosphere in comparison with Venus can be found here.