Ionospheric physics deals with the basic structure and variability of plasma within the upper atmospheres of the Earth and planets. Comparative studies foster both exploration and synthesis within diverse settings. In this paper we examine observations of the ionospheric profiles on Mars obtained on 9--27 March 1999 by the radio science experiment onboard the Mars Global Surveyor satellite, and compare their day-to-day variability with same-day observations on Earth. Using photochemical-equilibrium arguments applicable to the peak electron density layer on Mars and the E-layer on Earth, we find basic agreement in scaling laws between the planets, and in the details of correlations with simultaneous solar flux variations during a period of pronounced solar activity. We ascribe the residual variabilities (larger on Mars than Earth) to both observational uncertainties and to nonsolar mechanisms in need of further study on both planets.