Venus does not have a magnetic field because:
Its iron core is solid (not likely), or
it rotates too slowly (possible), or
it is currently undergoing a polarity reversal (also possible but unlikely), or
the absence of volcanic activity on Venus is allowing the temperature of the mantle to rise, which is neutralizing the thermal gradient within its outer (liquid) core. Therefore, the convection in the outer core, which induces the magnetic field, cannot occur under present conditions. (possible, but controversial).
The environmental consequences on the surface of Venus caused by the absence of a magnetic field include:
Cosmic rays and solar wind can enter the atmosphere without being diverted as on the Earth.
The rocks exposed at the surface may contain cosmogenic radionuclides formed by nuclear spallation reactions.
The rocks in the crust of Venus also do not contain remainant magnetization.
Navigation by means of a magnetic compass is not possible on the surface of Venus.
Auroral displays do not occur.
Energetic cosmic rays and ultraviolet radiation may break chemical bonds of compounds in the atmosphere thereby facilitating reactions that otherwise would not occur.
