  Earth possesses a persistent, internally-generated magnetic field, whereas no
trace of a dynamo has been detected on Venus, at present or in the past,
although a high surface temperature and recent resurfacing events may have
removed paleomagnetic evidence. Whether or not a terrestrial body can sustain
an internally generated magnetic field by convection inside its metallic fluid
core is determined in part by its initial thermodynamic state and its
compositional structure, both of which are in turn set by the processes of
accretion and differentiation. Here we show that the cores of Earth- and
Venus-like planets should grow with stable compositional stratification unless
disturbed by late energetic impacts. They do so because higher abundances of
light elements are incorporated into the liquid metal that sinks to form the
core as the temperatures and pressures of metal-silicate equilibration increase
during accretion. We model this process and determine that this establishes a
stable stratification that resists convection and inhibits the onset of a
geodynamo. However, if a late energetic impact occurs, it could mechanically
stir the core creating a single homogenous region within which a long-lasting
geodynamo would operate. While Earth's accretion has been punctuated by a late
giant impact with likely enough energy to mix the core (e.g. the impact that
formed the Moon), we hypothesize that the accretion of Venus is characterized
by the absence of such energetic giant impacts and the preservation of its
primordial stratifications.
