APATITE ON VENUS: SURFACE-ATMOSPHERE REACTION, CRYSTAL CHEMISTRY, AND RADAR REMOTE SENSING

The volatile element constituents of apatite can respond to its chemical surroundings, be they igneous, metasomatic, groundwater, or even planetary atmospheres. Thus, one can expect apatite on a hot planet’s surface, like Venus, to approach equilibrium with its atmosphere. Venus’s surface is composed mostly of basalt, and likely contains fluorapatite (by analogy with other planetary basalts). It is in contact with Venus’ Cl-bearing CO₂-dominant atmosphere at ~740K and 92 bars pressure, and would react over time to become chlorapatite.

This chemical transformation can be detected remotely because chlorapatite can undergo a phase transition from ferroelectric (at lower T) to dielectric. A ferroelectric substance maintains an electrical dipole that can be reversed by an applied electrical charge. In apatite’s crystal structure, the halogens and OH sit in ‘open channels’ parallel to the c axis. OH^- and F^- ions are small and occupy a high-symmetry site; Cl^- ions are too large to fit on that site, and are displaced from it along c. This displacement prouces chlorapatites’ electrical dipole (along c). If a strong electric charge is applied, the Cl^- ions can be forced to move across the high-symmetry site, i.e. the chlorapatite is ferroelectric. At high temperatures the Cl^- ions can jump easily across the high-symmetry site, and chlorapatite becomes dielectric (electrically normal). Such a phase transformation is marked by a spike in electrical permittivity (i.e., dielectric constant), and is thus detectable by radar either as a spike in backscatter or a dip in emissivity.

On Venus’ equatorial highlands, radar observations (principally from the Magellan orbiter) show increasing backscatter going from low to high elevations (decreasing temperature), followed by a sharp decrease at the highest elevations. This pattern implies the presence of a material with a ferroelectric->dielectric phase transition at ~700K, such as occurs in chlorapatite, and no other common material. Young volcanos in equatorial regions do not show this pattern of radar backscatter, which suggests that the chlorapatite develops over time.