Preparing for X/Ka Band radio occultations of Venus with VERITAS and
EnVision: Retrieving Sulfur Species Abundances
Abstract
The recently selected VERITAS and EnVision missions will fly with X and
Ka Band telecommunications channels, permitting dual band radio
occultations of the Venus atmosphere. While signal absorption
measurements during S and X Band radio occultations of Venus in the past
have been used to retrieve vertical abundances of
H2SO4 vapor as a function of latitude,
Ka Band links have yet to be employed to sound the neutral atmosphere.
Laboratory measurements and propagation models of the Venus atmosphere
suggest that H2SO4 cloud aerosols/vapor
and SO2 absorb radio signals differently between X and
Ka bands, permitting inversion of their abundance profiles down to the
attenuation limit near 45 km. Such measurements would be of great value
to the study of Venus atmospheric chemistry and dynamics. While the bulk
abundance of SO2 at the cloud base has been inferred
from microwave/infrared radiometry and from X Band occultations, Ka Band
measurements could be used to derive vertically resolved profiles at the
deepest altitudes yet, spanning a region where the abundance of
SO2 changes by several orders of magnitude. Vertical
profiles of lower cloud bulk density could also be achieved at higher
resolution than any prior remote measurements. This presentation will
discuss approaches to retrieving H2SO4
vapor/aerosol, and SO2 abundances using dual X/Ka Band
radio occultations of the Venus neutral atmosphere.
H2SO4 vapor can be retrieved with very
high accuracy, surpassing that of prior single frequency occultations.
Due to the relatively low (high) X (Ka) Band opacity of both
SO2 and H2SO4 aerosols,
retrievals of these species from dual band occultations are highly
degenerate. To improve accuracy, we find that it is necessary to
incorporate the results of chemical and dynamical modeling as prior
information. At lower latitudes and in regions of high abundance,
preliminary results suggest retrievals of SO2 profiles
can be accomplished within 15-20% uncertainty. This error increases at
higher latitudes, where models of cloud bulk density span a wider range
of predictions. We compare the effects of various assumptions on the
accuracy of the resulting retrievals, and discuss prospects for coupling
a chemical/dynamics model to the retrieval process.