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Imaging Venus' surface at night in the near-IR from above its clouds: New analytical models for the effective spatial resolution, illustrated with new Parker Solar Probe data

Anthony B. Davis

TL;DR

The paper develops an analytical framework for the atmospheric point-spread function (APSF) to quantify the effective spatial resolution of Venus nightside surface imaging through cloud scattering in the near-IR, using a blend of ballistic sub-cloud propagation and diffusive cloud treatments. It presents three progressively sophisticated models—treating the clouds as a thin diffusing plate, then as a two-layer finite-thickness medium with diffusion/P1 radiative transfer—to predict the ESR, finding FWHM values around 130 km across 1.0–1.2 μm. The framework is extended to Parker Solar Probe’s WISPR instrument, with empirical and theoretical footprints converging to about 150–162 km at 0.8 μm, validating the diffusion-based approach in a flyby context. Collectively, the work clarifies why Venus nightside imaging yields ESR near 100–130 km rather than the much smaller ~50 km values sometimes cited, and provides practical expressions linking atmospheric properties to image sharpness for current and future missions.

Abstract

There are a handful of spectral windows in the near-IR through which we can see down to Venus' surface on the night side of the planet. The surface of our sister planet has thus been imaged by sensors on Venus-orbiting platforms (Venus Express, Akatsuki) and during fly-by with missions to other planets (Galileo, Cassini). The most tantalizing finding, so far, is the hint of possible active volcanism. However, the thermal radiation emitted by Venus' searing surface (c. 475 degrees C) has to get through the opaque clouds between 50 and 70 km altitude, as well as the sub-cloud atmosphere. In the clouds, the light is not absorbed but scattered, indeed, many times. This results in blurring the surface imagery to the point where the smallest discernible feature is roughly 100 km in size, full-width half-max (FWHM), and this has been reproduced using numerical models. We propose a new analytical modeling framework for predicting the width of the atmospheric point-spread function (APSF) that determines the effective resolution of surface imaging from space. Our best estimates of the APSF width for the 1-to-1.2 micron spectral range are clustered around 130 km FWHM. Interestingly, this is somewhat larger than the accepted value of 100 km based on both visual image inspection and detailed numerical simulations. Lastly, we apply the new modeling framework to the fly-by imaging by the Parker Solar Probe in a somewhat shorter wavelength band.

Imaging Venus' surface at night in the near-IR from above its clouds: New analytical models for the effective spatial resolution, illustrated with new Parker Solar Probe data

TL;DR

The paper develops an analytical framework for the atmospheric point-spread function (APSF) to quantify the effective spatial resolution of Venus nightside surface imaging through cloud scattering in the near-IR, using a blend of ballistic sub-cloud propagation and diffusive cloud treatments. It presents three progressively sophisticated models—treating the clouds as a thin diffusing plate, then as a two-layer finite-thickness medium with diffusion/P1 radiative transfer—to predict the ESR, finding FWHM values around 130 km across 1.0–1.2 μm. The framework is extended to Parker Solar Probe’s WISPR instrument, with empirical and theoretical footprints converging to about 150–162 km at 0.8 μm, validating the diffusion-based approach in a flyby context. Collectively, the work clarifies why Venus nightside imaging yields ESR near 100–130 km rather than the much smaller ~50 km values sometimes cited, and provides practical expressions linking atmospheric properties to image sharpness for current and future missions.

Abstract

There are a handful of spectral windows in the near-IR through which we can see down to Venus' surface on the night side of the planet. The surface of our sister planet has thus been imaged by sensors on Venus-orbiting platforms (Venus Express, Akatsuki) and during fly-by with missions to other planets (Galileo, Cassini). The most tantalizing finding, so far, is the hint of possible active volcanism. However, the thermal radiation emitted by Venus' searing surface (c. 475 degrees C) has to get through the opaque clouds between 50 and 70 km altitude, as well as the sub-cloud atmosphere. In the clouds, the light is not absorbed but scattered, indeed, many times. This results in blurring the surface imagery to the point where the smallest discernible feature is roughly 100 km in size, full-width half-max (FWHM), and this has been reproduced using numerical models. We propose a new analytical modeling framework for predicting the width of the atmospheric point-spread function (APSF) that determines the effective resolution of surface imaging from space. Our best estimates of the APSF width for the 1-to-1.2 micron spectral range are clustered around 130 km FWHM. Interestingly, this is somewhat larger than the accepted value of 100 km based on both visual image inspection and detailed numerical simulations. Lastly, we apply the new modeling framework to the fly-by imaging by the Parker Solar Probe in a somewhat shorter wavelength band.
Paper Structure (13 sections, 27 equations, 4 figures, 1 table)

This paper contains 13 sections, 27 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction & Overview
  2. Cloud layer as a diffusing plate at 50 km altitude
  3. Cloud layer as a multiple scattering optical medium
  4. Extension to flyby imaging by the Parker Solar Probe
  5. Empirical estimate of the effective surface footprint of WISPR cameras
  6. Theoretical estimate of WISPR's anticipated spatial resolution
  7. Summary
  8. Two P$_1$ approximation models for multiple scattering in the WISPR/PSP application in §\ref{['sec:PSP_flyby']}
  9. Previous investigations of nightside Venus surface imaging from space in the NIR
  10. Definition of the APSF in radiative transfer (RT)
  11. Definition of the AMTF in the diffusion/P$_1$ approximation to RT
  12. Diffuse point-source on a reflective boundary
  13. Collimated point-source opposite a reflective boundary

Figures (4)

  • Figure 1: Graphical summary of the investigation by DavisEtAl_PSS_2024 of the feasibility of sharp nighttime NIR imaging of Venus' thermally glowing surface from a platform located just below the clouds.
  • Figure 2: Schematic for the estimation of the effective resolution of surface features achievable from space. Top panel: As a first-order approximation, the clouds act as a diffusing plate; thus, the smallest resolvable feature depends only on cloud-base height (left-hand side), which is $\approx$50 km. Factoring in the extinction across the sub-cloud Rayleigh-scattering atmosphere (right-hand side), the effective resolution becomes somewhat sharper since near-nadir rays weigh in more heavily on average. Bottom panel: Conceptualization of the cloud layer as a diffusing plate is upgraded to a strongly scattering medium of finite thickness where transmitted light is dispersed away from the horizontal position of the source by a predictable RMS distance. Effective surface resolution from above the clouds is then the RMS sum of two horizontal transport mechanisms: multiple scattering inside the cloud, and attenuated ballistic propagation below the cloud. See text for further details.
  • Figure 3: (a) WISPR/PSP image of the nighttime side of Venus.(b) Venus topography map from Magellan projected on to the side-viewed sphere.
  • Figure 4: Empirical estimate of the effective spatial resolution of WISPR cameras when imaging Venus' surface at 0.8 $\mu$m, as determined by the APSF and described in main text.