Long-term variations of Venus' 365-nm albedo observed by Venus Express, Akatsuki, MESSENGER, and Hubble Space Telescope

Abstract: An unknown absorber near the cloud top level of Venus generates a broad absorption feature from the ultraviolet (UV) to visible, peaking around 360 nm, and therefore plays a critical role in the solar energy absorption. We present a quantitative study on the variability of the cloud albedo at 365 nm and its impact on Venus' solar heating rates based on an analysis of Venus Express and Akatsuki's UV images, and Hubble Space Telescope and MESSENGER's UV spectral data; in this analysis the calibration correction factor of the UV images of Venus Express (VMC) is updated relative to the Hubble and MESSENGER albedo measurements. Our results indicate that the 365-nm albedo varied by a factor of 2 from 2006 to 2017 over the entire planet, producing a 25-40% change in the low latitude solar heating rate according to our radiative transfer calculations. Thus, the cloud top level atmosphere should have experienced considerable solar heating variations over this period. Our global circulation model calculations show that this variable solar heating rate may explain the observed variations of zonal wind from 2006 to 2017. Overlaps in the timescale of the long-term UV albedo and the solar activity variations make it plausible that solar extreme UV intensity and cosmic-ray variations influenced the observed albedo trends. The albedo variations might also be linked with temporal variations of the upper cloud SO2 gas abundance, which affects the H2SO4-H2O aerosol formation.

• The paper demonstrates that Venus’ 365-nm albedo varied by a factor of 2 over 11 years, affecting solar heating rates by 25%–40%.
• It employs multi-mission observations from Venus Express, Akatsuki, MESSENGER, and Hubble alongside advanced radiative transfer models to analyze cloud dynamics.
• The study suggests that albedo fluctuations and the unknown absorber may drive changes in zonal wind speeds, offering new insights into Venus’ super-rotation.

Insights into Venus' Long-term 365-nm Albedo Variations

The paper "Long-term variations of Venus' 365-nm albedo observed by Venus Express, Akatsuki, MESSENGER, and Hubble Space Telescope" provides a comprehensive analysis of the temporal changes in the cloud albedo of Venus at the 365-nm wavelength. This wavelength is critical due to the presence of an unknown absorber in Venus' atmosphere that influences solar energy absorption. The paper spans observations made with multiple instruments across different missions, utilizing sophisticated radiative transfer models to ascertain the implications of these albedo variations on Venus' atmospheric dynamics.

Key Observations and Findings

The study documents substantial variability in the albedo of Venus at 365 nm over an 11-year period. This variability, detected across instruments such as Venus Express, Akatsuki, MESSENGER, and Hubble Space Telescope, includes a notable reduction and subsequent recovery in albedo across the observation period. Numerical data indicates that the albedo at this wavelength varied by a factor of 2 during this time. Such variations substantially impact the solar heating rates at Venus' low latitudes. Radiative transfer calculations suggest these albedo changes correspond to solar heating rate alterations between 25% and 40%, signifying significant atmospheric heating variation at Venus' cloud tops.

The Role of the Unknown Absorber

The mysterious absorber near Venus' cloud tops, which absorbs solar radiation prominently from the UV to visible spectrum, forms a central element of the study. Despite ongoing research, the precise chemical composition of this absorber remains unidentified, though it significantly influences the energy budget of Venus' atmosphere. The study offers innovative insights into how this unknown absorber's variability, mirrored in the observed albedo changes, may affect atmospheric dynamics, including zonal wind patterns and general circulation.

Implications on Venus' Atmospheric Dynamics

The researchers postulate that the observed variability in solar heating, driven by changes in the 365-nm albedo, can explain fluctuations in Venus' zonal wind speeds. This is particularly significant given that the super-rotation of Venus' atmosphere is a unique characteristic distinguishing it from other terrestrial planets. The simulation results employing the IPSL Venus GCM model reveal a potential correlation between solar heating rates and zonal wind speed, where increased solar heating may accelerate atmospheric winds through enhanced meridional transport and thermal tides.

Theoretical and Practical Implications

Theoretically, this study underscores the role of variable solar energy input in modulating Venus' atmospheric circulation, providing a new dimension for understanding teleconnections between solar energy variability and atmospheric dynamics. Practically, extending these findings could prompt a refinement of current atmospheric models for Venus, enhancing future observational strategies.

Future Research Directions

Future work could build upon these findings by exploring the interplay between Venus' atmospheric chemistry, cloud dynamics, and energy balance in more depth. Additionally, further refinement in identifying the chemical composition of the unknown absorber could illuminate its role in Venusian atmospheric phenomena. Continuing long-term observations and modeling will be critical in tracing the influence of solar cycles on Venus' albedo variability and associated atmospheric changes.

In conclusion, the presented work offers a significant contribution to our understanding of Venus' atmospheric dynamics, particularly emphasizing the intricate linkages between albedo variations and atmospheric processes. This study not only enhances our comprehension of Venus but also allows researchers to draw parallels in planetary atmosphere studies across the solar system.