Solar system

Abstract:

Context . Venus exhibits strong and changing contrasts at ultraviolet wavelengths. They appear to be related to the clouds and the dynamics in the cloud layer, but to date their origin continues to be unknown. Aims . We investigate the nature of the UV contrasts exhibited by Venus’ clouds by examining possible correlations between the thermal structure inferred from radio occultation data and UV brightness from imagery data, both observed with Venus Express. Methods . We analysed Venus Express images obtained from 11 hours before to a few hours after the time of radio occultation measurements of the same area. We accounted for the advection of clouds by zonal and meridional winds and applied a phase angle correction to compensate for the changing viewing geometry. Results . We find a possible anti-correlation between UV brightness and atmospheric temperature around an altitude of 67 km for low latitudes, with a one percent probability of this finding being due to chance (p value = 0.01). Heating in this altitude and latitude region due to an increase in the UV absorber has been predicted by radiative forcing studies. The predictions roughly match our observed temperature amplitude between UV-dark and UV-bright regions. Conclusions . This could be the first observational evidence of a direct link between UV brightness and atmospheric temperature in the 65–70 km altitude region in the clouds of Venus.

Abstract:

The Cassini spacecraft observed Titan from 2004 to 2017, capturing key atmospheric features, including the tilt of the middle atmosphere and the formation and breakup of winter polar vortices. We analyze low spectral resolution infrared observations from Cassini’s Composite Infrared Spectrometer (CIRS), which provide excellent spatial and temporal coverage and the best horizontal spatial resolution of any of the CIRS observations. With approximately 4 times higher meridional resolution than previous studies, we map the stratospheric temperature for almost half a Titan year. We determine the evolution of Titan’s stratospheric tilt, finding that it is most constant in the inertial frame, directed 120° ± 6° west of the Titan–Sun vector at the northern spring equinox, with seasonal oscillations in the tilt magnitude between around 2 .° 5 and 8°. Using the high meridional resolution temperature field, we reveal finer details in the zonal wind and potential vorticity. In addition to the strong winter zonal jet, a weaker zonal jet in Titan’s summer hemisphere is observed, and there is a suggestion that the main winter hemisphere jet briefly splits into two. We also present the strongest evidence yet that Titan’s polar vortex is annular for part of its life cycle.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>Using a newly developed ‘holistic’ atmospheric model of the aerosol structure in Uranus’s atmosphere, based upon observations made by HST/STIS, Gemini/NIFS and IRTF/SpeX from 2000 – 2009, we make a new estimate the bolometric Bond albedo of Uranus during this time of A* = 0.338 ± 0.011, with a phase integral of q* = 1.36 ± 0.03. Then, using a simple seasonal model, developed to be consistent with the disc-integrated blue and green magnitude data from the Lowell Observatory from 1950 – 2016, we model how Uranus’s reflectivity and heat budget vary during its orbit and determine new orbital-mean average values for the bolometric Bond albedo of $\overline{A^*} = 0.349 \pm 0.016$ and for the absorbed solar flux of $\overline{P_\mathrm{in}}=0.604 \pm 0.027$ W m−2. Assuming the outgoing thermal flux to be $\overline{P_\mathrm{out}}=0.693 \pm 0.013$ W m−2, as previously determined from Voyager 2 observations, we arrive at a new estimate of Uranus’s average heat flux budget of Pout/Pin = 1.15 ± 0.06, finding considerable variation with time due to Uranus’s significant orbital eccentricity of 0.046. This leads the flux budget to vary from Pout/Pin = 1.03 near perihelion, to 1.24 near aphelion. We conclude that although Pout/Pin is considerably smaller than for the other giant planets, Uranus is not in thermal equilibrium with the Sun.</jats:p>