Glory revealed in disk-integrated photometry of Venus

Abstract

Context. Reflected light from a spatially unresolved planet yields unique insight into the overall optical properties of the planet cover. Glories are optical phenomena caused by light that is backscattered within spherical droplets following a narrow distribution of sizes; they are well known on Earth as localised features above liquid clouds. Aims. Here we report the first evidence for a glory in the disk-integrated photometry of Venus and, in turn, of any planet. Methods. We used previously published phase curves of the planet that were reproduced over the full range of phase angles with model predictions based on a realistic description of the Venus atmosphere. We assumed that the optical properties of the planet as a whole can be described by a uniform and stable cloud cover, an assumption that agrees well with observational evidence. Results. We specifically show that the measured phase curves mimic the scattering properties of the Venus upper-cloud micron-sized aerosols, also at the small phase angles at which the glory occurs, and that the glory contrast is consistent with what is expected after multiple scattering of photons. In the optical, the planet appears to be brighter at phase angles of ∼11-13 • than at full illumination; it undergoes a maximum dimming of up to ∼10% at phases in between. Conclusions. Glories might potentially indicate spherical droplets and, thus, extant liquid clouds in the atmospheres of exoplanets. A prospective detection will require exquisite photometry at the small planet-star separations of the glory phase angles.

Venus’ clouds as inferred from the phase curves acquired by IR1 and IR2 on board Akatsuki

“…Firstly, a glory feature in disk-integrated photometry data (Mallama et al, 2006), similar to our IR1 data, is identified by García Muñoz et al (2014). In this paper, we focused on Mallama et al's I-band data (the same wavelength with the IR1 data) but the glory in their data was more noticeable in shorter wavelengths (García Muñoz et al, 2014).…”

“…The agreement between the observed data and model computation is found to be fairly good, although there is substantial scattering in the observed data at small phase angles. The model curve indicates a shallow minimum around a $ 10°, which was identified as a glory feature in I band (García Muñoz et al, 2014).…”

Inferring heat recirculation and albedo for exoplanetary atmospheres: Comparing optical phase curves and secondary eclipse data

“…Ground-based data as well as spacecraft observations (e.g., Venus Express, Voyager 1 and 2, Pioneer 10 and 11) have been used to investigate, e.g., Venus (e.g., Arking & Potter 1968;García Muñoz et al 2014;Petrova et al 2015), Titan (e.g., Rages et al 1983), Jupiter (e.g., Tomasko et al 1978;Smith & Tomasko 1984), Saturn (e.g., Tomasko & Doose 1984) or Uranus (Rages et al 1991;Pryor et al 1997).…”

Long-term Variations of Venus’s 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope

“…Spectral data were convolved using the filter transmittance function of the 365- nm channel of VMC, and we applied the same photometric correction as for the images. In order to take into account UVI's 2011 data, we calculated whole-disk albedo (Sromovsky et al 2001;García Muñoz et al 2014, 2017, without photometric correction due to the small apparent size of Venus (Section 3.2). Radiative transfer model calculations were performed using the model and gaseous database in Lee et al (2015bLee et al ( , 2016) (Section 3.3), to estimate the abundance of the unknown absorber that explains the observed 365-nm albedo, and to calculate solar heating rates.…”

“…r o is the radius range in which the measured radiance are summed considering the point spread function of the instrument (5 pixels) that is the required radius of aperture photometry of UVI star flux analysis. The whole-disk albedo can be expressed as A g Φ(α), where A g is geometric albedo, and Φ(α) is the phase law of Venus, describing the disk-integrated scattering efficiency as a function of phase angle (García Muñoz et al 2014, 2017.…”