Stationary waves and slowly moving features in the night upper clouds of Venus

At the cloud top level of Venus (65-70 km altitude) the atmosphere rotates 60 times faster than the underlying surface, a phenomenon known as superrotation. Whereas on Venus's dayside the cloud top motions are well determined and Venus general circulation models predict a mean zonal flow at the upper clouds similar on both day and nightside, the nightside circulation remains poorly studied except for the polar region. Here we report global measurements of the nightside circulation at the upper cloud level. We tracked individual features in thermal emission images at 3.8 and 5.0 $\mathrm{\mu m}$ obtained between 2006 and 2008 by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M) onboard Venus Express and in 2015 by ground-based measurements with the Medium-Resolution 0.8-5.5 Micron Spectrograph and Imager (SpeX) at the National Aeronautics and Space Administration Infrared Telescope Facility (NASA/IRTF). The zonal motions range from -110 to -60 m s$^{-1}$, consistent with those found for the dayside but with larger dispersion. Slow motions (-50 to -20 m s$^{-1}$) were also found and remain unexplained. In addition, abundant stationary wave patterns with zonal speeds from -10 to +10 m s$^{-1}$ dominate the night upper clouds and concentrate over the regions of higher surface elevation.Comment: 15 pages, 4 figures, 6 supplementary figure

Spatial and Temporal Variability of the 365‐nm Albedo of Venus Observed by the Camera on Board Venus Express

We mapped the distribution of the 365‐nm albedo of the Venus atmosphere over the years 2006–2014, using images acquired by the Venus Monitoring Camera (VMC) on board Venus Express. We selected all images with a global view of Venus to investigate how the albedo depends on longitude. Bertaux et al. (2016, https://doi.org/10.1002/2015JE004958) reported a peak in albedo around 100° longitude and speculated on an association with the Aphrodite Terra mountains. We show that this peak is most likely an artifact, resulting from long‐term albedo variations coupled with considerable temporal gaps in data sampling over longitude. We also used a subset of images to investigate how the albedo depends on local time, selecting only south pole viewing images of the dayside (local times 7–17 hr). Akatsuki observed mountain‐induced waves in the late afternoon at 283 nm and 10 μm (Fukuhara et al., 2017, https://doi.org/10.1038/ngeo2873). We expect that the presence of such waves may introduce 365‐nm albedo variations with a periodicity of one solar day (116.75 Earth days). We searched for such a periodicity peak at 15:30–16:00 local time and low latitudes but did not find it. In conclusion, we find that temporal albedo variations, both short and long term, dominate any systematic variations with longitude and local time. The nature of VMC dayside observations limits regular data sampling along longitudes, so longitudinal variations, if they exist, are difficult to extract. We conclude that any influence by the Venus surface on 365‐nm albedo is negligible within this data set.TU Berlin, Open-Access-Mittel – 2020EC/H2020/841432/EU/Cloud Worlds: from Venus to Exoplanet/CLOUDWORLD

Thermal Infrared Imaging Experiments of C-Type Asteroid 162173 Ryugu on Hayabusa2

The thermal infrared imager TIR onboard Hayabusa2 has been developed to investigate thermo-physical properties of C-type, near-Earth asteroid 162173 Ryugu. TIR is one of the remote science instruments on Hayabusa2 designed to understand the nature of a volatile-rich solar system small body, but it also has significant mission objectives to provide information on surface physical properties and conditions for sampling site selection as well as the assessment of safe landing operations. TIR is based on a two-dimensional uncooled micro-bolometer array inherited from the Longwave Infrared Camera LIR on Akatsuki (Fukuhara et al., 2011). TIR takes images of thermal infrared emission in 8 to 12 μm with a field of view of 16×12∘ and a spatial resolution of 0.05∘ per pixel. TIR covers the temperature range from 150 to 460 K, including the well calibrated range from 230 to 420 K. Temperature accuracy is within 2 K or better for summed images, and the relative accuracy or noise equivalent temperature difference (NETD) at each of pixels is 0.4 K or lower for the well-calibrated temperature range. TIR takes a couple of images with shutter open and closed, the corresponding dark frame, and provides a true thermal image by dark frame subtraction. Data processing involves summation of multiple images, image processing including the StarPixel compression (Hihara et al., 2014), and transfer to the data recorder in the spacecraft digital electronics (DE). We report the scientific and mission objectives of TIR, the requirements and constraints for the instrument specifications, the designed instrumentation and the pre-flight and in-flight performances of TIR, as well as its observation plan during the Hayabusa2 mission

AKATSUKI-IR2 reveals unexpected opacity disruption affecting Venus's lower clouds every 9 days

The images of AKATSUKI acquired with the camera IR2 at 1.74-2.3 µm report the discovery of an equatorial disruption or “front” in the opacity of the lower clouds of Venus at 50 km between 30ºN¿30ºS. This feature appears on the night every 9 terrestrial days during more than 8 months, and introduces a dramatic and abrupt increase of the cloud opacity and reducing the thermal radiance in a factor of about 1:2 from its brightest to the darkest side.Peer ReviewedPostprint (published version

Active Membrane Fluctuations Studied by Micropipet Aspiration

We present a detailed analysis of the micropipet experiments recently reported in J-B. Manneville et al., Phys. Rev. Lett. 82, 4356--4359 (1999), including a derivation of the expected behaviour of the membrane tension as a function of the areal strain in the case of an active membrane, i.e., containing a nonequilibrium noise source. We give a general expression, which takes into account the effect of active centers both directly on the membrane, and on the embedding fluid dynamics, keeping track of the coupling between the density of active centers and the membrane curvature. The data of the micropipet experiments are well reproduced by the new expressions. In particular, we show that a natural choice of the parameters quantifying the strength of the active noise explains both the large amplitude of the observed effects and its remarkable insensitivity to the active-center density in the investigated range. [Submitted to Phys Rev E, 22 March 2001]Comment: 14 pages, 5 encapsulated Postscript figure

Cross calibration between Hayabusa2/ONC-T and OSIRIS-REx/MapCam for comparative analyses between asteroids Ryugu and Bennu

Proximity observations of (162173) Ryugu by the telescopic Optical Navigation Camera onboard Hayabusa2 and (101955) Bennu by MapCam onboard Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer found opposite spectral trends of space weathering on these carbonaceous asteroids. Whether the space weathering trends on these asteroids evolved from the same starting spectra would place an important constraint for understanding their relation. However, systematic error between data obtained by the two imagers needed to be reduced for accurate comparison. To resolve this problem, we cross calibrated albedo and color data using the Moon as the common standard. We show that the cross-calibrated reflectance can be obtained by upscaling the pre-cross-calibrated reflectance of Bennu by 12 +/- 2% at v-band, reducing the systematic errors down to 2%. The cross-calibrated data show that Bennu is brighter by 16 +/- 2% at v-band and bluer in spectral slope by 0.19 +/- 0.05 (/um) than Ryugu. The spectra of fresh craters on Ryugu and Bennu before cross calibration appeared to follow two parallel trend lines with offset, but they converged to a single trend after cross calibration. Such a post-cross-calibration perspective raise the possibility that Ryugu and Bennu evolved from materials with similar visible spectra but evolved in diverging directions by space weathering. The divergent evolution can be caused by the difference in space weathering dose/process and/or composition of the starting material. Thus, comparing the composition of samples returned from Ryugu and Bennu may change the way we interpret the spectral variation of C-complex asteroids

An artificial impact on the asteroid (162173) Ryugu formed a crater in the gravity-dominated regime

The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble-pile structure. We describe an impact experiment on Ryugu using Hayabusa2’s Small Carry-on Impactor. The impact produced an artificial crater with a diameter >10 meters, which has a semicircular shape, an elevated rim, and a central pit. Images of the impact and resulting ejecta were recorded by the Deployable CAMera 3 for >8 minutes, showing the growth of an ejecta curtain (the outer edge of the ejecta) and deposition of ejecta onto the surface. The ejecta curtain was asymmetric and heterogeneous and it never fully detached from the surface. The crater formed in the gravity-dominated regime; in other words, crater growth was limited by gravity not surface strength. We discuss implications for Ryugu’s surface age