Venus Nightside Research Articles

Use of VEGA data to analyse balloon options for possible subsequent long endurance Venus cloud layer missions

Abstract The vertical motions and buoyancy variations of the two VEGA super-pressure balloons, flown in the middle cloud layer of Venus, are discussed. Using data derived from these 1985 nightside flights, estimates are made of the energy required to operate some alternative balloon platform schemes under consideration for future-proposed Venus-atmosphere in situ science missions. Despite the dissimilarity of these alternative platform schemes, the energy inputs required to operate each scheme on the Venus nightside are found to be similar. Estimates of the associated mass penalties of the associated energy sources are also made. Further investigation of a vertical propulsive assist scheme is recommended.

Direct detection of atomic oxygen on the dayside and nightside of Venus

Atomic oxygen is a key species in the mesosphere and thermosphere of Venus. It peaks in the transition region between the two dominant atmospheric circulation patterns, the retrograde super-rotating zonal flow below 70 km and the subsolar to antisolar flow above 120 km altitude. However, past and current detection methods are indirect and based on measurements of other molecules in combination with photochemical models. Here, we show direct detection of atomic oxygen on the dayside as well as on the nightside of Venus by measuring its ground-state transition at 4.74 THz (63.2 µm). The atomic oxygen is concentrated at altitudes around 100 km with a maximum column density on the dayside where it is generated by photolysis of carbon dioxide and carbon monoxide. This method enables detailed investigations of the Venusian atmosphere in the region between the two atmospheric circulation patterns in support of future space missions to Venus.

A WISPR of the Venus Surface: Analysis of the Venus Nightside Thermal Emission at Optical Wavelengths

Parker Solar Probe (PSP) conducted several flybys of Venus while using Venus’ gravity for orbital adjustments to enable its daring passes of the Sun. During these flybys, PSP turned to image the nightside of Venus using the Wide-field Imager for Solar PRobe (WISPR) optical telescopes, which unexpectedly observed Venus’ surface through its thick and cloudy atmosphere in a theorized, but until-then unobserved near-visible spectral window below 0.8 μm. We use observations taken during PSP’s fourth Venus gravity assist flyby to examine the origin of the Venus nightside flux and confirm the presence of this new atmospheric window through which to observe the surface geology of Venus. The WISPR images are well explained by emission from the hot Venus surface escaping through a new atmospheric window in the optical with an overlying emission component from the atmosphere at the limb that is consistent with O2 nightglow. The surface thermal emission correlates strongly with surface elevation (via temperature) and emission angle. Tessera and plains units have distinct WISPR brightness values. Controlling for elevation, Ovda Regio tessera is brighter than Thetis Regio; likewise, the volcanic plains of Sogolon Planitia are brighter than the surrounding regional plains units. WISPR brightness at 0.8 μm is predicted to be positively correlated to FeO content in minerals; thus, the brighter units may have a different starting composition, be less weathered, or have larger particle sizes.

Minor species in Venus’ night side troposphere as observed by VIRTIS-H/Venus Express

The 2.3μm spectral window has been used to constrain the composition of the lower atmosphere (in the 30–40 km altitude range) on the night side of Venus for more than thirty years. Here, we present a follow-up study of Marcq et al. (2008), but using the full VIRTIS-H/Venus Express data archive as well as an updated radiative transfer forward model. We are able to confirm a latitudinal increase of CO of about 30% between 0° and 60°N, as well as an anti-correlated vertical shift of OCS profile by about −1km in the same latitude range. Both variations are about twice smaller in the southern hemisphere. Correlations of low latitude CO and OCS variations with zonally shifted surface elevation is tentatively found. These results are consistent with CO and OCS variations resulting from the competition between local thermochemistry and a Hadley-cell-like general circulation, albeit influenced by the orography. Finally, no evidence for spatial variations of water vapor (combined H2O and HDO) or sulfur dioxide could be evidenced in this data set; better constraining possible variations of these species would require future missions to include infrared spectrometers operating at a spectral resolving power higher than ∼104, such as VenSpec-H onboard EnVision. *Plain Language Summary Remotely measuring the composition of the Venusian atmosphere below the clouds is challenging, yet yields invaluable insights about the atmospheric chemistry, circulation and interaction with the surface and interior of the planet. The VIRTIS-H instrument on board ESA’s Venus Express orbiter (2006–2014) provides a rich data set in this regard, thanks to its ability to observe and analyze, on the night side of the planet, the infrared radiation emitted by the deep atmospheric layers. The results of our analyses confirm the previously observed trends for the variations of two trace gases (carbon monoxide and carbonyl sulfide) with latitude, explained by the combined effects of chemical reactions and transport by the atmospheric circulation. Variations of carbon monoxide may also be linked to the variations of ground elevation, confirming the link between surface topography and atmospheric circulation. However, we were unable to separate the signature of heavy water vapor from ordinary water vapor or to detect any variations in sulfur dioxide, both of which require more powerful infrared instruments such as those planned on future Venus orbiters such as ESA’s EnVision.

O2 (а1Δg) Airglow at 1.27 μM and upper Mesosphere Dynamics on the Night Side of Venus

This research studies the O2 (a1 Δg) nightglow distribution in 1.27 μm to understand the dynamics of the atmosphere of Venus. Several factors were considered in the retrieval process, such as thermal emission of the lower atmosphere, reflection by the clouds. Results show deviation from SS-AS circulation mode: the area where horizontal flows from the dayside converge and where oxygen recombines and emits shifts from the midnight to 22–23 hours local time. This shift is caused by solar-induced thermal tide on Venus nightside. Some conclusions about the upper mesosphere dynamics are also presented.

Venus’ Cloud-Tracked Winds Using Ground- and Space-Based Observations with TNG/NICS and VEx/VIRTIS

Characterizing the wind speeds of Venus and their variability at multiple vertical levels is essential for a better understanding of the atmospheric superrotation, constraining the role of large-scale planetary waves in the maintenance of this superrotation, and in studying how the wind field affects clouds’ distribution. Here, we present cloud-tracked wind results of the Venus nightside, obtained with unprecedented quality using ground-based observations during July 2012 with the near-infrared camera and spectrograph (NICS) of the Telescopio Nazionale Galileo (TNG) in La Palma. These observations were performed during 3 consecutive days for periods of 2.5 h starting just before dawn, sensing the nightside lower clouds of Venus close to 48 km of altitude with images taken at continuum K filter at 2.28 μm. Our observations cover a period of time when ESA’s Venus Express was not able to observe these deeper clouds of Venus due to a failure in the infrared channel of its imaging spectrometer, VIRTIS-M, and the dates were chosen to coordinate these ground-based observations with Venus Express’ observations of the dayside cloud tops (at about 70 km) with images at 380 nm acquired with the imaging spectrometer VIRTIS-M. Thanks to the quality and spatial resolution of TNG/NICS images and the use of an accurate technique of template matching to perform cloud tracking, we present the most detailed and complete profile of wind speeds ever performed using ground-based observations of Venus. The vertical shear of the wind was also obtained for the first time, obtained by the combination of ground-based and space-based observations, during the Venus Express mission since the year 2008, when the infrared channel of VIRTIS-M stopped working. Our observations exhibit day-to-day changes in the nightside lower clouds, the probable manifestation of the cloud discontinuity, no relevant variations in the zonal winds, and an accurate characterization of their decay towards the poles, along with the meridional circulation. Finally, we also present the latitudinal profiles of zonal winds, meridional winds, and vertical shear of the zonal wind between the upper clouds’ top and lower clouds, confirming previous findings by Venus Express.

A Comparative Study of Magnetic Flux Ropes in the Nightside Induced Magnetosphere of Mars and Venus

AbstractWe analyzed Mars Atmosphere and Volatile Evolution (MAVEN) and Venus Express (VEX) data of magnetic flux ropes observed in both Mars and Venus nightside induced magnetosphere, in order to understand their statistical characteristics and possible formation processes. We statistically identified 382 (286) events at Mars (Venus) via the minimum variance analysis and investigated the flux rope properties including their geometrical configurations of axial core field direction and spatial distributions. Interestingly, there is no significant difference with respect to the variety of the flux rope axial orientation between Mars and Venus, indicating that about at least one fourth of events do not necessitate magnetotail reconnection for their formations. However, the spatial distribution shows that flux ropes at Venus tend to be observed near the central plasma sheet, whereas those at Mars are more spread out. Moreover, the events tend to be observed more frequently in the −E hemisphere, where the solar wind electric field (ESW) is pointing toward the planet, rather than in +E hemisphere, where the ESW direction is away from the planet. The geographical distribution shows that flux ropes at Mars tend to be observed more frequently in the southern hemisphere where the strong crustal magnetic fields are primarily distributed. Therefore, such a tendency indicates that the Martian crustal magnetic fields could play significant roles on generating flux ropes in the nightside magnetosphere of Mars.

Winds in the Lower Cloud Level on the Nightside of Venus from VIRTIS-M (Venus Express) 1.74 μm Images

The horizontal wind velocity vectors at the lower cloud layer were retrieved by tracking the displacement of cloud features using the 1.74 µm images of the full Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS-M) dataset. This layer was found to be in a superrotation mode with a westward mean speed of 60–63 m s−1 in the latitude range of 0–60° S, with a 1–5 m s−1 westward deceleration across the nightside. Meridional motion is significantly weaker, at 0–2 m s−1; it is equatorward at latitudes higher than 20° S, and changes its direction to poleward in the equatorial region with a simultaneous increase of wind speed. It was assumed that higher levels of the atmosphere are traced in the equatorial region and a fragment of the poleward branch of the direct lower cloud Hadley cell is observed. The fragment of the equatorward branch reveals itself in the middle latitudes. A diurnal variation of the meridional wind speed was found, as east of 21 h local time, the direction changes from equatorward to poleward in latitudes lower than 20° S. Significant correlation with surface topography was not found, except for a slight decrease of zonal wind speed, which was connected to the volcanic area of Imdr Regio.

Global maps of Venus nightside mean infrared thermal emissions obtained by VIRTIS on Venus Express

One of the striking features about Venus atmosphere is its temporal variability and dynamics, with a chaotic polar vortex, large-scale atmospheric waves, sheared features and variable winds that depend on local time and possibly orographic features. The aim of this research is to combine data accumulated over several years and obtain a global mean state of the atmosphere focusing in the global structure of the clouds using the cloud opacity and upper cloud temperatures.We have first produced global maps using the integrated radiance through the infrared atmospheric windows centred around 1.74 μm and 2.25 μm, that show the spatial variations of the cloud opacity in the lower clouds around 44–48 km altitude and also provide an indirect estimation of the possible particle size. We have also produced similar global maps using the brightness temperatures seen in the thermal region at 3.8 μm and 5.0 μm, which provide direct indication of the temperatures at the top of the clouds around 60–70 km altitude.These maps have been generated using the complete dataset of the Visible and InfraRed Thermal Imaging Spectrometer mapping channel (VIRTIS-M) on board Venus Express, with a wide spatial and long temporal coverage in the period from May 2006 until October 2008.Our results provide a global view of the cloud opacity, particle size and upper cloud temperatures at both hemispheres, showing the main different dynamical regions of the planet. The profiles obtained also provide the detailed dependencies with latitude, local time and longitude, diagnostic of the global circulation flow and dynamics at various altitude layers, from about 44 up to 70 km over the surface.

Venus nightside surface temperature

First global map of Venus nightside surface temperature using Akatsuki infrared measurements reveals hot Venus surface with an average surface temperature of about 698 K. Surface temperatures do not show any significant variation with changing latitudes because only a small amount (~2.5%) of solar energy reaches the surface. Surface temperatures are relatively colder at higher altitude regions as compared to lower altitude regions. However, the major temperature variation on Venus surface is governed by various lithospheric heat transport mechanisms. On a global scale, surface temperatures show a spatial variation of about 230 K.

Nightside Winds at the Lower Clouds of Venus with Akatsuki/IR2: Longitudinal, Local Time, and Decadal Variations from Comparison with Previous Measurements

Abstract We present measurements of the wind speeds at the nightside lower clouds of Venus from observations by JAXA’s mission Akatsuki during 2016, complemented by new wind measurements from ground-based observations acquired with the TNG/Near Infrared Camera Spectrometer (NICS) in 2012 and IRTF/SpeX in 2015 and 2017. The zonal and meridional components of the winds were measured via cloud tracking on a total of 466 Akatsuki images of Venus acquired by the camera IR2 using the 2.26 μm filter, with spatial resolutions ranging from 10 to 80 km per pixel and covering 2016 March 22 to October 31. More than 149,000 wind vectors were obtained using an automatic template-matching technique, and 2947 wind vectors were inferred using a manual procedure. The meridional profiles for both components of the winds are found to be consistent with results from the Venus Express mission during 2006–2008, although stronger wind variability is found for the zonal component at equatorial latitudes where Akatsuki observations have better viewing geometry than Venus Express. The zonal winds at low latitudes also suggest a zonal variability that could be associated with solar tides or vertically propagating orographic waves. Finally, the combination of our wind measurements from TNG/NICS, IRTF/SpeX, and Akatsuki images with previously published data from 1978 to 2017 suggests variations of up to 30 m s−1 in the winds at the nightside lower clouds of Venus.

Effects of the solar wind and the solar EUV flux on O+ escape rates from Venus

We investigate dependences of O+ escape rates from Venus both on the solar wind and the solar extreme ultraviolet (EUV) flux by using the 8.5-year dataset (May 2006 to December 2014) of the Ion Mass Analyzer and the magnetometer aboard Venus Express. We examine the O+ escape rates for 8 different conditions depending on the solar wind's dynamic pressure, the magnitude of the motional electric field, and the solar F10.7 index which is a good indicator for the solar EUV flux. We find that the O+ escape rates are mainly controlled by the motional electric field and the solar EUV flux in the magnetosheath. We suggest that the ion pickup by the motional electric field is the main ion escape process in the magnetosheath. On the other hand, in the induced magnetosphere, the O+ escape rates are controlled by not only the solar wind and the solar EUV flux but also return flows. We find that the return flows become dominant in the magnetotail in the solar maximum period, which results in reducing the net escape rates in the induced magnetosphere. As a result, net escape rates tend to become larger in the solar minimum period than those in the solar maximum period. Further analysis of the magnetic field shows that the return flows are preferably observed when the magnetic field component along the Venus-Sun line reverses multiple times in the magnetotail. We suggest that ionospheric irregular structures or IMF sector boundary crossings form the fine scale anti-parallel draping pattern of the interplanetary magnetic field (IMF), and this causes ion acceleration towards Venus due to the magnetic tension force or magnetic reconnections. Another possibility is precipitations of O+ pickup ions on the nightside of Venus under the fine structure of IMF. Since the return flows significantly affect the O+ escape rates from Venus, it is important to study not only long-term transition of ion outflow from Venus but also return flows to estimate the net ion loss from Venus over the long history of the solar system.

Measurement of plasma channels in the Venus wake

Plasma channels have been suggested to account for the observation of ionospheric holes in the Venus nightside ionosphere. They are produced by the erosion of the solar wind on the magnetic polar regions of the Venus ionosphere, and are related to a sharp plasma transition that extends along the flanks of the wake and that is located downstream from the bow shock. The sharp plasma (intermediate) transition is a feature that has been detected with instruments onboard all the main spacecraft that have probed the Venus wake. From the early flyby transit of the Mariner 5 spacecraft to the later orbit crossings of the Venera 10, the Pioneer Venus and more recently the Venus Express there has been accumulated evidence of the presence of that transition by the flanks of the Venus wake. We report here measurements made with the ASPERA-4 instrument of the Venus Express that were conducted in orbit crossings by the midnight plane over a 4 year period (between 2006 and 2009), and that show different positions of the intermediate transition in the Venus wake at locations closer to the sun-Venus axis near solar minimum conditions by 2009. Measurements conducted in this latter time period also indicate that the intermediate transition by the midnight plane derives from the plasma channels and that is replaced by a broad velocity layer in orbits that probed farther away from that plane. It is argued that changes in the position of the intermediate transition with respect to the plasma channels are related to processes produced by the transport of solar wind momentum to the Venus polar ionosphere. The energy spectra of planetary ions measured in VEX orbits that probed by the midnight plane show momentum transport from the solar wind protons. This result is contrary to the conclusions reported by Collinson et al. (2014) in the sense that there is ¨no difference in the peak energy of the ionospheric outflow¨ when measured inside and outside an ionospheric hole.

Temperature estimation from hydroxyl airglow emission in the Venus night side mesosphere

The temperature of the night side of Venus at about 95 km has been determined by using spectral features of the hydroxyl airglow emission around 3 µm, recorded from July 2006 to July 2008 by VIRTIS onboard Venus Express. The retrieved temperatures vary from 145.5 to about 198.1 K with an average value of 176.3 ± 14.3 K and are in good agreement with previous ground-based and space observations. The variability with respect to latitude and local time has been studied, showing a minimum of temperature at equatorial latitudes, while temperature values increase toward mid latitudes with a local maximum at about 35°N. The present work provides an independent contribution to the temperature estimation in the transition region between the Venus upper mesosphere and the lower thermosphere, by using the OH emission as a thermometer, following the technique previously applied to the high-resolution O2(a1Δg) airglow emissions observed from ground.

Diurnal observations of HCl altitude variation in the 70–100 km mesosphere of Venus

First submm spectroscopic observations of the 625.9 GHz H35Cl absorption lines of the Venus dayside atmosphere were obtained with the James Clerk Maxwell Telescope (JCMT) on March 2, 2013. These data, which support retrieval of HCl altitude distributions in the Venus mesosphere (70–100 km), are presented here and compared with previously reported JCMT observations of Venus nightside HCl (Sandor et al., 2012). The measured dayside profile agrees with that of the nightside, indicating no diurnal variation is present. More specifically, the nightside spectra revealed a secular decrease of upper mesospheric HCl between observations one month apart, at fixed latitude and local time. The dayside profile reported here presents upper mesospheric abundances that are bracketed by the two previously measured nightside profiles, indicating that if diurnal variation is present, it must be weaker than the secular variations occurring at fixed local time. The previous study, which measured nightside HCl abundances above 85 km to be much smaller than predicted from photochemical modeling, suggested a dynamical explanation for the disagreement wherein nightside downwelling associated with the SubSolar to AntiSolar (SSAS) atmospheric circulation might suppress upper mesospheric abundances predicted purely from photochemistry. However a straightforward prediction from the proposed mechanism is that HCl abundance on the dayside, where the SSAS drives upward rather than downward transport should at least agree with, and perhaps exceed that of the photochemical model. The finding that dayside HCl abundance agrees with that of the nightside, hence also is much smaller than that of the model shows the SSAS hypothesis to be incorrect.

No statistical evidence of lightning in Venus night-side atmosphere from VIRTIS-Venus Express Visible observations

In this study we describe a dedicated analysis of luminous transient events on Venus night side atmosphere with the visible channel of the VIRTIS instrument (280–1100nm), this being the most comprehensive search of lightning conducted so far with Venus Express data. Our search results in thousands of signal detections, but unfortunately they can be all explained by cosmic rays impinging on the detector, and further statistical analysis shows that all of the events are randomly distributed along the spectral dimension, therefore not showing any clear evidence of signal coming from lightning emission in the Venus atmosphere. This does not exclude the existence of lightning, but imposes some constraints on their occurrence that are important for future research.

Observations of the nightside venusian hydrogen corona with SPICAV/VEX

Observations of the venusian hydrogen corona on the nightside (8PM local time) have been performed with SPICAV instrument on board Venus Express from 12 to 15 October 2011. These observations were associated with interplanetary observations obtained at apoapsis to subtract the interplanetary contribution. The observed brightness variation with altitude on the disk and above the limb shows that one part of the interplanetary emission is scattered out of the line of sight by the venusian hydrogen atoms above the CO2 absorption limb. The emission of the H corona beyond the cylindrical shadow is also scattered in the same way. Taking into account the geometry of the line of sight and this scattering in a radiative transfer model allows us to retrieve for the first time the vertical profile of the hydrogen density on the nightside of Venus from Lyman-α measurements. The derived cold hydrogen density at the exobase is ∼40 times larger than the dayside hydrogen density derived by the same instrument, showing an evening bulge in agreement with in situ measurements of Pioneer Venus. The hot hydrogen density derived at the nightside is ∼5 times larger than the average dayside hot hydrogen density found previously with SPICAV/Venus Express. A decrease of the hot hydrogen content by a factor ∼2 is observed during these three days of observations. This decrease could be explained by a decrease of the hot hydrogen production rate on the nightside driven by a variation of the dayside ionosphere.

Doppler winds mapped around the lower thermospheric terminator of Venus: 2012 solar transit observations from the James Clerk Maxwell Telescope

Doppler shifts of sub-millimeter 12CO (346GHz) and 13CO (330GHz) and millimeter 12CO (230GHz) line absorptions were mapped around the circum-disk terminator of Venus before, during, and after the June 5, 2012 solar transit, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of the solar transit 12CO thermal line absorptions yields cross-terminator winds in the Venus lower thermosphere (100–120km) over the local time (LT) and latitude extent of the atmospheric limb presented by the inferior conjunction, nightside apparent disk of Venus. The unique solar transit geometry provides enhanced spatial resolution of the terminator (0.2h in local time, LT) associated with solar illumination of this atmospheric limb region, and so provides the first characterization of the instantaneous distribution of cross terminator flow in the Venus lower thermosphere versus LT and latitude. Furthermore, by mapping Doppler winds over the nightside disk preceding and following the solar transit, we place the highly variable zonal and subsolar-to-antisolar (SSAS) circulation components of the nightside lower thermosphere (Clancy, R.T., Sandor, B.J., Moriarty-Schieven, G.H. [2012a]. Icarus 217, 794–812) in the context of the day-to-night cross terminator flow that drives this chaotic nightside dynamical regime. The solar transit observations indicate substantially supersonic (200–300m/s) day-to-night cross terminator winds that are significantly (by 50–150m/s) stronger over the evening versus the morning terminator. They also exhibit surprisingly large (50%) variations over a 1–2h timescale that challenge explanation. These behaviors likely contribute to both the variability and the apparent retrograde zonal component of circulation in the Venus nightside upper atmosphere. Hence, these observations support dynamical arguments for preferential deceleration of the morning sector SSAS circulation (e.g., Alexander, M.J. [1992]. Geophys. Res. Lett. 19, 2207–2210), as recently simulated in the Venus thermospheric general circulation model of Hoshino et al. (Hoshinom, N. et al. [2013]. J. Geophys. Res. 118, 2004–2015).

Thermal structure of Venus׳ nightside mesosphere as observed by infrared heterodyne spectroscopy at 10 μm

Ground-based heterodyne spectroscopy is used to observe the night side of Venus by probing single pressure broadened CO2 absorption lines. From the pressure induced line broadening, the predominant temperature at different altitude layers can be deduced. It is found, that heterodyne spectroscopy is sensitive to probe the mesosphere between ~60km and 90km with an altitude resolution of ~4.5km.During two observing campaigns in March and May 2012, four different locations on the planet were investigated. Herein, we report on the retrieval of vertical temperature profiles in the nightside atmosphere of Venus. Retrieval of atmospheric parameters is based on a Levenberg–Marquard χ2 optimization that iteratively compares observed data to telluric transmittance corrected Venus׳ top-of-atmosphere spectra calculated using a radiative transfer algorithm. The deduced profiles are compared to the Venus International Reference Atmosphere and some found to be in satisfactory agreement. Sub-Doppler resolution Infrared heterodyne observations can provide temperature measurements that complement existing sub-mm and space based observations.

Thermal structure of Venus nightside upper atmosphere measured by stellar occultations with SPICAV/Venus Express

The thermal structure of Venus upper atmosphere (90–140km) was investigated using stellar occultation measurements acquired by the SPICAV experiment on board Venus Express. The SPICAV ultraviolet channel provides CO2 local density and temperature vertical profiles with a vertical resolution of <7km of both the southern and the northern hemispheres on the nightside (18:00–06:00h local time). A permanent layer of warm air is observed at the mesopause in the altitude range 90–100km. Temperature then decreases with increasing altitude reaching a minimum value around 125km. Spatial and temporal changes in the thermal structure have been analyzed. Local time variations dominate the structure of Venus atmosphere at these altitudes: temperatures show an increase of ~20K on the morning side compared to the evening side. The homopause altitude was also determined; it varies between 119 and 138km of altitude, increasing from the evening side to the morning side. SPICAV temperature profiles were compared to several literature results from ground-based observations, previous spacecraft missions and the Venus Express mission.