The first NASA spacecraft to visit and explore planet Venus since the 1990s will be the Venus Emissivity, Radio science, InSAR, Topography, And Spectroscopy mission (VERITAS) orbiter. The Venus Emissivity Mapper (VEM) onboard the spacecraft is designed for surface mapping of Venus within dedicated atmospheric spectral windows. The instrument will provide global coverage for the detection of thermal emissions like volcanic activity, surface rock composition, water abundance, cloud formation and their dynamics by observing 14 narrow filter bands in the near-infrared to short-wave infrared (NIR, SWIR) range of 790 nm to 1510 nm. An almost identical instrument will be part of ESA’s recently announced EnVision mission to Venus, the VenSpec-M in the Venus Spectroscopy Suite (VenSpec). The utilized photodetector for both missions will be an InGaAs type imaging sensor with integrated thermoelectric (TE) cooling, comprising a 640x512 pixel array with 20 μm pixel pitch.
In general, a space environmental qualification of electronic devices combines its susceptibility to radiation induced single event effects (SEE) and the evaluation of permanent degradation effects due to total ionizing dose (TID) and displacement damage dose (DDD). Following a successful qualification test with heavy-ions focusing on SEE, our imaging sensor was subject to a proton irradiation test campaign at Helmholtz-Zentrum Berlin (HZB) for combined TID and DDD testing. To track the sensor evolution, we subdivided the proton fluence into 10 irradiation steps with intermediate measurements. The collected data provide information on the evolution of dark current, light sensitivity and pixels showing randomtelegraph- noise (RTN) on the sensor during a 5-year mission.
We report on the current Venus Emissivity Mapper (VEM) instrument design and development status onboard NASAs Venus Emissivity, Radio science, InSAR, Topography, And Spectroscopy (VERITAS) and ESAs EnVision orbiters. The VEM instrument is a push broom multispectral imager that comprises an optical system based on a sophisticated filter assembly with 14 spectral bands and an InGaAs detector with integrated thermoelectric cooler. A turn window mechanism and a two-staged baffle in front of the optics protect the instrument against contamination and straylight. The instruments nominal mass is approximately 6 kg. VEM opens the path for mapping Venus surface emission with a global coverage of >70%.
The Venus Emissivity Mapper (VEM) as part of NASAs Venus Emissivity, Radio science, InSAR, Topography, And Spectroscopy mission (VERITAS) is designed for mapping the surface of Venus within dedicated atmospheric spectral windows. The instrument will provide global coverage for detection of thermal emissions like volcanic activity, surface rock composition, water abundance and cloud formation as well as dynamics by observing 15 narrow filter bands in the near infrared to short wavelength infrared (NIR, SWIR) range of 862 nm to 1510 nm. An almost identical instrument will be part of ESAs EnVision mission to Venus, the VenSpec-M in the Venus Spectroscopy Suite (VenSpec). The utilized photodetector is an InGaAs type imaging sensor with integrated thermoelectric (TE) cooling. It comprises a 640x512 pixel array with 20 μm pixel pitch. Following the mission requirements we irradiated the detector with a set of ions of various stopping powers and range distributions from lower energy Argon (Ar) to higher energy Xenon (Xe). Therefore, exploiting the mentioned ions and proper tilt angles during irradiation, our data covers a Linear Energy Transfer (LET) range of 7 to 75 MeVcm2/mg which fulfills NASA/JPL led space qualification standards (up to 75 MeVcm2/mg) as well as ESA space qualification standards (up to 60 MeVcm2/mg) for heavy-ion irradiation. Our electrical setup consists of a dedicated over-current protection detecting high-current states occurring during irradiation steps and immediate power cycling to prevent physical damage of the device. From the event rates seen during the test we calculated the specific cross-sections and therefore can estimate the expected event rates at Venus during the mission. The detector showed saturated cross-sections below 1E-3 cm2 at 10°C with acceptable event rates for the highest LETs and our applications.
In June 2020 NASA has selected the VERTIAS Discovery mission to Venus for flight. The Venus Emissivity Mapper (VEM) provided by DLR together with the VISAR radar system provided by JPL are the core payload of the mission. VEM is the first flight instrument designed with a focus on mapping the surface of Venus using atmospheric windows around 1 μm wavelength. It will provide a global map of surface composition by observing with six narrow band filters from 0.86 to 1.18 μm. Continuous observation of Venus’ thermal emission will place tight constraints on current day volcanic activity. Eight additional channels provide measurements of atmospheric water vapor abundance as well as cloud microphysics and dynamics and permit accurate correction of atmospheric interference on the surface data. Combining VEM with a high-resolution radar mapper on the NASA VERITAS and ESA EnVision missions will provide key insights in the divergent evolution of Venus. After several years of pre-development including the setup of a laboratory prototype the implementation for flight has started with the qualification of the flight detectors, the review of all requirements flowdowns as well as the finalizing of spacecraft interfaces.
The Venus Emissivity Mapper (VEM) has a mature design with an existing laboratory prototype verifying an achievable instrument SNR of well above 1000 as well as a predicted error in the retrieval of relative emissivity of better than 1%. VEM will provide a global map of surface composition as well as redox state of the surface by observing the surface with six narrow band filters, ranging from 0.86 to 1.18 μm. Continuous observation of Venus’ thermal emission will place tight constraints on current day volcanic activity. Eight additional channels provide measurements of atmospheric water vapor abundance as well as cloud microphysics and dynamics and permit accurate correction of atmospheric interference on the surface data. A mission combining VEM with a high-resolution radar mapper such as the ESA EnVision or NASA VERITAS mission proposals will provide key insights in the divergent evolution of Venus. Here we discuss the approach and results of the evaluation of the VEM sensor’s radiometric performance by an analysis of the corresponding signal processing chain. The passage of a simulated radiometric scene signal of the planet Venus through the VEM optics, detector, analog and digital electronics has been evaluated based on a theoretical model of the system. The goal was to verify the overall system performance with respect to the scientific requirements. This is building on our preliminary evaluation of the VEM laboratory prototype and confirms that the VEM design has significant performance margins.
The VenSpec instrument suite is part of the payload for the ESA M5 mission proposal EnVision which is currently in a competitive Phase A study. VenSpec consists of three channels: VenSpec-M, VenSpec-H and VenSpec-U. VenSpec-M will provide near-global compositional data on rock types, weathering, and crustal evolution by mapping the Venus surface in five atmospheric windows. VenSpec-H will be dedicated to extremely high-resolution atmospheric measurements. The main objective of the VenSpec-H instrument is to detect and quantify SO2, H2O and HDO in the lower atmosphere, to enable characterization of volcanic plumes and other sources of gas exchange with the surface of Venus, complementing VenSAR and VenSpecM surface and SRS subsurface observations. VenSpec-U will monitor sulphured minor species (mainly SO and SO2) and the as yet unknown UV absorber in Venusian upper clouds and just above. In combination, VenSpec will provide unprecedented insights into the current state of Venus and its past evolution. VenSpec will perform a comprehensive search for volcanic activity by targeting atmospheric signatures, thermal signatures and compositional signatures, as well as a global map of surface composition.
The Planetary Spectroscopy Laboratory (PSL) of DLR in Berlin provides spectral measurements of primarily planetary analogues from the visible to the far-infrared range. PSL has supported the data analysis as well as the development and calibration of instruments for planetary missions from ESA, NASA and JAXA. For this purposes PSL provides reflection, transmission and emission spectroscopy of target materials. Currently PSL operates two identical Bruker Vertex 80V vacuum FTIR spectrometer, one spectrometer is equipped with aluminum mirrors optimized for the UV, visible and near-IR, the second features gold-coated mirrors for the near to far IR spectral range. External simulation chambers are attached to each of the instruments for emissivity measurements. The chamber at the near to far IR instruments allows emissivity measurements from 0.7-200 μm under vacuum for sample temperatures from 320K to above 900K, using an innovative induction system. The second chamber (purged with dry air and water cooled to ≤270K) allows emissivity measurements of samples with surface temperature from 290 to 420K. We measure bi-directional reflectance of samples, with variable incidence and emission angles between 13° and 85°. Samples are measured currently at room temperature and 170K, with a planned extension for temperatures below 100K. Bi-directional and hemispherical reflectance is measured under purging/vacuum conditions, covering the 0.2 to above 200 μm spectral range. Transmission of thin slabs, optical filters, optical windows, pellets, and others is measured in the complete spectral range from UV to FIR using a parallel beam configuration to avoid refraction.
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