The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) is part of the ESA-JAXA BepiColombo science payload en-route to Mercury. On April 10th 2020, BepiColombo flyby the Earth and obtained data from the Moon surface at a distance of around 700,000 km. The MERTIS thermal-infrared spectrometer (TIS) operating between 7 μm and 14 μm recorded more than 9,600 single hyperspectral observations of the Moon through its space baffle built for deep-space calibration. MERTIS has been designed to observe the surface of Mercury at a spatial resolution more than 1000 times better and at temperatures up to 2 times greater than that of the Moon. Therefore, lunar observations present a significant challenge for the instrument and the team. The standard acquisition procedure and the operations software were adapted to obtain data during the BepiColombo cruise phase. A specific calibration procedure and spatial binning have been developed to obtain the best radiometric data from the lunar surface. The calibrated data demonstrate the exceptional performance of the instrument, developed for a very different planetary object, comparable with ground-based measurements. The observations made during the cruise phase are very promising for future observations of Mercury with the space baffle during the fifth Mercury flyby scheduled for December 2024 and the nominal nadir viewing port (planet baffle) in orbit around Mercury.
KEYWORDS: Venus, Mesosphere, Spectroscopy, Infrared spectroscopy, Clouds, Air temperature, Space operations, Planets, Temperature metrology, Radiative transfer
Measurements made by MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) during the BepiColombo mission's first and second close flybys of Venus in October 2020 (FB1) and August 2021 (FB2) have provided a new and solid data base for the study of the planet's mesosphere at near equatorial latitudes. During the two FBs, the Pushbroom IR grating spectrometer (TIS) of the MERTIS instrument recorded a large number of spectra of planetary radiation from Venus in the 7-14 μm spectral range (715 - 1430 cm-1) for the first time since the Venera-15 Fourier spectrometer experiment FS-1/4 (PMV) in 1983. The MERTIS instrument was designed to study the hot surface of Mercury. Despite the much lower intensity of radiation from the Venusian mesosphere, it was able to demonstrate its suitability for studying the mesospheric temperature profile and various aspects of the mesospheric composition. In this paper, we demonstrate the capability of the MERTIS technology for the study of the Venusian atmosphere. We report on our technical and calibration approach and the main results of the two flybys. Our results on atmospheric temperature profiles and cloud parameters of the upper troposphere and mesosphere (60-75 km) of Venus are presented. We draw conclusions on the applicability and prospects of the developed technology for future targeted explorations of the Venusian mesosphere.
One of the main objectives of the NASA's VERITAS and the ESA's EnVision missions is to characterize the composition and origin of the major geologic terrains on Venus. Both missions carry the Venus Emissivity Mapper (VEM) – a multispectral imager - which will be able to observe the surface of Venus through five atmospheric windows with six bands, around the 1μm spectral range. This will enable the spectral characterization of the Venusian surface, as well as deduce the type of lava and likely alteration processes, providing new insights into the evolution of Venus. To improve our knowledge of the mineralogical information obtained from the 1μm spectral range, we are developing a series of "VEM emulator" (aka VEMulator). The first one was based on a commercial Raspberry PI HQ 12MP camera, containing the Sony IMX477 sensor, with a 35mm lens. Four filters with wavelengths of 860, 910, 990, 1100 nm could be attached in front of the lens similar to four of the six VEM mineralogy spectral bands. This instrument was deployed in summer 2022 on the Vulcano island in southern Italy as Venus analog site. Vulcano rocks display a diverse compositional variation from basaltic to rhyolitic, which makes this site an attractive analog to Venus. Currently, a new version of the VEMulator is being developed using the SCD Cardinal 1280 InGaAs detector – similar to the detector used in the VEM flight model. This VEMulator 2.0 will be used in Iceland, in 2023, for a VERITAS field campaign.
The BepiColombo spacecraft (ESA) on its way to Mercury has performed two flybys at Venus in October 2020 (FB1) and August 2021 (FB2). The pushbroom IR grating spectrometer (TIS) of the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument has recorded a large number of planetary radiation spectra of Venus in the spectral range 7-14 μm (715 - 1430 cm-1). MERTIS mid infrared spectra are the first spectrally resolved observations of Venus in the thermal spectral range longward of 5 μm since the Venera-15 Fourier spectrometer experiment FS-1/4 (PMV) in 1983. We report on the results of both flybys showing average radiance and brightness temperature spectra. Basing on a multi-channel radiative transfer simulation and retrieval algorithm we extract atmospheric temperature profiles and cloud parameters of Venus’ lower mesosphere (60-75 km). These results are compared to the former PMV studies.
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.
Venus is one of the less well studied planets in the inner solar system. Despite its mass being comparable to that of Earth, the development of its environmental conditions followed a completely different evolutionary path. Today's dense CO2 dominated atmosphere is characterized by extreme surface conditions of 92 bar and 735 K at the mean altitude of the planet's surface and a cloud layer enriched with sulfuric acid aerosols, which makes observations of the daytime surface in the visual spectral range impossible. This circumstance led to a long standstill for further space programs to Venus after the first space-based explorations of this planet in the 1970s through the mid-1990s, which was not broken until the launch of ESA's Venus Express (VEX) mission in 2005. Missions such as VEX, Akatsuki (JAXA), and BepiColombo (ESA; MERTIS, first Venus flyby) have made new VIS-IR spectroscopic measurements of Venus in recent years, contributing to the study of its atmosphere and surface. We report here selected results of these investigations and their comparison with earlier data. We derive from these discussion scientific and instrumental requirements for VIS-IR spectroscopic spaceborne measurements with focus to currently selected and future space missions such as EnVision (ESA) and VERITAS (NASA), and discuss their capabilities and limitations for atmospheric and surface studies.
The MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an instrument to study the mineralogy and temperature distribution of Mercury's surface in unprecedented detail. During the nominal mission, MERTIS will map the whole surface at 500 m scale, combining a push-broom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics, and in- ight calibration components for the wavelength range of 7-14 μm (TIS) and 7-40 μm (TIR). MERTIS observed the Moon in April 2020 and Venus in October 2020 under conditions different from nominal operation and performed surprisingly well. MERTIS returned several hundreds of thousands scientifically useful spectra from the two campaigns. MERTIS archival data are stored in Planetary Data System v4 format (PDS4), that describes 2 physical formats for each MERTIS channel. Each channel will is stored in Flexible Image Transport System (FITS) and in pure ASCII, to maximize both machine and human readability.
The ESA-JAXA BepiColombo Mercury mission was launched in October 2018. It performed successful flybys of the Earth-Moon system on April 10, 2020 and of Venus on October 15, 2020 during which the MERTIS instrument operated. MERTIS obtained a total of 1.2 million spectra during the Moon flyby from a distance of more than 700000km. Venus observations started at a distance of more than 1.3 Mio km recording more than 3 million spectra. For reference at Mercury MERTIS will observe the planet from a distance of less than 1500km. These data and the Near-Earth commissioning show that MERTIS performance exceeds requirements.
The MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) is an instrument to study the mineralogy and temperature distribution of Mercury’s surface in unprecedented detail. MERTIS was proposed in 2003 as payload of the Mercury Planetary Orbiter spacecraft of ESA-JAXA BepiColombo mission and will reach Mercury in 2025. MERTIS will map the whole surface at 500 m scale, combining a push-broom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics and in-flight calibration components for the whole wavelength range of 7- 14μm (TIS) and 7-40μm (TIR). MERTIS successfully completed its planned tests of the Near-Earth Commissioning Phase (NECP) between 13 and 14 November, collecting thousands of measurements of its internal calibration bodies and deep space. The data collected during NECP, are being used to verify the operational performances of onboard sub-modules, in particular the spectrometer and radiometer sensor sensitivity. A preliminary look at calibrated data shows a performance comparable with ground-based measurements and no appreciable performance loss or misalignment. The next important dates for MERTIS are the Earth/Moon flyby on 6 April 2020 and the first Venus flyby on 12 October 2020. Both those encounters will be important both for further instrument calibration refinement and for possible unprecedented measurement in the thermal infrared of the Moon and Venus.
KEYWORDS: Space operations, Spectroscopy, Mercury (planet), Calibration, Infrared spectroscopy, Data processing, Radiometry, Seaborgium, Data archive systems, Control systems
The MErcury Radiometer and Thermal Infrared Imaging Spectrometer (MERTIS) is an instrument to study mineralogy and temperature distribution of Mercury surface in unprecedented quality. MERTIS was proposed in 2003 as payload of the Mercury Planetary Orbiter spacecraft of ESA-JAXA BepiColombo mission and will reach Mercury in 2026. MERTIS will map the whole surface at 500m resolution combining a push-broom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics and in-flight calibration components for the whole wavelength range of 7-14μm (TIS) and 7-40μm (TIR). Currently we are developing and testing an ingestion, calibration and transformation pipeline for MERTIS data, from raw telemetry level data to calibrated product and high level derived product. Bepicolombo Science Ground Segment (BC-SGS or SGS) is embracing new technologies for the BepiColombo mission and follows the latest NASA/PDS format, the xml based PDS4. We adopt open source languages and well optimized libraries for the underlying processing. The data processing pipeline is fully containerized via Docker to be independent from transition between server/OSs/environment, drastically reducing the integration and testing time. Due to strict infrastructural constrains like spacecraft downlink bandwidth and onboard mass memory, the already complex observation scenario is subject to further optimizations. This complicates the reconstruction process for higher-level products like global maps of emissivity and thermal inertia.
The Venus Emissivity Mapper is the first flight instrument designed with a focus on mapping the surface of Venus using atmospheric windows around 1 μm. After several years of development 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%. With that it 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.
TROTIS (TROjan asteroid Thermal Infrared multi-Spectral imager) is a high spatial-resolution thermal imaging system optimized for targets in the outer solar system with heritage from the Miniaturized Asteroid thermal infrared Imager and Radiometer (MAIR) for the AIDA mission as well as Bepi-Colombo mission's MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS). TROTIS will provide unique science observations that will foster our understanding of Trojan asteroids. It will provide compositional information, thermal physical properties as well as help determine accurate shapes. In addition TROTIS can aid optical navigation, as it will be able to detect targets from any phase angle.
The MErcury Radiometer and Thermal infrared Imaging Spectrometer (MERTIS) is a highly integrated instrument to study mineralogy and temperature distribution of Mercury’s surface in unprecedented quality. MERTIS was proposed in 2003 as payload of the Mercury Planetary Orbiter spacecraft of the joint ESA-JAXA BepiColombo mission. With the planned launch on top of an Ariane 5 in October of 2018, the mission will soon start its 7 years journey to Mercury. On its way to Mercury, BepiColombo will have 2 flybys of Venus and one of the Earth-Moon system. MERTIS will obtain data during each of these flybys – for Venus the first mid-infrared spectral data since Venera 15 in 1983. After arrival at Mercury in 2025 MERTIS will globally map the surface composition with a resolution of 500m, and study surface temperature variations providing an insight into the thermo-physical properties of the surface. To achieve this, MERTIS combines a push-broom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics and in-flight calibration components for the whole wavelength range of 7-14 μm (TIS) and 7-40 μm (TIR), respectively. Instrument operations in the challenging environment at Mercury with power and data constraints require a sophisticated mapping scheme for the TIS observations, which also has to account for the MERTIS calibration needs. Execution of this scheme creates challenges for the operation of the instruments, data processing, and the creation of map products. Extensive onground testing and rehearsals during the Venus and Earth flybys will ensure flawless performance at Mercury.
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.
The Venus Emissivity Mapper (VEM) is the first flight instrument specially designed with a sole focus
on mapping the surface of Venus using the narrow atmospheric windows around 1μm. VEM will
provide a global map of surface composition as well as redox state of the surface, providing a
comprehensive picture of surface-atmosphere interaction on Venus. In addition, continuous observation
of the thermal emission of the Venus will provide tight constraints on current day volcanic activity.
These capabilities are complemented by measurements of atmospheric water vapor abundance as well as
cloud microphysics and dynamic. Atmospheric data will allow for the accurate correction of atmospheric
interference on the surface measurements and represent highly valuable science on their own. A mission
combining VEM with a high-resolution radar mapper such as the NASA VOX or the ESA EnVision
mission proposals in a low circular orbit will provide key insights in the divergent evolution of Venus.
Based on experience gained from using the VIRTIS instrument on Venus Express to observe the surface of Venus and the new high temperature laboratory experiments, we have developed the multispectral Venus Emissivity Mapper (VEM) to study the surface of Venus. VEM imposes minimal requirements on the spacecraft and mission design and can therefore be added to any future Venus mission. Ideally, the VEM instrument will be combined with a high-resolution radar mapper to provide accurate topographic information, as it will be the case for the NASA Discovery VERITAS mission or the ESA EnVision M5 proposal.
KEYWORDS: Venus, Sensors, Clouds, Spectrometers, Reflectivity, Signal to noise ratio, Minerals, Temperature metrology, Space operations, Optical benches
The permanent cloud cover of Venus prohibits observation of the surface with traditional imaging techniques most of the visible spectral range. Venus' CO2 atmosphere is transparent in small spectral windows near 1 micron. These windows have been successfully used from ground observers, during the flyby of the Galileo mission at Jupiter and most recently by the VMC and VIRTIS instruments on the ESA VenusExpress spacecraft. Studying surface composition based on only a small number of spectral channels in a very narrow spectral range is very challenging. The task is further complicated by the fact that Venus has an average surface temperature of 460°C. Spectral signatures of minerals are affected by temperature and therefore a comparison with mineral spectra obtained at room temperature can be misleading. We report here about first laboratory measurements of Venus analog materials obtained at Venus surface temperatures. The spectral signatures show clear temperature dependence. Based on the experience gained from using the VIRTIS instrument to observe the surface of Venus combined with the high temperature laboratory experiments we have developed the concept for the Venus Emissivity Mapper (VEM). VEM is a multi-spectral mapper dedicated to the task of multi-spectral mapping the surface of Venus. VEM imposes minimal requirements on the spacecraft and mission design and can therefore added to any future Venus mission. Ideally the VEM instrument is combined with a high resolution radar mapper to provide accurate topographic data.
The MErcury Radiometer and Thermal infrared Imaging Spectrometer (MERTIS) is part of the payload of the Mercury Planetary Orbiter spacecraft of the ESA-JAXA BepiColombo mission. MERTIS’s scientific goals are to infer rockforming minerals, to map surface composition, and to study surface temperature variations on Mercury. To achieve these science goals MERTIS combines a imaging spectrometer covering the wavelength range from 7-14 microns with a radiometer covering the wavelength range from 7-40 microns. MERTIS will map the whole surface of Mercury with a spatial resolution of 500m for the spectrometer channel and 2km for the radiometer channel. The MERTIS instrument had been proposed long before the NASA MESSENGER mission provided us with new insights into the innermost of the terrestrial planets. The discoveries of the MESSENGER fundamentally changed our view of Mercury. It revealed a surface that has been reshaped by volcanism over large parts of geological history. Volatile elements like sulfur have been detected with unexpectedly high abundances of up to 4%. MESSENGER imagined structures that are most likely formed by pyroclastic eruptions in recent geologic history. Among the most exciting discoveries of MESSENGER are hollows – bright irregularly shaped depressions that show sign of ongoing loss of material. Despite all this new results the MERTIS dataset remains unique and is now more important than ever. None of the instruments on the NASA MESSENGER mission covers the same spectral range or provides a measurement of the surface temperature. The MERTIS will complement the results of MESSENGER. MERTIS will for example be able to provide spatially resolved compositional information on the hollows and pyroclastic deposits – both among the most exciting discoveries by the MESSENGER mission for which the NASA mission can not provide compositional information.
The Mercury Radiometer and Thermal Infrared Imaging Spectrometer MERTIS on the joint ESA-JAXA mission
BepiColombo to Mercury is combining a spectrometer using an uncooled microbolometer in a pushbroom mode with a
highly miniaturized radiometer.
A full development model of MERTIS is now available. So, after three flybys of Mercury by the MESSENGER mission
and with the Planetary Emissivity Laboratory at DLR in Berlin that can routinely obtain infrared emission spectra at high
temperatures it is a good time to review the MERTIS science requirements and the performance in perspective of our
new knowledge of Mercury.
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