Mapping Venus in Emissivity: The Venus Emissivity Mapper, VenSpec-M on EnVision and VEM on VERITAS missions to Venus

Introduction: Understanding Venus’ surface composition and volcanic activity is central to addressing key questions about the planet’s evolution. These characteristics are tied to the degree of planetary differentiation Venus has undergone, as well as to post-formation geologic processes. Observations suggest Venus has been volcanically active in the past and is most likely active today. Venus Express revealed thermal anomalies and emissivity variations that suggested differences between fresh and weathered surface basaltic material, pointing to recently active hot spots^[1,2,3,4]. Supporting this, gravity, topography, and deformation features observed at these locations are consistent with mantle plume activity beneath the surface^[5,6]. Atmospheric sulfur dioxide (SO₂) fluctuations recorded by Pioneer Venus and Venus Express might reflect episodic volcanic outgassing^[7,8,9] and accelerate surface alteration. Estimates of lithospheric thickness, used as a proxy for surface heat flow, suggest values comparable to Earth's most volcanically active regions^[10]. Reanalysis of Magellan radar data has identified surface changes consistent with active eruptions^[11,12].
However, key questions about surface composition and the intensity of recent magmatic processes on Venus remain unanswered. This will soon change, as three missions - NASA’s VERITAS and DAVINCI, and ESA’s EnVision - will investigate Venus with unprecedented detail. All three missions will include instruments targeting the 1 µm spectral windows, where the FeO absorption feature, a key indicator of mafic mineralogy, can be detected^[13,14]. VERITAS and EnVision will carry Venus Emissivity Mappers (VEM on VERITAS, VenSpec-M on EnVision)^[15,16], while DAVINCI will image the surface in this spectral range during flybys using VISOR and descent using VenDI^[17].

The VenSpec-M/VEM Instrument: VenSpec-M/VEM builds on the heritage of VIRTIS on Venus Express. It is a push-broom multispectral imager designed to reveal the planet’s elusive surface characteristics and monitor its volcanic activity in the 1 μm region. The instrument incorporates a sophisticated 14-band filter assembly (ranging from 0.79 μm to 1.51 μm) and uses an InGaAs detector. By targeting specific spectral windows in Venus atmosphere, VenSpec-M/VEM will provide critical insights into surface composition and geological features not available with previous missions. On EnVision, VenSpec-M is part of the VenSpec Suite, working alongside VenSpec-H (high-resolution IR spectrometer)^[18] and VenSpec-U (UV spectral imager)^[19], together providing a comprehensive view of Venus surface-atmosphere processes.

VenSpec-M/VEM Science Goals: The instrument addresses Venus’ history and evolution through:

• Rock type mapping. Six narrow surface band filters, ranging from 0.86 to 1.18 µm, covering five windows in the Venus atmosphere, will be used to identify different rock types based on their Fe contents, potentially identifying continent-like rocks. This in turn would have important implications for Venus’ past, as it can suggest the past presence of liquid water on the planet’s surface.
• Search for active volcanism. The VenSpec-M/VEM instrument will detect active volcanic eruptions by analyzing surface thermal signatures in the five surface bands and two additional water vapor bands that are sensitive to water vapor from volcanic outgassing. Previous studies using the VIRTIS data focused on detecting eruptions with effusion rates in excess of 500 to 1000km³/yr. The improved signal-to-noise ratio of the VenSpec-M/VEM instrument will enable detection of eruptions with effusion rates one order of magnitude lower, significantly enhancing volcanic activity monitoring capabilities on Venus^[20].
• Combined atmospheric-surface investigations. The VenSpec suite is designed to observe volcanic plumes from their origin near the surface (VenSpec-M, VenSpec-H), through the middle atmosphere (VenSpec-H), and up to the cloud tops (VenSpec-U), providing a comprehensive view of gas exchanges and atmospheric processes.

VenSpec-M/VEM On-going Science Activities: A set of preparatory science activities are going on within the VenSpec-M, VEM and VenSpec Suite teams:

• Laboratory measurements. Spectral measurements are being acquired at DLR’s Planetary Spectroscopy Laboratory (PSL) to build a comprehensive dataset for the interpretation of VenSpec-M/VEM data. It will include measurements on basaltic and granitic samples; investigation of mineral mixtures in emissivity^[21]; study of the emissivity response of weathered vs. unweathered Venus analogs^[22]; and measurements on samples collected during field campaigns using a VEM instrument emulator to improve data interpretation and calibration techniques^[23]. Surface mapping performed by VEM combined with that of VenSpec-M on EnVision will look for emissivity changes and provide extended coverage of Venus surface.
  Moreover, a combined laboratory effort within the VenSpec Suite aims to support the overall VenSpec Science outcome through laboratory experiments that simulate Venus’ surface and atmospheric conditions. This effort fosters synergies between different laboratories and experimental approaches, providing essential reference data for instrument calibration and interpretation^[24].
• Atmospheric modelling. The team is investigating radiative transfer models^[25] to understand how emissivity translates into radiance at top of the atmosphere. This will provide calibration and error estimation support for the VenSpec-M/VEM instrument. Work on atmospheric modeling and radiative transfer is a coordinated effort within the VenSpec Suite.
• Interior modeling. Effects of mantle bulk composition on surface emissivity and geodynamic evolution connecting laboratory measurements and VenSpec-M/VEM data are being investigated using geodynamic models^[26]. VEM/VenSpec-M data and high-resolution gravity data ^[28] will be combined to assess the similarities in bulk composition between the plateaus and continents on Earth^[27].

Conclusion: The VenSpec-M/VEM instruments onboard the EnVision and VERITAS missions, alongside the rest of the payload, stand ready to advance our understanding of Venus. These missions hold the potential to uncover the mysteries of Venus' evolution, providing valuable insights into planetary differentiation and ongoing geological processes.

Acknowledgements: A portion of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract 80NM0020F0035 with NASA.

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