The Venus Emissivity Mapper (VEM) concept

Abstract

Based on experience gained from using the VIRTIS instrument on VenusExpress to observe the surface of Venus and the new high temperature laboratory experiments, we have developed the multi-spectral 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 is done on the VERITAS mission [1].The permanent cloud cover of Venus prohibits observation of the surface with traditional imaging techniques over most of the visible spectral range. Fortunately, Venus' CO 2 atmosphere is transparent in small spectral windows near 1 µm (Figure 1). Ground observers have successfully used these windows during the flyby of the Galileo mission at Jupiter and most recently for the VMC and VIRTIS instruments on the ESA Venus Express spacecraft. Observations have revealed compositional variations correlated with geological features, but existing data sets contain only a few channels. VEM offers an opportunity to gain significant information about surface iron-bearing mineralogy by virtue of having five different channels for surface observations. Surface mapping by VIRTIS on VEX: The VIRTIS instrument on the ESA mission Venus Express (VEX) was the first instrument to routinely map the surface of Venus using the near-infrared windows from orbit [3][4][5]. The instrument is the flight spare of the VIRTIS instrument on the ESA Rosetta comet encounter mission [6]. Originally designed to observe a very cold target far from the Sun, it was adapted to work in the Venus environment. The instrument's main purpose on VEX was to study the structure, dynamics and composition of the atmosphere in three dimensions. However, the idea of surface studies was introduced very late in the mission planning and VIRTIS was never specifically adapted for this purpose. For example, the wavelength coverage was not optimal and only the long wavelength flank of the main atmospheric window at 1.02µm could be imaged. Despite these issues, VIRTIS was an excellent proof-of-concept experiment and far exceeded our expectations. It provided significant new scientific results and showed, for example, that Venus had volcanic activity in the very recent geological past [7].The VEM concept: VEM is focused mainly on observing the surface, mapping in all near-IR atmospheric windows using filters with spectral characteristics optimized for the wavelengths and widths of those windows. It also observes bands necessary for correcting atmospheric effects [8]; these bands also provide valuable scientific data on cloud thickness, cloud opacity variations, and H 2 O abundance variations in the lowest 15 km of the atmosphere.VEM is a pushbroom multispectral imaging sys- Figure 2. Breadboard for VEM used for first testing.

Venus: The Atmosphere, Climate, Surface, Interior and Near-Space Environment of an Earth-Like Planet

“…In-situ measurements could provide new answers, with improved techniques for mapping from orbit to extend the data from landers and balloon-borne or aircraft platforms in the deep atmosphere to a global scale (e.g. Helbert et al 2016). These would contribute greatly to an understanding of the volatile inventory, especially for water, of the interior of Venus, and the rate at which volcanism supplies gases to the atmosphere and hence contributes to climate evolution.…”

Venus: key to understanding the evolution of terrestrial planets

“…Image credits: NASA/JPL Venus, thermal emission from the surface escapes to space in some of these spectral windows, allowing mapping of surface thermal emissivity, as demonstrated by the VIRTIS instrument on Venus Express [17]. A new instrument optimised for this observation could map mineralogy and also monitor the surface for volcanic activity [18].…”

The VenSpec suite on the ESA EnVision mission to Venus

“…The VenSpec-H electronics is self-contained, allowing mounting on the deck of the spacecraft itself, avoiding additional thermal input into the optical bench of the instrument. The VenSpec-M system design was discussed in detailed in as the Venus Emissivity Mapper for the NASA Discovery mission VERITAS [4,5], so only a top-level summary is provided here. VenSpec-M is a pushbroom multispectral imaging system.…”