The Venus Emissivity Mapper &#8211; Obtaining Global Mineralogy of Venus from Orbit on the ESA EnVision and NASA VERITAS missions to Venus.
Authors: Helbert Jörn , Dyar Darby , Walter Ingo , Ryan Conor , Marcq Emmanuel , Widemann Thomas , Reess Jean-Michel , Maturilli Alessandro , Müller Nils , Kappel David , Alemanno Giulia , Arnold Gabriele , D'Amore Mario , Rosas-Ortiz Yaquelin , Smrekar Suzanne , Ghail Richard 
Published: October 2020

Introduction: The Venus Emissivity Mapper is the first flight instrument designed to focus on mapping the surface of Venus using several atmospheric windows around 1 &#181;m. After years of development, VEM now has a mature design. An existing laboratory prototype has verified an achievable instrument SNR of well above 1000, as well as predicted error in retrieval of relative emissivity of better than 1%, assuming the availability of improved Venus topography.VEM science goals: The instrument will provide a global map of rock type from orbit, assessing iron contents and the redox state of the surface by observing the surface with six narrow band filters, ranging from 0.86 to 1.18 &#181;m. Three additional windows allow corrections for cloud composition and variability, two measure water abundance, and three compensate for stray light. Continuous observation of Venus&#8217; thermal emission will also place tighter constraints on current volcanic activity. Eight channels provide measurements of atmospheric water vapor abundance as well as cloud microphysics and dynamics, permitting accurate correction of atmospheric interference on the surface data.The instrument is currently part of two Phase A studies: is part of the payload of the NASA VERITAS mission as well as of the ESA EnVision mission, here as the VenSpec-M channel in the VenSpec spectrometer suite.&#160;Combining VEM with a high-resolution radar mapper will provide key insights into the divergent evolution of Venus and Earth. Flying VEM on more than one mission will enable a long timeline of monitoring for volcanic activity on Venus. Combined with the existing VenusExpress data, VEM enables detection and mapping of surface changes over decades.VEM Design: The VEM system design is a pushbroom multispectral imaging system. It leverages a proven measurement technique pioneered by VIRTIS on Venus Express (VEX). It also incorporates lessons learned from VIRTIS to achieve greatly improved sensitivity and spectral and spatial coverage:A filter array (rather than a grating) provides wavelength stability (band-center and width-scatter) ~5&#215; more stable and maximizes signal to the focal-plane array (FPA).
Spectral windows below 1&#181;m are covered for the first time.
A two-stage baffle decreases scattered light and improves sensitivity.
Use of an InGaAs detector with an integrated thermal electric cooler (TEC) eliminates the need for cryogenic cooling.
The design maturity, combined with a standard camera optical design, leads to low development risk.VEM prototyping: VEM has been under development for several years with significantly financial investment from DLR. Following creation of the first breadboard model in 2015 during Phase A for the NASA Discovery proposal VERITAS, a laboratory prototype (LP) of the VEM instrument has been developed. This prototype includes the development version of the VEM optics with a filter array with two active filter strips. The optics underwent a set of calibration measurements on sub-unit level at LATMOS prior to delivery to DLR. All key optical design parameters including transmission and wavelength coverage have been verified using the VEMO prototype.A first performance evaluation of the VEM prototype used two Venus analog samples heated to Venus surface temperatures. This was performed using the Venus simulations setup at the Planetary Spectroscopy Laboratory with the prototype mounted on the chamber. The retrieved emissivities match laboratory values, and uncertainty for a single unbinned exposure is 1000, as well as a predicted error in the retrieval of relative emissivity of better than 1%.