The Emirates Mars Ultraviolet Spectrometer (EMUS) for the EMM Mission

Abstract

The Emirates Mars Mission (EMM) Hope probe was launched on 20 July 2020 at 01:58 GST (Gulf Standard Time) and entered orbit around Mars on 9 Feb 2021 at 19:42 GST. The high-altitude orbit (19,970\xa0km periapse, 42,650\xa0km apoapse altitude, 25° inclination) with a 54.5 hour period enables a unique, synoptic, and nearly-continuous monitor of the Mars global climate. The Emirates Mars Ultraviolet Spectrometer (EMUS), one of three remote sensing instruments carried by Hope, is an imaging ultraviolet spectrograph, designed to investigate how conditions throughout the Mars atmosphere affect rates of atmospheric escape, and how key constituents in the exosphere behave temporally and spatially. EMUS will target two broad regions of the Mars upper atmosphere: 1)\xa0the thermosphere (100–200\xa0km altitude), observing UV dayglow emissions from hydrogen (102.6, 121.6\xa0nm), oxygen (130.4, 135.6\xa0nm), and carbon monoxide (140–170\xa0nm) and 2)\xa0the exosphere (above 200\xa0km altitude), observing bound and escaping hydrogen (121.6\xa0nm) and oxygen (130.4\xa0nm).EMUS achieves high sensitivity across a wavelength range of 100–170\xa0nm in a single optical channel by employing “area-division” or “split” coatings of silicon carbide (SiC) and aluminum magnesium fluoride (Al+MgF2) on each of its two optical elements. The EMUS detector consists of an open-face (windowless) microchannel plate (MCP) stack with a cesium iodide (CsI) photocathode and a photon-counting, cross-delay line (XDL) anode that enables spectral-spatial imaging. A single spherical telescope mirror with a 150\xa0mm focal length provides a 10.75° field of view along two science entrance slits, selectable with a rotational mechanism. The high and low resolution (HR, LR) slits have angular widths of 0.18° and 0.25° and spectral widths of 1.3\xa0nm and 1.8\xa0nm, respectively. The spectrograph uses a Rowland circle design, with a toroidally-figured diffraction grating with a laminar groove profile and a ruling density of 936\xa0gr\u2009mm−1 providing a reciprocal linear dispersion of 2.65\xa0nm\u2009mm−1. The total instrument mass is 22.3\xa0kg, and the orbit-average power is less than 15\xa0W.