content long 16-October-2018 07:28:36

ExoMars Trace Gas Orbiter Instruments

ACS - Atmospheric Chemistry Suite

The Atmospheric Chemistry Suite (ACS) is part of the instrument payload on the ExoMars Trace Gas Orbiter (TGO), launched in March 2016.

ACS accommodation on the TGO.
Credit: ESA / ATG medialab

ACS will be a set of three spectrometers that can measure the spectrum of sunlight across a wide range of wavelengths (near infrared, mid infrared, thermal infrared). This wavelength range will enable the detection of water, methane and a variety of minor atmospheric constituents as well as the study of the structure of the atmosphere and its photochemistry.

The measurements will be carried out in: solar occultation mode, i.e., the instrument points toward the Sun when the Martian atmosphere lies between the Sun and the Orbiter, at both sunrise and sunset; as well as in nadir mode, i.e., looking directly downward at the sunlight reflected from the surface of Mars; and also limb mode, i.e., looking towards the Martian horizon at the sunlight scattered from the atmosphere of Mars. The inclination of the TGO's orbital plane has been chosen to optimise the science that can be done with the instrument suite.

Atmospheric Chemistry Suite (ACS) modules. Credit: ESA/Roscosmos/ExoMars/ACS/IKI

ACS comprises three spectrometers integrated into a single unit together with a central electronics box:

  • ACS-NIR - an Echelle-Acousto-Optic Tuneable Filter (AOTF) spectrometer, which will operate in nadir, limb and solar occultation modes, detecting infrared light in the wavelength range from 0.7 µm - 1.7 µm.

  • ACS-MIR - an Echelle spectrometer, which will operate in solar occultation mode, covering the range from 2.3 µm - 4.6 µm.

  • ACS-TIR - a Fourier spectrometer with two channels, one covering 1.7 µm - 17 µm that will operate in nadir and solar occultation modes, and another covering 1.7 µm - 4 µm (optimised for 3.3 µm) that will operate in nadir mode.

ACS-NIR and ACS-MIR are designed to:

  • derive vertical profiles (atmospheric density and temperature) of carbon dioxide (CO2), and known minor species such as methane (CH4), water (H2O) and carbon monoxide (CO) at the evening and morning terminators in the 10 km - 80 km altitude range;
  • profile isotope ratios for HDO/H2O, 13CO2/CO2, CO18O/CO2, among others;
  • search for undetected species such as acetylene (C2H2), ethylene (C2H4), ethane (C2H6), sulphur dioxide (SO2), hydroperoxyl (HO2), hydrogen peroxide (H2O2), formaldehyde (H2CO), hydrogen chloride (HCl) and carbonyl sulphide (OCS);
  • obtain profiles of aerosols, distinguishing between dust and ice clouds;
  • monitor ozone using dayglow O2 emission;
  • perform a sensitive search for new hydroxide (OH), O2 and nitric oxide (NO) nightglow over a spectral range that is not covered by any other TGO spectrometer.

ACS-TIR is designed to:

  • derive a vertical thermal profile of the atmosphere from the surface to an altitude of 50 - 55 km with a resolution of a few km, using the CO2/15 µm band over the full orbit in nadir mode;
  • monitor atmospheric dust and condensation clouds;
  • measure the surface temperature;
  • map methane concentrations in the 3.3 µm band, on the day side, in nadir mode;
  • detect methane and other minor atmosphere constituents using occultation mode.
  • The separate components of ACS build on previous flight heritage. The MIR and NIR channels follow on from TIMM-2 (Phobos-Grunt), Rusalka (ISS) and SOIR (Venus Express). The TIR channel follows on from AOST (Phobos-Grunt) and PFS (Mars Express).


ACS participants
Principal Investigator
Oleg Korablev, Space Research Institute (IKI), Russian Academy of Sciences

Deputy Principal Investigator
Franck Montmessin, Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), France

Project Manager
A. Shakun, Space Research Institute (IKI), Russian Academy of Sciences
Participating countries
Russia, France, Germany, Italy
ESA contact
ExoMars instrument system engineer
Duncan Goulty, Directorate of Science and Robotic Exploration, European Space Agency


Last Update: 04 November 2016

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