Science Background
The Mars upper atmosphere is complex and distinct from that of Earth, and serves as a unique laboratory for studying atmospheric processes universal to all planets. Large scale circulation cells and small scale gravity waves, modulated by the dust cycle, input energy from below; while the solar irradiance and solar wind, modulated by the planet’s highly eccentric solar orbit, input energy from above. These competing processes significantly influence the global and vertical structure of the Mars upper atmosphere. This region of the atmosphere serves as the reservoir for escaping neutrals and ions, which have been carried away by the solar wind for millennia, a process that continues today. As such, the composition, structure and variability of the Mars upper atmosphere provide insight into how the planet has evolved over time and the fate of vast quantities of liquid water that once flowed on the surface of Mars as is evident in the geological record.
Over the past 5 years, the ESA Trace Gas Orbiter (TGO) and NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter made contemporaneous measurements of the Mars upper atmosphere. TGO and MAVEN have different science goals and are uniquely instrumented to achieve those goals. Thus, TGO and MAVEN observations are complementary and generally not redundant. When taken together, these missions can provide the most complete understanding of the Mars upper atmosphere ever realized.
MAVEN began science operations in November of 2014. It includes instruments for observing the solar wind, solar extreme ultraviolet (EUV) irradiance, atmospheric plasma and atmospheric neutral species. This project will primarily focus on the atmospheric neutral measurements, although the plasma and space environment measurements will be used as needed to constrain solar drivers for variability observed in the neutral atmosphere. MAVEN observes the Martian neutral atmosphere with three instruments: the Neutral Gas and Ion Mass Spectrometer (NGIMS), the Imaging Ultraviolet Spectrograph (IUVS), and the Extreme Ultraviolet Monitor (EUVM).
NGIMS measures neutral composition and density in situ with a maximum altitude above 300 km and minimum altitude of ~150 km (~200 km) before (after) August 2020 when the orbit periapsis altitude was raised to conserve fuel. Additionally, NGIMS was able to sample down to ~120 km through a series of nine deep dip campaigns, when the spacecraft’s periapsis altitude was lowered for approximately one week for each campaign. The deep dip campaigns were selected to sample a range of local times, latitudes, and seasons.
IUVS measures neutral composition and density remotely in primarily two different modes: airglow limb scans and stellar occultation. The airglow limb scans sense the atmosphere from ~80 km to ~160 km using a series of UV emission features and can measure the densities of neutral species on the dayside. The stellar occultations measure neutral species from below 90 km to 150 km on both the dayside and nightside of the planet.
EUVM measures neutral density from 120 km to 200 km with EUV solar occultations. The local times are inherently at the solar terminator (either dawn or dusk).
TGO started the science operation in April 2018. Its primary objective is the detection of trace gas and the study of their distributions. The spacecraft instruments are two spectrometers, one neutron detector, and a camera. The two spectrometers are the Atmospheric Chemistry Suite (ACS) and the Nadir and Occultation for MArs Discovery (NOMAD). Those spectrometers can measure spectral signatures of CO2 from the lower atmosphere into the upper thermosphere, CO into the upper mesosphere, and H2O into the upper mesosphere during southern summer. The spectrometers on TGO also monitor atmospheric dust, pressure, and temperature in the lower atmosphere. More work is required to understand the impact on the upper atmosphere of dust loading and the variability of other measured quantities in the lower atmosphere.
NOMAD is a suite of three spectrometers. Solar Occultation (SO), an infrared channel (covering wavelengths between 2.2 μm and 4.4 μm) dedicated to solar occultations; Limb, Nadir and Occultation (LNO), an infrared channel (2.2-3.8 μm) mainly dedicated to limb and nadir measurements; and Ultraviolet-Visible (UVIS), working in the 250-600 nm wavelength range and which performs solar occultations, limb, and nadir measurements. The LNO and UVIS channels monitor the lower atmosphere, while the SO channel measures the spectral signature of CO2 into the upper thermosphere (~190 km) and scans the terminator of Mars. SO can also measure the spectral signature of CO into the upper mesosphere (100 km) and H2O into the upper mesosphere during southern summer. The temperature is retrieved from the CO2 spectral signature into the upper thermosphere.
ACS is a suite of three spectrometers: Thermal InfraRed channel in honor of professor Vassilii Ivanovich Moroz (TIRVIM), an infrared channel dedicated to SO and nadir measurements (covering wavelengths between 7.7 μm and16.1 μm), Near InfraRed (NIR), an infrared channel (0.7-1.7 μm), and Middle InfraRed (MIR), an infrared channel (2.3-4.3 μm) both dedicated to solar occultation measurements. In thermal IR, TIRVIM monitored the lower atmosphere (0-50 km) when nadir pointed, but when in the solar occultation configuration, all ACS channels (NIR, MIR and TIRVIM) can probe the upper atmosphere. NIR senses the CO2 and H2O densities, and the temperature into the upper mesosphere (0-100 km). MIR senses the H2O density into the upper mesosphere and the CO2 density and the temperature into the upper thermosphere (~190 km), TIRVIM derives the CO2 density and temperature up to the thermosphere (~140-160 km).
The MAVEN and TGO observations are rarely co-located, thus providing a more global view of the Mars upper atmosphere when combined. This is useful for understanding atmospheric dynamics, where observations at a single location only tells part of the story. An example of this is thermospheric polar warming (TPW), where high altitude dust in the summer hemisphere, drives warming in the winter hemisphere. TPW has been observed in a number of studies, but simultaneous observations of the upper atmosphere in both hemispheres during TPW has never been attempted, yet it would reveal a more complete understanding of this phenomenon. Additional studies of dynamics enabled by multi-mission analysis include studying compositional changes that are tracers for dynamics, especially during dust storms, as well as gravity wave influences on thermospheric temperature.
Goals, Objectives, and Outputs
- Goal 1: Understand the structure and variability in the Mars upper atmosphere at a global-scale revealed in the MAVEN and TGO composite data.
- Goal 2: Compile the most comprehensive observational record of the neutral density and composition of the Mars upper atmosphere using MAVEN and TGO.
- Objective 1: Compare contemporaneous/simultaneous solar occultations measured by MAVEN EUVM and TGO and analyze observed differences for new insight.
- Objective 2: Compare composition and minor species measurements from MAVEN NGIMS and TGO from the homo-pause to the exobase.
- Objective 3: Determine factors used to scale the MAVEN and TGO datasets to each other to generate a composite dataset of atmospheric density and composition from the mesopause to the exobase.
- Objective 4: Analyze the composite dataset’s variations with latitude, local time and solar activity. Support the analysis with theoretical understanding, and global circulation models (GCMs) in particular.
- Objective 5: Identify discrepancies between MAVEN and TGO observations and recommend observational targets for future missions.