MEDA for Scientists(Mars Environmental Dynamics Analyzer)
MEDA is a suite of environmental sensors designed to record dust optical properties and six atmospheric parameters: wind speed/direction, pressure, relative humidity, air temperature, ground temperature, and radiation in discrete bands of the UV, visible, and IR ranges of the spectrum.
The radiation sensor is part of an assembly with two arrays of photodiodes that also capture low elevation angle scattered light and a sky-pointing camera; these data are combined to characterize the properties of atmospheric aerosols.
Systematic measurement is the main driver for MEDA operations. Over the entire mission's lifetime, with a configured cadence and frequency in accordance to resource availability, MEDA records data from all sensors. Implementation of this strategy is based on a high degree of autonomy in MEDA operations. MEDA wakes itself up each hour and after recording and storing data, goes to sleep independently of rover operations. It records data whether the rover is awake or not, and both day and night.
The main science objectives for the science team are:
- Signature of the Martian general and mesoscale circulation on phenomena near the surface (e.g., fronts, jets)
- Microscale weather systems (e.g., boundary layer turbulence, heat fluxes, eddies, dust devils)
- Local hydrological cycle (e.g., spatial and temporal variability, diffusive transport from regolith)
- Dust optical properties, photolysis rates, ozone column, and oxidant production
As an environmental instrument, MEDA's different sensors are in direct contact and exposed to the ambient:
- The radiation/dust sensor assembly is located on top of the rover deck.
- The pressure sensor is located inside the rover body and connected to the atmosphere through a dedicated pipe.
- All other sensors are located around the Remote Sensing Mast (RSM):
- Two wind sensor booms oriented at 120 degrees from each other measure winds approaching the RSM,
- Five sets of thermocouples measure the air temperature
- A thermal infrared sensor measures downward and upward thermal infrared radiation as well as surface skin temperature.
- The relative humidity sensor is also attached to the RSM.
A suite of sensors to characterize the Martian low atmosphere and dust properties
|Air Temperature:||Located around the RSM are three air temperature sensors (ATS). Two more are on the rover body to ensure that one of them is upwind. Placed on small thermal inertia forks, and outside the rover thermal boundary layers, these five sets of three thermocouples measure atmospheric temperature. The measurement range is 150 to 300 K, with a required accuracy of 5 K and a resolution of 0.1 K.
||Humidity:||Located at the RSM inside a protecting cylinder, a humidity sensor (HS) measures the relative humidity with an accuracy of 10% in the 200-323 K range and with a resolution of 1%. A dust filter surrounds the cylinder to protect the sensor from dust deposition.
||Pressure:||Located inside the rover body and connected to the external atmosphere via a tube, a pressure sensor (PS) collects pressure measurements. The tube exits the rover body through a small opening with protection against dust deposition. Its measurement range goes from 1 to 1150 Pa with an end-of-life accuracy of 20 Pa (calibration tests give values around 3 Pa) and a resolution of 0.5 Pa. As this component is in contact with the atmosphere, a HEPA filter is placed on the tube inlet to avoid contaminating the Martian environment.
||Radiation & Dust:||Located on the rover deck, the radiation and dust sensor (RDS) is composed of eight upward looking photodiodes in the following ranges:
The photodiodes face the zenith direction and have a field of view of 30 degrees except for the panchromatic one with its 180º field of view.
A second array of side-looking photodiodes at 880nm characterizes the low angle light scattering at different azimuth angles. They cover at least 270º or a circle with fields of view of ±15º and separated about 45º from each other.
A dedicated camera with ±60º around the zenith measures the intensity of the solar aureole.
The assembly of photodiodes and camera are placed on the rover deck without any dust protection. To mitigate dust degradation, magnetic rings are placed around each photodiode with the aim of maximizing their operational time. Nevertheless, to evaluate dust deposition degradation, images of the sensor are recorded periodically, with their readings compared with opacities from other Mars 2020 optical instruments such as MastCam-Z. A comparison of the differences in estimated sky opacities between MEDA and MastCam-Z permits evaluations of the level of dust deposition.
|Thermal Radiation:||Attached to the RSM and pointing to the front right side of the rover, the thermal infrared sensor (TIRS) measures the net infrared thermal radiation near the surface of Mars with a set of five thermopiles:
||Wind:||Attached to the RSM, two wind sensors (WS) measure wind speed and direction. These magnitudes are derived based on the information provided by six two-dimensional detectors on each boom. The detectors are located on six boards 60 degrees apart around the boom axis. Each records local speed and direction in the plane of the board. The combination of the 6 boards per boom serves to determine wind speed, as well as pitch and yaw angle of each boom relative to the flow direction.
The requirement is to determine horizontal wind speed with 2 m/sec accuracy in the range of 0 to 40 m/sec, with a resolution of 0.5 m/sec. The directional accuracy is expected to be better than 22.5 degrees. For vertical wind, the range is 0 to 10 m/sec, and the accuracy and resolution are the same as for horizontal wind.
The booms hold the detectors out of the RSM's thermal boundary layer, aiming to minimize the wind flow perturbation by the RSM at the boom tip where the these detectors are located. The two booms are separated in azimuth to help ensure that at least one of them will record clean wind data when the other is in the wake of the rover. The figure above shows the booms' relative position.
To correct for the perturbations at the booms by the RSM and the rover on the environmental temperature and wind field, a variety of numerical analyses and wind tunnel tests are used during calibration under Mars conditions. Numerical simulations are used to obtain results where tests conditions cannot be reproduced on Earth.