The SMOS mission is a direct response to the current lack of global observations of soil moisture and ocean salinity, which are needed to further our knowledge of the water cycle, and to contribute to better weather, extreme-event and seasonal-climate forecasting. Variability in soil moisture and ocean salinity is due to the continuous exchange of water between the oceans, atmosphere and land – Earth's water cycle.

See a summary of the SMOS mission in the following infographic:

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Soil moisture

The variability in soil moisture is mainly governed by different rates of evaporation and precipitation The importance of estimating soil moisture in the root zone is paramount for improving short- and medium-term meteorological modelling, hydrological modelling, the monitoring of plant growth, as well as contributing to the forecasting of hazardous events such as floods.

The amount of water held in soil, is of course, crucial for primary production but it is also intrinsically linked to our weather and climate. This is because soil moisture is a key variable controlling the exchange of water and heat energy between the land and the atmosphere. Precipitation, soil moisture, percolation, run-off, evaporation from the soil, and plant transpiration are all components of the terrestrial part of the water cycle. There is, therefore, a direct link between soil moisture and atmospheric humidity because dry soil contributes little or no moisture to the atmosphere and saturated soil contributes a lot. Moreover, since soil moisture is linked to evaporation it is also important in governing the distribution of heat flux from the land to the atmosphere so that areas of high soil moisture not only raise atmospheric humidity but also lower temperatures locally.

Ocean salinity

Between latitudes of 35°N and 35°S, the Earth receives more heat from the Sun than it loses to space. Poleward of these latitudes it loses more heat than it receives. The tropics would keep getting hotter and the poles would keep getting cooler if heat were not carried from the tropics by wind and ocean currents. Ocean currents are driven by temperature and salinity variations in the seawater.

Knowledge of the distribution of salt in the global ocean and its annual and inter-annual variability are crucial in understanding the role of the ocean in the climate system. Ocean circulation is mainly driven by the water and heat flux through the atmosphere-ocean interface, but salinity is also fundamental in determining ocean density and hence thermohaline circulation. In the surface waters of the oceans, temperature and salinity alone control the density of seawater – the colder and saltier the water, the denser it is. As water evaporates from the ocean, the salinity increases and the surface layer becomes denser. In contrast, precipitation results in reduced density, and stratification of the ocean. The processes of seawater freezing and melting are also responsible for increasing and decreasing the salinity of the polar oceans, respectively. As sea-ice forms during winter, the freezing process extracts fresh water in the form of ice, leaving behind dense, cold, salty surface water.

If the density of the surface layer of seawater is increased sufficiently, the water column becomes gravitationally unstable and the denser water sinks. This process is a key to the temperature- and salinity-driven global ocean circulation. This conveyor-belt-like circulation is an important component of the Earth's heat engine, and crucial in regulating the weather and climate. Ocean salinity is also linked to the oceanic carbon cycle, as it plays a part in establishing the chemical equilibrium, which in turn regulates the CO2 uptake and release. Therefore the assimilation of sea surface salinity measurements into global ocean bio-geo-chemical models could improve estimates of the absorption of CO2 by the oceans.

Cryosphere

Observations of ice caps provide a prediction tool for the greenhouse effect since the sea ice extent responds early to altered climatic conditions. Sea ice also insulates the ocean, reducing heat loss and forms an active barrier between the ocean and atmosphere, modulating the exchange of gases such as water vapour, CH4 and CO2. As the sea freezes, the water around the ice becomes more salty which alters the density of the seawater and can impact ocean circulation.

Furthermore, sea ice information is important for the fishing, shipping and oil and gas industries working in the Arctic region.

To observe the changing Arctic environment measurements of temperature, total sea ice extent and concentration, sea ice thickness and volume, the timing of sea ice advance, retreat and ice accumulation and melt season lengths are vital.

Mission requirement

• Soil moisture: Accuracy of 4% volumetric soil moisture; Spatial resolution 35-50 km; Revisit time 1-3 days
• Ocean salinity: Accuracy of 0.5-1.5 practical salinity units (psu) for a single observation; Accuracy of 0.1 psu for a 10-30 day average for an open ocean area of 200 x 200 km
• Sea ice thickness: Estimates up to a maximum value of 50 cm; revisit time 1 day (polar region)

The SMOS satellite consists of the platform and the payload, the MIRAS (Microwave Imaging Radiometer using Aperture Synthesis) instrument, which is mounted on a standard spacecraft platform called Proteus.

The Proteus platform was developed by the French space agency CNES (Centre National d'Etudes Spatiales) and Alcatel Alenia Space.

The Proteus platform acts as a service module accommodating all the subsystems that are required for the satellite to function.

Proteus uses a GPS receiver for orbit determination and control, which provides satellite position information, and a hydrazine monopropellant system for four 1-Newton thrusters that are mounted on the base of the spacecraft for satellite manoeuvres. A sun-synchronous, dawn-dusk orbit is required to obtain the optimum science data.

Learn more about SMOS satellite design in the SMOS multimedia book:

The SMOS mission is supported on the ground by a number of facilities that are required to operate the instrument and satellite, and to acquire and process the data from MIRAS (Microwave Imaging Radiometer using Aperture Synthesis) instrument.

Learn more about SMOS mission management in the SMOS multimedia book: