Water in the soil and salt in the oceans may seem to be unconnected, however, both variables are intrinsically linked to the Earth's water cycle and climate (ESA Website, Earth Explorers (SMOS)).
The SMOS mission is a direct response to the current lack of global observations of soil moisture and ocean salinity which is needed to further our knowledge of the water cycle, and to contribute to better weather and extreme-event and seasonal-climate forecasting.
The variability in soil moisture is mainly governed by different rates of evaporation and precipitation so that severe drought can result in features such as hard, dry, cracked soil, while floods and landslides can be a consequence of very heavy rainfall. Less obvious perhaps is the fact that some areas of the Earth's oceans are significantly saltier than others. Changes in the salinity of surface seawater are brought about by the addition or removal of freshwater, mainly through evaporation and precipitation, but also, in polar regions, by the freezing and melting of ice. Variability in soil moisture and ocean salinity is due to the continuous exchange of water between the oceans, the atmosphere and the land - the Earth's water cycle.
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.
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.
The principal objective of the SMOS mission is to provide global maps of soil moisture and ocean salinity of specified accuracy, sensitivity, spatial resolution, spatial and temporal coverage. In addition, the mission is expected to provide useful data for cryospheric studies.
A novel instrument, MIRAS (Microwave Imaging Radiometer with Aperture Synthesis), as shown in Fig. 1, has been especially developed to make these observations and to demonstrate the use of this new radiometer concept and its capability of observing both soil moisture and ocean salinity by capturing images of emitted microwave radiation in the protected frequency band between 1400 and 1427 MHz (L-Band). SMOS will carry the first-ever, polar-orbiting, space-borne, 2-D interfer-ometric radiometer.
Moisture is a measure of the amount of water within a given volume of material and is usually expressed as a percentage. From space, the SMOS instrument can
measure as little as 4% moisture in soil on the surface of the Earth - which is about the same as being able to detect less than one teaspoonful of water mixed with a handful of dry soil.
Salinity describes the concentration of dissolved salts in water. It measures in practical salinity units (psu), which expresses a conductivity ratio. The average salinity of the oceans is 35 psu, which is equivalent to 35 grams of salt in 1 litre of water. SMOS aims to observe salinity down to 0.1 psu (averaged over 10-30 days and an area of 200 km x 200 km) - which is about the same as detecting 0.1 gram of salt in a litre of water.
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