This is a NERC science project searching for tiny, microscopic particles in the ocean. These small particles are the “Dark Matter” of ocean optics. If the theories behind light in the ocean are correct, there should be millions of them floating around in the water. But, until now, they have never been seen in their natural environment….
Satellite remote sensing imagery is used to gather all sorts of information about the the oceans. In visible light images of sea surface reflectance (see left), the sea is often observed to have bright patches. To interpret the remote sensing imagery we need to know what causes these bright patches and what they can tell us about what’s going on in the water.
The conventional view is that these bright areas are caused by particles in the sea water reflecting sunlight back to the satellites sensors. More specifically, it is backscattering of light from really tiny particles suspended in the seawater. What do we mean by really tiny? Really tiny is less than a micron in size. You can find out just how small 1 micron actually is on our FAQ page.
The analogy we often use to explain the influence of suspended particles on light underwater is that these small particles act like mist – imagine driving along on a misty night and putting your vehicle’s headlamps on full beams. You are almost blinded by the light from your headlamps backscattered to you by the tiny droplets of mist. It is thought that all the millions of submicron particles floating around in seawater act on the underwater light field in much the same way, preventing light from travelling very far underwater by backscattering it towards the source. Until very recently it has been difficult to make reliable measurements of the size and abundance of these very small particles in the sea itself. We really don’t know if all the backscattering of light is actually caused by millions of submicron particles behaving like underwater mist … or by some other mechanism.
By combining novel technologies for non-disruptive underwater imaging of small particles with traditional techniques for measuring the optical properties of seawater and the mass of particles suspended in it, this research project aims to find out what role these tiny particles play in ocean optics. We aim to:
Produce new in situ particle size distributions extending into the potentially optically important sub-micron range for the first time.
Our specific objectives are to:
1) Extend in situ measurements of the size distribution of suspended particles in sea water down to the sub-micron scale
2) Determine the abundance of sub-micron particles in the sea and how this varies in space and time
3) To make measurements of seawaters inherent optical properties (IOP) and to determine the effect of the size distribution of particles in the sea on the IOPs down to sub micron level.
4) To establish the sensitivity of remote sensing reflectance to the particle size distributions using radiative transfer theories and the links which we will forge between the size distributions and IOPs.