John. F Kennedy gracefully describes our love of the ocean at the America’s Cup Dinner, Rhode Island, 14 September 1962.

“I really don’t know why it is that all of us are so committed to the sea, except I think it is because in addition to the fact that the sea changes and the light changes, and ships change, it is because we all came from the sea. And it is an interesting biological fact that all of us have, in our veins the exact same percentage of salt in our blood that exists in the ocean, and, therefore, we have salt in our blood, in our sweat, in our tears. We are tied to the ocean. And when we go back to the sea, whether it is to sail or to watch it we are going back from whence we came”

15 September 1962  The President and Mrs. Kennedy view the first of the 1962 America’s Cup races aboard the USS Joseph P. Kennedy Jr., off Newport, Rhode Island. Photograph by Robert Knudsen, White House, in the John F. Kennedy Presidential Library and Museum, Boston.

It may well be a case of not letting a few facts get in the way of a good speech but J.F.K’s fact accuracy is a little misleading. The percentage of salt in blood is generally quoted at 0.9% while the vast majority of seawater has a salinity of between 3.1% and 3.8%.

On the 10 June, 2011, Aquarius, a $287 million satellite that will map the saltiness of seawater went into orbit.

Artists impression of Aquarius in orbit.

Scientists have been able to measure ocean salinity for decades by lowering instruments from ships or by deploying robotic floats, but the technology to sense this property from orbit is a recent innovation.

The mission is led by the space agencies of America and Argentina (Nasa and Conae). Salinity is of interest to researchers because it is both a determinant of ocean behaviour and a tool to diagnose what might be happening in the climate system.

By monitoring changes in the amount of dissolved salts at the surface, scientists can see also where water is being evaporated and precipitated.

Evaporation at the surface increases salinity by leaving behind salts as the water moves into the atmosphere. When it rains, the surface is diluted and salinity drops. In this sense, the amount of salt present is a tracer for the global water cycle. It is thought something like 80% of this cycle – which moves fresh water from the ocean to the atmosphere, to the land, and back to the ocean again – occurs out over the sea.

Nasa has an Earth Science division, which has 13 missions in orbit right now and about half of them measure ocean quantities – sea-surface temperature, ocean winds, sea level, ocean colour, and the changing mass of the oceans. The key missing piece that’s really in demand by the ocean science community is ocean salinity.

If your looking for something different to do and save a few bob some day, collect a litre of salt water near you and either leave it on a black bag to evaporate or boil it if your too impatient, and see if you get between 31 and 38 grams for your litre. That will depend where you live of course, see recent chart below for global ocean salinity levels.