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We're getting close! Isn't it 21DD? There it is!

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DD for Dr. D? He has his own boat dock?

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Come over here! Welcome to the MAURI, Tidewater Community College's research vessel!

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Dr. D, we learned about waves and tides, but we need to know more about oceans.

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Let's start at the very beginning. You know how oceans formed?

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You mean they weren't always here?

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No, not always. When the Earth began about 4.6 billion years ago,

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volcanic eruptions helped to form an atmosphere. That included water vapor.

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So were the oceans created when it rained?

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That's right. About 3.8 billion years ago, the lower areas of the Earth's crust called basins filled with rainwater.

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If the ocean is made from rainwater, why is it salty?

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Well, in the beginning it wasn't very salty.

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So how did it get that way?

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One way is from freshwater rivers. As they flow towards the oceans,

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rivers dissolve minerals such as salt from the land, adding salt to the ocean.

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But how can the ocean be more salty than the rivers?

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When water evaporates from the oceans, it takes almost pure water away, leaving the salt behind.

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Then for millions of years, the rivers carried salt into the oceans, and evaporation leaves it there.

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Do the oceans continue to get saltier?

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No, and they're not getting any less salty either.

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We say saltiness, or salinity, has reached equilibrium.

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Why is the salinity of seawater so important?

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One reason is that it's a major factor in creating ocean currents.

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Let's do an experiment.

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We're going to pour salty water into fresh water. Let's see what happens.

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The salty water went to the bottom. Why is that?

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Salty water sinks to the bottom because it's denser than fresh water.

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A gallon of salt water weighs more than a gallon of fresh water.

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Jacob, test it out.

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They both feel the same.

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Salt water from the ocean is only about 3% heavier than fresh water.

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But even a small difference in density makes a real difference in currents.

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Is there anything other than salinity that determines the density of water?

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Yes, the temperature does.

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I think we're going to have to learn more about how density makes currents.

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I have a friend named Dr. Martens who can help you.

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He's doing research in an underwater laboratory called Aquarius.

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I'll give you his contact information.

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Thanks, Dr. D.