Origins of Oceans
Ice geysers erupt on Enceladus, an inner moon of Saturn. Instruments on the Cassini spacecraft detected the icy plumes during close encounters with Enceladus in November 2005. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA (http://apod.nasa.gov/apod/ap071013.html)
Where does water come from? How did it get to Earth? How old are Earth's oceans? Why do oceans exist? Do other planets have oceans? These were the questions a multidisciplinary panel of scientists moderated by IfA's Gareth Wynn-Williams tackled at the Frontiers of Astronomy Community Event, "Origins of Oceans," held on March 3 on the UH Manoa campus.
Let's take the questions one at a time:
Where does water come from? (Robert Jedicke, IfA): The Big Bang created all the hydrogen in the Universe about 13.7 billion years ago. Oxygen, the other element needed to create water, is the result of fusion inside massive stars that first turn hydrogen into helium, and then create the elements carbon, neon, oxygen, silicon, and iron. When these stars die in a supernova explosion, they seed the surrounding area of space with these elements and with even heavier elements, such as lead and uranium, created by the explosion itself. Water, known chemically as dihydrogen monoxide (H2O), forms in interstellar space.
How did it get to Earth? (Karen Meech, IfA): Some water was captured from the solar nebula within the first 10 million years of our solar system. It was included in the planetesimals, the small rocks that coalesced into planets and other solar system bodies. The hot early Earth may also have made its own water through chemical reactions. The inner solar system is generally hot and dry, and though water covers a great deal of Earth's surface, less than one percent of Earth's mass is water. Further out, beyond the "snowline," gases in the solar nebula, including water vapor, froze into tiny particles. Some of these particles became comets, and while some of Earth's water may have come from comets, we know that this could only be a small portion, since the ratio of deuterium (a heavy isotope of hydrogen) to regular hydrogen in Earth's water is quite different from that measured on comets.
How old are Earth's oceans? (Stephen Mojzsis, geologist at the University of Colorado): Oceans first appeared on Earth more than four billion years ago, not long after the planet's formation. We know this because rocks 3.8 billion years old (as determined by radioactive decay) contain sediment deposited by water. But though the very young Earth had oceans, these oceans were very different from the ones we have now. They were rich in iron and appeared green, because Earth's atmosphere had no free oxygen, so the iron couldn't rust. Later, after photosynthesis put oxygen in the air, the oceans turned blue.
Why do oceans exist? (Michael Mottl, UH Department of Oceanography): Mottl, an oceanographer, turned this question on its head, asking, why do we have dry land on Earth? He noted that if the amount of land were evenly distributed, all land on Earth would disappear under a mile and half of water. Earth has dry land because it has oceans and two kinds of crust. The oceanic crust, such as that around Hawaii, is made of basalt, is very dense, and is only 6 km (about 4 miles) thick. The continental crust is made of granite, is less dense, and is 35 km (about 22 miles) thick. Because of plate tectonics, the ocean plate goes under the continental plate, lifting up the land.
Do other planets have oceans? (Torrence Johnson, a planetary scientist based at NASA's Jet Propulsion Laboratory): As far as we know, no other planet in our solar system has oceans, but some moons may. Data from the Voyager and Galileo spacecraft show that one of Jupiter's large moons, Europa, and two of Saturn's moons, Titan and Enceladus, have water. At the cold temperatures that far from the Sun, water is generally in the form of ice. However, there is strong evidence that beneath Europa's icy surface there may be a global liquid ocean. On Titan, where the surface temperature is –300 degrees F (–200 C), "rocks" are actually boulders made of ice, and the "lakes" and "seas" detected by radar on the Cassini spacecraft are probably liquid hydrocarbons (mostly liquid methane). But there may be liquid water far below Titan's surface, where radioactivity generates heat. Near the south pole of Enceladus, Cassini has observed geysers of water vapor that may indicate a subsurface liquid layer heated by internal tides caused by the gravity of Saturn. These moons may support primitive life forms because they have liquid water and energy sources. It is also possible that moons orbiting giant Jupiter-like planets around other stars may harbor such potentially habitable environments.