Lindy Elkins-Tanton on the Origin of Earth’s Water

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The planets formed out of a cloud of gas and dust around the nascent Sun. Within the so-called snow line, it was too hot for liquid water to exist. Since the Earth lies well within this line, why does it have water? Did it somehow manage to retain water from the outset or did it acquire its water later? In the podcast, Lindy Elkins-Tanton explains how these two scenarios might have played out but she says the evidence strongly favors one of these theories.

Elkins-Tanton has concentrated much of her research career on the formation and evolution of planets, and especially the role of water. She is a Professor in the School of Earth and Space Exploration at Arizona State University and Principal Investigator of the NASA Psyche mission.

Podcast Illustrations

Images courtesy of Lindy Elkins-Tanton unless otherwise indicated.

Artist’s impression of a region of a giant molecular cloud that is collapsing to form a protoplanetary disk of gas and dust. A star forms at the center of the disk, and planets form out of the disk.

NASA/JPL CalTech

Plot of various solar system materials. The plot shows that the isotope ratios of hydrogen (D/H) and nitrogen (15N/14N) on Earth are very different from those of comets but quite similar to those of a certain class of meteorites called enstatite chondrites. If comets had delivered Earth’s water, they would have changed its nitrogen isotopic ratios as well as its hydrogen isotopic ratios. Isotopic fingerprinting strongly suggests that Earth’s water has come from the enstatite chondrites.

Marty, B. (2012), Earth and Planetary Science Letters 313, 56

Infrared image of a giant molecular cloud in our galaxy. Regions of such clouds collapse to form new stars, such as those that appear as blue dots in the image. This cloud is 6,500 light-years away and spans an area of sky equivalent to four full moons.

Spitzer Space Telescope, NASA/JPL-Caltech/L. Allen & X. Koenig (Harvard-Smithsonian CfA)

Microwave image of a protoplanetary disk surrounding the young star HL Tauri. The disk includes gaps possibly cleared by amalgamation onto newly-forming planets. This image was taken by the Atacama Large Millimeter/submillimeter Array interferometer, consisting of 66 radio telescopes in the Atacama Desert of northern Chile.

ALMA (ESO/NAOJ/NRAO)

Pillistfer enstatite chondrite. Such rocky meteorites carry water as part of their solid minerals. Such hydrous minerals liberate their water when heated or melted.

Two examples of hydrous minerals are chlorite (left) and serpentinite (right).

Graph of the water and carbon content of many kinds of meteorites. The dashed lines show how much water would be expelled onto the surface of an Earth-sized planet formed from material with the initial water content on the left-hand axis. For example, if meteorites with 1,000 ppm of water, just a tenth of a percent, formed a planet and were processed through solidification of a magma ocean, that planet would retain water inside and also create an ocean 5 km deep over its entire surface. In the podcast, Elkins-Tanton points out that this implies that any given rocky planet anywhere in the universe may have started with enough water to have oceans. This suggests that almost every rocky planet probably had some period of time when it was habitable, that is, when it had liquid water.