Passage IV

NATURAL SCIENCE: Passage A is from the article “Just Add Water” by Jan Zalasiewicz and Mark Williams. Passage B is rom the article "Plate Tectonics Spotted on Europa" by Thomas Sumner.

Passage A by Jan Zalasiewicz and Mark Williams

A look at our neighbours Mars and Venus shows how lucky Earth has been. They too had surface water in the early days, perhaps even large oceans. On frozen Mars today we see ancient shorelines more than 3 billion years old, and detect clays formed in water. Soon, though, Mars lost most of its atmosphere and protective magnetic field, and its water vapour leaked away. Venus is an inferno surrounded by suffocating clouds of sulphuric acid now, but probe measurements show it too once had abundant liquid water, until rising levels of water vapour and carbon dioxide led to a runaway greenhouse effect that boiled it off.

What made Earth different? The key is probably plate tectonics. The movement of segments of Earth's uppermost layer is unique, we think, among the rocky planets of the solar system. They crash against each other, buckling, rising or driving down into the planet's hot mantle. There is some evidence such tectonics tried to start up on Mars, but if so it didn't last long. On Earth, it has created natural depressions: ocean basins, underlain by dense newly forming crust, that hold deeper waters; and shallow seas on the lighter, more ancient crust of the continents. The bottom of these containers is cracked at the subduction zones where water-soaked plates slide down into the mantle. That water is mostly wrung back out to emerge as volcanic steam In mountain ranges.

This constant cycling of water, and the unlikely coexistence of wet and dry surfaces is, it turns out, crucial. Water evaporating from the oceans condenses as rain and chemically attacks the land, modulating atmospheric composition and global temperature. The atmosphere thus formed has a lid—a "cold trap" made by the chill of the stratosphere—that freezes water vapour out and stops it escaping into space. Below this lid, almost uncannily, all three phases of water—solid, liquid and gas—coexist almost all of the time: the only planetary surface known where this has been sustained for any long period.

To complete this remarkable planetary machine, plate tectonics itself needs water to function: water lubricates descending tectonic plates and softens mantle minerals so they melt more easily. Geochemist Francis Albarede of the Ecole Normale Superieure in Lyon, France, thinks that water's arrival from outer space kick-started the plate-tectonic motor 3 billion years ago.

Passage B by Thomas Sumner

Plate tectonics churns the icy exterior of Jupiter's moon Europa, researchers reported in 2014. The finding marks the first evidence of plate tectonics beyond Earth.

"Earth is not unique—we've found another body in the solar system with plate tectonics," says planetary scientist Simon Kattenhorn of the University of Idaho in Moscow. "This tells us that this process can happen on more than just rocky planets like Earth."

Previous observations have seen surface reshaping, such as volcanic activity, on other planetary bodies including Saturn's moon Titan. However, Kattenhorn says, Europa is the first found with a patchwork of drifting tectonic plates.

The rising and sinking ice slabs on Europa's surface may provide a mechanism for nutrients to move from the moon's surface to its subsurface ocean, Kattenhorn argues. Such transport would bolster the likelihood that this ocean hosts life. Astrobiologist Britney Schmidt of Georgia Tech in Atlanta says the mechanism is "very exciting for Europa's chances for supporting life."

Though the moon formed over 4 billion years ago, at the same time as the rest of the solar system, Europa's icy surface is surprisingly young. Based on the moon's small number of impact craters, scientists estimate Europa's surface to be just 40 million to 90 million years old. Dark bands crisscross the moon where warm, fresh ice wells up to the frigid surface, but a mystery remained: Where is the old material?

Two years ago, Kattenhorn and coauthor Louise Prockter of Johns Hopkins University spotted something odd as they scoured a Louisiana-sized portion of Europa mapped by NASA'S Galileo spacecraft in 1998. In the moon's northern hemisphere, a 20, 000-square-kilometer hunk of landscape was missing. Like a torn photograph placed so that the pieces overlap, Europa's crisscrossing surface fractures didn't properly line up.

The researchers propose that this discrepancy marks where two massive ice slabs smashed together, with one sinking under the other and blending into the moon's warmer interior ice. The action resembles a subduction zone on Earth, where one slab of crust—or tectonic plate—slides beneath another.

Passage A: @2014 Reed Business Infommation-UK. All rights reserved. Distributed by Tribune Content Agency, LLC.

Passage B: Copyright @2014. Reprinted with permission of Science News.