Monthly Archives: February 2016

New Horizons mission, Pluto

Pluto: The Gift that Keeps on Giving

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Nitrogen Glaciers on Pluto Studded with Water-ice Hills

Mountains on Pluto
This image shows the inset in context next to a larger view of Pluto’s encounter hemisphere. The inset was obtained by the Multispectral Visible Imaging Camera (MVIC) instrument on New Horizons.  The image resolution is about 1,050 feet (320 meters) per pixel. The image measures a little over 300 miles (almost 500 kilometers) long and about 210 miles (340 kilometers) wide. It was obtained at a range of approximately 9,950 miles (16,000 kilometers) from Pluto, about 12 minutes before the spacecraft’s closest approach to Pluto on July 14, 2015.
Courtesy NASA/JHU-APL, SWRI/New Horizons mission.

The King of the Kuiper Belt Objects continues to deliver its secrets, data bit by data bit as the New Horizons spacecraft slowly radios its mother lode of science from the July 14th flyby back to Earth. The latest thing it’s showing us is a series of chunky hills made of water ice. They ride along on the nitrogen glaciers that cover Sputnik Planum. That’s the ice plain that we see at the “heart” of the heart-shaped Tombaugh Regio.

How Do Water Mountains Form on Pluto?

Okay, so we know that nitrogen ice dominates Pluto’s surface.  So, how do water-ice mountains get into the picture? It turns out they’re jabbing up from the Planum because of the differences between the two types of ice that are there. Water ice is less dense than the nitrogen-rich ice. That means that, like the way ice cubes float in a glass of water or iced tea, the water ice mountains are floating in a sea of frozen nitrogen. They’re moving more like icebergs do in Earth’s Arctic Ocean.

The next question is, if they’re floating like icebergs, where do they come from? The nearby water ice mountains ringing the Planum may provide clues. “Chains” of these drifting hills get in the way of the surface glaciers as they flow. Eventually some of the hills enter the cellular terrain of central Sputnik Planum. That’s when the motion of the nitrogen ice takes over and pushes them out to the edges of the surface cell. What New Horizons is showing us are 20-kilometer-long ice mountain “ranges” being shoved around by the action of nitrogen ice. Imagine a 20-kilometer stretch of the Colorado Rockies or the Himalayas being pushed around to get an idea of the geological action taking place on Pluto.

This is all incredibly exciting — one year after New Horizons formally began its “close fly-by” mission operations, it’s telling an amazing story about this world that is, by all rights, one of the most interesting planetary bodies in the solar system.

Moon, moon formation

Creating the Moon in a Head-on Collision

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The Moon’s Catastrophic Birth

Two worlds collide to make the Moon
The extremely similar chemical composition of rocks on the Earth and moon helped scientists determine that a head-on collision, not a glancing blow, took place between Earth and Theia. Copyright William K. Hartmann

The more we learn about the early solar system, the more chaotic those times seem. It was a busy time. Newborn planets were jostling around, some of them were migrating outwards, and others — such as Earth — were ground zero for ongoing collisions and impacts. One of those collisions formed the Moon. For a long time, planetary scientists cited the glancing blow by a planetary embryo called Theia as the event that made the Moon. But, it turns out that the collision was more of a head-on smack-up than they suspected. Instead of sideswiping Earth at a 45-degree angle, Theia hit Earth squarely in the gizzards.

Analysis of a Head-on Collision

The clues that helped scientists figure out the details of this collision after the fact lies locked away in rocks from both Earth and the Moon. You analyze the chemical compositions of rocks to find out their formation history and subsequent erosion or bombardment. The Apollo missions brought back Moon rocks for just that reason — so that scientists could study their chemical make-up and compare it to Earth’s. In 2014, scientists who did chemical analysis of Moon rocks reported that the Moon’s rocks have their own unique chemical signature — a ratio of oxygen isotopes — that is different from Earth rocks. However, that conclusion is under debate, and more recent studies show there’s very, very little difference between the oxygen isotopes in Moon and Earth rocks.

The scientists who did this work are geochemists and cosmochemists based at the University of California at Los Angeles (UCLA). They used an instrument called a mass spectrometer to analyze the chemical composition of both sets of rocks. That’s when they found that the oxygen signatures in the rocks were the same. And, that tells a different story than one of Theia simply giving Earth a glancing blow in the collisional chaos of the early solar system.

Why is this?

If Earth and Theia had simply “bumped” alongside each other and went their merry way, the oxygen ratios would be slightly different. Instead, this “sameness” argues for head-on encounter where the materials from both worlds were mixed quite thoroughly. That idea is not new — a number of planetary scientists have suggested such a direct crash scenario for several years now.

Here’s more to think about: Theia didn’t survive the collision. Most of its body was mixed in with Earth and the Moon. That’s where the similarity in isotope ratios come in. It’s like mixing batter for a white cake and a chocolate cake together — if they’re mixed well, the batter is going to have the same elements, even if you bake two cakes in two different pans.

Now, the sad thing is (for Theia) that up until the point of impact, Theia was a growing world. It could have been (as the character Terry says in On the Waterfront) a contender. It could have been a planet. Not a big one, maybe something the size of Mars. And, our solar system would have had another rocky planet on its hands. Instead, Theia had that encounter with the newborn Earth and the rest is geologic and lunar history.