Charon

“Tides” on the Moon?

The cause of tides in Earth’s oceans is well understood, mostly caused by the gravitational pull of our nearest neighbor, the Moon. What many don’t realize is that Earth raises tides on the Moon, too, although instead of ocean tides, these tides affect the Moon’s rocky crust. The evidence for this tidal effect is seen in images taken by NASA’s Lunar Reconnaissance Orbiter (LRO), which has been observing the Moon’s surface at high resolution for the past six years.

Between 1969 and 1972, the Apollo missions discovered features called “lobate scarps,” long ridge- or cliff-like faults that stretch up to six miles (ten kilometers) in length, slightly smaller than similar features seen on Mercury. The locate scarps are thought to result from global contraction, which caused the lunar crust to wrinkle up like the skin of a drying apple. The Apollo surveys uncovered about 70 lobate scarps, and shortly after LRO went into orbit in 2009, it discovered 14 more.

Global contraction alone would result in a random orientation of the scarps. However, after six years of observation covering more than three-fourths of the lunar surface, LRO has detected more than 3,000 scarps, and the new data showing a more global distribution and the orientation of the scarps indicates that something influenced their alignment—a gravitational pull that caused a predictable pattern of orientation. This could only be caused by Earth’s gravity. In a paper published in the October issue of the journal Geology, lead author Thomas Watters of the Smithsonian Institution, says, “The agreement between the mapped fault orientations and the fault orientations predicted by the modeled tidal and contractional forces is pretty striking.” There is a tide in the affairs of the Moon after all! –Bing Quock

Heading to Mars

“Orion is a key piece of the flexible architecture that will enable humanity to set foot on the Red Planet, and we are committed to building the spacecraft and other elements necessary to make this a reality.”

That was NASA administrator Charles Bolden this week describing another step forward in getting humans to Mars. The timing is excellent. With the upcoming film The Martian getting the general public excited in human deep space exploration, NASA committed this week to testing the Orion capsule in two exploration missions (EM-1, an unmanned space flight, and EM-2, with astronauts) with a budget of $6.7 billion dollars.

The plan is consistent with funding levels in the president’s budget request, NASA reports. Orion engineers now are executing a rigorous review of the spacecraft’s engineering design and technical progress of the vehicle systems and subsystems. In the next few months, this critical design review will demonstrate Orion is ready to proceed to full-scale fabrication, assembly, integration, and testing.

“We’re committing to this funding and readiness level to stay on the journey we’ve outlined to get to Mars,” confirms NASA’s Robert Lightfoot. –Molly Michelson

Subsurface Ocean on Enceladus

Saturn’s moon, Enceladus has a moon-wide ocean underneath its icy crust, according to a paper published in the online journal Icarus this week.

It has been long suspected that the small moon harbored at least pockets of water under its surface—a source that could fuel the geysers of water vapor, icy particles, and simple organic molecules erupting from the moon’s south pole. First imaged by NASA’s Cassini mission in 2005, the geysers needed some supply of water. But how much water does the tiny moon harbor?

“This was a hard problem that required years of observations, and calculations involving a diverse collection of disciplines, but we are confident we finally got it right,” said Peter Thomas of Cornell University.

Answering the question involved analyzing seven years’ worth of images collected by Cassini, carefully mapping the positions of the moon’s surface features in order to measure extremely precise changes in its rotation. Because Enceladus is not a perfect sphere and its orbit around Saturn is not a perfect circle (causing it to go slightly faster or slower during portions of its orbit), the giant planet subtly rocks Enceladus back and forth as it rotates. The team measured this slight wobble, called libration, to about a tenth of a degree.

“If the surface and core were rigidly connected, the core would provide so much dead weight that the wobble would be far smaller than we observe it to be,” explained Matthew Tiscareno, also at Cornell. The conclusion? A liquid ocean must be present, separating the moon’s surface from its core.

The mechanisms keeping the ocean from freezing are not yet entirely understood, although it is currently thought that tidal forces may play a role as they do on Jupiter’s moon Europa—basically the interior of the moon may be warmed by the friction of Saturn’s gravity deforming Enceladus’s rocky core.

The Cassini mission is approaching its 2017 termination, but the craft is scheduled to make its deepest-ever dive through Enceladus’s active icy plumes on October 28, passing a mere 30 miles (50 kilometers) above the moon’s surface. –Elise Ricard

Mystery of Mordor

By this time you’ve probably heard about the #PlutoFlyBy. On July 14, NASA’s New Horizons spacecraft passed less than 8,000 miles (13,000 kilometers) from Pluto at a speed of nearly 30,000 miles (50,000 kilometers) per hour. Because of the spacecraft’s construction, it could either gather or transmit data at any given moment—never both at the same time. During the flyby, scientists rightly chose to focus on gathering as much data and taking as many pictures as they could. Over the Labor Day weekend, New Horizons began the 16-month-long process of uploading all of that information back to Earth.

As the new data arrives and scientists dig into the secrets of the dwarf planet Pluto (including its oddly young surface populated by strange features with pop-culture names), they will also be turning their eyes to Pluto’s companion world: Charon.

Charon is technically considered a moon of Pluto, but many experts consider them a binary system, since the center of gravity between them is an invisible point in space which they both orbit. Binary systems of stars are incredibly common, much more so than solitary stars like our Sun, but this is our first chance to witness the interaction of a binary dwarf planet system right here in the Kuiper Belt.

A common feature of stellar binaries is their tendency to share material, as gravity from one star pulls gasses from the other. A team at Southwest Research Institute in Colorado recently suggested that this same process may account for the large dark region at Charon’s North Pole, whimsically nicknamed Mordor. The team proposes that nitrogen, methane, and carbon monoxide from Pluto’s atmosphere could be temporarily trapped on Charon’s pole and frozen solid to the surface: Charon’s temperatures range from 60°F to 140°F (15°C to 60° C) above absolute zero. Then, when the pole tips toward the Sun again and heats up (slightly), that material sublimates directly back into gaseous form and escapes into space.

But in the meantime, some of those gases are transformed by sunlight into organic compounds called “tholins” that remain solid on the ground. Over time, these tholins built up to create the vast dark patch that dominates our pictures of this icy world. Tholins come in a variety of colors: Pluto’s are reddish, but scientists have yet to figure out why these particular compounds and not others are being produced on Charon.

(BTW, following up on Bing Quock’s post last week about new images from Pluto, several new photographs were released yesterday from the New Horizons closest perch on July 14, including the one above of Norgay and Hillary Montes. For more information and images, click here.)

As more data comes in, scientists hope to better understand these strange outer worlds. For now, the secrets of Mordor remain a mystery. –Colin Elliott

Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

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