A MESSENGER to Mercury reveals a strange little planet

Mercury is the smallest of the planets, and despite being known from antiquity, due to its proximity to the Sun, it has proven hard to study. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is designed to fill in many of the gaps in our knowledge of our small planetary cousin. Two new papers from the MESSENGER team respectively highlight the surface and internal structure of Mercury, from detailed observations of the northern hemisphere.

By mapping the fluctuations in Mercury's gravitational field, the researchers measured the variations in mass, indicating how the planet's interior structure differs from place to place. In addition, the spacecraft probed the forces shaping the planet's surface using laser altitude measurements as it flew over the surface. These results show Mercury to be a recently active world with an interior markedly different from the other terrestrial planets—Venus, Earth, and Mars—as well as from Earth's Moon. Specifically, the team found evidence for a solid shell just outside the planet's core, a feature found on no other world.

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Recent formations say the Moon’s not (tectonically) dead yet!

Earth's Moon appears seismically quiet: its major volcanic and tectonic activity is confined to its distant past, as evidenced by the lack of new large-scale features on the surface. However, recent images from the Lunar Reconnaissance Orbiter (LRO) have revealed smaller features that had escaped earlier notice. Several regions exhibit small ravines known as graben that are free of cratering or other marring, which indicates relatively recent formation.

A new paper in Nature Geoscience (by Thomas R. Watters, Mark S. Robinson, Maria E. Banks, Thanh Tran, and Brett W. Denevi) suggests these shallow graben may have formed within the last 50 million years. While this activity is not precisely new, it postdates the last major tectonic activity, which ended roughly 1.2 billion years ago. Since graben form under extension—the stretching of rock by internal pressures—the authors argue that the Moon's interior may still have a significant molten component, and that its cooling and contraction is producing new features on small scales.

More significantly for the Moon's history, they propose it may not have been fully molten when it formed, as that would produce compression, which tends to erase graben

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Faint sunlight enough to drive weather, clouds on Saturn’s moon Titan

Titan, Saturn's largest moon, can seem like Earth viewed through a funhouse mirror. Far colder than our home, water on the surface exists only in a frozen state, while its liquid lakes and clouds are made of methane, and there's an apparent methane cycle playing the role of our water cycle. Now, a detailed model suggests that daily and yearly variations in the moon's temperature may produce distinct layers in the lowest region of Titan's atmosphere, similar to stratification seen in Earth's troposphere.

Benjamin Charnay and Sébastien Lebonnois of the Laboratoire de Météorologie Dynamique in Paris, France have developed a three-dimensional general circulation model (GCM) of Titan's thick atmosphere. Based on data from the Huygens lander and Cassini radio data, the model shows a weak amount of convection, varying over the course of Titan's day. As Saturn is much farther from the Sun than Earth is, the difference between day and night on Titan is comparatively small, but the amount of surface heating in the researchers' model is sufficient to drive cloud formation and wind patterns.

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Figuring out why most of Titan’s methane lakes have northern exposures

Saturn’s moon Titan is one of the most intriguing bodies in our solar system. Its dense atmosphere and lakes of liquid methane make it both beautiful and bizarre, as well as a tantalizing target for those seeking extraterrestrial life. To me, though, the most amazing thing (so far) has been the revelation that is Titan’s meteorology. There’s something extraordinary about imagining liquid methane falling as rain on another world—it’s so similar to our experience, yet so very different. Earth has a familiar hydrologic cycle; Titan has an alien methane cycle.

In a letter published in Nature, researchers describe a model that successfully simulates some key aspects of Titan’s weather. The model offers possible explanations for some of the moon's quirky features that have long been puzzling.

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The quasicrystal that fell to Earth

The 2011 Nobel Prize in chemistry was awarded to Dan Schechtman for his discovery of quasicrystals, materials that do not have the regular lattice structure of crystalline solids. Schechtman produced quasicrystals in the laboratory in 1982, but until 2008 nobody had found a naturally occurring quasicrystal. Now researchers in Italy and the United States have examined the rock that contained these natural quasicrystals and determined it may actually be part of a meteorite.

Normal crystalline solids have atoms or molecules arranged in cubes, hexagons, or other regular repeating patterns. Quasicrystals exhibit different symmetries that never precisely repeat: pentagons, icosahedrons, and so forth. Schechtman and researchers after him produced these quasi-periodic lattices by melting materials under high pressure, then cooling them quickly in a process known as quenching.

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Kepler-20′s oddball planet assortment challenges models of planet formation

NASA's press conference on the newly discovered Earth-sized planets has just concluded, and most of the time was spent reiterating what was in our earlier report. Most of the value of listening in came from hearing the genuine enthusiasm of the scientists involved; lead author Francois Fressin of the Harvard-Smithsonian Center for Astrophysics highlighted the findings' significance by excitedly stating, "Here is where the era of exo-Earths has begun."

A lot of time was spent discussing how long it would take to figure out the masses, and thus composition, of the planets. It may happen as soon as next year: the instruments that might be sensitive enough to detect the influence of these planets on their host star will be coming on line in that time frame, and some of the Kepler team is also working on those projects. There was something said along the lines of "you can bet Kepler-20 will be one of the first places we point them."

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Kepler team spots Earth-sized planets orbiting sun-like star

Just two weeks after the confirmation of a planet that's within the habitable zone of a distant star, the Kepler team is back with the discovery of two Earth-sized planets orbiting in what is now a five-planet system (three other planets orbiting the star, Kepler-20, had been spotted earlier). Although these planets are much too hot to support liquid water, one of them (Kepler-20e) is the smallest exoplanet yet detected.

Kepler-20 was already a busy star system, with three small planets orbiting close in to the star: Kepler-20b is about twice the size of Earth and orbits once every 3.7 days; Kepler-20c is three times Earth's radius and orbits every 11 days; and Kepler-20d is 2.75 Earth radii with an orbit of 77.6 days. If that seems somewhat tightly packed, the new finds actually jam a couple more planets within the orbit of Kepler-20d. Kepler-20e has an orbit of six days, while Kepler-20f takes 19.6 days to orbit its host star.

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Gypsum deposit on Mars provides definitive evidence of water

The Mars Science Laboratory Curiosity is on its way to the red planet, scheduled for a landing in August. In the meantime, the Opportunity rover, which has been operating for nearly eight years, is still sending back scientific results. Its latest, announced at the annual meeting of the American Geophysical Union, provides a clear indication that water once flowed through underground fissures, giving us a better picture of Mars' geological history.

The findings are based on a vein of material that's picked up the name "Homestake," found near the rim of the Endeavour Crater. The material is only a couple centimeters across, but about 50cm long. Readings with a spectrometer indicated it was a form of calcium sulfate, and that it contained significant amounts of water. Gypsum crystals are formed when calcium sulfate associates with water; they tend to dissolver readily, but Mars' dry climate allows them to be stable. The material has been spotted elsewhere on Mars, in the form of sand dunes in the northern polar region.

Gypsum is formed when calcium-containing water comes in contact with sulfates, either from rocks or via volcanic activity. The calcium sulfate precipitates, capturing water molecules within the crystal structure in the process. The gypsum vein clearly indicates that water was flowing through through cracks below the surface of Mars at some point in its past; the rover's scientific lead, Cornell's Steve Squyres, calls it a "slam-dunk story."

By now, the evidence for liquid water on Mars' surface in its past is so extensive that we can probably treat it as a fact. But, so far at least, we haven't found a way to determine whether that water ever supported living creatures. Still, the Homestake find is another indication that Mars' waters were present in a variety of environments, both on the surface and percolating beneath it.

(Incidentally, many of you may be familiar with a common form of gypsum: drywall. If we ever want to put a housing development on Mars, we maybe able to locally source some of the material.)

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