Getting plate tectonics going
Earth's plate tectonics seem to be unique in the solar system. This constant production and recycling of a mostly crystallized outer layer not only speeds up the cooling of a rocky planet but seems to play a critical role in long-term climate stabilization and the resupply of chemical energy to the surface. But a key factor is the speed of plate subduction (where crust plunges back into the planetary mantle). A new study by Sobolev and Brown in Nature proposes a special connection between this speed and the lubricating nature of sediments coming from exposed continents on a world. Sedimentary layers are weaker and wetter than oceanic crust. Early continents, and global glaciation episodes (snowball Earth events) might have provided the eroded material to make sediments that helped kick start plate tectonics as we see it today - starting about 0.75 billion years ago.
Lurking at the Moon's south pole
The Aitken-Basin, a 2,500 km diameter impact feature at the Moon's southern polar region, is a fascinating window into the early solar system and to decoding the composition of the lunar environment. A new study led by Peter James at Baylor University* uses exquisitely sensitive data from NASA's Gravity Recovery And Interior Laboratory (GRAIL) mission (together with topographic data from the Lunar Reconnaissance Orbiter) to reveal an unexpectedly large mass anomaly, buried under the crater surface. This excess mass is pulling the basin floor down by about a kilometer from what would be expected. What this mass is exactly is not yet known. One distinct possibility is that it's the dispersed remains of a nickel-iron core from the giant object that likely slid into the Moon to make the original impact structure some 4 billion years ago. Altogether the mass amounts to some 2x1018 kilograms. If correct it means that the Moon has a seriously metal bottom.
The hyperactive comets
The precise origin of Earth's water remains somewhat mysterious. A key gauge of its history comes from the ratio of deuterium to hydrogen isotopes (D/H) in our oceans. Water that forms in very cold interplanetary or interstellar environments is thought to incorporate more of the heavy deuterium than in warmer conditions. Most cometary bodies studied to date have D/H ratios 2 to 3 times higher than in terrestrial water, suggesting that at most 10% of Earth's water was delivered by comets. Now a study by Dariusz et al. of a small number of hyperactive comets (which release more water vapor than other comets when near the Sun due to icy grains lofted into their temporary atmospheres) indicates D/H ratios similar to terrestrial values. One interpretation is that hyperactive comets are giving us a more accurate measurement of their internal water content, compared to other comets where the water is just coming from the near surface, and not from ejected icy grains. If that's true then all comets may be back in contention as suppliers of Earth's water.
*This post originally and incorrectly identified the institution as "Baylor College." We regret the error.