Pebble Atmospheres

The loss of atmospheric components over time is a major issue for rocky planets, especially those that might be capable of harboring life. Earth does it slowly, a world like Mars has done it faster. A new study by Howe et al. in the Astrophysical Journal digs into the details of a slightly different scenario: Atmospheric loss for young Earth-analog worlds with initially massive atmospheres (dominated by molecular hydrogen and helium), formed out of a so-called “pebble-accretion” mechanism. They examine loss by mechanisms like Jeans escape, hydrodynamic escape, stellar winds, impact erosion, photoionization and photodissociation. Intriguingly, photodissociation by lower energy ultraviolet light (the splitting of molecules that allows lighter elements to escape to space) seems like it could account for 90% of atmospheric loss, with impact erosion (from asteroid collisions) amounting to around 5%. The catch is that these worlds don’t lose much atmosphere. It seems hard to explain how pebble accreted worlds could ever lose enough of their primordial ingredients to wind up looking like the Earth.

Ocean Energy

What happens to a terrestrial planet’s oceans when the planet heats up? Global warming on Earth is, unfortunately, providing a set of answers. A new statistical study by Hu et al. in Science Advances uses three decades of data, together with computer modeling to probe water circulation, finding that the kinetic energy of major currents in Earth’s oceans has grown by some 15% per decade. Much of this could be driven by the documented increase in ocean wind energies. But exactly what is going on is still unclear – changing patterns of circulation could also contribute to the way measurements are shifting, and what is seen is a trend on top of a range of variations. Nonetheless, from a physics standpoint it all makes sense – rising temperatures in a system must end up manifested in the energetics of that system. 

Wasp Clocks

Some 12,000 years ago there was another period of rising sea-level on Earth, as it emerged from an ice-age. As it happened certain northern coastal areas in Australia were inundated, disrupting the human societies there. Now a remarkably ingenious technique for dating ancient rock art in the region has been used to show that a unique style of artwork – with slender human figures called Gwions – coincides with that period. This artwork doesn’t use charcoal, which would otherwise provide carbon for radioisotope dating. But researchers have discovered that wasps built nests of mud on the cave walls – before they were painted, and afterwards.  The wasps also incorporated particles of carbon from the environment into their sticky mud constructions. Dating that carbon provides a ‘before and after’ range of times for the paintings themselves. That range sits right on 12,000 years ago and suggests that the unusual art could have been influenced by disrupted environments and cultural dynamics as the world around the artists was transformed.