February 2018 Letter from the Director
Lots of news to report from DTM in February, at least as far as press releases about newly published papers by DTM scientists are concerned.
A year and a half ago, former DTM postdoc Guillem Anglada-Escudé, working with DTM staff scientist Paul Butler and a large international team, reported detection of a planet orbiting the star nearest our own Sun, Proxima Centauri. Proxima Centauri is a cool red dwarf star, so even though the planet Proxima b is on a tight orbit, its surface temperature could be in the range to support the presence of liquid water on its surface, believed by many to be one of the key ingredients necessary for the development of life. Red dwarf stars, however, are known for their frequent violent behavior. Using data from the Atacama Large Millimeter Array (ALMA) in Chile, DTM's NSF fellow Meredith MacGregor and staff scientist Alycia Weinberger observed a massive solar flare on Proxima Centauri. Energetic flares of this type emit huge amounts of ultraviolet light and X-rays. For the press release for their paper, Carnegie's Natasha Metzler paraphrased the title from Judith Viorst's children's book in noting that the size of this flare would have made it a "terrible, horrible, no good, very bad day" for any form of life at the surface of Proxima b. As the exploration for life elsewhere in the galaxy continues, the flare discovery by MacGregor and Weinberger provides yet more evidence of the special characteristics of Earth. Earth is not only at just the right distance from the Sun to support surface life, but the Sun is a relatively stable star, so Earth's persistent magnetic field is strong enough to protect Earth's surface from the energetic particles that are emitted from the Sun. Given that it took nearly 4 billion years for complex, multi-cellular life, to evolve on Earth, finding the planetary conditions that can provide long-term stable environments may be a critical component in the search for life elsewhere.
An artist's impression of a flare from Proxima Centauri, modeled after the loops of glowing hot gas seen in the largest solar flares. An artist's impression of the exoplanet Proxima b is shown in the foreground. Proxima b orbits its star 20 times closer than the Earth orbits the Sun. A flare 10 times larger than a major solar flare would blast Proxima b with 4,000 times more radiation than the Earth gets from our Sun's flares. Credit: Roberto Molar Candanosa / Carnegie Institution for Science, NASA/SDO, NASA/JPL.
Explaining a Lunar Oasis
Besides solar flares, another known cause of extinctions are large meteorite impacts, none likely bigger than the one that ejected enough material into space to form Earth's moon. DTM postdoctoral fellow Miki Nakajima has been exploring how water and other volatile elements might have been retained in the cloud of dust and gas created by that impact. One would think that the conversion of the huge amounts of kinetic energy into thermal energy during collisions of this magnitude boiled all the water and other volatile elements out of the planet. Earth indeed is deficient in water by about a factor of 30 compared to the more volatile-rich meteorites believed to represent the building blocks of the planet. The Moon is even more depleted in water than Earth, but past work by DTM staff scientist Erik Hauri working with colleague Alberto Saal of Brown University has shown the lunar interior to contain substantially higher contents of water than previously suspected. Nakajima's theoretical studies of the disk of dust and gas that orbited Earth in the aftermath of the moon-forming collision suggests that conditions were not so extreme as to cause all the water to disassociate into hydrogen and oxygen. Because water molecules would diffuse through the disk much more slowly than hydrogen atoms, a good portion of the water could be retained in the materials that would eventually coalesce to form the Moon.
A video simulation shows the canonical model of the Moon’s formation, in which the proto-Earth was hit by a Mars-sized object between 4.4. and 4.5 billion years ago. Animation courtesy of Miki Nakajima and Dave Stevenson.
Memories of Earth's Birth
The violence of impacts of this magnitude are beyond human comprehension, and it turns out that our intuition about their effects isn't so good either, as the retention of water during Moon formation suggests. Another study published in Nature by DTM postdoctoral fellow Brad Peters, DTM chemistry lab manager Mary Horan, myself, and James Day of Scripps Institution of Oceanography reports preservation of an isotopic signal in modern day volcanic rocks that likely dates back to the time of the giant impact. Peters' work focused on recent volcanism at Reunion Island, in the Indian Ocean, that is the current location of the volcanic hotspot responsible for the Deccan Traps flood basalt, and quite possibly extinction of the dinosaurs, 66 million years ago. Through extremely careful analytical work, Peters resolved very small variations in the relative abundance of the mass 142 isotope of the rare earth element neodymium. These changes reflect the radioactive decay of 146-samarium, an isotope that was only present for the first half billion years of Earth history. Preservation of this signature in very young volcanic rocks shows that there are regions in Earth's interior that still remember the events associated with Earth formation, providing a new look at the efficiency of mixing of Earth's interior by 4.4 billion years of mantle convection.
Sunrise over Piton des Neiges, an extinct stratovolcano rising more than 10,000 feet above the Indian Ocean on La Réunion. Photo courtesy Bradley Peters.
Clearing Skies on a Brown Dwarf
If there is one phase of planet evolution that we understand the least, it's the recovery of the planet from the energy of its own creation. Nakajima's study investigates this era theoretically while Peters' study looks for remnants of the consequences of early planetary differentiation that survived to the present. Another approach to this problem is being pursued by DTM's Sagan postdoctoral fellow Jonathan Gagné working with former DTM Hubble fellow Jacqueline Faherty and colleagues. Gagné's work is focused on brown dwarfs, those objects bigger than the biggest planet, but too small to sustain the nuclear fusion that powers a star. The work described in the new paper examined an unusually red brown dwarf that the authors were able to associate with a group of similar stars that appear to be gravitationally unconnected with any solar system. The advantage to observing such free-floating objects is twofold. First, there is no need to mask out the overpowering light from a nearby bright star when directly observing a free-floating object. Second, this group of brown dwarfs formed roughly 150 million years ago, and so gives a direct observational window into the time frame of planet formation being investigated in Nakajima's and Peters' work. Their observations of this object defined its radius, mass, and temperature. A key result from this study is that it allowed them to define the time in the gas giant's evolution when its temperature cooled sufficiently for the atmosphere to transition from cloudy to cloud-free. Their approach thus allows the opportunity to directly observe a step in planetary evolution that likely also occurred on Jupiter and Saturn, but over 4 billion years ago.
An artist's conception of a brown dwarf. Image is courtesy of NASA/JPL, slightly modified by Jonathan Gagné.
Although all these results were published recently, the work going into them took considerably longer. Nature does not easily give up its secrets, but with hard work, novel approaches, and asking the right questions, when those secrets are revealed, they provide fascinating glimpses into the processes that produced and modify the natural world around us.