I. Climate Change
1. Clues Found That Earth May Have a Thermostat Set to “Habitable”
Weathering of rocks can control Earth’s temperature over geologic timescales, new geochemical data suggest.
2. Why Are Arctic Rivers Rising in Winter?
Increased glacial melt is boosting winter streamflows by filling aquifers, a new study on an Alaskan river suggests.
II. Hazards & Disasters
1. Big Space Rock to Pass near Earth on Friday
An asteroid named for Florence Nightingale will make its closest approach to our planet since 1890 but will remain a safe distance away.
2. Deciphering Deluges
New modeling approach reexamines two key assumptions about flooding.
III. Ocean Sciences
1. The Legacy of the 1992 Nicaragua Tsunami
A powerful tsunami struck Nicaragua’s Pacific coast 25 years ago. In its wake emerged the first coordinated collaboration among international tsunami scientists.
1. A Grand Tour of the Ocean Basins
A new teaching resource facilitates plate tectonic studies using a Google Earth virtual guided tour of ocean basins around the world.
1. Microbes May Thrive in Subsea Permafrost Long After Flooding
Two cores from the East Siberian Arctic Shelf reveal how microbial communities develop over thousands of years as submarine permafrost slowly thaws.
VI. Geophysical Research Letters
1. In situ detection of boron by ChemCam on Mars
We report the first in situ detection of boron on Mars. Boron has been detected in Gale crater at levels <0.05 wt % B by the NASA Curiosity rover ChemCam instrument in calcium-sulfate-filled fractures, which formed in a late-stage groundwater circulating mainly in phyllosilicate-rich bedrock interpreted as lacustrine in origin. We consider two main groundwater-driven hypotheses to explain the presence of boron in the veins: leaching of borates out of bedrock or the redistribution of borate by dissolution of borate-bearing evaporite deposits. Our results suggest that an evaporation mechanism is most likely, implying that Gale groundwaters were mildly alkaline. On Earth, boron may be a necessary component for the origin of life; on Mars, its presence suggests that subsurface groundwater conditions could have supported prebiotic chemical reactions if organics were also present and provides additional support for the past habitability of Gale crater.
2. Reconstructing the past climate at Gale crater, Mars, from hydrological modeling of late-stage lakes
The sedimentary deposits in Gale crater may preserve one of the best records of the early Martian climate during the Late Noachian and Early Hesperian. Surface and orbital observations support the presence of two periods of lake stability in Gale crater—prior to the formation of the sedimentary mound during the Late Noachian and after the formation and erosion of the mound to its present state in the Early Hesperian. Here we use hydrological models and late-stage lake levels at Gale, to reconstruct the climate of Mars after mound formation and erosion to its present state. Using Earth analog climates, we show that the late-stage lakes require wetter interludes characterized by semiarid climates after the transition to arid conditions in the Hesperian. These climates are much wetter than is thought to characterize much of the Hesperian and are more similar to estimates of the Late Noachian climate.
3. Observations and modeling of ocean-induced melt beneath Petermann Glacier Ice Shelf in northwestern Greenland
We update observationally based estimates of subaqueous melt, Qm, beneath Petermann Glacier Ice Shelf (PGIS), Greenland, and model its sensitivity to oceanic thermal forcing, TF, and subglacial runoff, Qsg, using the Massachusetts Institute of Technology general circulation model (MITgcm), in a two-dimensional domain, with 20 m vertical and 40 m horizontal resolution at the grounding line. We adjust the drag coefficient to match the observationally based Qm. With the inclusion of Qsg, the maximum melt rate ( ) is 2 times larger in summer and 1/3 larger annually than in winter. increases above linear with TF and below linear with Qsg. We estimate that increased by 24% (+8.1 m/yr) beneath PGIS from the 1990s to the 2000s from a 0.21°C warming in ocean temperature and a doubling in Qsg, hence contributing to its thinning. If the PGIS is removed, we estimate that the modeled melt rate near the grounding line will increase 13–16 times.
4. Exploiting LF/MF signals of opportunity for lower ionospheric remote sensing
We introduce a method to diagnose and track the D region ionosphere (60–100 km). This region is important for long-distance terrestrial communication and is impacted by a variety of geophysical phenomena, but it is traditionally very difficult to detect. Modern remote sensing methods used to study the D region are predominately near the very low frequency (VLF, 3–30 kHz) band, with some work also done in the high-frequency and very high frequency bands (HF/VHF, 3–300 MHz). However, the frequency band between VLF and HF has been largely ignored as a diagnostic tool for the ionosphere. In this paper, we evaluate the use of 300 kHz radio reflections as a diagnostic tool for characterizing the D region of the ionosphere. We present radio receiver data, analyze diurnal trends in the signal from these transmitters, and identify ionospheric disturbances impacting LF/MF propagation. We find that 300 kHz remote sensing may allow a unique method for Dregion diagnostics compared to both the VLF and HF/VHF frequency bands, due to a more direct ionospheric reflection coefficient calculation method with high temporal resolution without the use of forward modeling.
5. Slow slip events and the 2016 Te AraroaMw 7.1 earthquake interaction: Northern Hikurangi subduction, New Zealand
Following a sequence of three Slow Slip Events (SSEs) on the northern Hikurangi Margin, between June 2015 and August 2016, a Mw 7.1 earthquake struck ~30 km offshore of the East Cape region in the North Island of New Zealand on the 2 September 2016 (NZ local time). The earthquake was also followed by a transient deformation event (SSE or afterslip) northeast of the North Island, closer to the earthquake source area. We use data from New Zealand's continuous Global Positioning System networks to invert for the SSE slip distribution and evolution on the Hikurangi subduction interface. Our slip inversion results show an increasing amplitude of the slow slip toward the Te Araroa earthquake foreshock and main shock area, suggesting a possible triggering of the Mw 7.1 earthquake by the later stage of the slow slip sequence.
6. On production of gamma rays and relativistic runaway electron avalanches from Martian dust storms
Production of runaway electron avalanches and gamma rays originating inside Martian dust storms are studied using Monte Carlo simulations. In the absence of in situ measurements, we use theoretical predictions of electric fields inside dust storms. Electrons are produced through the relativistic runaway electron avalanches process, and energetic photons are results of the bremsstrahlung scattering of the electrons with the air. Characteristic lengths of the runaway electron avalanche for different electric fields and the energy spectrum of electrons are derived and compared to their terrestrial counterparts. It is found that it is possible for Martian dust storms to develop energetic electron avalanches and produce large fluxes of gamma ray photons similar to terrestrial gamma ray flashes from Earth's thunderstorms. The phenomenon could be called Martian gamma ray flash, and due to the very thin atmosphere on Mars, it can be observed by both ground-based instruments or satellites orbiting the planet.
VII. AGU Blogs
1. Record-low 2016 Antarctic sea ice due to ‘perfect storm’ of tropical, polar conditions
While winter sea ice in the Arctic is declining so dramatically that ships can now navigate those waters without any icebreaker escort, the scene in the Southern Hemisphere is very different. Sea ice area around Antarctica has actually increased slightly in winter — that is, until last year.
2. Low-frequency sea sounds ring clear at high altitudes
A new study shows microphones suspended from helium balloons in the stratosphere can detect low-frequency sounds from ocean waves. The new method shows promise for detecting acoustic signals from natural disasters and nuclear explosions that cannot always be reliably detected by sensors on the ground, according to the study’s authors.