Planetary scientists would be thrilled if they could peel the Earth like an orange and look at what lies beneath the thin crust. We live on the planet's cold surface, but the Earth is a solid body and the surface is continually deformed, split, wrinkled and ruptured by the roiling of warmer layers beneath it.
The contrast between the surface and the depth is nowhere starker -- or more important -- than in Antarctica. What is causing the mysterious line of volcanoes that emerge from the ice sheet there, and what does it mean for the future of the ice?
"Our understanding of what's going on is really hampered because we can't see the geology," said Andrew Lloyd, a graduate student in earth and planetary sciences in Arts & Sciences at Washington University in St. Louis. "We have to turn to geophysical methods, such as seismology, to learn more," he said.
Lloyd helped deploy research seismometers across the West Antarctic Rift System and Marie Byrd Land in the austral summer of 2009-10. He then returned in late 2011 and snowmobiled more than 1,000 miles, living in a Scott tent, to recover the precious data.
The recordings the instruments made of the reverberations of distant earthquakes from January 2010 to January 2012 were used to create maps of seismic velocities beneath the rift valley. An analysis of the maps was published online in the Journal of Geophysical Research: Solid Earth on Nov. 12, 2015 (doi:10.1002/2015JB012455).
This is the first time seismologists have been able to deploy instruments rugged enough to survive a winter in this part of the frozen continent, and so this is the first detailed look at the Earth beneath this region.
Not surprisingly, the maps show a giant blob of superheated rock about 60 miles beneath Mount Sidley, the last of a chain of volcanic mountains in Marie Byrd Land at one end of the transect. More surprisingly, they reveal hot rock beneath the Bentley Subglacial Trench, a deep basin at the other end of the transect.
The Bentley Subglacial Trench is part of the West Antarctic Rift System and hot rock beneath the region indicates that this part of the rift system was active quite recently.
Detecting an "earthquake" on Venus would seem to be an impossible task. The planet's surface is a hostile zone of crushing pressure and scorching temperatures--about 874 degrees F, hot enough to melt lead--that would destroy any of the normal instruments used to gauge seismic activity. But conditions in Venus' atmosphere are much more hospitable, and it is here that researchers hope to deploy an array of balloons or satellites that could detect Venusian seismic activity--using sound.
These kinds of low frequency or infrasonic sound waves, much lower than an audible voice, are already measured on Earth. The rumbling or "hum" can be generated by sources as diverse as volcanoes, earthquakes, ocean storms and meteor air blasts. In recent years, says Los Alamos National Laboratory researcher Stephen Arrowsmith, infrasonic observations have undergone a renaissance of sorts, especially as a relatively inexpensive way to monitor atmospheric nuclear weapons tests. But last year, a team of experts convened by the Keck Institute for Space Studies began thinking of ways to use infrasonic observations to get a better look at the geological dynamics of Venus.
At about 50-60 kilometers above Venus' surface, the temperature and pressure conditions are much more like those on Earth, albeit with a denser atmosphere. This dense atmosphere helps translate any seismic waves into infrasonic waves that can be detected with instruments floating above the planet's surface, says Jim Cutts, a Jet Propulsion Laboratory researcher who participated in the Keck conference. Infrasonic waves can be "felt" as either fluctuations in pressure, or as light emissions called airglow, or electron disruptions in Venus' upper atmosphere.
Arrowsmith and colleagues say that barometric pressure changes might be detected with a series of balloons in the Venus cloud layer at 55 kilometers above the surface, such as those launched by the Soviet Union in Venus' atmosphere in the 1980s. In a second talk, Philippe Lognonné and colleagues discuss a complementary way to analyze the planet's infrasonic waves, using orbiting satellites to detect airglow. In both cases, the first goal will be determine what the noise-to-signal ratio might be for these two techniques. The researchers want to know if the instruments onboard a balloon or satellite will be sensitive enough to detect and identify a seismic signal in the midst of other infrasonic waves, and how large of a seismic event might be detected by these observations.
Seismic model of Mars: Effects of hydration
Authors:
Zharkov et al
Abstract:
The arguments according to which the Martian minerals are assumed to contain large amount of water in the mantle minerals are given. As for the Earth, these minerals may constitute about 60 wt% of the Martian mantle, and can be considered as main components in their zones. In the mantle of the Earth the molecular concentration of Fe is about 10%, and for the mantle of Mars - about 20%. Taking into account twofold increase of Fe in Martian silicates in comparison with the terrestrial minerals, we have extrapolated the available partial experimental data of the hydration effect on the compressional and shear velocities of seismic waves in forsterite (olivine) and its high pressure phases - wadsleyite and ringwoodite for Martian conditions. The presence of water in the mantle of Mars may lead to the noticeable widening of the olivine-wadsleite phase transition zone, thus the determination of the olivine-wadsleite phase transition width by seismological methods could get a direct indication on the presence of water in the mantle of Mars. To find out real estimates of water content in the mantle of Mars is a task for the future seismic missions. The results of this article are important for InSight mission that will land a geophysical station on Mars in 2016.
