The origin of deep ocean microseisms in the North Atlantic Ocean Kedar, Sharon; Longuet-Higgins, Michael; Webb, Frank ...
Proceedings of the Royal Society. A, Mathematical, physical, and engineering sciences,
03/2008, Letnik:
464, Številka:
2091
Journal Article
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Oceanic microseisms are small oscillations of the ground, in the frequency range of 0.05-0.3 Hz, associated with the occurrence of energetic ocean waves of half the corresponding frequency. In 1950, ...Longuet-Higgins suggested in a landmark theoretical paper that (i) microseisms originate from surface pressure oscillations caused by the interaction between oppositely travelling components with the same frequency in the ocean wave spectrum, (ii) these pressure oscillations generate seismic Stoneley waves on the ocean bottom, and (iii) when the ocean depth is comparable with the acoustic wavelength in water, compressibility must be considered. The efficiency of microseism generation thus depends on both the wave frequency and the depth of water. While the theory provided an estimate of the magnitude of the corresponding microseisms in a compressible ocean, its predictions of microseism amplitude heretofore have never been tested quantitatively. In this paper, we show a strong agreement between observed microseism and calculated amplitudes obtained by applying Longuet-Higgins' theory to hindcast ocean wave spectra from the North Atlantic Ocean. The calculated vertical displacements are compared with seismic data collected at stations in North America, Greenland, Iceland and Europe. This modelling identifies a particularly energetic source area stretching from the Labrador Sea to south of Iceland, where wind patterns are especially conducive to generating oppositely travelling waves of same period, and the ocean depth is favourable for efficient microseism generation through the 'organ pipe' resonance of the compression waves, as predicted by the theory. This correspondence between observations and the model predictions demonstrates that deep ocean nonlinear wave-wave interactions are sufficiently energetic to account for much of the observed seismic amplitudes in North America, Greenland and Iceland.
The InSight lander will deliver geophysical instruments to Mars in 2018, including seismometers installed directly on the surface (Seismic Experiment for Interior Structure, SEIS). Routine operations ...will be split into two services, the Mars Structure Service (MSS) and Marsquake Service (MQS), which will be responsible, respectively, for defining the structure models and seismicity catalogs from the mission. The MSS will deliver a series of products before the landing, during the operations, and finally to the Planetary Data System (PDS) archive. Prior to the mission, we assembled a suite of
a priori
models of Mars, based on estimates of bulk composition and thermal profiles. Initial models during the mission will rely on modeling surface waves and impact-generated body waves independent of prior knowledge of structure. Later modeling will include simultaneous inversion of seismic observations for source and structural parameters. We use Bayesian inversion techniques to obtain robust probability distribution functions of interior structure parameters. Shallow structure will be characterized using the hammering of the heatflow probe mole, as well as measurements of surface wave ellipticity. Crustal scale structure will be constrained by measurements of receiver function and broadband Rayleigh wave ellipticity measurements. Core interacting body wave phases should be observable above modeled martian noise levels, allowing us to constrain deep structure. Normal modes of Mars should also be observable and can be used to estimate the globally averaged 1D structure, while combination with results from the InSight radio science mission and orbital observations will allow for constraint of deeper structure.
Geophysical measurements can reveal the structures and thermal states of icy ocean worlds. The interior density, temperature, sound speed, and electrical conductivity thus characterize their ...habitability. We explore the variability and correlation of these parameters using 1‐D internal structure models. We invoke thermodynamic consistency using available thermodynamics of aqueous MgSO4, NaCl (as seawater), and NH3; pure water ice phases I, II, III, V, and VI; silicates; and any metallic core that may be present. Model results suggest, for Europa, that combinations of geophysical parameters might be used to distinguish an oxidized ocean dominated by MgSO4 from a more reduced ocean dominated by NaCl. In contrast with Jupiter's icy ocean moons, Titan and Enceladus have low‐density rocky interiors, with minimal or no metallic core. The low‐density rocky core of Enceladus may comprise hydrated minerals or anhydrous minerals with high porosity. Cassini gravity data for Titan indicate a high tidal potential Love number (k2>0.6), which requires a dense internal ocean (ρocean>1,200 kg m−3) and icy lithosphere thinner than 100 km. In that case, Titan may have little or no high‐pressure ice, or a surprisingly deep water‐rock interface more than 500 km below the surface, covered only by ice VI. Ganymede's water‐rock interface is the deepest among known ocean worlds, at around 800 km. Its ocean may contain multiple phases of high‐pressure ice, which will become buoyant if the ocean is sufficiently salty. Callisto's interior structure may be intermediate to those of Titan and Europa, with a water‐rock interface 250 km below the surface covered by ice V but not ice VI.
Plain Language Summary
Seismometers, magnetometers, and other tools may be used in the future to glimpse the insides of ocean worlds‐moons of Jupiter and Saturn that have lots of liquid water under their icy surfaces. These measurements could reveal whether water and rock interact to produce chemical conditions that on Earth support life, how much life such chemical activity might support, and how long that activity has persisted through time. The pressures and temperatures in these extraterrestrial oceans differ from those in Earth's oceans, so only just now are the needed tools and data becoming available to predict what future measurements might reveal. In this work, we investigated the interior structures of icy ocean worlds based on available information—mainly NASA's Galileo and Cassini missions—and used chemical data to test what ocean and rock compositions are possible. Our calculations make predictions for Saturn's moons: Titan should not have an iron core, and its ocean may contain little or no high‐pressure ice. Fluids may flow through the whole of the rock core of Enceladus because Cassini gravity measurements seem to point to a porous interior. Geophysical investigations could test whether the ocean in Jupiter's moon Europa has a composition like Earth's or may instead by very acidic if water and rock have not interacted much. Our calculations predict that these two scenarios can create unique combinations of measurable properties that can be probed by future missions using seismology, magnetic field measurements, and other means.
Key Points
We examine possible ice thicknesses, mineralogy, and porosity in icy ocean worlds consistent with spacecraft and thermodynamic data
We identify available and needed thermodynamics of ices, oceans, silicates, and metals
We examine the influences of ocean composition and depth‐dependent ocean density on tidal dissipation
Seismic data will be a vital geophysical constraint on internal structure of Europa if we land instruments on the surface. Quantifying expected seismic activity on Europa both in terms of large, ...recognizable signals and ambient background noise is important for understanding dynamics of the moon, as well as interpretation of potential future data. Seismic energy sources will likely include cracking in the ice shell and turbulent motion in the oceans. We define a range of models of seismic activity in Europa's ice shell by assuming each model follows a Gutenberg‐Richter relationship with varying parameters. A range of cumulative seismic moment release between 1016 and 1018 Nm/yr is defined by scaling tidal dissipation energy to tectonic events on the Earth's moon. Random catalogs are generated and used to create synthetic continuous noise records through numerical wave propagation in thermodynamically self‐consistent models of the interior structure of Europa. Spectral characteristics of the noise are calculated by determining probabilistic power spectral densities of the synthetic records. While the range of seismicity models predicts noise levels that vary by 80 dB, we show that most noise estimates are below the self‐noise floor of high‐frequency geophones but may be recorded by more sensitive instruments. The largest expected signals exceed background noise by ∼50 dB. Noise records may allow for constraints on interior structure through autocorrelation. Models of seismic noise generated by pressure variations at the base of the ice shell due to turbulent motions in the subsurface ocean may also generate observable seismic noise.
Plain Language Summary
In this study, we are looking at sources that vibrate the outer ice shell of Europa and produce energy recorded by a seismometer. We are interested in this because seismology has been the best tool for determining the interior structure of the Earth; therefore, we want to consider how much energy we expect to go into seismic activity on Europa. In this study, we simulate long seismic recordings assuming that icequakes behave statistically similar to earthquakes. This predicts how frequently we expect events of different sizes. By scaling the total energy released observed on the Earth's moon by the much higher tidal energy available to Europa, we predict a range of simulated event catalogs. With those catalogs, we simulate the seismic waves recorded at a seismometer. This lets us determine how large events are likely to be, and also what the more or less continuous background noise from many small events will look like. We also examine a technique that can use an approach called autocorrelation to pull signals out of the noise, which in our simulated records show a clear energy arrival representing energy reflected from the ocean bottom. We conclude that a simple instrument does not have enough sensitivity to reliably record either the large signals or the background noise on Europa's surface, but a more sensitive instrument may record the background noise for periods shorter than 10 s, as well as very likely recording signals from larger events expected to occur a few times per week of observation.
Key Points
Seismic activity level and ambient seismic noise due to tidal cracking in the ice on Europa are estimated
Activity and noise are modeled with a Gutenberg‐Richter relationship using numerical wave propagation
Autocorrelation of seismic noise may potentially be used to determine structure in the absence of large events
Seismic Wave Propagation in Icy Ocean Worlds Stähler, Simon C.; Panning, Mark P.; Vance, Steven D. ...
Journal of geophysical research. Planets,
January 2018, 2018-01-00, 20180101, Letnik:
123, Številka:
1
Journal Article
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Seismology was developed on Earth and shaped our model of the Earth's interior over the twentieth century. With the exception of the Philae lander, all in situ extraterrestrial seismological effort ...to date was limited to other terrestrial planets. All have in common a rigid crust above a solid mantle. The coming years may see the installation of seismometers on Europa, Titan, and Enceladus, so it is necessary to adapt seismological concepts to the setting of worlds with global oceans covered in ice. Here we use waveform analyses to identify and classify wave types, developing a lexicon for icy ocean world seismology intended to be useful to both seismologists and planetary scientists. We use results from spectral‐element simulations of broadband seismic wavefields to adapt seismological concepts to icy ocean worlds. We present a concise naming scheme for seismic waves and an overview of the features of the seismic wavefield on Europa, Titan, Ganymede, and Enceladus. In close connection with geophysical interior models, we analyze simulated seismic measurements of Europa and Titan that might be used to constrain geochemical parameters governing the habitability of a sub‐ice ocean.
Plain Language Summary
Icy ocean worlds, like Europa or Titan harbor an ocean below a solid ice layer. This ocean may be habitable but is difficult to study from orbit. We demonstrated that surface‐installed seismometers are able to measure ice thickness and ocean depth directly and help constrain ocean temperature and chemistry, which are both critical for potential habitability. This paper tries to bridge the gap between methods of seismology on Earth and potential icy moon seismology by adapting common concepts to this setting. Using seismic wavefield simulations on high‐performance computers, we showcase a few tests for ice thickness, ocean depth, location of seismic events, and the existence of high‐pressure ice layers below an ocean. The paper focuses on Europa and Titan, with a more general description of Ganymede and Enceladus. The seismic waveform databases are made available to the seismological and planetary community to allow other researchers to build their own studies on icy ocean world seismology.
Key Points
Prepares for icy moon seismology by proposing a phase‐naming scheme
Observation of a few magnitude 3 events allows estimates of ice thickness and ocean depth
The existence of high‐pressure ice phases can be inferred from spectral analysis
•A deck seismometer on a “Curiosity” engineering model showed clear tectonic events.•The deck seismometer had high coherence with an instrument on the ground.•Seismometers with minimal accommodation ...may meet science needs for future landers.
Seismic measurements are an important tool for exploration of planetary interiors, but may not be included in missions due to perceived complexity in placement of sensitive instruments on the surface. To help address this concern, we assess the fidelity of recordings of ground motion by an instrument placed on the deck of the engineering model of the Mars Science Laboratory compared with an identical instrument placed on the ground directly beneath. Comparison of the recordings reveals clear recordings of teleseismic earthquakes on both instruments. The transfer function between the instruments demonstrates the deck instrument is affected by resonance frequencies of the lander, and does not faithfully record ground motion at these frequencies or higher. In addition, additional decoherence is observed near 1 Hz during periods of strong airflow due to air conditioning cycling. However, excellent coherence and a transfer function near 1 can be observed in the important seismic band between 2 and 30 s at all times and extending up to the lander resonances during the night time when air conditioning was not running. This suggests a deck-mounted seismic instrument may be able to provide valuable science return without requiring additional deployment complexity.
In December 2018, the NASA InSight lander successfully placed a seismometer on the surface of Mars. Alongside, a hammering device was deployed at the landing site that penetrated into the ground to ...attempt the first measurements of the planetary heat flow of Mars. The hammering of the heat probe generated repeated seismic signals that were registered by the seismometer and can potentially be used to image the shallow subsurface just below the lander. However, the broad frequency content of the seismic signals generated by the hammering extends beyond the Nyquist frequency governed by the seismometer's sampling rate of 100 samples per second. Here, we propose an algorithm to reconstruct the seismic signals beyond the classical sampling limits. We exploit the structure in the data due to thousands of repeated, only gradually varying hammering signals as the heat probe slowly penetrates into the ground. In addition, we make use of the fact that repeated hammering signals are sub‐sampled differently due to the unsynchronized timing between the hammer strikes and the seismometer recordings. This allows us to reconstruct signals beyond the classical Nyquist frequency limit by enforcing a sparsity constraint on the signal in a modified Radon transform domain. In addition, the proposed method reduces uncorrelated noise in the recorded data. Using both synthetic data and actual data recorded on Mars, we show how the proposed algorithm can be used to reconstruct the high‐frequency hammering signal at very high resolution.
Key Points
Hammering of the InSight heat probe generates high‐frequency seismic signals that exceed the Nyquist frequency of the seismometer
We developed a new data acquisition and reconstruction workflow that allows for the recovery of the full‐bandwidth hammering signals
During hammering, we deliberately turned off the seismometer's anti‐aliasing filters and reconstructed the aliased signal using a sparseness‐promoting algorithm
In response to NASA's announced requirement for Earth hazard monitoring sensor-web technology, a multidisciplinary team involving sensor-network experts (Washington State University), space ...scientists (JPL), and Earth scientists (USGS Cascade Volcano Observatory (CVO)), have developed a prototype of dynamic and scalable hazard monitoring sensor-web and applied it to volcano monitoring. The combined Optimized Autonomous Space - In-situ Sensor-web (OASIS) has two-way communication capability between ground and space assets, uses both space and ground data for optimal allocation of limited bandwidth resources on the ground, and uses smart management of competing demands for limited space assets. It also enables scalability and seamless infusion of future space and in-situ assets into the sensor-web. The space and in-situ control components of the system are integrated such that each element is capable of autonomously tasking the other. The ground in-situ was deployed into the craters and around the flanks of Mount St. Helens in July 2009, and linked to the command and control of the Earth Observing One (EO-1) satellite.
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