43 research outputs found
The influence of gravity on granular impacts II. A gravity-scaled collision model for slow interactions
Slow interactions on small body surfaces occur both naturally and through human intervention. The resettling of grains and boulders following a cratering event, as well as observations made during small body missions, can provide clues regarding the material properties and the physical evolution of a surface. In order to analyze such events, it is necessary to understand how gravity influences granular behavior. In this work, we study slow impacts into granular materials for different collision velocities and gravity levels. Our objectives are to develop a model that describes penetration depth in terms of the dimensionless Froude number and to use this model to understand the relationship between collision behavior, collision velocity, and gravity. We use the soft-sphere discrete element method to simulate impacts into glass beads under gravitational accelerations ranging from 9.81 m/s^2 to 0.001 m/s^2. We quantify collision behavior using the peak acceleration, the penetration depth, and the collision duration of the projectile, and we compare the collision behavior for impacts within a Froude number range of 0 to 10. The measured penetration depth and collision duration for low-velocity collisions are comparable when the impact parameters are scaled by the Froude number, and the presented model predicts the collision behavior well within the tested Froude number range. If the impact Froude number is low (0 < Fr < 1.5), the collision occurs in a regime that is dominated by a depth-dependent quasi-static friction force. If the impact Froude number is high enough (1.5 < Fr < 10), the collision enters a second regime that is dominated by inertial drag. The presented collision model can be used to constrain the properties of a granular surface material using the penetration depth measurement from a single impact event. If the projectile size, the collision velocity, the gravity level, and the final penetration depth are known and the material density is estimated, then the internal friction angle of the material can be deduced
Low-velocity impacts into granular material: application to small-body landing
With the flourishing number of small body missions that involve surface interactions,
understanding the mechanics of spacecraft - surface interactions is crucial for improving our
knowledge about the landing phases of space missions, for preparing spacecraft operations,
and for interpreting the results of measurements made during the surface interactions. Given
their regolith-covered surfaces, the process of landing on a small body can be considered as
an impact at low-velocity onto a granular material in reduced-gravity.
In order to study the influence of the surface material, projectile shape, and gravity on
the collision dynamics we used two experimental configurations (one for terrestrial gravity
experiments and one for reduced-gravity experiments) to perform low-velocity collisions into
different types of granular materials: quartz sand, and two different sizes of glass beads (1.5
and 5 mm diameter). Both a spherical and a cubic projectile (with varying impact orientation)
were used.
The experimental data support a drag model for the impact dynamics composed of both a
hydrodynamic drag force and quasi-static resistance force. The hydrodynamic and quasi-static
contributions are related to the material frictional properties, the projectile geometry, and the
gravity.
The transition from a quasi-static to a hydrodynamical regime is shown to occur at lower
impact velocities in reduced-gravity trials than in terrestrial gravity trials, indicating that
regolith has a more fluid-like behaviour in low-gravity. The reduced quasi-static regime of a
granular material under low-gravity conditions leads to a reduction in the strength, resulting
in a decreased resistance to penetration and larger penetration depths
The Polarization of Ambient Noise on Mars
Seismic noise recorded at the surface of Mars has been monitored since February 2019,
using the InSight seismometers. This noise can reach â200 dB. It is 500 times lower than on Earth at night and it increases of 30 dB during the day. We analyze its polarization as a function of time and frequency in the band 0.03â1 Hz. We use the degree of polarization to extract signals with stable polarization independent of their amplitude and type of polarization. We detect polarized signals at all frequencies and all times. Glitches correspond to linear polarized signals which are more abundant during the night. For signals with elliptical polarization, the ellipse is in the horizontal plane below 0.3 Hz. In the 0.3-1Hz high frequency band (HF) and except in the evening, the ellipse is in the vertical plane and the major axis is tilted. While polarization azimuths are different in the two frequency bands, they both vary as a function of local hour and season. They are also correlated with wind direction, particularly during the daytime. We investigate possible aseismic and seismic origins of the polarized signals. Lander or tether noise can be discarded. Pressure fluctuations transported by wind may explain part of the HF polarization but not the tilt of the ellipse. This tilt can be obtained if the source is an acoustic emission coming from high altitude at critical angle. Finally, in the evening when the wind is low, the measured polarized signals may correspond to the seismic wavefield of the Mars background noise
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Preparing for InSight: An Invitation to Participate in a Blind Test for Martian Seismicity
The InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) lander will deploy a seismic monitoring package on Mars in November 2018. In prepara-
tion for the data return, we prepared a blind test in which we invite participants to detect and characterize seismicity included in a synthetic dataset of continuous waveforms from a single station that mimics both the streams of data that will be available from InSight, as well as expected tectonic and impact seismicity and noise conditions on Mars. We expect that the test will ultimately improve and extend the current set of methods that the InSight team plan to use in routine analysis of the Martian dataset
The Marsquake Service: Securing Daily Analysis of SEIS Data and Building the Martian Seismicity Catalogue for InSight
Abstract The InSight mission expects to operate a geophysical observatory on Mars for at least two Earth years from late 2018. InSight includes a seismometer package, SEIS. The Marsquake Service (MQS) is created to provide a first manual review of the seismic data returned from Mars. The MQS will detect, locate, quantify and classify seismic events, whether tectonic or impact in origin. A suite of new and adapted methodologies have been developed to allow location and quantification of seismic events at the global scale using a single station, and a software framework has been developed that supports these methods. This paper describes the expected signals that will be recorded by SEIS, the methods used for their identification and interpretation, and reviews the planned MQS operational procedures. For each seismic event, the MQS will locate events using all available body and surface phases, using the best estimates of the Martian structure, which will become more accurate as more Martian marsquakes are identified and located. The MQS will curate the Mars seismicity catalogue, with all events being relocated to use revised suites of structure models as they are introduced
Pre-mission InSights on the Interior of Mars
Abstract The Interior exploration using Seismic Investigations, Geodesy, and Heat Trans-
port (InSight) Mission will focus on Marsâ interior structure and evolution. The basic structure of crust, mantle, and core form soon after accretion. Understanding the early differentiation process on Mars and how it relates to bulk composition is key to improving our understanding of this process on rocky bodies in our solar system, as well as in other solar systems. Current knowledge of differentiation derives largely from the layers observed via seismology on the Moon. However, the Moonâs much smaller diameter make it a poor analog with respect to interior pressure and phase changes. In this paper we review the current knowledge of the thickness of the crust, the diameter and state of the core, seismic attenuation, heat flow, and interior composition. InSight will conduct the first seismic and heat flow measurements of Mars, as well as more precise geodesy. These data reduce uncertainty in crustal thickness, core size and state, heat flow, seismic activity and meteorite impact rates by a factor of 3â10Ă relative to previous estimates. Based on modeling of seismic wave propagation, we can further constrain interior temperature, composition, and the location of phase changes. By combining heat flow and a well constrained value of crustal thickness, we can estimate the distribution of heat producing elements between the crust and mantle. All of these quantities are key inputs to models of interior convection and thermal evolution that predict the processes that control subsurface temperature, rates of volcanism, plume distribution and stability, and convective state. Collectively these factors offer strong controls on the overall evolution of the geology and habitability of Mars
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Planned Products of the Mars Structure Service for the InSight Mission to Mars
Abstract 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
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Initial results from the InSight mission on Mars
NASAâs InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planetâs surface geology and volatile
processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September
2019, 174 seismic events have been recorded by the landerâs seismometer, including over 20 events of moment magnitude Mw
= 3â4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indi-
cate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below
approximately Mw = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes
exceeding Mw = 4 have been observed. The landerâs other instrumentsâtwo cameras, atmospheric pressure, temperature and
wind sensors, a magnetometer and a radiometerâhave yielded much more than the intended supporting data for seismometer
noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital
estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity
waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by
further measurements by the InSight lander
Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data
Marsâs seismic activity and noise have been monitored since January 2019 by the seismometer of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander. At night, Mars is extremely quiet; seismic noise is about 500 times lower than Earthâs microseismic noise at periods between 4 s and 30 s. The recorded seismic noise increases during the day due to ground deformations induced by convective atmospheric vortices and ground-transferred wind-generated
lander noise. Here we constrain properties of the crust beneath InSight, using signals from atmospheric vortices and from the
hammering of InSightâs Heat Flow and Physical Properties (HP3) instrument, as well as the three largest Marsquakes detected
as of September 2019. From receiver function analysis, we infer that the uppermost 8â11 km of the crust is highly altered and/
or fractured. We measure the crustal diffusivity and intrinsic attenuation using multiscattering analysis and find that seismic
attenuation is about three times larger than on the Moon, which suggests that the crust contains small amounts of volatiles
SEIS: Insightâs Seismic Experiment for Internal Structure of Mars
By the end of 2018, 42 years after the landing of the two Viking seismometers
on Mars, InSight will deploy onto Marsâ surface the SEIS (Seismic Experiment for Internal
Structure) instrument; a six-axes seismometer equipped with both a long-period three-axes
Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These
six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz,
with possible extension to longer periods. Data will be transmitted in the form of three
continuous VBB components at 2 sample per second (sps), an estimation of the short period
energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at
10 sps. The continuous streams will be augmented by requested event data with sample
rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Vikingâs Mars
seismic monitoring by a factor of ⌠2500 at 1 Hz and ⌠200 000 at 0.1 Hz. An additional
major improvement is that, contrary to Viking, the seismometers will be deployed via a
robotic arm directly onto Marsâ surface and will be protected against temperature and wind
by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is
reasonable to infer a moment magnitude detection threshold of Mw ⌠3 at 40⊠epicentral
distance and a potential to detect several tens of quakes and about five impacts per year. In
this paper, we first describe the science goals of the experiment and the rationale used to
define its requirements. We then provide a detailed description of the hardware, from the
sensors to the deployment system and associated performance, including transfer functions
of the seismic sensors and temperature sensors. We conclude by describing the experiment
ground segment, including data processing services, outreach and education networks and
provide a description of the format to be used for future data distribution