The 25-km Discontinuity: Implications for Lunar History

Simmons, Gene; Todd, Terry; Wang, Herbert F.

doi:10.1126/science.182.4108.158

Cited by 36 publications

(23 citation statements)

References 11 publications

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“…Simmons et al (1973) have discussed this problem in depth, and it is possible that the interface also represents a compositional change. The issue remains unresolved.…”

Section: Implications Of the Seismic Resultsmentioning

confidence: 99%

“…The favored hypothesis at this point is that cratering effects have produced a complex series of cracks and fissures in a layer of extremely dry, volatile-poor, outgassed rock. Below a certain depth, 1 to 20 km, either no cracks were formed because meteorite impact disruption did not extend that far into the moon, or pressure and subsequent processes have annealed or replaced most of the cracked material (Simmons et al, 1973).…”

Section: Seismogram Characteristicsmentioning

confidence: 99%

“…These two seismic models differ substantially in several ways, and an attempt to delineate the source of the differences and reconcile the two models will be made in Chapter 3, analyzing the latest results from each group. A few other researchers have made contributions to the structural models (Simmons et al, 1973;Burkhard and Jackson, 1975;Nyland and Roebrock, 1975;Voss et al, 1976;Jarosch (1977). They will be discussed in later sections.…”

Section: 2 Review Of Previous Workmentioning

confidence: 99%

“…high attenuation suggests high temperature) quantitative models depend critically on laboratory measurements of velocity attenuation, and density as a function of pressure, temperature, physical structure and volatile content in rocks of candidate lunar compositions. Much work has been accomplished in this area (Tittman et al, 1976(Tittman et al, , 1977(Tittman et al, , 1978Schreiber, 1977;Kanamori et al, 1972;Mizutani et al, 1977;Talwani et al, 1974;Todd et al, 1972Todd et al, , 1973Warren et al, 1973;Chung, 1970Chung, , 1971Frisillo and Barsch, 1972), but there are still many pressing questions. Given this situation, the most reasonable approach is to examine specific compositional and temperature models, use what rock physics data is available and determine if the seismic results can be satisfied within the allowable uncertainties.…”

mentioning

confidence: 99%

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Lunar seismology: The internal structure of the Moon

Goins¹,

Dainty²,

Toksöz³

1981

J. Geophys. Res.

147

104

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Tables A1–A4 and Figures A1–A3 are available with entire articleon microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐003;$1.00. Payment must accompany order. The arrival times of direct P and S waves, measured on seismograms recorded from natural lunar seismic events, have been analyzed using linearized inversion and parameter search methods to simultaneously determine event locations, origin times, and structural parameters (seismic velocities). Polarization‐filtered record section plots are correlated with theoretical travel time curves to identify later phases and obtain additional structural and velocity information. Shear wave amplitudes plotted as a function of distance provide data on the existence and magnitude of seismic velocity gradients in the interior. These studies are used to delineate the structure of the moon below the crust to a depth of about 1100 km. This structure has been divided into an upper and a lower mantle. The upper mantle, from 60‐ to 400‐km depth, has average seismic wave velocities of Vp = 7.70 ±0.15 km/s and Vs = 4.45 ± 0.05 km/s with corresponding Q values (taken from the literature) of Qp ∼ 5000 and Qs ∼ 3000. The shear wave velocity decreases with a negative gradient of at most −6 × 10−4 km/s/km, implying a Vs variation of from 4.57 km/s at 60‐km depth to 4.37 km/s at 400‐km depth; this decrease can be accounted for by increasing temperature, and so no major compositional gradient is required in the upper mantle. A small negative P wave velocity gradient may also be present. Between 400‐ and 480‐km depth there is a transition zone with a sharply decreasing shear wave velocity and possibly an accompanying small decrease in Vp. The dominant shear velocity decrease may occur at a 480‐km interface and may represent a compositional change, although the effects of increased temperature cannot be totally ruled out. The lower mantle extends from 480‐km depth to at least 1100‐km depth; few seismic waves recorded by the lunar network penetrate below 1100 km. The average velocities are Vp = 7.6 ± 0.6 km/s and Vs = 4.2 ± 0.1 km/s. Q values (taken from the literature) are Qp ∼ 1500 and Qs ∼ 1000. Below 1100 km there is some indication that the attenuation may increase still further for shear waves, with Qs dropping to a few hundreds or less. This seismic model of the moon is well constrained, with uncertainties on the above values given explicitly by the analysis methods, and so it serves as a strong control on the present‐day lunar compositional and thermal structure.

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“…Simmons et al (1973) have discussed this problem in depth, and it is possible that the interface also represents a compositional change. The issue remains unresolved.…”

Section: Implications Of the Seismic Resultsmentioning

confidence: 99%

Section: Seismogram Characteristicsmentioning

confidence: 99%

Section: 2 Review Of Previous Workmentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

Lunar seismology: The internal structure of the Moon

Goins¹,

Dainty²,

Toksöz³

1981

J. Geophys. Res.

147

104

View full text Add to dashboard Cite

show abstract

“…Simmons et al (2) pointed out that the multiply impacted near-surface rocks on the Moon were fractured to depths of tens of kilometers and the fracture density controls the near-surf ace seismic velocity structure. Dvorak and Phillips (3) discovered cracks beneath young fresh craters that were unfilled on the Moon, or filled with air or water on the Earth gave rise to significant (negative) gravity anomalies.…”

Section: Introductionmentioning

confidence: 99%

Depth of Cracking beneath Impact Craters: New Constraint for Impact Velocity

Ahrens¹,

Xia²,

Coker³

2002

AIP Conference Proceedings

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Abstract. Both small-scale impact craters in the laboratory and less than 5 km in diameter bowlshaped craters on the Earth are strength (of rock) controlled. In the strength regime, crater volumes are nearly proportional to impactor kinetic energy. The depth of the cracked rock zone beneath such craters depends on both impactor energy and velocity. Thus determination of the maximum zone of cracking constrains impact velocity. We show this dependency for small-scale laboratory craters where the cracked zone is delineated via ultrasonic methods. The 1 km-deep cracked zone beneath Meteor Crater is found to be consistent with the crater scaling of Schmidt (1) and previous shock attenuation calculations. INTRODUCTIONImpact-induced fractures in rock have long been recognized beneath terrestrial impact craters. Simmons et al. (2) pointed out that the multiply impacted near-surface rocks on the Moon were fractured to depths of tens of kilometers and the fracture density controls the near-surf ace seismic velocity structure. Dvorak and Phillips (3) discovered cracks beneath young fresh craters that were unfilled on the Moon, or filled with air or water on the Earth gave rise to significant (negative) gravity anomalies. Ahrens and Rubin (4) demonstrated that the seismic velocity of rocks beneath small laboratory, strength-limited, craters increased with depth. This results from the near-surface zone of cracking beneath craters.The intensity of cracking decreases with increasing depth reflecting the decreasing dynamic tensile stresses with depth in the rock. The volume of strengthcontrolled impact craters has been demonstrated to be nearly proportional to impactor energy for soft and hard rocks (5).

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Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review

Pettinelli

Cosciotti

Paolo

et al. 2015

Reviews of Geophysics

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The first European mission dedicated to the exploration of Jupiter and its icy moons (JUpiter ICy moons Explorer-JUICE) will be launched in 2022 and will reach its final destination in 2030. The main goals of this mission are to understand the internal structure of the icy crusts of three Galilean satellites (Europa, Ganymede, and Callisto) and, ultimately, to detect Europa's subsurface ocean, which is believed to be the closest to the surface among those hypothesized to exist on these moons. JUICE will be equipped with the 9 MHz subsurface-penetrating radar RIME (Radar for Icy Moon Exploration), which is designed to image the ice down to a depth of 9 km. Moreover, a parallel mission to Europa, which will host onboard REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) equipped with 9MHz and 60MHz antennas, has been recently approved by NASA. The success of these experiments strongly relies on the accurate prediction of the radar performance and on the optimal processing and interpretation of radar echoes that, in turn, depend on the dielectric properties of the materials composing the icy satellite crusts. In the present review we report a complete range of potential ice types that may occur on these icy satellites to understand how they may affect the results of the proposed missions. First, we discuss the experimental results on pure and doped water ice in the framework of the Jaccard theory, highlighting the critical aspects in terms of a lack of standard laboratory procedures and inconsistency in data interpretation. We then describe the dielectric behavior of extraterrestrial ice analogs like hydrates and icy mixtures, carbon dioxide ice and ammonia ice. Building on this review, we have selected the most suitable data to compute dielectric attenuation, velocity, vertical resolution, and reflection coefficients for such icy moon environments, with the final goal being to estimate the potential capabilities of the radar missions as a function of the frequency and temperature ranges of interest for the subsurface sounders. We present the different subsurface scenarios and associated radar signal attenuation models that have been proposed so far to simulate the structure of the crust of Europa and discuss the physical and geological nature of various dielectric targets potentially detectable with RIME. Finally, we briefly highlight several unresolved issues that should be addressed, in near future, to improve our capability to produce realistic electromagnetic models of icy moon crusts. The present review is of interest for the geophysical exploration of all solar system bodies, including the Earth, where ice can be present at the surface or at relatively shallow depths.

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The 25-km Discontinuity: Implications for Lunar History

Cited by 36 publications

References 11 publications

Lunar seismology: The internal structure of the Moon

Lunar seismology: The internal structure of the Moon

Depth of Cracking beneath Impact Craters: New Constraint for Impact Velocity

Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review

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