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Volumn 339, Issue 6117, 2013, Pages 296-300

Bright and dark polar deposits on Mercury: Evidence for surface volatiles

Author keywords

[No Author keywords available]

Indexed keywords

ASTEROID; BACKSCATTER; ICE; INSULATION; MERCURY (PLANET); ORGANIC MATTER; POLAR REGION; RADAR; SURFACE LAYER; SURFACE REFLECTANCE; VOLATILE SUBSTANCE;

EID: 84872493452     PISSN: 00368075     EISSN: 10959203     Source Type: Journal    
DOI: 10.1126/science.1229764     Document Type: Article
Times cited : (144)

References (34)
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    • note
    • The MLA is a time-of-flight laser range finder that uses direct detection and pulse-edge timing to determine precisely the range from the MESSENGER spacecraft to Mercury's surface. MLA's laser transmitter emits 6-ns-long pulses at an 8-Hz rate with 20 mJ of energy at a wavelength of 1064 nm. Return echoes are collected by an array of four refractive telescopes and are detected with a single silicon avalanche photodiode detector. The timing of laser pulses is measured with a set of time-to-digital converters linked to a crystal oscillator for which the frequency is monitored from Earth.
  • 15
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    • note
    • The MLA measures the threshold crossing times of the received pulses at two discriminator voltages simultaneously, a low threshold for maximum sensitivity and a threshold about twice as high to give four sample points of the received pulse waveform. A laser pulse may result in triggers at one or both thresholds or not at all. Ranging with low-threshold detections is possible at ranges up to 1500 km, but steady returns that cross both low and high thresholds are obtained mostly at altitudes less than ∼600 km and with near-nadir (<20°) incidence. When a pulse is detected by a pair of discriminators, its energy and duration may be inferred from a model waveform that accounts for the dispersion in time of return pulses as a result of surface slope and/or roughness. To estimate the pulse energy, we adopted a simple triangular model that fits the rising and falling edges of the trigger at each threshold. This model generates values nearly equal to a Gaussian model for well-constrained pulses. Energy is a nonlinear function of pulse timing measurements and tends to have a long-tailed or approximate log-normal distribution, as illustrated in the supplementary materials.
  • 17
    • 84872487023 scopus 로고    scopus 로고
    • note
    • The lidar link equation is Erx = Etxhr(Ar/R2)(rs/p), where Erx is the received signal pulse energy, Etx is the transmitted laser pulse energy, hr is the receiver optics transmission, Ar is the receiver telescope aperture area, R is range, and rs is the target surface reflectivity (relative to Lambertian). The ratio rs of reflected energy to incoming energy (i.e., irradiance/solar flux, often simply written I/F) would be unity for a perfect diffusive reflector for which the transmitter and receiver orientation are perpendicular to the surface. Mercury's reflectivity at optical wavelengths normally lies in a range from 0.08 to 0.12 (30-32), but because of the opposition effect (33) the average 1064-nm reflectance is about 50% higher, or about 0.17.
  • 19
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    • note
    • The topography derived from 700 MLA profiles (29 March 2011 to 1 May 2012) provides a near-complete topographic map of the northern hemisphere northward to 84°N at a resolution of 0.5 km. Craters Prokofiev and Kandinsky were sampled by several off-nadir profiles, from which radial averages of topography were constructed and used to fill in the unsampled interior after adding pseudo-random noise, with a root variance of 70 m, and decimating and interpolating with the blockmedian and surface programs of the Generic Mapping Tools (http://gmt.soest.hawaii.edu). We modeled the average and maximum illumination conditions over a Mercury day by using an approach (20) developed to assess illumination conditions of polar regions of the Moon.
  • 22
    • 84872500021 scopus 로고    scopus 로고
    • note
    • s < 0.3.
  • 23
    • 84872494025 scopus 로고    scopus 로고
    • note
    • Diameters of large craters were fit to the maximum MLA topographic contours of the rims, whereas the diameters of smaller craters were estimated from Mercury Dual Imaging System (34) image mosaics. Locations are less certain for smaller features inadequately sampled by MLA. Diameters of craters sampled ranged from 7 to 108 km, not including the 320-km-diameter Goethe basin. Not included are several degraded and partially flooded craters, such as a 133-km-diameter degraded crater that encloses Purcell but for which the relief does not create an area of permanent shadow.
  • 25
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    • 10.1126/science.1231106
    • D. A. Paige et al., Science 339, 300 (2013); 10.1126/science.1231106.
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* 이 정보는 Elsevier사의 SCOPUS DB에서 KISTI가 분석하여 추출한 것입니다.