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Träfflista för sökning "WFRF:(Lemmon Mark T.) "

Sökning: WFRF:(Lemmon Mark T.)

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1.
  • Hamilton, Victoria E., et al. (författare)
  • Observations and preliminary science results from the first 100 sols of MSL Rover Environmental Monitoring Station ground temperature sensor measurements at Gale Crater
  • 2014
  • Ingår i: Journal of Geophysical Research. - 0148-0227. ; 119:4, s. 745-770
  • Tidskriftsartikel (refereegranskat)abstract
    • We describe preliminary results from the first 100 sols of ground temperature measurements along the Mars Science Laboratory's traverse from Bradbury Landing to Rocknest in Gale. The ground temperature data show long-term increases in mean temperature that are consistent with seasonal evolution. Deviations from expected temperature trends within the diurnal cycle are observed and may be attributed to rover and environmental effects. Fits to measured diurnal temperature amplitudes using a thermal model suggest that the observed surfaces have thermal inertias in the range of 265-375?J m-2 K-1 s-1/2, which are within the range of values determined from orbital measurements and are consistent with the inertias predicted from the observed particle sizes on the uppermost surface near the rover. Ground temperatures at Gale Crater appear to warm earlier and cool later than predicted by the model, suggesting that there are multiple unaccounted for physical conditions or processes in our models. Where the Mars Science Laboratory (MSL) descent engines removed a mobile layer of dust and fine sediments from over rockier material, the diurnal temperature profile is closer to that expected for a homogeneous surface, suggesting that the mobile materials on the uppermost surface may be partially responsible for the mismatch between observed temperatures and those predicted for materials having a single thermal inertia. Models of local stratigraphy also implicate thermophysical heterogeneity at the uppermost surface as a potential contributor to the observed diurnal temperature cycle. Key Points Diurnal ground temperatures vary with location Diurnal temperature curves are not well matched by a homogeneous thermal model GTS data are consistent with a varied stratigraphy and thermophysical properties.
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2.
  • Kahanpää, Henrik, et al. (författare)
  • Convective vortices and dust devils at the MSL landing site : annual variability
  • 2016
  • Ingår i: Journal of Geophysical Research - Planets. - 2169-9097. ; 121:8, s. 1514-1549
  • Tidskriftsartikel (refereegranskat)abstract
    • Two hundred fifty-two transient drops in atmospheric pressure, likely caused by passing convective vortices, were detected by the Rover Environmental Monitoring Station instrument during the first Martian year of the Mars Science Laboratory (MSL) landed mission. These events resembled the vortex signatures detected by the previous Mars landers Pathfinder and Phoenix; however, the MSL observations contained fewer pressure drops greater than 1.5 Pa and none greater than 3.0 Pa. Apparently, these vortices were generally not lifting dust as only one probable dust devil has been observed visually by MSL. The obvious explanation for this is the smaller number of strong vortices with large central pressure drops since according to Arvidson et al. [2014] ample dust seems to be present on the surface. The annual variation in the number of detected convective vortices followed approximately the variation in Dust Devil Activity (DDA) predicted by the MarsWRF numerical climate model. This result does not prove, however, that the amount of dust lifted by dust devils would depend linearly on DDA, as is assumed in several numerical models of the Martian atmosphere, since dust devils are only the most intense fraction of all convective vortices on Mars, and the amount of dust that can be lifted by a dust devil depends on its central pressure drop. Sol-to-sol variations in the number of vortices were usually small. However, on 1 Martian solar day a sudden increase in vortex activity, related to a dust storm front, was detected. 
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4.
  • Moore, Casey A., et al. (författare)
  • A Full Martian Year of Line-of-Sight Extinction within Gale Crater, Mars as Acquired by the MSL Navcam through sol 900
  • 2016
  • Ingår i: Icarus (New York, N.Y. 1962). - 0019-1035. ; 264, s. 102-108
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on line-of-sight extinction in northern Gale Crater, Mars as seen by the Mars Science Laboratory (MSL) rover, Curiosity from sol 100 to sol 900; a little more than an entire martian year. Navcam images oriented due north, which show the distant crater rim, the near ground and the sky allow the extinction due to dust within the crater to be determined. This line-of sight extinction is compared to a complementary dataset of column extinctions derived from Mastcam. The line-of-sight extinction within the crater is less than the column extinction for the majority of the martian year. This implies that the relatively low mixing ratio of dust within the crater as compared to the atmosphere above the crater rim persists through most of the year. This suggests relatively little mixing between the atmosphere above the crater and the atmosphere inside the crater and suggests that northern Gale Crater is a net sink of dust in the current era. The data does however show a yearly convergence of the line-of-sight extinction and the column-averaged extinction around Ls = 270° – 290°. This suggests that air above the crater mixes with air in the crater at this time, as predicted by mesoscale models. Matching line-of-sight and column extinction values are also seen around Ls ≈ 135°, a season that has only been observed once in this dataset, this is particularly interesting as the Rover Environmental Monitoring Station onboard Curiosity reports increased convective boundary layer heights in the same season.
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5.
  • Moores, John E., et al. (författare)
  • Atmospheric movies acquired at the Mars Science Laboratory landing site : Cloud Morphology, Frequency and Significance to the Gale Crater Water Cycle and Phoenix Mission Results
  • 2015
  • Ingår i: Advances in Space Research. - 0273-1177. ; 55:9, s. 2217-2238
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the first 360 sols (LS 150° to 5°), representing just over half a Martian year, of atmospheric monitoring movies acquired using the NavCam imager from the Mars Science Laboratory (MSL) Rover Curiosity. Such movies reveal faint clouds that are difficult to discern in single images. The data set acquired was divided into two different classifications depending upon the orientation and intent of the observation. Up to sol 360, 73 Zenith Movies and 79 Supra-Horizon Movies have been acquired and time-variable features could be discerned in 25 of each. The data set from MSL is compared to similar observations made by the Surface Stereo Imager (SSI) onboard the Phoenix Lander and suggests a much drier environment at Gale Crater (4.6°S) during this season than was observed in Green Valley (68.2°N) as would be expected based on latitude and the global water cycle. The optical depth of the variable component of clouds seen in images with features are up to 0.047 ± 0.009 with a granularity to the features observed which averages 3.8 degrees. MCS also observes clouds during the same period of comparable optical depth at 30 and 50 km that would suggest a cloud spacing of 2.0 to 3.3 km. Multiple motions visible in atmospheric movies support the presence of two distinct layers of clouds. At Gale Crater, these clouds are likely caused by atmospheric waves given the regular spacing of features observed in many Zenith movies and decreased spacing towards the horizon in sunset movies consistent with clouds forming at a constant elevation. Reanalysis of Phoenix data in the light of the NavCam equatorial dataset suggests that clouds may have been more frequent in the earlier portion of the Phoenix mission than was previously thought.
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6.
  • Moores, John E., et al. (författare)
  • Observational evidence of a suppressed planetary boundary layer in northern Gale Crater, Mars as seen by the Navcam instrument onboard the Mars Science Laboratory rover
  • 2015
  • Ingår i: Icarus (New York, N.Y. 1962). - 0019-1035. ; 249, s. 129-142
  • Tidskriftsartikel (refereegranskat)abstract
    • The Navigation Cameras (Navcam) of the Mars Science Laboratory rover, Curiosity, have been used to examine two aspects of the planetary boundary layer: vertical dust distribution and dust devil frequency. The vertical distribution of dust may be obtained by using observations of the distant crater rim to derive a line-of-sight optical depth within Gale Crater and comparing this optical depth to column optical depths obtained using Mastcam observations of the solar disc. The line of sight method consistently produces lower extinctions within the crater compared to the bulk atmosphere. This suggests a relatively stable atmosphere in which dust may settle out leaving the air within the crater clearer than air above and explains the correlation in observed column opacity between the floor of Gale Crater and the higher elevation Meridiani Planum. In the case of dust devils, despite an extensive campaign only one optically thick vortex (τ=1.5±0.5×10-3) was observed compared to 149 pressure events > 0.5Pa observed in REMS pressure data. Correcting for temporal coverage by REMS and geographic coverage by Navcam still suggests 104 vortices should have been viewable, suggesting that most vortices are dustless. Additionally, the most intense pressure excursions observed on other landing sites (pressure drop >2.5Pa) are lacking from the observations by the REMS instrument. Taken together, these observations are consistent with pre-landing circulation modeling of the crater showing a suppressed, shallow boundary layer. They are further consistent with geological observations of dust that suggests the northern portion of the crater is a sink for dust in the current era.
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7.
  • Smith, Michael D., et al. (författare)
  • Aerosol optical depth as observed by the Mars Science Laboratory REMS UV photodiodes
  • 2016
  • Ingår i: Icarus (New York, N.Y. 1962). - 0019-1035. ; 280, s. 234-248
  • Tidskriftsartikel (refereegranskat)abstract
    • Systematic observations taken by the REMS UV photodiodes on a daily basis throughout the landed Mars Science Laboratory mission provide a highly useful tool for characterizing aerosols above Gale Crater. Radiative transfer modeling is used to model the approximately 1.75 Mars Years of observations taken to date taking into account multiple scattering from aerosols and the extended field of view of the REMS UV photodiodes. The retrievals show in detail the annual cycle of aerosol optical depth, which is punctuated with numerous short timescale events of increased optical depth. Dust deposition onto the photodiodes is accounted for by comparison with aerosol optical depth derived from direct imaging of the Sun by Mastcam. The effect of dust on the photodiodes is noticeable, but does not dominate the signal. Cleaning of dust from the photodiodes was observed in the season around Ls=270°, but not during other seasons. Systematic deviations in the residuals from the retrieval fit are indicative of changes in aerosol effective particle size, with larger particles present during periods of increased optical depth. This seasonal dependence of aerosol particle size is expected as dust activity injects larger particles into the air, while larger aerosols settle out of the atmosphere more quickly leading to a smaller average particle size over time.
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9.
  • Webster, Christopher R., et al. (författare)
  • Mars methane detection and variability at Gale crater
  • 2015
  • Ingår i: Science. - 0036-8075. ; 347:6220, s. 415-417
  • Tidskriftsartikel (refereegranskat)abstract
    • Reports of plumes or patches of methane in the Martian atmosphere that vary over monthly timescales have defied explanation to date. From in situ measurements made over a 20-month period by the Tunable Laser Spectrometer (TLS) of the Sample Analysis at Mars (SAM) instrument suite on Curiosity at Gale Crater, we report detection of background levels of atmospheric methane of mean value 0.69 ± 0.25 ppbv at the 95% confidence interval (CI). This abundance is lower than model estimates of ultraviolet (UV) degradation of accreted interplanetary dust particles (IDP’s) or carbonaceous chondrite material. Additionally, in four sequential measurements spanning a 60-sol period, we observed elevated levels of methane of 7.2 ± 2.1 (95% CI) ppbv implying that Mars is episodically producing methane from an additional unknown source.
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