PRELIMINARY DISTRIBUTION AND STATISTICS OF “BACILLI” … · southern section of Sputnik...
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PRELIMINARY DISTRIBUTION AND STATISTICS OF “BACILLI” IN THE SOUTHERN REGION OF SPUTNIK PLANITIA. J. W. Conrad 1 , N. Abu-Hashmeh 1 , F. Nimmo 1 , O. L. White 2 , K. Singer 3 , A. D. Howard 4 , T. R. Lauer 5 , R. P. Binzel 6 , J. M. Moore 2 , S. A. Stern 3 , C. B. Olkin 3 , H. A. Weaver 7 , K. Ennico 2 , L. A. Young 3 , J. R. Spencer 3 and the New Horizons Geology, Geophysics and Imaging Theme Team. 1 Dept. Earth and Planetary Sciences, University of California Santa Cruz, 1156 High St, Santa Cruz, CA 95064, USA ([email protected]), 2 NASA Ames, 3 SwRI, 4 Univeristy of Virginia, 5 NOAO, 6 MIT, 7 Johns Hopkins APL. Summary: We have mapped the distribution of pit- ted terrain in the southern portion of Sputnik Planitia and tabulated the morphometry of these features. The orientation may be slightly influenced by the topo- graphic slope of the surrounding nitrogen ice. Introduction: The New Horizons mission revealed many novel features on Pluto [1]. The dominant feature of the encounter hemisphere is the informally named Sputnik Planitia. Current evidence points towards this feature being a basin, probably impact related [2, 3], filled with soft nitrogen ice. In the northern, probably deeper, sections the ice is currently convecting [4, 5], forming cells on the order of a few tens of kilometers in width. In the southern part sublimation appears to play a larger role in surface processes [6, 7] The dominant sur- face features in this region are large pits. These pits have been given the informal name of “bacilli”. Figure 1 shows the distribution of bacilli mapped to date in the southern section of Sputnik Planitia. Figure 1: Distribution of mapped bacilli within the high reso- lution strip images of Sputnik Planitia. Yellow represent standard bacilli (fig. 2A), blue coalesced pits (fig. 2C), and red long pits or pit chains (fig. 2D). Bacilli Types: The typical range of bacilli morphol- ogies is shown in figure 2. It is unknown whether the observed range of bacilli types is the result of an evolu- tionary sequence or the result of different local condi- tions (e.g heat flux and/or shear rate). Typical depths of bacilli are 0.15-0.2 km [7]. Figure 2. Close-up of various bacilli types. Scale bar applies to all 5 sections. A) Standard Bacilli. B) Thin Strands. C) Co- alesced Pits. D) Long Pits. E) Massive bacilli where the un- derlying basin floor may be exposed. Orientation: Bacilli are primarily N-S aligned with a much smaller secondary population that are E-W aligned (fig. 3). The smaller E-W population is primar- ily those whose orientation is controlled by the proxim- ity of nearby bacilli and undisturbed nitrogen ice. Figure 3: Orientation Distribution of various bacilli types. Note the small E-W population in the Standard Bacilli. It appears likely that for the coalesced pits and long pits, their orientation is controlled by how they form. Both seem to be formed by the coalescence of standard bacilli, which seems to be most easily accomplished in the N-S direction. Additionally, long pits seem to avoid growth perpendicular to the long axis.. Aspect Ratio: Since these features can be approxi- mately described as ellipses, the distribution of aspect 1767.pdf Lunar and Planetary Science XLVIII (2017)
Transcript of PRELIMINARY DISTRIBUTION AND STATISTICS OF “BACILLI” … · southern section of Sputnik...
PRELIMINARY DISTRIBUTION AND STATISTICS OF “BACILLI” IN THE SOUTHERN REGION OF SPUTNIK PLANITIA. J. W. Conrad1, N. Abu-Hashmeh1, F. Nimmo1, O. L. White2, K. Singer3, A. D. Howard4, T. R. Lauer5, R. P. Binzel6, J. M. Moore2, S. A. Stern3, C. B. Olkin3, H. A. Weaver7, K. Ennico2, L. A. Young3, J. R. Spencer3 and the New Horizons Geology, Geophysics and Imaging Theme Team. 1Dept. Earth and Planetary Sciences, University of California Santa Cruz, 1156 High St, Santa Cruz, CA 95064, USA ([email protected]), 2NASA Ames, 3SwRI, 4Univeristy of Virginia, 5NOAO, 6MIT, 7Johns Hopkins APL.
Summary: We have mapped the distribution of pit-
ted terrain in the southern portion of Sputnik Planitia and tabulated the morphometry of these features. The orientation may be slightly influenced by the topo-graphic slope of the surrounding nitrogen ice.
Introduction: The New Horizons mission revealed many novel features on Pluto [1]. The dominant feature of the encounter hemisphere is the informally named Sputnik Planitia. Current evidence points towards this feature being a basin, probably impact related [2, 3], filled with soft nitrogen ice.
In the northern, probably deeper, sections the ice is currently convecting [4, 5], forming cells on the order of a few tens of kilometers in width.
In the southern part sublimation appears to play a larger role in surface processes [6, 7] The dominant sur-face features in this region are large pits. These pits have been given the informal name of “bacilli”. Figure 1 shows the distribution of bacilli mapped to date in the southern section of Sputnik Planitia.
Figure 1: Distribution of mapped bacilli within the high reso-lution strip images of Sputnik Planitia. Yellow represent standard bacilli (fig. 2A), blue coalesced pits (fig. 2C), and red long pits or pit chains (fig. 2D).
Bacilli Types: The typical range of bacilli morphol-ogies is shown in figure 2. It is unknown whether the observed range of bacilli types is the result of an evolu-tionary sequence or the result of different local condi-tions (e.g heat flux and/or shear rate). Typical depths of bacilli are 0.15-0.2 km [7].
Figure 2. Close-up of various bacilli types. Scale bar applies to all 5 sections. A) Standard Bacilli. B) Thin Strands. C) Co-alesced Pits. D) Long Pits. E) Massive bacilli where the un-derlying basin floor may be exposed.
Orientation: Bacilli are primarily N-S aligned with
a much smaller secondary population that are E-W aligned (fig. 3). The smaller E-W population is primar-ily those whose orientation is controlled by the proxim-ity of nearby bacilli and undisturbed nitrogen ice.
Figure 3: Orientation Distribution of various bacilli types. Note the small E-W population in the Standard Bacilli.
It appears likely that for the coalesced pits and long
pits, their orientation is controlled by how they form. Both seem to be formed by the coalescence of standard bacilli, which seems to be most easily accomplished in the N-S direction. Additionally, long pits seem to avoid growth perpendicular to the long axis..
Aspect Ratio: Since these features can be approxi-mately described as ellipses, the distribution of aspect
1767.pdfLunar and Planetary Science XLVIII (2017)
ratios (the ratio of the semiminor axis over the semima-jor axis, fig. 4) reveals if there are any asymmetries pre-sent.
Figure 4: Distribution of aspect ratios showing that most ba-cilli (yellow) are more equant and that the coalesce pits and long pits (blue/red) are highly elliptical. Generally the bacilli are slightly elliptical, with a mean aspect ratio of ~0.7. As expected, coalesced pits and long pits are more elliptical.
Size: Bacilli vary in size, but are all typically within the same order of magnitude (fig. 5). The average equiv-alent radius of a standard bacilli is on the order of a kil-ometer. There is no obvious trend in either orientation or size with latitude, but perhaps a weak correlation be-tween orientation and aspect ratio.
Figure 5: Distribution of Equivalent Radius (square root of the bacilli area). The distribution of the standard bacilli is nearly Gaussian while the distribution of the other two types is controlled by the number of combined bacilli.
It should be noted that there are a few outliers in the
long pit category which reach lengths of around ten kil-ometers. These are rare, but show a kind of end stage in the easily recognizable population.
Slope Dependence: One of the factors that may act to shape bacilli is the regional topographic slope. Nitro-gen ice has a rather low viscosity [8, 9] and flows rap-idly on geological timescales. This flow may shear the bacilli in the same direction. The direction of this flow
is controlled by the topography and while the topogra-phy under the nitrogen ice is unknown the topography of the nitrogen ice has been constrained using stereo im-ages [7].
We compare the direction of slope to the orientation of the standard bacilli (fig. 6). There may be a slight cor-relation with the slope, although the dominant orienta-tion is still N-S. This correlation could imply that re-gional flow of the nitrogen ice has a second-order effect on the growth of these features [6].
Figure 6: Map of averaged standard bacilli orientation (white) and topographic slope (red) in 2° squares over the area of Sputnik Planitia. Inset is a distribution of the angle of differ-ence between the slope and orientation.
Future Work: The preliminary mapping presented
here is ongoing, for instance, bacilli in the Hillary Mon-tes region may provide further tests of what controls their orientation.. In addition the counting area can be expanded by using signal processing techniques on the lower resolution images.
Conclusion: The bacilli exhibit a primarily north-south orientation (perhaps because of insolation-driven sublimation; [6]), an elliptical shape, and a median ra-dius of about a kilometer There also may be a weak de-pendence on the topography of Sputnik Planitia. Char-acterizing the morphometry of the bacilli is the first stage to understanding how they form and evolve.
References: [1] Stern S. A. et al. (2015) Science,
350, aad1815. [2] Johnson B. C. et al. (2016) Geophys. Res. Lett, 43, 10,068-10,077. [3] Nimmo F. et al. (2016) Nature, 540, 94-96. [4] McKinnon W. B. et al. (2016) Nature, 534, 82-85. [5] Trowbridge A. J. (2016) Nature, 534, 79-81. [6] Moore J. M. et al. (2016) Icarus, doi:10.1016/j.icarus.2016.08.025. [7] White O. L. et al. (2017) Icarus, in press. [8] Eluszkiewicz, J. and Steven-son D. J. (1990) Geophys. Res. Lett., 17, 1753–1756. [9] Yamashita, Y. et al. (2010) Icarus, 207, 972–977.
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