7. CONCLUSIONES, RECOMENDACIONES Y MEDIDAS PREVENTIVAS
7.3 Medidas Preventivas
[87] The Phoenix mission has collected a unique set of in
situ meteorological data from the Martian arctic [Taylor et al., 2009] and with this data set, the existence of convec- tive vortices and dust devils in the north polar regions of Mars has been confirmed. During the Phoenix mission, 502 vortex identifications with DP > 0.3 Pa were found, with most events occurring in the noon hours. The largest recorded pressure drop caused by a vortex event was 3.6 Pa and was captured on sol 95 (Ls= 120). Most events have aG
duration around 15 s and a correlation between high wind speeds and large pressure events is seen.
[88] The diurnal distribution of the convective vortices is
bell shaped with most events occurring in the noon hours, consistent with the late afternoon collapse of boundary layer turbulence [Tamppari et al., 2008; Tyler et al., 2008]. A general increase with major peaks in the convective vortex activity is seen from sols 75–151 (Ls= 111–148), indicating
that the dust devil season continues after Ls = 148 at the
Phoenix landing site. During sols 75–151, we also see an increase in the number of pressure events with large DP. This correlates with changes in midsol surface heat flux [Davy et al., 2010] and increasing wind speeds as observed by the Telltale [Holstein‐Rathlou et al., 2010]. An inverse relationship between boundary layer depth and number of vortices is also seen. Comparisons with MARCI imaging show that the convective vortex and dust devil activity seems to be stimulated along weak fronts and is controlled mainly by active weather events passing by; the same per- iods where a correlation between vortex activity and Telltale wind data shows increased vortex density. This is different from the lower latitudes on Mars [Fisher et al., 2005; Cantor et al., 2006; Greeley et al., 2006] where the dust devil activity is mainly controlled by local forcing.
[89] Assuming cyclostrophic balance, tangential wind
speeds for the found pressure events agree with previous observations [Metzger et al., 1999; Renno et al., 2000] and yields a maximum wind perturbation of less than 30 m s−1. This is below the general Martian dust lifting threshold [Greeley and Iversen, 1985; Greeley et al., 2003] and sup- ports the presence of other dust lifting effects [Greeley et al., 2003, 2006; Merrison et al., 2007; Wurm et al., 2008].
[90] Acknowledgments. We thank the Phoenix engineering and sci-
ELLEHOJ ET AL.: DUST DEVILS AT THE PHOENIX LANDING SITE E00E16 E00E16
participation in the Phoenix mission is greatly appreciated. Canadian uni- versity participation was supported by Canadian Space Agency grants and contracts. We thank an anonymous reviewer for constructive criticisms and suggestions improving the manuscript substantially. Thanks to Carlos Lange for his help with the vortex shedding and to Søren Larsen for sug- gestions to the manuscript. L. Tamppari’s contribution to the research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. The Phoe- nix mission was led by Peter Smith of the University of Arizona, on behalf of NASA and was managed by NASA’s Jet Propulsion Laboratory, Cali- fornia Institute of Technology. The spacecraft was developed by Lockheed Martin Space Systems.
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L. Drube, M. D. Ellehoj, and M. B. Madsen, Niels Bohr Institute, University of Copenhagen, Copenhagen DK‐1165, Denmark. (ellehoj@ gfy.ku.dk)
D. Fisher, Geological Survey of Canada, 580 Booth, Ottawa, ON K1A 0E4, Canada.
B. T. Gheynani, P. A. Taylor, W. Weng, and J. Whiteway, Centre for Research in Earth and Space Science, York University, Toronto, ON M3J 1P3, Canada.
H. P. Gunnlaugsson and C. Holstein‐Rathlou, Institute of Physics and Astronomy, University of Aarhus, Århus DK‐8000, Denmark.
A.‐M. Harri, H. Kahanpää, and J. Polkko, Finnish Meteorological Institute, P.O. Box 503, FIN‐00101 Helsinki, Finland.
P. H. Smith, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85719, USA.
L. K. Tamppari, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
ELLEHOJ ET AL.: DUST DEVILS AT THE PHOENIX LANDING SITE E00E16 E00E16