Regular articleConvective vortices on Mars: a reanalysis of Viking Lander 2 meteorological data, sols 1–60
Introduction
On 7 August 1976 Viking Lander 2 touched down at Utopia Planitia, Mars. The primary aim of both Viking Landers was to look for the presence of organic life on the surface of Mars; however, each Lander also had a comprehensive meteorological package to monitor the martian atmospheric conditions (Hess et al., 1977). The meteorological package consisted of pressure, temperature, and wind sensors, enabling the Landers to perform the first detailed in situ investigation of martian weather. Data were logged for the full mission at a variety of sampling rates. High data rates were used early on in the mission (one sample every 8 s), but this moved to a lower rate (one sample per minute) after sol 60. The meteorological results from the Viking Landers highlighted diurnal variations in pressure and temperature. The ambient wind speeds were generally below 10 ms−1. The largest pressure variation was between the two landing sites, explained by the difference in latitude and altitude. Apart from this variation the long-term pressure was relatively constant at approximately 6 mbar. Large temperature variations were seen to be commonplace on Mars, with a diurnal range of approximately 50 K. This diurnal temperature variation is essentially a consequence of the low thermal inertia of the surface and the atmosphere.
Thermal or convective vortices develop in all kind of conditions at the base of convective plumes, but dust devils become visible only when over dry or dusty conditions. These convective plumes form when the adiabatic lapse rate (Γd) is less than the virtual temperature lapse rate (Γv). Therefore, if Γd < Γv, then the atmosphere is unstable and convection can occur (Jacobson, 1999). Only a sense of rotation is then required to form a convective vortex. The vorticity or sense of rotation is governed by surface features and random surface vorticity and not the Coriolis force Carroll and Ryan 1970, Sinclair 1966. The final structure typically consists of a centrally rotating low-pressure core with a diameter on Mars ranging from a couple of metres to a kilometre, with heights of up to a few kilometres (Thomas and Gierasch, 1985).
A dust devil, whether on Earth or Mars, is a specific case of a convective vortex; in this case the internal wind speed of the vortex is required to be higher than the threshold wind speed of the regolith particle size. If this threshold wind speed is reached (around 5 and 30 ms−1 for Earth and Mars, respectively; Greeley and Iversen, 1985, Eq 3.16) then the surface regolith particles will be lofted, making the vortex visible, although regolith is not required for vortex formation. The lofting of particulate matter may help sustain the vortex by providing a method of transporting heat into the lower air column through contact with the lofted hotter particles (Sinclair, 1966).
Dust devils, when detected, will produce changes in all of the key meteorological parameters measured by the Viking Landers, namely pressure, temperature, and wind. These thermal vortices may also leave tracks or marks on the surface where they have lofted the surface regolith; these were first seen and identified from Viking orbital photography (Thomas and Gierasch, 1985). The two Viking Lander meteorological instruments did indeed detect possible convective vortices, first reported by Ryan and Lucich (1983), who gave an indication of annual occurrence statistics. Mars Pathfinder also detected 79 convective vortices inferred from pressure variations (Murphy and Nelli, 2002). Dust devils have also been detected in some of the Mars Pathfinder wide angle camera images (Metzger and Carr, 1999). It is clear therefore that dust devils occur on Mars, and their relevance to the martian global dust cycle is currently a topic of scientific interest Metzger and Carr 1999, Tratt et al 2001, Balme et al 2002.
Section snippets
Convective vortex detection
Convective vortices can be detected by a characteristic behaviour in their meteorological parameters. This signature or change in meteorological parameters could include a change in wind speed and direction, a rise in temperature, or a drop in pressure. The last two parameters will only be truly characteristic if the sensor suite encounters the vortex core.
Modelling exists to approximate empirically the wind speed and direction changes characteristic of atmospheric vortices. Wind speed is
Results
Ryan and Lucich (1983) analysed 52 sols in the winter and 23 sols in the spring, 8% and 70% of the sols respectively contained vortices. They also provided seasonal statistics on the formation of vortices. The analysis contained in this paper does not cover the seasonal variation which was analysed by Ryan and Lucich, but provides a more in-depth analysis of diurnal statistics in the first 60 sols of Viking Lander 2, which will complement the findings of Ryan and Lucich.
Threshold values used in
Conclusions
Convective vortices and dust devils have been seen on Mars both in orbital data and in meteorological data, as well as Lander images. Previous estimates of dust devil activity (Ryan and Lucich, 1983) at the Viking Lander sites have provided information about the seasonal behaviour. To add to this data set, Viking Lander 2 meteorological data have been analysed for the sols 1–60, which have a sufficiently high sampling rate for reliable detection of short-term meteorological phenomena.
Acknowledgements
Jim Murphy (New Mexico State University) is thanked for providing the Viking Lander 2 meteorological data. David Tratt (Jet Propulsion Laboratory) is thanked for kindly providing access to the MATADOR wind data. We greatly appreciated valuable revisions from two anonymous reviewers. Martin Towner acknowledges the financial support of the UK Particle Physics and Astronomy Research Council (PPARC).
References (24)
Automatic phase pickerstheir present use and future prospects
Bull. Seismol. Soc. Am.
(1982)- Balme, M., Greeley, R., Mickelson, B., Iversen, J., Beardmore, G., Branson, D., Metzger, S., 2002. Dust devils on Mars:...
- et al.
Atmospheric vortices and dust devil rotation
J. Geophys. Res.
(1970) - et al.
Atmospheric measurements on Marsthe Viking meteorological experiment
Bulletin American Meteorological Society
(1976) - Faber, T.E., 1995. Fluid dynamics for physicists, Cambridge University...
- et al.
Erosion rates on Mars and implications for climate changeconstraints from the Mars pathfinder landing site
J. Geophys. Res.
(2000) - Greeley, R., Iversen, J.D., 1985. Wind as a geological process on Earth, Mars, Venus, and Titan, Cambridge University...
- et al.
Threshold windspeeds for sand on Marswind tunnel simulations
Geophys. Res. Letters
(1980) - Hecht, M., Tratt, D., Catling, D., Samulon, R., 2001. MATADOR Dust Devil...
- et al.
Meteorological results from the surface of MarsViking 1 & 2
J. Geophys. Res.
(1977)
Mechanisms of tornado funnel formation
The Physics of Fluids
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