Notes on the Martian yellow clouds

Abstract

The so‐called yellow clouds which occur in the Martian atmosphere are generally believed to consist of granular material which has been swept from the surface by atmospheric winds. Three questions relating to these clouds are considered in this paper: the surface winds required to initiate grain motion, the particle grain sizes which may be primarily responsible for the surface obscuration, and the effects such material movement may have on the surface. It is found that unless the Martian surface is extremely rough, which appears unlikely, the wind velocities necessary to initiate grain movement are considerably greater than on the earth. If the surface atmospheric pressure is 80 mb, the required velocities do not generally exceed those observed. This is not the case if the surface pressure is 25 mb. Compatibility, for 25 mb, can be achieved, however, if the required high‐velocity winds are of such short duration as to have escaped detection or if they are associated with relatively small cyclonic disturbances. It is also found that the vertical wind velocities required to maintain particles aloft are less than those required in the terrestrial atmosphere over a rather large grain size range (1 to 300 μ for 80 mb; 4 to 200 μ for 25 mb) and that particles significantly larger than generally quoted in the literature could be responsible for most surface obscurations. The possible effects of yellow cloud formation on the surface are noted. In particular, the presence or absence of dune formations can be used as an indirect indication of the surface grain size.

Multiyear Mars Orbiter Camera (MOC) observations of repeated Martian weather phenomena during the northern summer season

“…Second, no dust devil has been observed to be associated with the formation of any dust storm, while dust cells are always associated with dust storms. This suggests that contrary to a popular hypothesis [Ryan, 1964], dust devils, although nearly ubiquitous , are not directly responsible for initiating dust storms.…”

Dust devil speeds, directions of motion and general characteristics observed by the Mars Express High Resolution Stereo Camera

“…They are seen as bright features illuminated by the Sun and emphasized by an elongated dark shadow. The existence of such atmospheric vortices was predicted for Mars (Ryan, 1964;Neubauer, 1966;Gierasch and Goody, 1973) before they were first detected in Viking images (Thomas and Gierasch, 1985). Both Viking orbiters imaged dust devils, and data recorded by the two Viking landers suggested the passage of several vortices that were likely dust devils (Ryan and Lucich, 1983;Ringrose et al, 2003).…”

Thermal structure of the Martian atmosphere during the dissipation of the dust storm of 1971

“…The terminal velocity required in (16) is assumed to be given by the StokesCunningham relation for spherical particles (Cunningham, 1910;Green and Lane, 1957;Ryan, 1964) 1871 d where pp is dust particle density, g is the Martian gravitational acceleration, d is particle diameter, 77 is molecular viscosity, and X is molecular mean free path. The pressure dependence of k is approximated by…”