Orbit‐Spin Coupling and the Triggering of the Martian Planet‐Encircling Dust Storm of 2018. Issue 6 (30th May 2020)

• Record Type: Journal Article
• Authors: Shirley, James H.; McKim, Richard J.; Battalio, J. Michael; Kass, David M.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 6(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-05-30
• Subjects: Mars atmosphere -- Mars dust storm -- orbit‐spin coupling -- S2D -- S2S; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2019JE006077
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The Martian global dust storm (GDS) of 2018 began soon after the southern spring equinox, which is quite early in the dust storm season. The origins of early‐season GDS, including those of 1977, 2001, and now 2018, have been mysterious, as atmospheric dynamical investigations and numerical modeling experiments have been unable to explain or reproduce the timing of these events. We employ a newly expanded catalog of historic Martian GDS for our investigation, which includes 2018 and the telescopically observed equinoctial dust storms of 1877 and 1909. All of the GDS of this catalog took place either (1) when orbit‐spin coupling torques on the Martian atmosphere were near peak values or (2) near times when the orbit‐spin coupling torques were changing most rapidly. The second category, here termed "Mode 2, " includes all six of the equinoctial GDS of the historic record, including 2018. Recognition of the existence of two triggering modes for GDS occurrence leads to a significant improvement in temporal resolution for both hindcasting and forecasting. Orbit‐spin coupling now provides explanations for the late‐season inception dates of the 1924 and 1973 storms, as well as for the equinoctial events. We provide conditional forecasts, with sub‐seasonal time resolution, for GDS occurrence and non‐occurrence in Mars years 35 through 40. We introduce a detailed working hypothesis for the genesis of equinoctial GDS that may be validated through numerical modeling. The characteristic timescale for frictional damping of an intensified Martian Hadley circulation is estimated to be O(10) sols. Plain Language Summary: Spectacular planet‐encircling dust storms occur in some Mars years, but not in others. Five Mars years (and 11 Earth years) elapsed between the two most recent global dust storms (GDS) of 2007 and 2018. Until recently, the fundamental causes of the inter‐annual variability of GDS occurrence were completely unknown. Since 2015, a series of studies have shown a strong relationship linking changes in the orbital motion of Mars with the occurrence of GDS. GDS preferentially occur when Mars is gaining orbital angular momentum, at the expense of other members of the solar system family, during the southern summer dust storm season on Mars. The underlying orbit‐spin coupling hypothesis identifies a torque on the Mars atmosphere that varies strongly with time. GDS tend to happen at times when the torque reaches peak values and also at times when the torque is changing most rapidly. The GDS of 2018 falls in the latter category. Compared to past investigations, much higher precision is attained, in this study, with respect to the question of the timing of these storms within a given Mars year. The new results are important for at least two reasons. First, they will permit more accurate Martian weather forecasts to be made for future years. These may be helpful in the planning of future Mars missions. The results in addition lend impetus to a new scientific question of some interest: Does orbit‐spin coupling give rise to variability with time of the atmospheric circulations of planets other than Mars? Key Points: We introduce an expanded catalog of Martian planet‐encircling dust storms including pre‐space‐age observations and the 2018 storm Global dust storms occur near times when orbit‐spin coupling torques are peaking and near times when torques are changing most rapidly The 2018 storm began during the first of two torque episodes that were each separately capable of triggering a global dust storm that year