MRO MARCI Observations of the Evolution of the 2018 Planet-Encircling Dust Event

Planet-encircling dust events (PEDE) are among the rarest of Martian atmospheric phenomena. Only eight confirmed cases, with several additional unconfirmed candidates have been observed since the start of the 20th century. The MRO-MARCI was able to monitor the entire evolution of the latest PEDE. The onset of the PEDE occurred on 30 May 2018 (L_s 184.4), nearly 6 Mars years since the 2007 PEDE. The 2018 PEDE was initiated in the Acidalia-Cydonia region following the merger of two eastward propagating, circumpolar regional storms that had dipped south into the Acidalia trough (May 21-29, L_s 179.9-184.0). Matching the early southern Spring/ northern Autumn onset of the 2001 PEDE, the 2018 event distinguished itself from the majority of other planet-encircling events in a few key ways: (1) The early onset and expansion occurred in the northern hemisphere. (2) The Arabia Terra region was an active dust-lifting center, a region that had been a dust sink for most of the past ten Mars years. (3) The intermittent nature of the expansion in both the northern and southern hemispheres. There were three rapid expansion periods corresponding with the merging of lifting center storms. The first was the merger of the main-body of the dust event with northern mid-latitude storms from June 4-8 (L_s 187.1-189.9) that resulted in an eastward expansion that almost reached planet-encircling status on June 10 (L_s 191.0). Second, the slow southward cross-equatorial expansion of the main-body of the dust event along the Acidalia storm-track on June 2 (L_s 186.2) which merged with storm activity along the seasonal south polar cap edge in Noachis on June 8 (L_s 189.7), resulting in rapid east and west expansion along the cap edge through June 11 (L_s 191.6). It also resulted in the initiation of a new dust-lifting center in the southern hemisphere, the Noachis-Hellas region. And third was the merger of the Noachis-Hellas dust lifting-center with the Claritas-Daedalia-Solis storm, which was critical in the storm activity reaching PEDE status in the southern mid-latitudes by June 20 (L_s 196.4). The decay phase began around July 1 (L_s 203). Atmospheric dust levels returned to within seasonal nominal levels around L_s 250, signaling the end of the PEDE. The longer-term daily global monitoring from Mars orbit is critical to understanding the evolution of PEDEs.