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We present the first Faraday rotation measure (RM) grid study of anindividual low-mass cluster -- the Fornax cluster -- which is presentlyundergoing a series of mergers. Exploiting commissioning data for thePOlarisation Sky Survey of the Universe's Magnetism (POSSUM) covering a$\sim34$ square degree sky area using the Australian Square Kilometre ArrayPathfinder (ASKAP), we achieve an RM grid density of $\sim25$ RMs per squaredegree from a 280 MHz band centred at 887 MHz, which is similar to expectationsfor forthcoming GHz-frequency all-sky surveys. We thereby probe the extendedmagnetoionic structure of the cluster in unprecedented detail. We find that thescatter in the Faraday RM of confirmed background sources is increased by$16.8\pm2.4$ rad m$^{-2}$ within 1 degree (360 kpc) projected distance to thecluster centre, which is 2--4 times more extended than the presently-detectableX-ray-emitting intracluster medium (ICM). The Faraday-active plasma is moremassive than the X-ray-emitting ICM, with an average density that broadlymatches expectations for the Warm-Hot Intergalactic Medium. The morphology ofthe Faraday depth enhancement exhibits the classic morphology of anastrophysical bow shock on the southwest side of the main Fornax cluster, andan extended, swept-back wake on the northeastern side. Our favoured explanationis an ongoing merger between the main cluster and a sub-cluster to thesouthwest. The shock's Mach angle and stand-off distance lead to aself-consistent transonic merger speed with Mach 1.06. The region hosting theFaraday depth enhancement shows a decrement in both total and polarisedintensity. We fail to identify a satisfactory explanation for this; furtherobservations are warranted. Generally, our study illustrates the scientificreturns that can be expected from all-sky grids of discrete sources generatedby forthcoming all-sky radio surveys.

Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster