This article presents a new explanation for the emergence after 1877 of public and expert fascination with a single observed feature of the planet Mars: its network of ‘canals’. Both the nature of these canals and their widespread notoriety emerged, it is argued, from a novel partnership between two practices in their ascendancy in the last two decades of the nineteenth century: astrophysics, and the global telegraphic distribution of news. New transmission technologies—in particular submarine telegraphy and its consequent media genre, the globally-networked newspaper—are shown to have become fundamentally embedded within the working practices of remote astrophysical observatories, entangling professional spaces of observation with public forms of mass media. These novel collaborations gave rise to a new type of ‘event astronomy’, as exemplified by the close working relationship forged between the enterprising Harvard astronomer William Henry Pickering and the preeminent English-language publisher of international news, the $New\; York\; Herald$. Pickering’s telegrams to the $Herald$ sent from his remote mountain outstation in Arequipa, Peru, are shown to be at the heart of the ‘great Mars boom’ of August 1892, with significant consequences for emerging and contested accounts of the red planet. By tracing the particular transmission effects typical to this new kind of astronomical work, I show how the material, temporal, and linguistic constraints imposed by telegraphic news distribution shaped and bounded what could be said about, and therefore what could be known about, Mars.