From its earliest history, the moon was thought to be less geologically active than the Earth. But what is that stuff its plumes have been seen venting? TheBigRetort uncovers the pages of a forgotten lunar history:
Paisley Herald and Renfrewshire Advertiser - Saturday 09 January 1869
LUNAR VOLCANOES. On this subject the Academy of Sciences has received curious communication from Dr Montucci. The facts of the case are these: —At the request of the Bureau des Longitudes, the Messageries Imperiales had established a temporary observatory on Sarah Island, opposite Aden, for the purpose of viewing the eclipse of the 18th of August last.
The sky happened to be rather cloudy on that day, and the observer, M. De Crety, could not properly watch the phenomena until after the totality, when the weather cleared up. By that time one-third of the sun's disc was already uncovered, and M. De Crety then perceived three protuberances, not on the sun's limb, but on the moon's, a thing unheard of until then.
They were in the shape of three triangles with their bases attached to the border of our satellite, which they never quitted.
'I observed,' says M. de Crety, 'three luminous protuberances on the moons limb; they were feebly illuminated, and resembled the tops of mountains receiving light from the solar rays.’
‘Fifteen minutes later, two-thirds of the sun's disc having emerged from the moon, the same appendages were seen more strongly illuminated, and better distinguishable from the lunar disc; their summits had the appearance of metal in a state of fusion.’
‘After another quarter of an hour, the central protuberance diminished in altitude,' &c.
From this description Dr Montucci concludes that these excrescences have been either gaseous or formed of solid matter in a state of great division, as ashes; and, admitting that an optical illusion is here out of the question, the observer having made seven diagrams of the phenomenon, he endeavours to explain the mystery by supposing that at the time of the eclipse there was a chain of volcanoes in a state of activity on the posterior hemisphere of the moon, and close to its border, and that what was seen was simply the smoke or ashes-ejected from the craters.
He shows by calculation that an observer's eye could just skim the crest of a lunar mountain 18,000 feet high (there are much larger ones), situated on the posterior surface at a distance of five degrees from the border, so that the whole jet might be seen, while the crater would be out of sight. This, of course, is an extreme case, but the volcano might be larger or smaller, nearer the border or farther off, without endangering the principle on which his explanation rests.