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Let's go to Mars

Herschel gives Mars seasons, and inhabitants

Herschel put the tilt of Mars near 29 degrees, read the shrinking polar caps as seasons, and concluded its inhabitants probably lived much as we do.

A plate of eight numbered ink circles in two rows, lettered Fig. 1 to Fig. 4 above and 5 to 8 below, each a small disc of Mars stippled with dark markings. Most carry a small white polar cap outlined at the top or bottom edge and lettered a or b; the first disc has a straight line ruled across it from B to A, the second is drawn as a gibbous phase with a solid black crescent down its left side, and two stray letters from a further row are clipped at the bottom edge.
Eight of the twenty-four Mars drawings from William Herschel's 1784 memoir on the polar regions of the planet, reproduced in Camille Flammarion's La planète Mars of 1892. Fig. 1 records the two bright spots of 17 April 1777; figs. 2 to 8 run from 13 March to 3 July 1781, with the polar spots lettered a and b on most of the discs. · Image: Camille Flammarion

Between 1777 and 1783 William Herschel observed Mars at four oppositions and recorded white polar patches at three, beginning as a musician in Bath who ground his own mirrors and finishing as George III's court astronomer at Datchet. In 1784 the Royal Society printed the result: On the remarkable appearances at the polar regions of the planet Mars, the inclination of its axis, the position of its poles, and its spheroidical figure.

The caps behaved reciprocally. When the southern patch was large the northern was small, and each in turn shrank and regrew over months. From the way the markings wheeled across the disc, Herschel put the axial tilt at 28 degrees 42 minutes, about three and a half degrees above the true value of 25.19 degrees. He timed the rotation at 24 hours 39 minutes 21.67 seconds, two minutes long against the modern sidereal figure. He measured the disc as flattened at the poles, and ascribed bright belts that appeared and faded to clouds and vapours in an atmosphere.

At the oppositions of 1704 and 1719 Giacomo Maraldi, at the Paris Observatory, had shown that the southern cap wobbled as the planet turned, so it was not centred on the pole of rotation, and could vanish altogether and return. Maraldi did not say what it was. Herschel supplied the ice and the mechanism: an axial tilt close to Earth's, producing seasons that drive snow and ice on and off the poles.

Mars, he concluded, "has a considerable but moderate atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to ours." Surface pressure averages about 6 millibars, under one per cent of sea level on Earth, and the air is roughly 95 per cent carbon dioxide. The caps are water ice mixed with frozen carbon dioxide, which sublimes each spring and freezes back out each autumn. No instrument of 1784 could measure that pressure. The closing line entered the literature alongside the correct numbers.