Evolving Views of Mars

Even to the naked eye, Mars clearly varies in brightness over months and years. Mars is roughly 50 percent farther away from the Sun than the Earth, and its distance from us depends on which side of the Sun each planet is on and the details of their elliptical orbits. At its closest,9 Mars is only 55 million kilometers away and, at its farthest, it’s 400 million kilometers away. This variation cor­responds to a factor of 50 in apparent brightness and a factor of 7 in angular size. Only the brightness variation is visible to the naked eye; a telescope is needed to resolve Mars into a pale red disk. Even when it looms closest in the sky, Mars is just 25 arc seconds across, or seventy times smaller than the full Moon.

Following the invention of the telescope, the view of Mars evolved relatively slowly. Galileo began observing Mars in Septem­ber 1610.10 He noticed that it changed in angular size and he specu­lated that the planet had phases. The Dutch astronomer Christian Huygens was first to draw a sketch with surface features, in partic­ular the dark area or “mare” called Syrtis Major. Huygens thought Mars might be inhabited, perhaps by intelligent creatures. In the middle of the seventeenth century, Giovanni Cassini and Huygens first spotted the pale polar caps of Mars,11 and in the early eigh­teenth century Cassini’s nephew Giacomo Maraldi saw variations in the polar caps that he speculated were due to water freezing and melting during the Martian seasons, although he could not rule out varying clouds.12 William Herschel used his state-of-the-art tele­scopes for a period of more than eight years beginning in 1777 to bolster the interpretation that the poles were made of frozen water. He had measured the tilt of Mars’s spin axis relative to the plane of its orbit so knew it had similar seasons to the Earth. He had also read Huygens’s posthumous book Cosmotheoros in which the Dutchman speculated about life in the Solar System. In an address to the Royal Society in London, Herschel asserted boldly: “These alterations we can hardly ascribe to any other cause than the vari­able disposition of clouds and vapors floating in the atmosphere of the planet. . . . Mars has a considerable but modest atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to our own.”13 With respected scientists setting up the ex­pectation of life on Mars so long ago, it’s not surprising that the idea had taken deep root by the modern age.

Telescope design continued to improve through the nineteenth century, allowing telescopes to make sharper images and resolve smaller features on Mars. In 1863, the Jesuit astronomer Angelo Secchi saw the maria appear to change in color; he fancifully drew them as green, yellow, blue, and brown at different times. He also saw two dark, linear features that he referred to as canali, which is Italian for grooves or channels.14 It was a fateful choice of words, because the literal English translation as canals suggests construc­tion by a technological civilization.