But the Moon, Mercury, and Mars have all been shown, on the ground of their small size, to be worlds unfit for habitation; the satellites of Jupiter are, therefore, all rejected on the same score. Nor can the greater nearness of their immediate primary compensate for their remoteness from the Sun. It is true that Jupiter presents to Ganymede a disc with more than 200 times the apparent area that the Sun presents to the Earth, but to make up for the falling-off of the solar radiation, each unit of this area should radiate about 1/250th as much heat as each unit of the Sun's surface. In other words, the absolute surface temperature of Jupiter should be 1/4th that of the Sun, or about 1550° C., and this is higher than can be admitted. The Sun and Jupiter together cannot put Ganymede in as favourable a position as Mars, much less as favourable as the Earth. The case of Jupiter carries with it those of Saturn, Uranus, and Neptune. All three, from their high albedoes and low densities, are still in a vaporous condition; still in some sort, semi-Suns; sources of a certain amount of heat, and not recipients merely. The days are yet far distant when a solid crust can form on any one of them, and the water condense from the steamy atmosphere to form oceans, seas, and rivers. Not till then, if at all, when water as a liquid, water that flows, is present, can life begin to appear and enter on its long course of change. CHAPTER XI WHEN THE MAJOR PLANETS COOL The question has been asked: "It is evident that life cannot exist at the present time on the outer planets, since they are in a highly heated and quasi-solar condition; but when they cool down, as cool they must, and a solid crust is formed, may not a time come when they will be habitable? It seems impossible to think that worlds so beautiful to our eyes and so vast in scale are destined never to be peopled by intelligent beings." It is clearly difficult to answer satisfactorily a question that requires so deep a plunge into the recesses of the unknown future; yet, so far as our knowledge goes, there is no reason to think that Jupiter will be more habitable then than it is now. The difficulty of the small supply of light and heat received from the Sun would apparently still remain, if indeed, the cooling of the Sun itself would not increase it. We do not know of any means by which our Sun could so increase its radiation as to supply to Jupiter from 25 to 30 times as much heat as it now receives, and this would be necessary to place it in the same favoured condition as the Earth. If so great a change were to take place in the Sun, life would be scorched out of existence on all planets nearer than Jupiter, and, similarly, if the solar emission were increased to meet the necessities of Uranus or Neptune, even Jupiter would fall a victim.