If we could go to the moon, or to Mercury, Venus, or Mars, we may be certain that upon reaching any of those globes we should find ourselves upon a solid surface, probably composed of rock not unlike the rocky crust of the earth; but with Jupiter the case would evidently be very different.  As already remarked, the mean density of that planet is only one quarter of the earth’s density, or only one third greater than the density of water.  Consequently the visitor, in attempting to set foot upon Jupiter, might find no solid supporting surface, but would be in a situation as embarrassing as that of Milton’s Satan when he undertook to cross the domain of Chaos: 

    “Fluttering his pinions vain, plumb down he drops,
    Ten thousand fathom deep, and to this hour
    Down had been falling had not, by ill chance,
    The strong rebuff of some tumultuous cloud. 
    Instinct with fire and niter, hurried him
    As many miles aloft; that fury stayed,
    Quenched in a boggy Syrtis, neither sea
    Nor good dry land, nigh foundered, as he fares,
    Treading the crude consistence, half on foot,
    Half flying.”

The probability that nothing resembling a solid crust, nor, perhaps, even a liquid shell, would be found at the visible surface of Jupiter, is increased by considering that the surface density must be much less than the mean density of the planet taken as a whole, and since the latter but little exceeds the density of water, it is likely that at the surface everything is in a state resembling that of cloud or smoke.  Our imaginary visitor upon reaching Jupiter would, under the influence of the planet’s strong force of gravity, drop out of sight, with the speed of a shot, swallowed up in the vast atmosphere of probably hot, and perhaps partially incandescent, gases.  When he had sunk—­supposing his identity could be preserved—­to a depth of thousands of miles he might not yet have found any solid part of the planet; and, perchance, there is no solid nucleus even at the very center.

The cloudy aspect of Jupiter immediately strikes the telescopic observer.  The huge planet is filled with color, and with the animation of constant movement, but there is no appearance of markings, like those on Mars, recalling the look of the earth.  There are no white polar caps, and no shadings that suggest the outlines of continents and oceans.  What every observer, even with the smallest telescope, perceives at once is a pair of strongly defined dark belts, one on either side of, and both parallel to, the planet’s equator.  These belts are dark compared with the equatorial band between them and with the general surface of the planet toward the north and the south, but they are not of a gray or neutral shade.  On the contrary, they show decided, and, at times, brilliant colors, usually of a reddish tone.  More delicate tints, sometimes a fine pink, salmon, or even light green, are occasionally to be seen about the equatorial zone, and the borders of the belts, while near the poles the surface is shadowed with bluish gray, imperceptibly deepening from the lighter hues of the equator.