The surface temperatures of the smaller planets and most moons in our solar system can exhibit tremendous differences over very short distances. This is because these bodies have very tenuous atmospheres, which allow the surface to quickly heat up when illuminated by the Sun and to quickly cool down when the Sun disappears from view. The gravity of objects like Mercury and our moon, for example, is not strong enough to have held onto any atmosphere that may have existed eons earlier. On these worlds, the temperature on the sunlit side may be 250 degrees F on the moon and perhaps 750 degrees F on Mercury - too hot to be suitable for life-forms on Earth. In contrast, on the dark side of the Mercury or even in areas of shadow during the day, the temperatures can be more than 150 degrees below zero F - too cold for most all life-forms that exist on our planet. During the daytime on Mercury, if you were traversing a crater near the north pole, the temperature sensor on your spacesuit or your rover vehicle (a Mercury for sure) could drop from degrees 600 F to 10 degrees F, over a distance of maybe a few hundred meters, once you moved from direct daylight into a permanently-shadowed crater.

Despite the nearness of Mercury to our Sun, it's possible that ice may exist in craters near the poles. Ice may also be a feature of some of our moon's polar craters. Planetary missions employing radar sensors and neutron spectrometers have indicated that ice could be buried beneath layers of dust on Mercury and our moon.The flanks and bottoms of a few of the deeper craters at polar latitudes have never been exposed to the Sun's rays, and therefore, the temperatures are always well below the freezing point. NASA's Lunar Prospector Mission has provided data that suggests upwards of 500 billion tons of water ice could be hidden in permanently-shadowed regions near our moon's poles! It's envisioned that this ice could be utilized by lunar bases in the future.

Since Earth has such a robust atmosphere, we're not subject to the wild temperature fluctuations that must be endured by anyone who visits places like Mercury or our moon. Nonetheless, in desert areas, where the water vapor is negligible and the relative humidity may be near 5%, surface temperatures can exceed 150 degrees F. At night, the dry air permits air temperatures to drop to temperatures as much as 50 degrees F below the daytime readings, and surface temperatures may be 75 degrees F cooler at night than they are during the day. However, even though shadows give welcomed and sometimes life-saving shade to desert residents, the contrast in temperature is small compared to the Sun/shadow contrasts on our moon. If the temperature is 130 degrees F on a rock surface in the desert, in the shadow of the rock, the temperature might be as much as 40 degrees cooler (90 degrees F), for instance. This is still a rather substantial temperature difference over such a small distance.

Since water has a much higher thermal inertia (resistance to temperature changes) than does land, temperature contrasts in and around water are generally considerably less than they are on land. As we move deeper into the ocean, the temperature drops off, and because there's less solar heating, temperature fluctuations are typically smaller too. However, it just so happens that the bottom of the ocean floor is often where the greatest temperature gradients on our planet are found.

Temperatures of water gushing from vents at the bottom of the ocean floor have been found to be as high as 760 degrees F!. Deep sea hydrothermal vents usually form along mid-ocean ridges - the volcanic undersea mountain ranges where new sea floor is created. They're basically underwater geysers. The scalding water from these vents slowly mixes with the very cold bottom water forming a warm plume of mineral-laden water, rising from the bottom of the ocean. Some of the minerals fall out along-side of the vents, creating unusual looking deposits (often consisting of metal sulfides). The minerals and chemicals exiting from the vents support an ecosystem of little known and often bizarre creatures that's completely independent of the Sun's warmth and energy. Instead of photosynthesis, life here depends on chemosynthesis. Some of these vents host what may be the highest concentration of invertebrate life on our planet. From one series of vents alone, hundreds of new species have been discovered.

When the superheated water from the vents comes in contact with the surrounding ocean water, which is only a few degrees F above freezing, the mixing fluids give rise to billowing black columns known as "black smokers." This is a place where the water temperatures can abruptly change over an extremely short distance - the ocean bottom water is near the temperature where its density is greatest (about 39 degrees F or 4 degrees C). Bottom water becomes stratified in horizontal layers, and at the high pressure, high density and low temperature of the ambient sea water on ocean bottoms, mixing rates with the superheated fluids emerging from the vents are very slow. Thus, gradients between the hot and cold fluids can be very sudden. In fact, a type of worm, Alvinellids, which averages about 3 inches in length, has been observed in water where the temperature difference between one end and its other is 140 degrees F - the steepest temperature gradient on Earth! How these things can live under such extreme conditions is a question that has biologists scratching their heads!

Another location where large gradients may exists is at the bottom of Lake Vostok in Antarctica, which is about the size of Lake Ontario and nearly 500 m deep (twice as deep as Lake Ontario). It's speculated that hydrothermal fluids may be spewing out of vents on the lake bottom. However, since thick ice covers almost the entire lake, it's hard to explore, and it's not known yet if the crust here is very old or relatively fresh. NASA has expressed an interest in exploring Lake Vostok because of its similarity to Jupiter's moon, Europa. This moon appears to have a liquid ocean that's covered by an ice sheet perhaps tens of kilometers in thickness. Some scientists hypothesize that if Europa has hydrothermal vents existing at the bottom of its icy ocean, the molecular building blocks for life may be found clinging to these vents.

For more about this see the October 2000 issue of the National Geographic Magazine (Deep Sea Vents). Also see the following web site for deep sea hydrothermal vents;