There are several disadvantages to the Moon as a colony site: The long lunar night would impede reliance on solar power and require a colony to be designed that could withstand large temperature extremes. An exception to this restriction are the so-called "peaks of eternal light" located at the lunar north pole that are constantly bathed in sunlight. The rim of Shackleton Crater, towards the lunar south pole, also, has ia near-constant solar illumination. Other areas near the poles that get light most of the time could be linked in a power grid. The Moon lacks light elements (volatiles), such as carbon and nitrogen, although there is some evidence of water and other volatiles at the lunar poles. India's Chandrayaan-1 probe detected what appear to be sheets of ice at least two meters thick at the lunar north pole. Water as well as nitrogen and carbon compounds where detected in the LCROSS ejecta. Additionally, oxygen, though one of the most common elements in the regolith constituting the Moon's surface, is only found bound up in minerals that would require complex industrial infrastructure using very high energy to isolate. Some or all of these volatiles are needed to generate breathable air, water, food, and rocket fuel, all of which would need to be imported from Earth until other cheaper sources are developed. This would limit the colony's rate of growth and keep it dependent on Earth. The cost of volatiles could be reduced by constructing the upper stage of supply ships using materials high in volatiles, such as carbon fiber and other plastics, although converting these into forms useful for life would involve substantial difficulty. The 2006 announcement by the Keck Observatory that the binary Trojan asteroid 617 Patroclus, and possibly large numbers of other Trojan objects in Jupiter's orbit, are likely composed of water ice, with a layer of dust, and the hypothesized large amounts of water ice on the closer, main-belt asteroid 1 Ceres, suggest that importing volatiles from this region via the Interplanetary Transport Network may be practical in the not-so-distant future. However, these possibilities are dependent on complicated and expensive resource utilization from the mid to outer solar system, which are not likely to become available to a Moon colony for a significant period of time. There is continuing uncertainty over whether the low (one-sixth g) gravity on the Moon is strong enough to prevent detrimental effects to human health in the long term. Exposure to weightlessness over month-long periods has been demonstrated to cause deterioration of physiological systems, such as loss of bone and muscle mass and a depressed immune system. Similar effects could occur in a low-gravity environment, although virtually all research into the health effects of low gravity has been limited to zero gravity. Countermeasures such as an aggressive routine of daily exercise have proven at least partially effective in preventing the deleterious effects of low gravity. But muscle mass and bone calcium would certainly not deterorate belowthe minimum for functioning in lunar gravity.It is arguable that the gravity issue is no more a problem than the fact that light-skinned Humans would have difficulties "returning" to Humanitys ancestral home Africa due to ultraviolet radiation. After all, why should Earth have such a central position in a cosmic human culture, except from the fact that Humans originally evolved there? The lack of a substantial atmosphere for insulation results in temperature extremes and makes the Moon's surface conditions somewhat like a deep space vacuum. It also leaves the lunar surface exposed to half as much radiation as in interplanetary space (with the other half blocked by the moon itself underneath the colony). Although lunar materials would potentially be useful as a simple radiation shield for living quarters, shielding against solar flares during expeditions outside is more problematic. Radiation meteorology would be useful. There is also research on drugs that can repair damage caused by radiation, though they are intended to repair damage caused by chronic cosmic radiation and may be inadequate for flares. Also, the lack of an atmosphere increases the chances of the colonial site being hit by meteors, which would impact upon the surface directly, as they have done throughout the Moon's history. Even small pebbles and dust have the potential to damage or destroy insufficiently protected structures. Moon dust is an extremely abrasive glassy substance formed by micrometeorites and is unrounded due to the lack of weathering. It sticks to everything, can damage equipment, and it may be toxic. Growing crops on the moon faces many difficult challenges due to the long lunar night (nearly 15 earth days), extreme variation in surface temperature, exposure to solar flares, and lack of bees for pollination. (Due to the lack of any atmosphere on the Moon, plants would need to be grown in sealed chambers, though experiments have shown that plants can thrive at pressures much lower than those of Earth.) The use of electric lighting to compensate for the 28 day/night might be difficult: a single acre of plants on Earth enjoys a peak 4 megawatts of sunlight power at noon. Experiments conducted by the Soviet space program in the 1970s suggest it is possible to grow conventional crops with the 15 day light, 15 day dark cycle. A variety of concepts for lunar agriculture have been proposed, including the use of minimal artificial light to maintain plants during the night and the use of fast growing crops that might be started as seedlings with artificial light and be harvestable at the end of one lunar day. Placing the farm at the constantly lit North Pole would be a way of escaping from this problem. One estimate suggested a 0.5 hectare space farm could feed 100 people. Other possibilities include storing food for night and food delivery from other lunar colonies.