There is a good reason for colonizing another planet, which is to avoid extinction in case the Earth is hit by a 10km or larger asteroid, as has happened many times in the Earth's history. Colonization of Mercury appears to be a very real and practical possibility, whereas colonization of Mars or the other planets, moons or asteroids is really more in the realm of fantasy. The first thought about Mercury is that it would be too hot and dry, because the equatorial surface temperature ranges between 90K and 700K as the planet rotates. But an analysis of temperature vs. latitude and depth shows that the temperature is nearly constant at room temperature (295K+/-1K) in underground rings circling the planet's poles, and deeper than .7 meter below the surface. Similar results are found using numerical techniques in a BepiColombo report and an Icarus paper, Vol. 141, 179–193 (1999). Researchers at Arecibo observatory have found high radar-reflective areas near Mercury's poles in permanently shadowed crater bottoms, and water ice is considered the most likely explanation. The wavelength dependence of radar signals indicates a minimum of several meters of fairly clean ice over a total area of 10¹⁰m², and considering that many of the craters are 1-2 km deep, the average ice depth is probably around 10-100 meters. This results in a volume of 10¹¹-10¹²m³ (.26x10¹⁴-.26x10¹⁵gallons), and since ice has a density of 10³kg/m³, this leads to an estimate of 10¹⁴-10¹⁵kg of ice.

Agriculture would be possible if 10¹⁴-10¹⁵ kg of water is really present, which would provide food and oxygen, and consume the carbon dioxide we exhale. All human habitation and agriculture would be underground to avoid temperature extremes, ionizing radiation, and the loss of oxygen, water and carbon dioxide to the surface. Filtered light could be used for crops, but it is likely that rapidly growing crop varieties could be developed which would take advantage of the high light intensity and the long Mercury day, where sunrise to sunset lasts for 88 Earth days. X-ray spectrometry data and gamma-ray spectrometry data from the Mercury MESSENGER mission show the following average composition of Mercury's soil compared to Earth:

Average Element Concentration (%weight)

Planet\Element	O	Si	Mg	Al	S	Ca	Ti	Fe	Cl	Cr	Mn	K	Th	U	C	N	Na	H
Mercury (top .1mm)	43?	25	12.5	6	2.5	5.5	.2	2.5	< .2	< .5	< .5	.1	.00002	.00001	?	?	?	?
Earth (continental crust)	47	28	2.5	8	.04	4	.5	5	.02	.02	.1	2	.0007	.0002	.1	.002	2	.1

It is not known for sure that there is enough of certain elements to grow crops, particularly the volatile elements such as C, N, Na, H which cannot be measured by these methods. However, the fact that Mercury's K/Th ratio is higher than Earth strongly suggests that Mercury's volatile elements have not been boiled away at some point in the planet's history. The S abundance supports this conclusion. The K abundance also supports this conclusion, being similar to the abundance on Earth as a whole. However, K abundance is low compared to that in the Earth's continental crust, which might be a problem for plant growth. It is also possible that some of the elements are locked up in minerals which cannot be metabolized by plants. The MESSENGER probe is currently gathering ultraviolet and infrared spectrometry data and other data about the soil and the high radar-reflective areas, so some of these questions might be answered soon.

Several other aspects of Mercury make it a good prospect for a colony. One very important advantage is the high solar light intensity, which is stronger than on Earth by a factor of 10.6 at perihelion and 4.6 at aphelion. This strong light intensity would provide virtually unlimited power via electronic solar arrays, and the resulting vertical temperature gradients of ~200K/m would provide even more unlimited power via thermal solar arrays. With such an unlimited and inexpensive power source, almost anything needed for survival could be produced. The gravity on Mercury is 38% that of Earth, which is strong enough to avoid the reduction in bone mass that occurs in very low gravity and weightless environments. There are no temperature variations over periods longer than the Mercury day (like Earth's seasons), which avoids the need for heating/cooling equipment within the 295+/-1K underground rings mentioned above. This occurs because Mercury's orbit is synchronized with its rotation such that 0deg and 180deg longitudes always experience midnight and noon at perihelion whereas 90deg and 270deg longitudes always experience midnight and noon at aphelion. The rings would be about 5000km long, similar to the diameter of the planet. They would be only 20-60km wide because of horizontal temperature gradients of .035-.097K/km. This results in a total area of about 40x5000=200,000km² of 295+/-1K temperature around each pole. The rings could also be extended hundreds of floors downwards, essentially by making underground skyscrapers. And the entire area between the rings and the poles could also be populated (albeit more sparsely) simply by using abundant solar power. Now, an underground existence may sound undesirable to many people. However, that fact is that most people spend 95% of their lives indoors, and from a quality-of-life perspective there is little difference between indoors above ground and indoors below ground. And the colony could still have natural areas, trees, flowers, parks, lakes, wild animals, etcetera. In fact it would probably need all of these things to maintain the ecosystem. The only difference from Earth is that they would be in man-made underground greenhouses instead of on the planet surface.

Mars automatically comes to mind when discussing planetary colonization, and manned missions to Mars have been the long term focus of US space exploration plans since 2004. But despite all the hype, Mars is really a poor prospect for colonization. The solar light intensity on Mars is .43 that of Earth, which makes solar power and agriculture much less practical than on Mercury. The gravity of Mars is 38% of Earth, essentially equal to Mercury. The magnetic field of Mars is .1% of Earth, and its atmosphere density is 2% that of Earth, so protection from ionizing radiation would require underground habitation, the same as on Mercury. The average equatorial surface temperature of Mars is about 228K (-45C, -50F), which would be the constant temperature underground. And of course the temperature gets much lower away from the equator. Such low temperatures can be withstood by machines such as the Spirit and Opportunity Mars rovers, but not by people. Low temperatures combined with limited solar power capacity would make human habitation of Mars impossible without using nuclear power, and this power source is likely to be much less feasible on Mars than on Earth. The other planets, moons and asteroids have even worse drawbacks than Mars.

Asteroid impacts of 5km diameter or greater occur roughly once every 10 million years, and those of 10km or greater occur roughly once every 100 million years. In the past 540 million years there have been 5 extinction events where more than 50% of the Earth's species were killed off, including the Permian-Triassic extinction where 90% of the species were lost. Most scientists think that some of these were caused by asteriod impacts. A well known example where this has been proven conclusively is the Chicxulub impact which resulted from a 10km asteroid impact at the Cretaceous-Tertiary boundary 65 million years ago and caused the extinction of 70% of the Earth's species, including the dinosaurs. Even larger impacts have occured at earlier times, of which only a few are known because their impact craters get erased by the Earth's geological processes over time. It is thought that a 20km or larger asteroid would cause the extinction of all higher order animals and plants, leaving only microorganisms. While the likelihood of such an event is very small in any given year, it could happen at any time, and it is almost guaranteed to happen eventually.

Given the facts above, it appears that the focus of US space exploration plans should be shifted from Mars to Mercury. It is very unlikely that Mercury would ever be a practical source of minerals to be transported back to Earth, or that it would ever have any other Earth-serving economic value. But there is a good chance that Mercury could host a self-sustaining human colony which would protect humanity from extinction in the event of a catastrophic asteroid impact. A Mercury colony appears to be a real possibilty using current technology, not a fantasy for the distant future. In fact, before the distant future is upon us, humans will probably consume our energy and mineral resources to the extent that such an effort would be impossible. A Mercury colony is something we should be pursuing now.