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THE ICE MAN COMETH: THE ICY RESERVOIRS OF THE SOLAR SYSTEM
Our sun is a giant campfire in the center of the Solar System. Some planets (Mercury, Venus) are too close to the campfire and are overheated. Some planets (Earth and perhaps Mars) are at the “just right” distance from the campfire and receive enough solar radiation to give them an average temperature compatible with self-sustaining life. But the bulk of the System’s extra-solar mass is located farther out in the frozen outer region where the average temperature is well below the freezing point of water. This situation is not entirely bad. In the outer Solar System the water has not evaporated away. There are comets and proto-comets in three distinct regions that contain vast quantities of water, a reservoir that could, in principle, be tapped as a major source of water for planet-scale engineering products.
In the inner Solar System, water and ice are fairly rare and valuable commodities. As an example, the recent discovery of ice deposits at the south pole of the Moon means that a future lunar colony should not need to import water from Earth for life support. Mars is very dry, and its atmosphere is very thin. A substantial reservoir of water, ice, and gases would need to be found if Mars were to be terraformed to give it Earth-like oceans and a more substantial atmosphere. It is therefore interesting that there are reservoirs of water ice in the Solar System. A hypothetical “Ice Man” terraformer need only do some rearranging to bring needed water to the dry regions of the inner Solar System. This is particularly relevant to recent works of science fiction describing the “belters” who have colonized the asteroid belt, or works describing the terraforming of Mars and other bodies by bombarding them with icy objects from the outer Solar System.
This column draws much of its information from a report presented at the General Assembly of the International Astronomical Union in 2009 by Professor David Jewitt of UCLA, in which he reviewed the interesting picture of the outer Solar System that is emerging from recent astronomical observations.
The study of comets is steadily increasing our knowledge about the reservoirs of ice in the outer Solar System. Comets are of interest because they are easily observed, and they are objects that have been dislodged by some random gravitational encounter from a quasi-stable orbit in their reservoir region and caused to dive inward toward the Sun. Observations of their orbital motion and out-gassing reveal their composition and give clues to the environment in their region of origin.
Three regions of the Solar System that are sources of comets need to be considered separately: the Oort cloud, the Kuiper belt, and the outer asteroid belt. To discuss these, it’s convenient to characterize them with a number called “the Tisserand parameter.” The Tisserand parameter TJ, for Jupiter-dominated orbits, is defined as TJ = R + 2 sqrt[(1- e2)/R] cos uC, where R = aJ/aC, aJ and aC are the semi-major orbital axes of Jupiter and the comet of interest, e is the eccentricity of the comet’s orbit, and uC is its inclination with respect to the plane of the ecliptic of the Solar System.
In simple three-body gravitational interactions, the Tisserand parameter does not change. Therefore, even if the comet is “knocked around” by gravitational pushes from the planets and the Sun, the Tisserand parameter stays relatively constant, and can be used as a characterization of any cometary orbit. For the orbit of Jupiter TJ = 3 exactly. Comets from the Oort cloud have TJ < 2, comets from the Kuiper belt have 2 ≤ TJ ≤ 3, and comets from the outer asteroid belt have TJ > 3. The web site http://sajri.astronomy.cz /asteroidgroups/groups.htm has some very interesting plots of Solar System objects grouped by their Tisserand parameter and other criteria. It shows the major grouping of known objects and a few nonconformist “outliers” that do not fit with the others.
The existence of the Oort cloud (composed of objects with TJ < 2) was first deduced by J. H. Oort in 1950, based on a rather narrow peak in the distribution of binding energies of long-period comets. The Oort cloud is a spherical “cloud” rather than a disk-like “belt” because evidence indicates that comets originating there are randomly oriented and uncorrelated with the plane of the ecliptic. Oort interpreted the narrowness of the binding energy peak as indicating that the majority of comets originating in the Oort cloud were “first-arrivals” that had not made many previous passes through the Solar System. Recent estimates of the cometary bodies in the Oort cloud place the number at between 1011 and 1012 such objects and a total mass in the range of 0.1 to 1.0 Earth-masses.
The Kuiper belt (composed of objects with 2 ≤ TJ ≤ 3) was only discovered in 1992. It lies in the region beyond Neptune in which a very large number of small objects have recently been discovered. The total mass of these objects is estimated to be around 0.1 Earth-masses. It is believed that in the early Solar System there were 100 to 1000 times more such objects, but that some dramatic planetary rearrangement (see “The Nice Way to Make a Solar System,” March 2010, my column on the Nice model) caused most of them to be ejected, perhaps at the time of the Late Heavy Bombardment of the inner Solar System.
Objects in the Kuiper belt fall into several classes: the classical objects with small orbital eccentricities, the resonance objects (the most famous of which is Pluto) which may safely cross the orbit of Neptune because they are in resonant phase with the planet, and the scattered objects having large orbital eccentricities and rather unstable orbits, presumably due to past gravitational scatterings. The centaur objects, with orbits between Neptune and Jupiter and interacting strongly with both, are not part of the Kuiper belt but are thought to be recent escapees from it.
The outer asteroid belt (composed of objects with TJ > 3) is a source of a few comets that look a lot like ordinary asteroids, but with the comae and tails of comets. It’s rather surprising that objects that have resided for billions of years in the rather warm asteroid belt, with temperatures of around 150 K at 3 Earth-orbit radii (AU), have retained enough ice to produce cometary behavior. Some chondrite meteorites from this region show evidence of the action of liquid water, including brine pockets with gas bubbles and millimeter-scale salt (NaCl) crystals.
So how could some potential planetary engineers gain access to all of this ice and water? Briefly, by arranging for a comet to impact a body of interest (the Moon, Mars, etc.). This could be done by identifying candidate comets in the three reservoir regions with orbits that could be modified with minimum expenditure of energy to deliver their water to the receiving body. In cases where there was a pre-existing colony on that body, the choreography of this operation could be interesting and demanding. If the receiving body had some atmosphere (e. g., Mars), it might be possible to put the comet into a braking orbit that after many passes delivered the water to the ground without much impact. But with a receiving body like the Moon that had no atmosphere at all, a direct collision would be needed, and the impact site would have to be carefully placed well away from the sites of colonization and infrastructure.
Is such terraforming as simple as arranging a visit from “the Ice Man” in the form of a comet impact? Probably not. The icy mass of a comet contains many condensables besides water. The most abundant cometary chemicals aside from water are likely to be simple hydrogen-carbon-nitrogen combinations like ammonia (NH3), methane (CH4), and hydrogen cyanide (HCN). The latter may create the most problems for the terraforming engineer. Somehow, the cyanide in the comet water, if it is sufficiently abundant, will have to be neutralized to protect astronauts and colonists. One can imagine bio-engineered organisms that are released after a comet strike to accomplish the conversion of ammonia and hydrogen cyanide to free nitrogen, carbon dioxide, and water.
But anyhow, it appears that there is a vast reservoir of water in the Solar System that could be used for making the Moon and Mars more habitable, or even Earth-like. As we physicists say, there are no fundamental problems. It’s just a matter of solving a few engineering problems. n
AV Columns Online: Electronic reprints of over 150 “The Alternate View” columns by John G. Cramer, previously published in Analog, are available online at: http://www. npl.washington.edu/av.
The Nice Model:
David Jewitt, “Icy Bodies in the New Solar System”, arXiv preprint 0912.2070v1 [astro-ph.EP].
Copyright © 2010 John G. Cramer
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