Meteorites, asteroids and comets
Unexpectedly, many meteorites, of every type, contain minerals that formed through reaction with liquid water (a phenomenon known as ‘aqueous alteration’). In some cases the minerals hydrated before the particles which make up the meteorites coalesced. Water vapour was suspended in space, wetting the grain surfaces and making them stickier, thereby accelerating the process by which grains accreted. Another interesting fact is that the amount of water tends to be in inverse proportion to the chondrules [see part 3]. Some meteorites that lack chondrules consist entirely of aqueous minerals. Apparently, the source of the heat that prevented hydration was the chondrules themselves. If conditions were cool enough, hydration almost always occurred.
Hydrated minerals have also been detected remotely in comets and asteroids and directly recovered from the short-period comet Tempel 1, investigated by NASA’s Deep Impact probe. Comets, of course, contain copious amounts of frozen water.
Interplanetary space seems to have been wet. Evidence for this doesn’t just come from asteroids and comets. With the exception of Mercury, which may simply have been too hot, all the terrestrial planets show signs of having once been drenched by water.
Perhaps the most surprising instance is Venus. Although the planet today has a hot dry surface and is shrouded under clouds of carbon dioxide and sulphuric acid, the high ratio of deuterium to hydrogen in its atmosphere suggests that it once hosted a substantial ocean, subsequently evaporated (or blasted) away. Deuterium, an isotope of hydrogen, can combine with oxygen to produce a heavy form of water, and the inference is that ultraviolet radiation from the sun split the evaporated water into hydrogen, deuterium and oxygen. The lightest gas, hydrogen, escaped into space, as did most of the deuterium, but a proportion remained in the atmosphere. The heavier oxygen oxidised the crust.
Until recently, the Moon was believed to be devoid of water. Then in September 2009 it was announced that unequivocal traces of water had been found on the surface, some of it ascribed to the effect of solar-wind protons reacting with the oxygen bound up in minerals. That hypothesis has since been disproved – the water appears to have been very ancient. The following month the LCROSS (Lunar Crater Observation and Sensing Satellite) mission discovered larger quantities of water when it drove a spacecraft into a crater close to the permanently shadowed south pole. Five months after that, it was announced that millions of tons of ice were hidden deep within craters around the northern pole. The polar ice also had to be very ancient. Finally, in May 2011 came the news that water had been discovered in volcanic melt inclusions – tiny pockets of magma that were trapped in the growing crystals while the magma was as yet unerupted – showing that the interior also contained appreciable amounts. Indeed, the proportions were similar to those within the Earth’s upper mantle.
Oceans of water cover most of the Earth’s surface, to an average depth of almost 4 km. According to the nebula hypothesis, Earth, like Venus, should not have had oceans to start with, since it lies within the ‘snow line’, within which the infant Sun’s heat would have prevented volatiles from condensing into liquid. Yet water has been abundant on or in the Earth from as far back as datable minerals can take us, in geological time as early as 4.4 billion years ago. At the beginning of the Archaean, around 3.9 billion years ago, the entire planet was under water, and it was to remain largely submerged for another 1,300 million years (Flament et al 2008). Water has dominated the planet throughout its known history.
Mars’s early history is no less puzzling than Earth’s and Venus’s. Its surface is both cold and dry, yet there is evidence of former water wherever one looks. The ancient impact-gouged depression in its northern hemisphere once contained an ocean more than 400 metres deep, covering a third of the planet’s surface. Deltas and valley networks – the ancient conduits of water from the highlands – fringe the basin. Within its hollow one can still see the faint outlines of smaller craters whose walls were eroded by the ocean and whose floors received thick sheets of diluvial sediment. In other regions, ejecta splashes surround the craters, showing that the ground had (or was shock-heated to) a mud-like consistency. When asteroids bombarded the planet, the surface was wet. Condensing clouds continued to rain on the lowlands for many years, repeating their cycles of evaporation, re-precipitation and runoff, until gradually the water seeped into the ground. There it is now locked up as subsurface ice.
The Kuiper Belt
The furthest of the eight planets is Neptune, named after the Roman god of the Ocean. Beyond Neptune is the Kuiper Belt, a region of small diffuse ice bodies between 30 and 55 AU (Earth distances) from the Sun. The space occupied is greater than that containing all the planets, so the belt represents far more than merely the edge of the solar system. In the standard model, the belt is presumed to be the volatile-rich remains of the protoplanetary disc, but the actual story appears to have been more complicated. Complications include:
- its fragmentary nature – it is estimated to contain more than 100,000 objects over 50 km in size and, wildly contrary to computer models, quadrillions of objects 10-100 metres in size (Cooray 2006);
- its low overall density – this is not satisfactorily explained by the nebula hypothesis and is known as the ‘missing mass problem’, though the problem may be partly alleviated by the quantity of the 10-100 metre-size objects;
- the ‘surprisingly high level of dynamical excitation’ of the objects – they have highly elliptical orbits at various angles to the ecliptic plane, not, as expected, circular orbits all close to the plane;
- the existence of more ice bodies, known as the ‘scattered disc’, that extend in similarly erratic orbits beyond the Kuiper Belt and are essentially a continuation of it.
The largest Kuiper Belt Objects (KBOs) are Eris, Pluto, Makemake and Haumea, all classified as dwarf planets. The icy moons of Neptune and Uranus may also have been former members of the Kuiper Belt, as may some of the Centaurs. Just as with the asteroid belt, the vast number of bodies is thought to reflect the outcome of collisions between larger bodies. Thus the present state of the Kuiper Belt does not reflect its primeval state, and its more recent history may be one of disaggregation rather than aggregation.
The composition of the KBOs has to be inferred from their surface composition. This is not a straightforward matter, since a variety of events and influences has undoubtedly complicated the chemistry, such as interaction with the interstellar medium and polymer-producing cosmic rays. In addition, a number of very ancient glaciations in Earth history suggest that the solar system may have passed through an interstellar molecular cloud, consisting of molecules and compounds of hydrogen, carbon, nitrogen and oxygen.
The little we know about KBO surfaces conforms with such a scenario. In simple terms, the surfaces of the largest bodies are mainly nitrogen and methane (CH4), whereas the surfaces of the small to medium-sized objects are mainly water-ice. Since most of the smaller bodies are fragments of larger ones and therefore younger, it is the surfaces of the smaller bodies that more closely approximate the interior composition. KBOs therefore probably consist predominantly of water. Some of the water ice is crystalline and must have formed in temperatures well above those now prevailing. This may not have been that long ago, for cosmic rays will reduce crystalline ice to an amorphous state within 0.1–1.0 million years.
The substantial amount of ammonia (NH3) and methane in the atmospheres of Jupiter, Saturn, Uranus and Neptune may also be residue from a molecular cloud. They also contain large quantities of water (as do all moons large and cold enough to retain such volatiles).
Traditions about the creation of celestial water
In view of the problems associated with the nebula hypothesis, it is reasonable to ask whether a creation-based approach might not offer a better interpretation. The obvious alternative would be to understand the Kuiper Belt as the remnant of a created aqueous cocoon around the solar system.
It is noteworthy that many pre-scientific peoples had a tradition that a celestial ocean existed above the terrestrial one. The Egyptians, for example, visualised the sun as travelling through the sky in a boat. The creation myth of Babylon, Enuma Elish, visualised the goddess of the deep being split in two to form an upper ocean and a lower. According to the Hebrews, the space encompassing the solar system was created by separating the primordial deep into two bodies of water, one under the firmament and the other above it.
Thus it may be possible to solve some of the mysteries of the solar system’s past by combining ancient tradition and modern astronomical knowledge. Initially these waters existed in the gas phase, forming a protective, nebulous, slowly rotating circumambient shell not unlike the spherical shape postulated for the ‘Oort cloud’. Over time, much of this water diffused inwards under the influence of the Sun’s gravity, the shell contracting into an annular disc. By 4.568 Ga ago in geological time interplanetary space may have hosted a substantial volume of water. In the course of diffusion, some of this water showered onto the planets – hence the large volume of water attracted by Mars in its Noachian period and the evidence of ubiquitous water elsewhere. Further out, cooling and electrostatic sticking caused the droplets to consolidate into small bodies of ice.
Creation theory postulates a nebulous spherical envelope, evolution theory, a spherical cloud. For ease of calculation, and because of the ‘metre-size barrier’, simulations within the latter framework begin at the point where orbiting objects are 1–10 km in size. The bodies merge and grow on relatively short timescales, with the orbits of the smallest increasing in eccentricity, after which accretion proceeds more slowly. Only a few objects reach the size of Pluto. Collisions between the smaller bodies remaining then produce debris instead of mergers, grinding away until eventually 90% or more of the initial material is eliminated. Such reconstructions leading up to the Kuiper Belt’s present form, which seem reasonable enough, are equally valid within the framework of creation theory.
The Deluge waters from above
Shock fronts from planetary explosions will have entrained water as well as rock fragments. If so, when the fragments hit Venus, Mars and Earth, the bombardment would have been accompanied by heavy rainfall. Earth’s case was slightly different, since its dry upper atmosphere had previously absorbed the water diffusing through space, retarding precipitation. As asteroids ripped through the atmosphere, the stored water added to the deluge. At the same time the pillars supporting the dry land collapsed. Vast amounts of subterranean water surged to the surface. By the end of the cataclysm the whole planet was submerged, and, as we noted, it was to remain mostly submerged throughout the Archaean.
There is a strong argument, therefore, that much of the water discovered around the lunar poles in 2009 was water from the event popularly known as Noah’s Flood.