Unexpectedly, many meteorites, of every type, contain minerals that formed through reaction with liquid water, such as clays and carbonates (a phenomenon known as ‘aqueous alteration’). In at least some of the chondrite-type meteorites the minerals hydrated before the parent body 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 in the rock. 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.

As well as being detected remotely in asteroids, hydrated minerals have also been 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 sulfuric 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, mostly escaped into space, the deuterium mostly remained in the atmosphere, and the heavier oxygen oxidised the crust.

The Earth’s surface is predominantly ocean, with the average depth of water being 3,800 metres. According to the nebula hypothesis Earth, like Venus, should not have oceans, since it lies within the ‘snow line’ and the Sun’s heat would have prevented it from condensing in the regions where the rocky planets are. Yet water has been abundant on or in the Earth from as far back as datable minerals can take us, as early as 4.4 billion years ago in conventional time. 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). What is the explanation for this unusual history?

The Moon, until recently, was believed to be devoid of water, but traces have since been found in volcanic glasses. In September 2009 it was announced that unequivocal traces of water had also been found on the surface, some of it probably due to the reaction of solar-wind hydrogen with the oxygen bound up in minerals. The following month the LCROSS (Lunar Crater Observation and Sensing Satellite) mission discovered larger quantities when it drove a spacecraft into a crater close to the permanently shadowed south pole. In March 2010 it was announced that the craters around the north pole hold millions of tons of frozen water – a lot more than could be attributed to solar-wind hydrogen. The ice locked up at the poles must be very ancient.

Mars’s early history is no less puzzling than Earth’s and Venus’s. For most of its history its surface was dry, yet there is evidence of water wherever one looks. The ancient impact-gouged depression in its northern hemisphere once contained an ocean more than 400 metres deep, and within it can still be seen the faint outlines of smaller craters whose walls were eroded by the water and whose floors received great thicknesses of diluvial sediment. When asteroids subjected the surface to intense bombardment, there was already copious water about. Condensing from the resultant steam, torrential rain flooded the newly formed lowlands. Further cycles of evaporation, re-precipitation and runoff continued in the millennia following until all the water had seeped permanently below ground.

Water, or evidence of water in the distant past, also occurs in the outer solar system, even as far as the Kuiper Belt.

The Kuiper Belt is a region of small diffuse ice bodies bodies between 30 and 55 AU from the Sun. The space occupied is greater in extent than the space containing all the planets. In the standard model, the belt is interpreted as the volatile-rich remains of the protoplanetary disc. However, as with other parts of the solar system, this is a presupposition, not an inference, for the actual story appears 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 seems now to be largely counteracted 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 ‘scattered disc objects’ that extend in similarly erratic orbits beyond the Kuiper Belt.

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 is one of disaggregation rather than aggregation.

The internal composition of the KBOs has to be inferred from their surface compositions. This is not a straightforward matter, since a variety of events and influences has undoubtedly complicated their chemistry, such as interaction with the interstellar medium and polymer-producing cosmic rays. In simple terms, the surfaces of the largest bodies are mainly methane, 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. At one time, therefore, KBOs probably consisted 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.

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 it as the remnant of a created aqueous cocoon around the solar system. This would accord with the belief of pre-scientific peoples 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. Did this idea go back to a common tradition? According to the Hebrews, who preserved the least mythologised account, 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. Presumably, these waters existed initially in the gas phase, forming a protective, nebulous, 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 radioisotope time interplanetary space may have hosted a substantial volume of water. In the course of diffusion, some of this water would have 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 gradually 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, simulations within the latter framework begin at the point where orbiting objects are 1 m up to 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 seem reasonable enough, and are also valid within the framework of creation theory.

Shock fronts from planetary explosions are likely to 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 that had been diffusing through space, thereby retarding precipitation. As asteroids ripped through the atmosphere, the stored water now 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.

Much of the water discovered around the lunar poles in 2009 was water from the event popularly known as Noah’s Flood.