Not so long ago, scientists believed that most of the water in the Solar System was confined to Earth. This view, however, has become somewhat dated. From ancient water-carved landscapes on Mars, erupting geysers at Ceres, sub-surface oceans within Europa, or icy crater floors on our own Moon, water appears to be in abundance in many shapes and forms. We now know that the oceans, responsible for our planet’s pale blue appearance from space, constitute only a small fraction of the water present within our planetary system. The latest findings even suggest H2O forms in the dust particles that float between the planets, indicating a completely different water-rich history of the Solar System.
|Earth. Image credit: NASA Earth Observatory, Robert Simmon.|
The first few planetary exploration missions revealed the apparently barren and arid landscapes on the Moon, Mars and Mercury. Mars lies on the outer edge of the Goldilocks zone of our system, the region in which surface water can exist in liquid form. It was not for several decades that the true extent of the red planet’s past warm and wet environment, indicated by its extensive channel and valley networks, was uncovered. The discovery of water ice, on both the Martian and lunar surfaces, has since forced scientists to completely re-establish the chemical compound’s role within the inner solar system.
But liquid water is not just limited to the past. In mid-January of this year, researchers working on the Hershel spacecraft confirmed the existence of active jets spouting water vapour from opposite sides of dwarf planet, Ceres, the largest object in the asteroid belt. Long suspected to possess an active geology from previous observations, ESA’s direct detection marked the first definitive discovery of water in the asteroid belt, making the arrival of NASA’s Dawn spacecraft next year all the more exciting.
Ceres is not the only object in the Solar System known to contain such mysterious jets; the Cassini flybys in 2005 found similar geysers on the south pole of Enceladus, the sixth largest Moon of Saturn. At the end of 2013, studies with the Hubble Space Telescope also revealed water vapour plumes on the smallest Galilean satellite, Europa.
Beyond the orbit of the asteroid belt is the snow line, the boundary which defines where it is cold enough for hydrogen compounds – such as water, ammonia and methane – to condense into solid ice grains. However, many of the icy moons and large trans-Neptunian objects, far beyond this line, are believed to contain global oceans beneath their thick, icy crusts, which fuel these active jets. Tidal forces, created from the gravitational pull of their parent bodies, stretch and squeeze the hard outer layer, which can cause cracks to form. The resulting friction generates heat, which can then stimulate plumes of vapour and ice to spew out. To put these unseen oceans in perspective, Europa’s diameter is only one quarter that of the Earth’s, but its total volume of water is thought to amount to 2-3 times the Earth’s supply.
So where did Earth’s water come from? The vast amount was initially believed to have originated through the impact of comets, but more recent studies suggest that asteroids and protoplanets were more probably responsible. Last year, researchers found that the water on the moon and Earth arose from the same source, implying the Earth was already wet 4.5 billion years ago when the colossal impact that formed the Moon occurred.
Another potential water source are interplanetary dust particles, micrometre-sized grains which form from comets, asteroids and debris left over from the formation of the Solar System. Recently, H2O was discovered trapped inside the dust, which is created through solar wind irradiation of the silicate material. The Sun’s wind, predominantly comprising of ionised hydrogen atoms, bombards the silicates, knocking the atoms out of order and leaving behind oxygen to more readily react with hydrogen and form water molecules.
|Interplanetary dust particles carrying water produced via solar wind and organic matter interaction. Image credit: John Bradley.|
Tens of thousands of tonnes of interplanetary dust rain down on the Earth, and other Solar System bodies, every year. This could provide an explanation as to why the virtually atmosphere-less moon contains water, but would not account for the large volume on Earth. However, the water-rich grains, which were already known to contain organic matter, could be viewed as potential sources for delivering the key precursors for life. This infers that important molecules are continually showering all of the planetary bodies, and presumably those in other systems across the Galaxy.
Whatever its origins, it is clear that water is plentiful throughout the Solar System; what we do know is that it has likely travelled a long way to get to our taps. Perhaps more poignantly, we should also take note of its production and loss as evident on other Solar System bodies. Earth is not the only planet residing within the Goldilocks zone of our system, but so too are Venus and Mars. The past warm and wet climate of Mars, now too cold to maintain liquid water, reminds us of its fragility, and further, the responsibility we have in protecting our own planet’s limited supply.