EXTRATERRESTRIAL HYDROGEOLOGY

The nascent field of extraterrestrial hydrogeology is rapidly coming into its own. Recent missions to Mars have confirmed the existence of water and related fluvial processes that imply the existence of an Earth-like hydrological cycle in the geologic past. Present-day Mars still cycles appreciable amounts of water, and beneath its subsurface cryosphere may exist groundwater processes similar to Earth. There is evidence that Venus may have also once had an Earth-like hydrological cycle, causing suppositions that water may still be beneath the surface today, albeit in a supercritical liquid phase. Mercury and our Moon have ice at their poles, most likely accumulated from cometary impacts. In the outer solar system, the Galilean satellites of Jupiter are water abundant, the most intriguing of which, Europa, may possess a vast ocean beneath its icy crust. Saturn’s moon, Titan, reveals the intriguing possibility of a hydrological cycle without water, instead involving methane and other hydrocarbons. Neptune’s satellite, Triton, may also cycle hydrocarbons, although with less available energy than Titan. Pluto, comets, and other icy bodies are shedding light on the origins of water in our solar system. In addition, recent data on extrasolar planets raises the possibility of other planets similar to Earth, waterworlds covered in immensely deep oceans, or gas giants with inner, hyper-pressurized oceans.

The variance of hydrological cycles in our own solar system is great, involving variations on the processes familiar to Earth (i.e., Mars), including other chemicals in a pseudo-hydrological cycle (i.e., Titan), or having water exist in a manner utterly alien to our planet (i.e., Neptune). Many of these variations have been proposed to be conducive with the evolution of life. It is an exciting opportunity for hydrogeology; by studying and understanding all the variations possible for a planetary hydrosphere, we ultimately improve our own understanding of the water-related processes here on Earth. This is likely to be manifest in all aspects of hydrologic science, ranging from improved measurement methods, to advanced conceptual models of fluid flow under widely varying geologic and atmospheric conditions, to opportunities to test geostatistical concepts applied to hydrogeology.