Geochemical calculations by researchers at Tokyo Institute of Technology to determine how the water content of Mars has changed over the past 4.5 billion years suggest as yet unidentified reservoirs of water on the planet.
A warmer more watery primordial Martian landscape more closely resembling Earth has long been suggested from geochemical and geological observations. However, as Hiroyuki Kurakawa and colleagues in Japan point out in their recent report "the timing, processes, and the amount of the water loss have been poorly constrained." Their latest studies using geochemical meteorite data to understand how the volume of water has changed on Mars over its history suggest there is more water present there now than has so far been observed.
Today Martian water is considered to exist chiefly as ice at the poles of the planet. However geological observations of rocks containing water laid sediments suggest that lakes and oceans once existed. Previous studies have focused on the volumes of lake-like geological structures to extrapolate how much water was previously present on Mars.
In contrast the researchers at Tokyo Institute of Technology and colleagues at Nagoya University and Kyushu University in Japan determined the water quantities over the course of the planet's history from ratios of the isotopes deuterium and hydrogen (D/H) in ancient meteorites. Deuterium and hydrogen exist in water at a standard ratio at equilibrium. However hydrogen is lighter and escapes more readily so that the changes in the D/H ratio over the course of time can be used to determine how much water has been lost.
They compared their results with previous geological estimates of the primordial water volume and found discrepancies in the figures that suggest the existence of as yet unidentified reservoirs of water on Mars at present. They hypothesise that these may be in the form of mid-latitude ice mantles or underground reservoirs.
As hydrogen from water molecules escape oxygen is left behind. These latest results also suggest greater quantities of this oxygen than current models account for.
Water on Mars
Water has been hypothesised to exist on Mars since ancient times when the white caps at the poles were correctly thought to be ice. Today the observed water content of Mars is low and in the form of ice. However the existence of geological structures resembling lakes and oceans and water lain residues and sediments suggest that Mars was once warm enough to host liquid water.
Measuring D/H values of different eras
Current ratios of hydrogen and its heavier isotope deuterium (D/H) on Mars are six times those on Earth. The D/H ratio during different eras was determined from analysing meteorite samples. Recent technical developments have allowed more accurate determination of D/H ratios with this approach. In addition while most meteorites have been aged as comparatively young – less than 1.3 billion years old, the recently analysed meteorite ALH84001 is aged 4.1 billion years old. For even older D/H ratios, the meteorite Yamato 980459 contains mineral inclusions from crystallised mantle that represents the Martian mantle around 4.5 billion years ago.
Calculating water loss
The water loss between two periods was calculated using the D/H ratios for different periods, the current estimated water on Mars and the so-called fractionation factor, which gives the equilibrium value for deuterium versus hydrogen content in the Martian water. The researchers calculated the water loss for two periods: 4.5-4.1 billion years ago and 4.1 billion years ago to the current day. The model gives greatly increased values for water loss between 4.5 and 4.1 billion years ago relative to the water lost since regardless of the value used to estimate the amount of water on Mars at present.
The missing oxygen
As hydrogen escapes the oxygen from the water molecules is left behind. Existing mechanisms for oxygen loss include interaction with solar winds, which is also linked to the magnetic fields present, and oxidation of surface minerals. However according to observations of mineral abundance and residual magnetism these mechanisms cannot account for the total oxygen lost over the course of Martian history.
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More information: H.Kurokawa, M.Sato, M.Ushioda, T.Matsuyama, R.Moriwaki, J.M.Dohm, T.Usui "Evolution of water reservoirs on Mars: Constraints from hydrogen isotopes in Martian meteorites." Earth and Planetary Science Letters 394 2014 179-185/.