Free Astronomy Magazine May-June 2015

12 MAY-JUNE 2015 SPACE CHRONICLES Mars: the planet that lost an ocean’s worth of water by ESO on large telescopes back on Earth.) In the study, the team mapped the distribution of H 2 O and HDO re- peatedly over nearly six Earth years — equal to about three Mars years — producing global snapshots of each, as well as their ratio. The maps reveal seasonal changes and microclimates, even though modern Mars is essentially a desert. Ulli Käufl of ESO, who was respon- sible for building one of the instru- ments used in this study and is a co-author of the new paper, adds: "I am again overwhelmed by how much power there is in remote sens- ing on other planets using astro- nomical telescopes: we found an ancient ocean more than 100 mil- lion kilometres away!" The team was especially interested in regions near the north and south poles, because the polar ice caps are the planet’s largest known res- ervoir of water. The water stored there is thought to document the evolution of Mars’s water from the wet Noachian period, which ended about 3.7 billion years ago, to the present. The new results show that atmo- spheric water in the near-polar re- gion was enriched in HDO by a factor of seven relative to Earth’s ocean water, implying that water in Mars’s permanent ice caps is en- riched eight-fold. Mars must have lost a volume of water 6.5 times larger than the present polar caps land, USA, and lead author of the new paper. “With this work, we can better understand the history of water on Mars.” The new estimate is based on de- tailed observations of two slightly different forms of water in Mars’s atmosphere. One is the familiar form of water, made with two hy- drogen atoms and one oxygen, H 2 O. The other is HDO, or semi- heavy water, a naturally occurring variation in which one hydrogen atom is replaced by a heavier form, called deuterium. As the deuterated form is heavier than normal water, it is less easily lost into space through evapora- tion. So, the greater the water loss from the planet, the greater the ratio of HDO to H 2 O in the water that remains. (In oceans on Earth there are about 3200 molecules of H 2 O for each HDO molecule.) By comparing the ratio of HDO to H 2 O, scientists can measure by how much the fraction of HDO has increased and thus determine how much water has escaped into space. This in turn allows the amount of water on Mars at earlier times to be estimated. (Although probes on the Martian surface and orbiting the planet can provide much more detailed in situ measurements, they are not suitable for monitor- ing the properties of the whole Martian atmosphere. This is best done using infrared spectrographs A primitive ocean on Mars held more water than Earth’s Arctic Ocean, and covered a greater portion of the planet’s surface than the Atlantic Ocean does on Earth, according to new results published online in Science on 5 March 2015. An inter- national team of scientists used ESO’s Very Large Telescope, along with instruments at the W. M. Keck Observatory and the NASA Infra- red Telescope Facility, to monitor the atmosphere of the planet and map out the properties of the water in different parts of Mars’s atmosphere over a six-year period. These new maps are the first of their kind. About four billion years ago, the young planet would have had enough water to cover its en- tire surface in a liquid layer about 140 metres deep, but it is more likely that the liquid would have pooled to form an ocean occupy- ing almost half of Mars’s northern hemisphere, and in some regions reaching depths greater than 1.6 ki- lometres. “Our study provides a solid esti- mate of how much water Mars once had, by determining how much water was lost to space,” said Geronimo Villanueva, a scientist working at NASA’s Goddard Space Flight Center in Greenbelt, Mary-