Ghosts of ancient oceans lingering in the atmosphere of Mars have led astronomers to speculate that the red planet may have once had proportionally more water than the Earth.
Astronomers used the orbiting Far Ultraviolet Spectroscopic Explorer to probe Mars' atmosphere for molecular remnants of primordial water, and compared their readings with prior data obtained using the Hubble Space Telescope and measurements of frozen water currently present on Mars to estimate the amount of water on Mars shortly after it formed.
"We calculate that if the initial quantity of water on Mars could have been evenly distributed across the planet somehow, it would have been equivalent to a global Martian ocean at least three-quarters of a mile deep," says Vladimir Krasnopolsky of the Catholic University of America. "This is 1.3 times more water per mass than the Earth."
Krasnopolsky authored a paper on the results in the Nov. 30 Science with Hopkins' Paul Feldman, chair of the Physics and Astronomy Department in the Krieger School of Arts and Sciences.
Mars has been haunted by water for many decades, from popular but inaccurate speculation that the surface features included canals early in the 20th century to recent imaging by NASA spacecrafts that showed a vast dry basin in the northern hemisphere and features resembling dry riverbeds. Water is a critical component for life, and recent images of Mars' geographical features have left some scientists wondering if the planet might once have been able to support life.
To further explore the history of water on Mars, scientists looked for signs of molecular hydrogen in the Martian atmosphere. Ultraviolet radiation from sunlight breaks water down into different components that include atomic hydrogen, H; and molecular hydrogen, H2.
"FUSE is ideal for studying molecular hydrogen in many astrophysical environments," Feldman says. "In our solar system, FUSE has already been used to study Jupiter and Saturn, which are made up primarily of hydrogen.
"By contrast, hydrogen is a minor component in the Martian atmosphere, but it's a very important tracer of atmospheric chemistry that is responsible for the stability of Mars' carbon dioxide atmosphere."
Krasnopolsky and Feldman used FUSE to determine that the level of H2 in the Martian atmosphere is about 15 parts per million. They compared that result to a finding Krasnopolsky obtained in 1997 using the Hubble Space Telescope. At that time, Krasnopolsky was assessing the abundance of another form of hydrogen known as deuterium in the Martian atmosphere.
Both the hydrogen found in H2 and deuterium can be part of a water molecule, and both can be produced when sunlight breaks down water. Scientists know the approximate ratio of each type of hydrogen in water molecules on Earth, and assume that the ratio between these two types of hydrogen was similar on Mars shortly after it formed. Currently, that ratio, known as the D to H ratio, is different on Mars because hydrogen tends to slip away from Mars more quickly than deuterium.
Krasnopolsky and Feldman analyzed the D to H ratio in the Martian atmosphere to get a feeling for how much more quickly hydrogen leaves the Martian atmosphere, increasing the ratio.
To work backward to an estimate of Mars' primordial waters, they also needed an estimate of the current volume of frozen water on Mars. They based that on measurements of Mars' polar ice caps taken by NASA's Mars Global Surveyor probe.
"It's possible this is an underestimate and that there's more water frozen in the Martian soil that we don't have a good feel for yet," Feldman says. "Even if it does turn out that we've underestimated, more water currently on Mars will only increase our estimates of the early Martian oceans."
Finally, Krasnopolsky and Feldman had to consider an era in Mars' history known as the hydrodynamic escape period. Lasting for just under a billion years after Mars' formation approximately 4.5 billion years ago, this period was characterized by a high rate of water loss driven in part by heat left over from the planet's inception.
When all these factors were considered, the amount of primordial water that seemed to be necessary to produce today's D to H ratio was surprisingly large.
This research was supported by NASA. FUSE is operated by Johns Hopkins University for NASA.