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Mars will be one of the first targets for NASA's James Webb Space Telescope as the orbiting observatory gathers more data to learn why the Red Planet lost so much water over its 4.5-billion-year history.

In the ancient past, Mars' surface likely hosted an ocean as deep as the Mediterranean Sea. As the planet's atmosphere thinned, however, most of the ocean was lost to space. The remainder of the water is locked in the Martian ice caps.

The Webb telescope — which NASA bills as a successor to the famed Hubble Space Telescope mdash; is expected to reach space in 2019. Mars will be visible to Webb in its first year of operations, between May and September 2020. [Webb Emerges from Giant Vacuum Chamber]

"Webb will return extremely interesting measurements of chemistry in the Martian atmosphere," Heidi Hammel, a planetary astronomer and executive vice president of the Association of Universities for Research in Astronomy in Washington, D.C., said in a statement. "And most importantly, these Mars data will be immediately available to the planetary community to enable them to plan even more detailed Mars observations with Webb in future cycles."

Hammel will lead the telescope's observations of Mars under a Guaranteed Time Observation (GTO) project. GTO dedicates research time to scientists such as Hammel who played a key role in the telescope's development. Hammel was named an interdisciplinary scientist for the Webb project in 2003."A respectable ocean"

Webb's observations will follow years of research examining Martian water loss and its changing environment. NASA's Mars Atmosphere and Volatile Evolution mission arrived at Mars in 2014 to examine the rate of atmospheric loss today. And in 2015, NASA released results from several telescopes that measured the atmospheric ratio of "normal" to "heavy" water molecules on Mars, in different seasons and locations.

NASA's James Webb Space Telescope is an $8.8 billion space observatory built to observe the infrared universe like never before. <a href="http://www.space.com/21232-nasa-james-webb-space-telescope-infographic.html">See how NASA's James Webb Space Telescope works in this Space.com infographic</a>
NASA's James Webb Space Telescope is an $8.8 billion space observatory built to observe the infrared universe like never before. See how NASA's James Webb Space Telescope works in this Space.com infographic
Credit: Karl Tate, SPACE.com Infographics Artist

Water is made up of hydrogen and oxygen, but hydrogen comes in different types, or isotopes. The heavier version of hydrogen — which has one proton and one neutron, rather than just a proton, in its nucleus — is called deuterium. The 2015 research suggests that deuterium, due to its heavier weight, remained on Mars even while the lighter hydrogen molecules were lost to space, according to a video NASA released in 2015.

Researchers suggested that pressure from charged particles in the solar wind blew the lighter hydrogen molecules out of Mars' atmosphere, because the planet has no global magnetic field to protect it. Additionally, the water molecules Mars had in its atmosphere likely broke apart under the sun's ultraviolet light.

Past infrared observations with the W. M. Keck Observatory, the NASA Infrared Telescope Facility and the European Southern Observatory's Very Large Telescope showed that the Martian polar caps are highly enriched in deuterium, supporting the theory that deuterium remained behind. Mars' frozen water has a ratio of 1 hydrogen molecule to 400 deuterium molecules — about eight times greater than the ratio in Earth's oceans, which is 1 hydrogen molecule to 3,200 deuterium molecules, the 2015 research showed.

"Now we know that Mars water is much more enriched than terrestrial ocean water in the heavy form of water, the deuterated form," Michael Mumma, a senior scientist at NASA's Goddard Space Flight Center in Maryland, said in the 2015 video. "Immediately that permits us to estimate the amount of water Mars has lost since it was young."

In the ancient past, Mumma added, Mars had an ocean that covered about 20 percent of the planet's surface area — "a respectable ocean," he said. The body of water was about 5,000 feet (1,500 meters) deep, on average. Today, only 13 percent of that ancient ocean remains, locked in the polar ice caps.

In separate observations, the Mars Curiosity rover, located at Gale Crater near the Martian equator, found that conditions were wet in that region for about 1.5 billion years. That period of time, Mumma said, "is already much longer than the period of time needed for life to develop on Earth." The infrared telescope observations suggested that "Mars must have been wet for a period even longer," he added. [Building the James Webb Space Telescope]

The Webb telescope, which is designed for infrared observations, will follow up on the normal-water and heavy-water observations performed by the other observatories, NASA officials said in the statement. It will watch the normal-water-to-heavy-water ratio during different seasons, and at different times and locations. 

Webb's observations will let researchers refine the measurements of heavy water at Mars and see how much water moves in the Martian water cycle between the atmosphere, the soil (regolith) and the polar ice.

Webb will be located far from Mars, in an orbit about 1 million miles (1.6 million kilometers) from Earth at a gravitationally stable spot in space between Earth and the sun called a Lagrange point. Unlike an orbiter, its vantage point will let Webb image the entire disk of Mars at once. 

Webb's observation has other benefits, too. Earth's atmosphere can interfere with observations, but Webb is far away. And it is designed to look at small differences in light wavelengths, NASA officials said in the statement. 

However, observing Mars will still be a challenge. Engineers will need to calibrate Webb's observations of the Red Planet to make sure the amount of light reaching its sensitive instruments will not overwhelm the telescope.

"Webb is designed to be able to detect extremely faint and distant targets, but Mars is bright and close," Geronimo Villanueva, the Mars lead on the GTO project and a planetary scientist at Goddard Space Flight Center, said in the statement. 

"Very importantly, observations of Mars will also test Webb's capabilities in tracking moving objects across the sky, which is of key importance when investigating our solar system," added Goddard research scientist Stefanie Milam, who is coordinating Webb's solar system program.

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