Mars scientist Philip R. Christensen. ASU photo. Download image

Philip R. Christensen, Regents' Professor of Geological Sciences in the School of Earth and Space Exploration at Arizona State University, has been given the G.K. Gilbert Award for 2008 by the Planetary Sciences Division of the Geological Society of America.

The prestigious award, given annually for outstanding contributions to the solution of fundamental problems in planetary geology, commemorates geologist Grove Karl Gilbert (1843–1918), one of the first scientists to study the geology of the American West.

This year's award, bestowed at the GSA's annual meeting in Houston this week, recognizes Christensen's work in the field of remote sensing of minerals on Mars using instruments that operate in the infrared part of the spectrum.

"The award is especially meaningful to me because I have long admired Gilbert and have been intrigued by many of the same scientific questions that he pursued throughout his career," says Christensen, who in addition to being a professor in the College of Liberal Arts and Sciences is also director of ASU’s Mars Space Flight Facility.

Mars explorer

In 2000 Christensen's Thermal Emission Spectrometer (TES), launched on NASA's Mars Global Surveyor, discovered a large deposit of the iron mineral hematite on Mars. Four years later, NASA sent Opportunity, one of its two Mars Exploration Rovers, to the site in Meridiani Planum found by TES. Upon landing, the rover confirmed the hematite discovery and explored sediments repeatedly soaked by shallow pools of water.

In addition, both rovers carry identical miniature versions of TES, which are used to scout for new rocks and minerals for the rovers to examine. Recently, the Mini-TES on the Spirit rover (which is exploring the Columbia Hills in Gusev Crater) pointed the way to a discovery of silica minerals. These appear to be relics of an ancient hot spring like those in Yellowstone National Park.

Christiansen is also the designer and principal investigator for the Thermal Emission Imaging System (THEMIS), a multi-band infrared camera working on NASA's Mars Odyssey orbiter. Among this instrument's discoveries are beds of chloride minerals in the ancient Martian highlands. These may be salt deposits laid down during a time when Mars was wetter and warmer than today, and they may offer clues about the existence of a Martian biosphere, either at present or in the past.

Says ASU professor Ronald Greeley, "I believe it's unprecedented in planetary science for a principal investigator to be operating four instruments concurrently: TES on Mars Orbiter, Mini-TES on Spirit and Opportunity, and THEMIS on Mars Odyssey."

Reading the scenery

Studying landscapes began early for Christensen. "I grew up in the West, having been born in Utah and lived in Kansas and California," he explains. "Each summer my family would drive across the West to visit our scattered relatives, and during those long drives I spent many hours looking out the window of our car at the mountains and landforms. I didn’t realize it at the time but I was becoming a geologist."

As with Gilbert decades before, Christensen was fascinated by western scenery and wondered at its formation and history. "My family liked to explore out-of-the-way places, and we probably traveled many of the same routes that Gilbert did, seeing landscapes that have not changed much since his time."

While Gilbert's career involved terrestrial geology almost entirely, he had links to what is now called planetary science. A hundred years ago, Gilbert was about the only geologist in the world arguing that craters on the moon were caused by the impacts of meteorites. The overwhelming consensus of geologists of his time and long afterward held that lunar craters were volcanic in origin, an idea that didn't finally disappear until the mid-1960s.

"When I was 12 my parents gave me a telescope and, again like Gilbert, I spent countless hours looking at the moon," Christensen says. "The only features I could see with my small telescope were the craters, and in reading the few books about the geology of the moon I quickly learned of his early hypothesis for crater formation and his role in shaping our understanding of the moon’s history."

As a graduate student in the late 1970s, Christensen's interests ranged far beyond the moon: He worked on NASA's Viking project, which sent two orbiters and two non-roving landers to Mars. Christensen earned a doctorate in geology from University of California, Los Angeles, and came to ASU in 1981.

Impact in Arizona

Arizona had drawn Gilbert as well, nearly a century before. Besides lunar craters, Gilbert also studied northern Arizona's Meteor Crater, then known as Coon Butte, making the first geological survey of it in 1891. Because Meteor Crater is surrounded by millions of fragmentary nickel-iron meteorites, Gilbert had a strong hunch it might be a terrestrial counterpart of the moon craters he saw by telescope.

Ironically, however, Gilbert's survey led him to conclude, mistakenly, that Meteor Crater was not the result of an impact.

Despite weeks of fieldwork, Gilbert could find no large remnant of the impacting body, which he thought should have survived the impact. Thus he decided reluctantly that Meteor Crater was an explosion hole blown open when volcanic heat turned groundwater to steam. It wasn't until the 1920s, after Gilbert had died, that scientists realized the shock of impact would vaporize most the impacting object.

"While Gilbert's volcanic interpretation was wrong," notes Christensen, "he did what every scientist should do. He threw away a favorite theory when the data didn't back it up."

Looking into the future of planetary geology, Christensen is excited. "In my lifetime our perception of Mars has changed from a point of light in the night sky to a complex planet we are coming to know as well as our own."

He adds, "The past 30 years have been a remarkable period in planetary exploration, and I consider myself to be very fortunate to have participated in this modern age discovery.

"The images we have of Mars now rival the views I had out the window of our family car."