In 2008, NASA’s Phoenix spacecraft is scheduled to land in the northern plains of Mars to determine if that environment could support life in the past or in geologically recent times.
Like a detective, NASA scientist and Phoenix co-investigator Carol Stoker will piece together clues radioed to Earth from the spacecraft to try to determine if conditions on Mars in the icy north could support life either today (unlikely, she says), or in the relatively recent past, when warmer conditions occurred.
“My role is to try to help pull the story together from all different instruments,” said Stoker, a scientist who works at NASA Ames Research Center in the heart of California’s Silicon Valley. Stoker will lead the science-working group that will evaluate the biological potential of the landing site.
“We are trying to learn whether conditions allowing life to grow ever occur at this site,” she explained. The possibility of life revolves around the question of whether or not liquid water forms in the region as a result of warming due to climate change, according to Stoker.
During the 90-day mission, Phoenix will deploy its robotic arm that can dig trenches as deep as 1.6 feet (one half meter) into the soil, or to the top of an ice boundary. According to Stoker, scientists are expecting to take a small sample from icy soil covered with a thin layer of dry soil.
To analyze soil samples collected by the robotic arm, Phoenix will carry eight “ovens” in a “portable laboratory.” Martian samples will be heated to release volatiles that can be examined for their chemical composition. Scientists will look for the building blocks of life that include carbon, hydrogen, nitrogen, oxygen and sulfur among other things.
NASA has scheduled the Phoenix spacecraft for launch in August 2007. In May 2008, Phoenix is to land in an ice-rich area in the northern polar region of Mars between 65 and 72-north latitude. Phoenix is not a ‘rover,’ and will remain where it lands.
“Why land in the northern plains?” Stoker said. “When Phoenix was being proposed, the Mars-orbiting Odyssey (http://mars.jpl.nasa.gov/odyssey/) spacecraft had just discovered that in the high latitude regions of Mars, at 60 degrees north and south, there is a high concentration of hydrogen molecules, indicating that there is up to 50 percent ice in the soil”, she explained.
Although there is ground ice in both the northern and southern high latitude regions, melting ice can’t form liquid water in the south because it is about 2.5 miles (4 kilometers) higher than the north and the atmospheric pressure is so low that liquid water can never form, while in the (lower) north melting ice can form liquid although it will still evaporate quickly.
The search for life is the main goal of the overall Mars exploration program. “But the strategy in searching for life is to first find evidence of a habitable environment,” Stoker said. “Recent missions — Mars Exploration Rovers — have explored environments that may have been habitable billions of years ago, but are not today. Phoenix is the first mission to explore an environment that might be capable of supporting modern life,” she explained.
Phoenix carries an instrument capable of detecting organic compounds, another indicator of habitable conditions. The last missions that could look for organic material on Mars were the Viking missions of 1976, according to Stoker. NASA sent two Viking landers to Mars in 1976, which landed in non-icy zones, but neither spacecraft found organics. “We think the ice may preserve organic compounds that are otherwise destroyed by harsh ultraviolet light and strong oxidants found on Mars,” Stoker said.
Also, Phoenix is destined to go to Mars’ north polar region because of two circumstances – the spacecraft’s launch date in August 2007, and Mars’ position in its orbit when the spacecraft is to land in 2008. At that time, the season will be summer in Mars’ northern region.
“Because Phoenix is a solar-powered mission, we had to land in spring or summer,” Stoker said. Scientists also had to decide how far north to land Phoenix. According to Stoker, at 75 to 80 degrees, on the landing date, the spacecraft would be in the seasonal polar cap region. Researchers decided to land Phoenix between 65 and 72 degrees north latitude, south of the northern polar cap but in water-rich terrain.
The polar layers of ice and soil form there over long periods, and the polar caps move due to changes in the tilt of the planet and the position in the orbit when summer occurs, Stoker observed. These and other factors affect regional climates on the red planet causing cycles of warming and cooling.
Carbon dioxide snow falls on the polar regions of Mars. “So, the northern winter is going to be minus 195 degrees Fahrenheit (minus 125 degrees Centigrade), a nasty place to be in the winter,” Stoker observed. “Given how nasty it is, why would we think that it could be habitable? This requires an understanding of how Mars’ climate changes in the polar regions.”
The climate on Mars is governed strongly by the intensity of sunlight energy hitting each area of Mars. Sunlight intensity changes over time in each region because Mars’ orbit changes, and the tilt of the planet’s axis of rotation changes.
“Currently, Mars is tilted 25.3 degrees, which is nearly the same as the tilt of the Earth’s axis,” Stoker said. This is just a coincidence, Stoker said. “If you were to take a snapshot in any other epoch of Mars’ history, the tilt of the axis could be anywhere from 15 degrees to 35 degrees.”
Globally, Mars does not get warmer as orbital parameters change, explained Stoker, but a particular location could be much warmer. “The change of the tilt of the axis is cyclical,” she said. “The time scale for an obliquity (tilt) cycle is about 100,000 years, and the size of the variation increases as you go back in time. Between five and 10 million years ago, the solar energy was almost three times higher in the northern plains than it is now.”
“You are talking about big changes between the warmest conditions experienced and the coldest conditions. Right now the northern plains, where Phoenix will land, are in one of the coldest periods,” Stoker observed.
Another factor Stoker noted is that the martian orbit is slightly noncircular. Currently, when it is summer in the north Mars is furthest from the sun, the time of year when Mars is closest to the sun changes over time and goes through a complete cycle in 50,000 years. So, in 25,000 years, the northern summer will occur when Mars is closest to the sun. That effect alone will increase the amount of sunlight in high northern latitudes by roughly 50 percent, according to Stoker.
“The near-surface ground ice at the Phoenix landing site could have become warm enough to melt as recently as 25,000 years ago, and these melting periods recur on 50,000-year cycles,” Stoker noted. “So, that region is possibly habitable for periods of time lasting a few thousand years, separated by periods where it is too cold, lasting roughly 50,000 years or so,” she added.
“Phoenix will be looking for evidence that the ice has melted in the past, but the evidence may not be easy to figure out,” Stoker said. “The most definitive thing is the amount of ice we find. What fraction of the material we find under the surface will be ice?”
Water can condense directly from vapor into the soil, but only enough to fill the pore spaces. The only way to produce a higher concentration of ice is for a melting cycle to occur, according to Stoker, and if there were enough ice, that fact would suggest the region must have been warmer in the past, she said.
Principal Investigator Peter H. Smith of the University of Arizona, Tucson, leads the Phoenix mission. Project management is being led by NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Lockheed Martin Space Systems, located near Denver, Colo., is designing and building the spacecraft and will provide Mars space flight operations for Phoenix. The University of Arizona, the Jet Propulsion Laboratory, the Canadian Space Agency, the University of Neuchatel (Switzerland), the University of Copenhagen and the Max Planck Institute in Germany built instruments for Phoenix. JPL is a division of the California Institute of Technology in Pasadena.
