Is the white stuff in the Martian soil ice or salt?
That's the question bedeviling scientists in the three weeks since the Phoenix lander began digging into Mars' north pole region to study whether the arctic could be habitable.
Shallow trenches excavated by the lander's backhoe-like robotic arm have turned up specks and at times even stripes of mysterious white material mixed in with the clumpy, reddish dirt.
Phoenix merged two previously dug trenches over the weekend into a single pit measuring a little over a foot long and 3 inches deep.
The new trench was excavated at the edge of a polygon-shaped pattern in the ground that may have been formed by the seasonal melting of underground ice.
New photos showed the exposed bright substance present only in the top part of the trench, suggesting it's not uniform throughout the excavation site.
Phoenix will take images of the trench dubbed "Dodo-Goldilocks" over the next few days to record any changes.
If it's ice, scientists expect it to sublimate — or go from solid to gas, bypassing the liquid stage — when exposed to the sun because of the planet's frigid temperatures and low atmospheric pressure.
"We think it's ice. But again, until we can see it disappear ... we're not guaranteed yet," mission scientist Ray Arvidson of Washington University in St. Louis said Monday.
Even if it's not ice, the discovery of salt would also be significant because it's normally formed when water evaporates in the soil.
Preliminary results from a bake-and-sniff experiment at low temperatures failed to turn up any trace of water or ice in the scoopful of soil that was delivered to the lander's test oven last week.
Scientists planned to heat the soil again this week to up to 1,800 degrees, said William Boynton of the University of Arizona in Tucson.
Phoenix landed in the Martian arctic plains on May 25 on a three-month, $420 million mission to study whether the polar environment could be favorable for primitive life to emerge. The lander's main job is to dig into an ice layer believed to exist a few inches from the surface.
The project is led by the University of Arizona and managed by NASA's Jet Propulsion Laboratory. The lander was built by Lockheed Martin Corp.
The Phoenix lander sent back the most detailed view of the Martian soil to date, showing clumps of fine grains mixed with possible minerals, scientists said Friday.
Much of the dirt that Phoenix scooped up and sprinkled on its microscope appeared as a reddish-orange hue that's typical of the Martian landscape.
Zooming in, scientists noticed green particles that could be olivine, a mineral usually associated with volcanic eruptions.
The soil also contained round, black glassy specks that could be volcanic glass, said mission scientist Tom Pike of the Imperial College in London.
It's too early to make any generalizations about Phoenix's landing site, but scientists were intrigued by the latest images.
"What we are looking at here is a part of history of Martian soil," Pike said at a news conference in Tucson, Ariz.
The mineral olivine was previously discovered by the rover Spirit, which has been roaming the Martian equatorial plains since 2004.
Since landing near Mars' north pole on May 25, Phoenix has been busy digging shallow trenches in the permafrost and delivering scoopfuls to its microscope and a test oven that is baking and sniffing the soil for traces of the chemical building blocks of life.
The first results from the oven experiment won't be available until next week.
Phoenix's main task during the three-month mission is to dig into an ice layer that's believed to be lurking inches below the surface.
The lander cannot directly detect life, but it will study whether the arctic region has the raw ingredients to support primitive life.
The $420 million mission is led by the University of Arizona and managed by NASA's Jet Propulsion Laboratory.