Bolstered by images showing water ice in the Martian soil, Phoenix team members are cracking on with plans to analyse the soil in-depth. The composition of the first soil sample delivered to an onboard oven may be known within days, and the lander is already attempting to collect a second sample.
On Thursday, images received from Phoenix showed that several clumps of soil that had appeared in an earlier image of a trench called Dodo-Goldilocks had apparently vaporised over the course of four days.
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鈥淪omething on the order of eight things [are] missing,鈥� team member Mark Lemmon said at a press briefing on Friday. 鈥淲e believe these things are water ice and in the process of sitting out in the cold but very dry atmosphere, they sublimated.鈥�
Patches of hard white material also faded over the four-day period. That suggests the white material is also water ice 鈥� and not salt or other materials, the team says.
Salt would not make such a disappearing act, and it is currently too warm on the northern plains of Mars for carbon dioxide ice to exist for any length of time. 鈥淚t would be the equivalent of having water ice on a 140掳 day on Earth,鈥� Lemmon said. 鈥淚t鈥檚 not going to be there very long 鈥� not long enough to take its picture.鈥�
The team had expected water ice to exist about 5 centimeters below the surface, where it was found, since NASA鈥檚 Mars Odyssey orbiter had previously found evidence of widespread subsurface ice around the poles.
Habitable environment
Also, when Phoenix peeked below its body shortly after landing, it saw a bright, hard surface 鈥� thought to be ice that had been exposed by its landing thrusters.
Discovering that similar patches of material are water ice is a great relief for the team. 鈥淥ne of our great fears was that we鈥檇 see ice 鈥� for example, under the lander 鈥� and the arm wouldn鈥檛 be able to reach it,鈥� said principal investigator Peter Smith of the University of Arizona in Tucson, US.
What鈥檚 next for the lander? 鈥淔inding out what is mixed in with the ice and to what extent it鈥檚 a habitable environment,鈥� Lemmon said.
That means trying to find clues that liquid water occasionally exists at the site 鈥� possibly as a result of periodic changes in the planet鈥檚 tilt that warm up the poles. Salts might provide such clues, since they might be left behind by the evaporation of liquid water, even in small amounts.
Extreme heat
Clues about whether the environment was ever habitable would come from organic materials 鈥� molecules containing carbon. These molecules would provide food for any life in the region, and would 鈥渉ave to be there if you鈥檙e going to have a habitable zone on Mars鈥�.
The team plans to look for such signs in soil samples taken from a variety of depths 鈥� beginning around the surface, then from lower down, around the boundary between soil and ice.
Soil collected from the surface at one trench is already being studied in one of eight ovens on an instrument called TEGA (Thermal Evolved Gas Analyzer). The soil fell into oven number 4 more than a week ago, but some software problems 鈥� now in the process of being fixed 鈥� caused delays in the experiment. As early as today, mission members may receive the results from that sample, though it is likely to take several more days to analyse them.
TEGA heats samples to 1000 掳C to study their composition. It takes advantage of the fact that hydrated minerals 鈥� which have been altered by water in the past 鈥� thermally decompose, or change into another form, when heated to a given temperature.
Second sample
This decomposition either gives off or takes in heat 鈥� changes that TEGA can study to decode the soil鈥檚 minerals, says team member Ray Arvidson of Washington University in St Louis, Missouri, US.
TEGA can also study the composition of any volatiles, such as ice, in the sample, which will vaporise when heated. The resulting gases can be separated according to their mass inside TEGA.
The soil in oven 4 is not expected to contain ice, however, since it sat on top of the oven for several days before falling in, allowing any ice it contained to sublimate away. But in the future, mission planners are hoping to deliver icy soil samples to TEGA ovens in less than 30 minutes, which should be fast enough to prevent the ice from vaporising.
Today, the lander is also trying to dig up soil to put into a second TEGA oven, called oven number 5. It is collecting the soil sample, dubbed 鈥淩osy Red,鈥� from the surface of a polygonal section of soil dubbed 鈥淐heshire Cat鈥�.
Hard ice
It began collecting the soil after digging a nearby trench called 鈥淪now White 2鈥�. About 5 cm below the surface of Snow White 2, the lander had to stop digging because it reached something too hard to dig through.
That hard material 鈥渓ooks like an ice layer and feels like an ice layer鈥�, Arvidson said. Curiously, it is more bluish than the white ice layer seen in the Dodo-Goldilocks trench. 鈥淲e鈥檝e got hard layers that are different 鈥� we hope to learn a lot from those differences,鈥� said Lemmon.
At some point, the team hopes to return to Snow White 2 to try dig into the ice a bit with various instruments on its robotic arm, including a scraper and a motorised grinder called a rasp. But the team does not expect to dig very far into the ice. 鈥淚ce at -80 or -90 掳C is as hard as a table top,鈥� Smith said.
The team is also preparing the first of four wet chemistry cells in MECA (Microscopy, Electrochemistry, and Conductivity Analyzer) to receive a soil sample. MECA will dissolve small amounts of soil in water to determines the pH, mineral abundance and conductivity of the soil.
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