CREDIT: JPL/Corby Waste
Artist's rendering of the Phoenix Mars Lander as the sun dips toward the martian horizon

After the landers of (Mars') summer have gone

Cue up Don Henley's "The Boys of Summer," tweak the lyrics a bit, and it's time to bid adieu to the Phoenix Mars Lander.

The craft's summer (Mars time) fling is over. Winter has set in around the planet's northern reaches, where Phoenix landed. The craft's solar panels no longer intercept enough sunlight to power the craft or keep it warm. So the National Aeronautics and Space Administration has ordered the Mars Reconnaissance Orbiter (MRO) -- the lander's radio relay station -- to stop listening for the its telltale signal.

But if the hardy little chemistry lab on a platter is now a memory, it lives on in the reams of data it's sent back on summer weather patterns, and especially on the soil and ice found at its landing site. Those may hold clues as to whether the landing site could become a temporary host for microbial life as the planet's orbit changes periodically, shifting the planet's climate much like changes in Earth's orbit triggers ice ages here.

Those changes on Mars have been neatly recorded by the MRO in rock formations far to southeast of the lander's site, in a region called Arabia Terra. The formal scientific results appear in the current edition of the journal Science. A version for the rest of us appears here.

The bottom line: Layered rock formations appear in four craters scattered across Arabia Terra. Some of the areas look as if they are petrified rice paddies. In reality, they appear to tell a tale of climate change occurring at time scales of roughly every 100,000 years and 1 million years.

These correspond to periodic cycles in Mars' orbit, which in turn affect the seasonal distribution of sunlight over its surface. One set of cycles occurs every 100,000 years, driven by wobbles in the planet's axis of rotation. That shifts Mars' tilt by tens of degrees and back from one period to the next. On Earth, by contrast, the comparable cycle shifts the planet's tilt by only 2.4 degrees roughly every 41,000 years.

Mars' 1-million-year cycle is traced to a second, more subtle shift in the tilt of its axis.

The pattern is most starkly revealed in Becquerel Crater, according the Caltech team, who analyzed the features in images from MRO's HiRISE camera. The crater sports a pattern where 66 layers, each roughly 10 feet thick, appear in 10-layer "bundles."

It's a bit like stacking two decks of play cards, and sandwiching a piece of cardboard between every 10th card.

When all is said and done, the crater is recording some 12 million years worth of climate shifts, says Kevin Lewis, a Caltech planetary scientist who wrote up the findings on behalf of himself and five colleagues who shared the labor.

So where does this leave the late, lamented Phoenix Mars Lander's contribution?

Peter Smith, the project's lead scientist, explains that first-cut studies of the soil the lander sampled revealed clays and carbonates -- minerals that on Earth form in the presence of liquid water.

During the mission, which enjoyed an extension along the way, the lander also picked up signs of salts and perchlorate. The salts, including potassium, sodium, magnesium, and chlorine, could easily be considered nutrients. On Earth, some microbes use perchlorate as a source of energy, in place of the sun, or heat from hydrothermal vents.

Dr. Smith suggests that even if the landing site is devoid of life now (a proposition the lander was not designed to test),  it conceivably could grow hospitable when Mars' tilt changes to bring warmer temperatures to planet's polar latitudes. More-detailed studies of the lander's data should help test that notion.

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