Phoenix Finis

NASA officially pulled the plug on the Phoenix mission this week. The Mars lander went silent early in November and finally, after remaining silent for a week, was declared dead. It was a natural and expected death brought on by the coldness and darkness of the Martian winter. Unlike the rovers or the spacecraft in orbit around Mars, Phoenix had a limited lifespan and the only question was how long it would hang on before cold and dark killed it.

It used solar panels to collect electricity to operate its computer and instruments, and to keep its batteries charged. As the Martial winter approached, the sun dropped lower and lower in the sky, making it difficult for Phoenix to collect enough energy. Eventually its batteries ran down to the point that it couldn't run its computer and communicate with Earth, and it fell silent. NASA gave it a week to see if it could "trickle-charge" its batteries into a last spasm of activity, but it remained stubbornly silent and was declared defunct..

It lasted about five months on the surface of Mars, though it was originally expected to last only three months. There is a technical possiblity that when the next Martian spring arrives the increasing amount of sunlight will recharge the battery and Phoenix will suddenly wake up from its coma, but it is extremely unlikely - it will spend the winter mantled in a coating of dry ice, which as you might imagine is not a good environment for electronic devices.

The first spacecraft designed to explicitly look for life on Mars were the Viking landers, which
touched down on Mars in 1976. They carried various experiments that scientists felt would answer once and for all the question of whether there were microbes or other forms of life in the Martian soil. The actual experiments produced odd, unexpected results that to a small group of scientists looked like evidence of life on Mars and looked like "funny soil chemisty" to the majority.

For years NASA and the ESA dispatched additional spacecraft to Mars, but instead of trying to
answer the Big Question ("Was there ever life on Mars?") these new spacecraft were designed to test a smaller but more basic question: "Were there ever conditions suitable for the formation of
life on Mars?" And that question can in turn be boiled down to this: "Did Mars ever have large amounts of liquid water on its surface that organisms could have grown in?"

It's really hard to say if you've found life or not. Even on Earth it's not always clear if a dark mark in a rock is a hardy algae, or just a smear of dirt. And on Mars, the peculiar soil chemistry made it difficult to find life by the usual means - dump some Martian dirt in a warm nutrient soup and see if it starts to produce gas. But it is easier to say if you've found evidence of liquid water or not, because liquid water leaves certain telltale clues behind, such as stratified and cross-bedded sediments, the presence of carbonates, the presence of certain chemicals like hematite and so forth, things that we know from prior experience require liquid water to form.

The two rovers, Spirit and Opportunity, were designed to carry out a detailed study of the Martian surface to look for such evidence of liquid water, and the evidence seems pretty firm that Mars did indeed have liquid water on its surface for a significantly long period of time. You might think that having found evidence for liquid water, the next question would be "Was there life in that liquid water?" But really the next cautious question is "Where did the water go, and why?"

It was always assumed that the water in the Martian northern ocean vanished by two main routes. Some of it would evaporate and in the from of water vapor be blown away from Mars by the solar wind, along with the remainder of Mars's atmosphere (one of the many downsides of a planet not having a magnetic field). But most of it would simply sink into the planet's porous regolith and freeze underground. Measurements taken from orbit hinted that this was the right answer, but until someone actually landed there and found said ice, it was nothing more than a hypothesis. So Phoenix was designed to land in the extreme northern part of Mars and determine by direct measurements if there was ice in the ground.

And it did. Phoenix found ice aplenty, both fairly solid chunks of ice and a sort of rime-and-dirt
mixture. This confirmed the theory that the water had largely frozen into the crust, and helped to confirm that there had once been a genuine Martian ocean. None of this proves that life existed, but it provides answers to basic questions like "Could life have existed?"

Phoenix also got to the bottom of the "funny soil chemistry" that so vexed the Viking landers. It
turns out that the Martian soil contains peroxides and perchlorates, which are powerful oxidizing
compounds. Many scientists, upon hearing about this, said "That's it, Mars must be sterile because these peroxides and perchlorates are the kiss of death for microorganisms." And indeed they are - usually. In fact, if you went just by the experience of the two rovers and Phoenix, you'd have to say that Mars was an extremely hostile environment for life: bombarded by ultraviolet radiation because there's no ozone layer, bombarded by cosmic rays and charged particles because there's no magnetic field, tortured by peroxides and wrecked by perchlorates; what chance could life have against such dreadful odds?

Then people doing research in the Atacama Desert of South America found a class of bacteria that actually eat and thrive on perchlorates found naturally in the dry, cold soil. This doesn't automatically mean that there are Martian bugs that use perchlorates as an energy source; it simply means that at least in the case of the Atacama Desert, perchlorates are not an automatic death sentence.

So, from the life point of view, we're back to square one. There was once liquid water, and that
liquid water is frozen into the rocky soil of the northern hemisphere. There are peroxides and perchlorates in the soil that normally kill bacteria, but there are bacteria that use them as food. So the life question still isn't settled, by any means. So that is Phoenix's legacy: the confirmation of ice, and a first assay of the strange chemistry of the Martian soil.

Every scientist and engineer ever involved with a Mars spacecraft probably has fantasized at least once about looking at a high-resolution photograph of the ground next to the spacecraft and seeing obvious evidence of life. Something lichen-like clinging to the side of a protected alcove, or a fossil of something with obvious radial symmetry. They never admit such fantasies because science makes people cautious and methodical, and because it's considered gauche to hope for such a find. But Phoenix did something almost as striking. It found the water, and where the water is, so also lies the hopes for finding life on Mars.