How NASA's Phoenix Mars Lander revolutionized our understanding of Martian habitability
Discovering the complex chemistry that makes Mars both promising and challenging for life
For centuries, we have looked at Mars as a world of possibility—a sibling planet that might harbor life. But is it a friendly sibling, or a harsh, sterile desert? The answer lies in understanding its habitability: the potential for an environment to support life as we know it. In 2008, NASA's Phoenix Mars Lander didn't go to find life itself. Instead, it went to dig into the icy Arctic soil and answer a more fundamental question: Could Mars have ever supported the most basic, microbial life? What it found forever changed our understanding of the Red Planet.
Before we can search for life, we must search for the conditions life requires. Scientists have a checklist for potential habitability, and Mars is the prime candidate for assessment.
Water is the universal solvent for biological chemistry. It transports nutrients, helps shape molecules, and facilitates metabolic reactions.
Life needs key chemical building blocks like carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS). It also needs a source of energy.
This includes a suitable temperature range, protection from harsh radiation, and a stable surface on which to exist.
Phoenix was sent to Mars's northern polar plains precisely because orbital data suggested vast quantities of water ice lay just beneath the surface. Its mission was to be a robotic geochemist, tasting the soil and ice to assess this checklist firsthand.
While Phoenix had several sophisticated instruments, its most profound discoveries about Martian chemistry came from a single, brilliant experiment: the Wet Chemistry Lab (WCL). This was the first time since the Viking landers in the 1970s that we mixed Martian soil with water from Earth.
The WCL was a miniature laboratory about the size of a mug. The process was meticulous:
Phoenix's 2.3-meter robotic arm scooped up a soil sample from a trench named "Snow White."
The arm carefully poured this fine, reddish soil into one of four "teacups" on the WCL instrument.
The WCL sealed the cup and injected 25 milliliters of purified water brought from Earth into the soil.
A paddle inside the cup stirred the mixture for several Martian days (sols), creating a Martian mud.
As the soil dissolved, a suite of 26 electrochemical sensors immersed in the slurry measured the types and concentrations of ions that leached into the water.
The data that streamed back to Earth was a shock. The Martian soil was not just salty; it was alkaline, with a pH between 8 and 9—similar to seawater or baking soda. But the real headline was the discovery of a game-changing ion: perchlorate (ClO₄⁻).
On Earth, most microbes find perchlorate toxic. Its presence was a major strike against the immediate habitability of that specific spot.
Perchlorate is a powerful antifreeze. When mixed with water, it can form a brine that stays liquid at temperatures as low as -70°C (-94°F).
The finding forced scientists to re-examine the results of the 1970s Viking landers, which found no organic molecules . It's now believed that the perchlorate in the soil, when heated by Viking's ovens, would have destroyed any organics present , hiding the very evidence Viking was looking for.
The WCL experiment painted a complex new picture: Mars was both less and more habitable than we thought—harsh for life, but potentially capable of harboring transient, liquid brines.
| Ion Detected | Concentration | Significance |
|---|---|---|
| Perchlorate (ClO₄⁻) | ~0.5% | Powerful antifreeze; toxic to most life. |
| Magnesium (Mg²⁺) | ~3000 ppm | A key nutrient for life; contributes to soil salinity. |
| Sodium (Na⁺) | ~1300 ppm | Makes the soil salty, similar to Earth's soils. |
| Chloride (Cl⁻) | ~7000 ppm | Confirms the presence of salt compounds. |
| pH Level | 8.3 | Slightly alkaline, similar to Earth's oceans. |
| Launch Date | August 4, 2007 |
|---|---|
| Landing Date | May 25, 2008 |
| Landing Site | Green Valley, Vastitas Borealis (68°N) |
| Mission Duration | 157 Martian days (sols) |
| Primary Goal | Study the history of water and assess habitability |
| Tool / Reagent | Function |
|---|---|
| Robotic Arm (RA) with Scoop | The "hand" of the mission; excavated ice and soil samples |
| Thermal and Evolved Gas Analyzer (TEGA) | Tiny ovens that baked samples; identified water vapor and CO₂ |
| Wet Chemistry Lab (WCL) | The "taste tester"; dissolved soil in water to analyze chemistry |
| Purified Water (from Earth) | The solvent for the WCL; used to leach soluble ions |
| Solar Panels & Batteries | Provided power; mission ended when winter deprived it of sunlight |
The Phoenix mission ultimately presented us with a paradox. It triumphantly confirmed the presence of water ice just below the surface and found the soil to be chemically similar to a backyard garden in its pH and nutrient content. Yet, it also discovered the toxic perchlorate, complicating the habitability question immensely.
Phoenix taught us that habitability isn't a simple "yes" or "no." It's a nuanced, shifting state. The Martian Arctic is not a permanently welcoming place for life, but it may have fleeting moments—warmer, wetter periods—where brines could liquefy and create transient oases. It forced us to stop looking for "Earth-like" life and start considering "Mars-adapted" life, perhaps hidden deep underground, away from the harsh surface conditions.
The Phoenix, true to its name, did not rise again after the Martian winter. But from its findings, a new, more sophisticated search for life was born, guiding the explorations of Curiosity and Perseverance rovers today . It proved that to find life, we must first understand the complex, and sometimes contradictory, chemistry of the worlds we explore.