Ten years ago this past week, a robot named Phoenix landed in the northern plains of Mars. The mission lasted a little over five months before the lander succumbed to the frigid martian winter, about two months longer than planned. Unlike Spirit and Opportunity, its roving older siblings near the equator, Phoenix was a stationary lander positioned at 68°N latitude—above the Arctic Circle on Earth (for any fellow Canadians out there, it’s about the same latitude as Inuvik). The goal of Phoenix was “to study the history of water and habitability potential in the martian arctic’s ice-rich soil.” And riches indeed it found there! Let’s take a look back at five big contributions to science made by the Phoenix mission in its brief time on Mars:
1. Ice, ice, baby!
Multiple observations from orbit had hinted at buried ice in the high latitudes of Mars. To investigate this, the Phoenix lander was equipped with a robotic arm capable of digging into the soil. However, before the lander even got a chance to use this arm, its landing thrusters did some of the work for it—blasting away a few inches of loose soil—and exposed what appeared to be ice.
This led to even more anticipation for trench digging. Of the dozen trenches Phoenix dug during its mission, two exposed hard, white material that disappeared in less than 4 sols (martian days). Scientists interpreted this to be ice that, once exposed to the temperature and pressure conditions at the martian surface, sublimated away. Sublimation is a process where ice goes directly from solid to vapour without going through the liquid phase. Liquid water is generally not stable on the surface of Mars due to its low temperature and atmospheric pressure, but…
2. Liquid water…
Shortly after landing, the camera on Phoenix’s robotic arm captured views of blobs of material on one of the landing struts. Over time, these blobs moved, darkened, and coalesced, behaving like droplets of liquid water. The hypothesis here was that these blobs “splashed up” on the struts when the descent thrusters melted the ice exposed upon landing mentioned above.
But if liquid water isn’t stable on the martian surface, how did Phoenix observe liquid water on Mars? The key here lies in salt. If you live anywhere that gets snow, you’re probably familiar with salt as a de-icer for roads, sidewalks, etc. Salt lowers the freezing point of water, allowing it to remain liquid at temperatures lower than that of non-salty water. For example, pure water freezes at 0°C/32°F, but ocean saltwater freezes around -2°C/28.4°F. While the de-icing salts you get at the hardware store lower the freezing point by a few degrees, more exotic salts can lower the freezing point as much as -70°C/-89°F! Phoenix discovered some of these exotic salts in the soil around the lander—in particular, magnesium perchlorate.
3. …with more than a dash of salt.
On Earth, perchlorate salts generally only form in trace amounts in nature. However, humans manufacture perchlorates in large amounts for use in things like rockets and gunpowder thanks to the fact that they explode upon heating. Perchlorates are also found in some cleaning agents, temporary adhesives, and batteries.
They are also toxic to humans.
Perchlorates affect the uptake of iodine by the thyroid gland, potentially leading to hypothyroidism. The small amounts humans tend to encounter on Earth generally don’t pose a significant health risk. But the soil at the Phoenix landing site contains about 0.5% perchlorates—and some observations suggest soils on Mars could contain up to 1% perchlorates—which is quite a bit in the relative sense. How this will affect future martian astronauts is something that will need to be heavily studied. (Perchlorates might also have affected the Viking lander experiments to look for organic material in the martian soil, as I discussed in a previous article.)
The presence of perchlorates on Mars does however provide some benefits. As an ingredient in rocket fuel, it could allow for the production of fuel on Mars rather than having to haul everything we’d need from Earth. Perchlorates are also used in chemical oxygen generators, which could come in handy for us to be able to breathe in our martian habitats.
(Perchlorates are also used in fireworks, so at least we’ll still be able to properly celebrate Canada Day and the 4th of July while we are slowly poisoned?)
4. Canadians discover snow on Mars
In what is perhaps the most fitting story in planetary exploration ever told, a Canadian built and operated instrument aboard Phoenix made the first-ever confirmation of snow on Mars.
Canada supplied the Meteorological Station aboard Phoenix, built by the same company that built the iconic Canadarm on the Space Shuttle and Canadarm2 aboard the International Space Station. The Meteorological Station included a laser system (LiDAR) to study martian clouds. By shooting this laser into the sky, scientists observed snow in the atmosphere about 4 kilometres (~2.5 miles) above the lander on multiple days. The snow vaporized before reaching the ground however, so don’t start dreaming of a dreamy snowscape on Mars just yet.
While snow might not reach the ground on Mars, frost certainly does. It’s easily visible from orbit, was viewed on the ground by the Viking landers in the 1970s, and is even viewable from a backyard telescope during martian winter! Phoenix managed to capture this cool view of early morning frost collecting on the mirror of its wind detection instrument:
5. The ravages of martian winters
Contact was lost with Phoenix on 2 November 2008, approximately five months after landing on Mars. This was expected as martian winter arrived. Temperatures plummeted and clouds thickened in the darkness of winter, reducing the amount of sunlight reaching the solar panels to fatally low levels. NASA/JPL continued efforts to contact the lander into the following spring without success, and officially declared Phoenix dead on 12 May 2010.
In death, Phoenix was still able to show us the harshness of winter. Photographs of the lander taken by the High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA’s Mars Reconnaissance Orbiter in July 2008 vs. May 2010 suggested that the western solar panel had fallen off the lander sometime between the acquisition of the two images. Scientists think this was due to the weight of over a foot of carbon dioxide frost collecting on the solar panels in winter.
Despite its (expectedly) short life compared to other successful surface missions thanks to the harshness of martian winter, Phoenix contributed key pieces of knowledge to our greater understanding of Mars. It may sit silently in the cold northern plains, slowly succumbing to the frost as the sols go by, but it will never be forgotten.