For the last six years, NASA’s Curiosity rover has been sampling the air on Mars. At regular intervals, curiosity takes a “breath” of Martian atmosphere. These air samples do not travel through a nose into a pair of lungs, but instead into an onboard lab called the Sample Analysis at Mars (SAM). Instruments within SAM study the composition of the air, observing which gases comprise Mars’ incredibly thin atmosphere. Think of it as a very advanced sense of smell. When you inhale, you have the ability to sense minute quantities of particles in the air. Curiosity’s atmospheric analysis is not so different. It can “smell” various quantities of gases. According to a new paper published this week, Curiosity smells something unexpected.

The results of sniffing surface-level atmosphere in the Gale crater are some of the first-ever direct measurements of the composition of and seasonal changes in Mars’ atmosphere. Our atmosphere on Earth is around 80% nitrogen (N2) and 20% Oxygen (O2) with other trace gases such as Carbon dioxide (CO2). On Mars, the atmosphere is quite different. Aside from being far thinner than Earth’s (about one hundredth the thickness), it is composed 95% by volume of carbon dioxide (CO2), 2.6% molecular nitrogen (N2), 1.9% argon (Ar), 0.16% molecular oxygen (O2), and 0.06% carbon monoxide (CO). Measurements from SAM has confirmed this makeup, but these are not the only data that have returned from Curiosity on this topic.

This mission found that most gases on Mars circulate in predictable patterns. For example, CO2 freezes over Mars’ North Pole in the winter, decreasing atmospheric concentrations in the winter which will rebound the following Summer. Data from SAM seem to confirm previous predictions and models. However, oxygen and methane do not seem to behave as expected.

In Summer, oxygen increases to a level 30% higher than anticipated, while in winter it drops to levels well below predicted values while methane spikes as high as 60% above predicted values in the summer. It is important to keep in mind that these are trace gases in Mars’ atmosphere. These changes do not represent complete turnovers in atmospheric composition. Methane is so rare that it is difficult to detect, yet changes in methane concentration repeat year after year.

Currently, atmospheric scientists have no explanation as to why Oxygen and Methane behave the way they do. On Earth, sources of oxygen and methane are typically biological. Most of Earth’s oxygen supply originates by the process of photosynthesis from plants and algae, while methane more commonly originates from animal sources.

Researchers who first reviewed the data wanted to make sure that the specific instrument within SAM that measures atmospheric gases, the Quadrupole Mass Spectrometer, was functioning properly. After ruling out possible malfunctions, they set about to attempt to explain why this oxygen spike seems to be occurring.

An initial hypothesis focused on the possibility that during the summer, water (H2O) or CO2 could be breaking apart in the atmosphere, leading to a surplus of gaseous oxygen. The water hypothesis has been ruled out because there is not enough water available to account for this. CO2 is not a possibility either. While CO2 can break apart and release oxygen (plants do it all the time), this process happens far too slowly without some sort of catalyst.

Another curious finding is that the oxygen spike is not repeatable. Some years it spikes higher than others. Also, methane in the atmosphere seems to spike along with it, suggesting to researchers that they may be linked somehow.

This confounding result excites many atmospheric scientists who are now on a quest to find the cause of these methane and oxygen spikes. While there are no good explanations at the moment, it is important to remember that curiosity is a first attempt to measure atmospheric composition on Mars across multiple seasons.

The more data collected increase the chance of discovering the mechanisms behind these observed shifts, whether they are atmospheric or geologic in origin. For now, they are but a tantalizing mystery.