Categorization of methane spikes
We focus on the six methane spikes reported by the TLS instrument (which is referred to as the “detector” in the following text) during the 4.6 years of the Curiosity mission through May 2017 (Fig. 1, Table S1). The six spikes can be categorized based on the seasons and the time of day of their detections. In terms of seasons, Spikes 1 and 6 were detected from late northern fall into winter. Spikes 2–5 were detected in northern spring. In terms of the time of day, Spikes 1 and 5 were detected in the early afternoon (“daytime”), and Spikes 2, 3, 4 and 6 were detected between midnight and early morning (“nighttime”). As a result, Spikes 1 and 6 share similar seasonal, regional and global circulation patterns, as do Spikes 2–5. Spikes 1 and 5 share similar diurnal crater circulation patterns, as do Spikes 2, 3, 4 and 6. The similarity in atmospheric circulation patterns also manifests itself in the subsequent emission region localization.
Current understanding of atmospheric circulation in Gale crater is primarily based on GCM and higher-resolution mesoscale simulations. We employ MarsWRF, a Mars GCM and mesoscale model to simulate atmospheric circulation at Gale crater (refer to the Methods section for detailed model configurations). Simulation results show that the circulation consists of three components – a global meridional overturning circulation, a regional circulation, and a crater-scale circulation. Figure 2 shows an example of near-surface winds simulated by MarsWRF. In northern winter, the rising branch of the global meridional overturning circulation is centered in the southern hemisphere. Prevailing winds at the topographic dichotomy next to Gale crater are towards the south and are particularly strong around 270° solar longitude when Spike 6 was detected. In northern spring, the large-scale prevailing winds at Gale crater are weak. The regional circulation is characterized by upslope northerlies along the topographic dichotomy in the afternoon, and downslope southerlies in the nighttime. The crater circulation is characterized by upslope winds along the inner crater rim and the slope of Mount Sharp in the afternoon, and downslope winds in the nighttime. The PBL thickness at Gale crater undergoes a daily cycle between a nighttime minimum thickness of tens of meters, and a daytime maximum thickness of about three kilometers, similar to previous findings in ref. 30.
Upstream emission regions
At every timestep in a back-trajectory simulation (which corresponds to an emission time), based on the instantaneous particle density in the PBL, STILT generates a “footprint” map in units of ppbv µmol-1 (ref. 27), which quantifies the contribution of unit methane emission from any emission site at that emission time to the methane mole fraction at the detector. The values of footprints are equal to the prospected methane mole fraction in the unit of ppbv above the ~ 0.41 ppbv background level induced by 1 µmol of methane emission. High footprints at a certain emission time indicate regions where the emission at that emission time casts strong influence over the detection, or in brief, the upstream regions. Time-integrated footprints measure the influence of a constant-flux emission on a detection, which show upstream regions at all possible emission times. Figure 3 shows the time-integrated footprint maps for Spikes 1 and 2. Refer to Fig. 5 and Fig. S5 for the footprints of Spikes 3–6.
Within Gale crater, the strongest footprint of Spike 1 is located to the north of the TLS detector (Fig. 3a), which is also the case for Spike 5 (Fig. S5g). The similarity in their footprints is consistent with the similarity in the early-afternoon crater-scale circulation patterns when these two spikes were detected (Fig. 1, Table S1). Despite the different seasons of these two detections, the prevailing local wind in the early afternoon comes from the north in both cases. For Spike 2, the strongest footprint lies on the entire northwestern crater floor (Fig. 3d), which is also the case for Spikes 3, 4, and 6 (Fig. 5d, Fig. S5a, d), despite some fine spatial patterns in the footprints of Spike 6. These four spikes were all detected in the nighttime when the PBL was shallow. The released particles are confined within the PBL, so they imprint almost equally strong footprints onto the entire northwestern crater floor as they are advected backwards in time. In the time-forward perspective, emission that occurs at different places on the northwestern crater floor in the nighttime casts almost equal influence over a nighttime detection.
Outside Gale crater, the strongest footprint for Spike 1 lies to the north of the crater, as a result of the prevailing northerlies in this season (Fig. 3b). This is also true for Spike 6 (Fig. 5e). This shows that for these two spikes, if a methane emission region exists in the neighborhood of Gale crater (but outside the crater), it is most likely located to the north of the crater. The locations of the upstream regions for Spike 2 are, however, less definitive. The strongest footprints for Spike 2 cover the regions in the first and third quadrants of Gale crater (Fig. 3e). This is also true for Spikes 3–5 (Fig. S5b, e, h). Despite this ambiguity, the strongest footprints for all the six spikes overlap in a region within 300 km to the north of Gale crater. It is noteworthy that the “E8” and “ESE” regions, suggested as the most likely emission regions for Spike 1 (ref. 13), do not bear strong footprints in our study and are hence not identified as the preferred upstream regions for Spike 1 (Fig. 3b).
Further zooming out to the hemispherical scale, the strongest footprints of Spike 1 extend from Elysium Planitia towards two directions – one to the north along the western side of Elysium Mons reaching Utopia Planitia, and the other to the east along the southern side of Elysium Mons reaching Amazonis Planitia (Fig. 3c). This is also true for Spike 6, although the northern branch appears more prominent (Fig. 5f). This suggests that these large-scale geographic units are more likely to be the emission regions than other large-scale geographic units for Spikes 1 and 6. For Spikes 2–5, the strong footprints cover many large-scale geographic units around Gale crater (Fig. 3f, Fig. S5c, f, i), with the aforementioned Elysium Planitia and Utopia Planitia included.
Minimum methane emission
Based on the footprints, the minimum amount of methane emission from any emission site required by the observed methane spikes can be calculated. TLS’s ~ 0.41 ppbv background level is first subtracted from the six methane spikes. The remainder of the signals must then be a consequence of recent emission. It is unknown whether the emission was continuous, intermittent, or episodic, but to put a lower bound on the required methane emission, we can assume the emission was instantaneous and occurred at the exact moment when an emission site had the strongest influence on a detection. Finally, dividing each methane signal by the maximum footprint at an emission site yields the minimum amount of methane emitted from that emission site required by the methane signal (Fig. 4). Regions that can produce the methane signals by emitting a small amount of methane are well correlated with the regions bearing strong footprints in Fig. 3.
TGO’s 0.06 ppbv upper limit on the background concentration, combined with the 330-year lifetime from standard photochemical models, implies that, on average, no more than 1.8×10− 4 ppbv of methane is replenished every year. During the 4.6 years of TLS operation, then, on average, no more than about 8.4×10− 4 ppbv has been replenished into the atmosphere. Assuming the six methane spikes result from six emission events, then, on average, each of them can emit no more than 1.4×10− 4 ppbv of methane; otherwise, they would have resulted in a significant, and observable rise in the background methane concentration. Only the blue and cyan areas in Fig. 4 are such qualified areas that are able to produce a methane spike with the observed mole fraction by emitting less than 1.4×10− 4 ppbv of methane. This means that without fast removal mechanisms that can significantly reduce the methane lifetime, the methane emission site needs to be located within the blue and cyan areas inside Gale crater. In fact, the assumed situation where only six methane emission events occurred during the 4.6 years and all of them were captured by the TLS measurements is almost impossible. The actual emission event frequency may be much higher than this, which will put a much lower upper bound on the amount of methane emitted by a single emission event. The qualified emission regions will then be confined within much smaller areas, such as the blue or even the dark blue areas on the northwestern crater floor in Fig. 4. However, this suggests a coincidence that Curiosity was sent to the vicinity of a methane emission hotspot. Another possibility that does not invoke the coincidence is that rapid methane removal mechanisms that are not known to date are at work. If the methane lifetime is shorter than 330 years, more methane can be emitted into the atmosphere every year without perturbing the background methane concentration, and the emission sites will have some freedom to be located at distant places outside Gale crater. (Refer to Fig. S10 for a more thorough analysis.)
Consecutive methane measurements
At ~ 266° solar longitude in Mars Year 33, two measurements were consecutively performed within a few hours. The first measurement started at ~ 01:30 local time and detected a 0.332 ppbv signal. Only a few hours later, the second measurement at ~ 06:30 local time detected Spike 6 with 5.55 ppbv. It is possible that the rapid increase in the ambient methane concentration is due to the change in wind direction. Figure 5 shows a comparison of the time-integrated footprints for Spike 6 and the background level. A significant difference can be found between the upstream regions within Gale crater (Fig. 5a, d). There are not significant differences between the upstream regions at larger scales (Fig. 5b, e, and Fig. 5c, f). On the northwestern crater floor, the upstream region of Spike 6, indicated by high footprint values, primarily lies to the west and the southwest of Curiosity rover, whereas the upstream region of the background level primarily lies to the northeast of the rover. Therefore, the region to the west and the southwest of Curiosity in northwestern Gale crater is identified as the most likely location of an emission site.
We note that this method based on consecutive methane measurements is able to precisely constrain the location of an emission site, but it requires consecutive measurements performed within a short period of time, optimally a few hours. Fortunately, the measurement strategy of TLS, which often performs paired measurements within a few hours, meets this requirement.
In conclusion, if we trust the methane abundances detected by both TLS and TGO and accept the 330-year methane lifetime from known photochemistry, our back-trajectory modeling for atmospheric transport strongly supports surface emission sites in the vicinity of the Curiosity rover in northwestern Gale crater. This may invoke a coincidence that we selected a landing site for Curiosity that is located next to an active methane emission site. Another possibility that does not invoke the coincidence is the existence of fast methane removal mechanisms that are unknown to date. Should future studies confirm the existence of heterogeneous pathways or other unknown photochemical processes for methane destruction, the methane emission sites can be located outside Gale crater, and most likely to the north of the crater.
Our study demonstrates the feasibility and the advantages of applying the inverse Lagrangian modeling technique to source localization problems on other planets. Methane abundance data from future in situ measurements, especially those collected in consecutive measurements performed within a few hours, would further improve the source localization.