An image of our ABB methane detector deployed at Gypsum Hill on Axel-Heiberg Island in Nunavut. Alex's work here showed that the variability in a measured methane signal might be able to tell us more about our distance from the source than the total amount of methane does. This is important for how we might prospect for methane seeps on Mars.
Oh, and look at that view!
Sometimes it's not just the results of our investigations that take our breath away.
by Alex Innanen
Almost three years ago now (and wow, time really flies) I spent three weeks in Nunavut, which you can read all about here. I talked a little in that post about why I went up and what sort of work I was doing there. But the work did not end when I landed back in Ottawa (or got back to Toronto after an extended weekend at the cottage). No, I then spent the next several months going “I guess I need to write this up in a paper somehow.” This was complicated by a few things – the fact I hadn’t ever written a paper based on fieldwork (nor read many), the fact that the results were not super clear cut, and some good old fashioned procrastination. But I ended up presenting the work a few times, including at my yearly research evaluation meetings and at a couple conferences, and it started to come together into some kind of story.
When I took methane measurements, I let the instrument ingest the air passing over for ten minutes, and the instrument took a measurement every second over this time period. This meant I ended up with what I took to calling a ‘spiky plot’ of hundreds of methane measurements over that ten-minute period. I noticed two things in these ‘spiky plots’. The first was that I could find the average methane concentration over that period, and that the average methane concentration tended to be highest right next to the source of the methane and drop off as I moved away downwind – typically the way you expect methane (or any gas) to work, which if nothing else meant the instrument was working. The other thing I noticed was that the variation in how spiky the spiky plot was was also higher right next to the methane source. That is to say, the methane signal varied over a much larger range when I was closest to the source, and had a much smaller range further away or upwind of the source. You can see this in the three graphs below which I took at one of the springs.
Three spiky plots. You can see that the upwind measurement has not only a lower average concentration (dashed line) but also is much, much less spiky (solid line) than the other two. Note that the y-axis is much larger on the 'Inside Wolf Spring' measurement because I saw such huge spikes of methane!
I saw this same phenomenon with the variability getting higher closer to the source even when I wasn’t moving in the exact same direction as the wind. At Wolf Spring I only moved in a (mostly) straight line in the wind direction, but at Gypsum Hill I took two sets of measurements – one along the wind direction, and one at a diagonal to the wind direction. This second set of measurements suggested that getting more data at various locations around the methane source could give us a clearer understanding of how methane behaves in a two-dimensional grid around such a source.
To that end, I sent the instrument back up to the arctic last summer in the company of an MSc student from McMaster with detailed instructions to get me a grid of measurements around Wolf Spring. My procrastination had achieved one thing – I was able to add this new dataset into my paper. And I’m glad I was! From the 2024 measurements I was able to see to impact both distance from the source and the angular distance I was from the wind direction had on the methane signal. (I’ve visualised the geometry simply below in case it’s not clear what I mean, where θ is that angular distance from the wind direction.)
Now, in 2022 I did not have any way of accurately measuring the wind direction. Instead I used a technique which is actually similar to how the Phoenix Lander did it, wherein I held up a roll of flagging tape and watched which way the wind blew it. In 2024 we were a bit more high-tech: the master’s student had access to a small weather station which gave me actual numbers for my wind direction. Knowing the position of the instrument at each measurement and the wind direction at the time of the measurement, I was able to get the distance from the source (d) and the angle of the instrument to the wind direction (θ) and combine these (d/cos(θ)) and compare this value to the average methane concentration and the variability in the measurements. I found that both fell off with increasing d/cos(θ) (or distance from the center of the methane plume), but that the variability actually fell off in a slightly more predictable way.
Okay, you may be thinking, this is all mildly interesting but what does this have to do with planetary science? Well, as has been discussed on this blog before, there’s a lot we don’t know about martian methane. One of the unanswered questions is where it’s coming from – both in the sense of what is producing it, but of more interest to this work, the actual location from which it is being emitted. We know that we see methane plumes on Mars, but we don’t know how long they last, how the behave or, again, where they’re coming from. If we did send an instrument to Mars to investigate this, we could use what I learned in the arctic to determine what that instrument should look like and also how we should use it to find the source of these methane plumes.
I learned that the variability is a better indicator of how close we are to a methane source. The variability I saw in my spiky plots is over very short timescales, thus our hypothetical instrument should be able to make high frequency measurements to capture changes over these short timescales. I also learned that knowing the wind direction is pretty important, so our instrument should be combined with some kind of wind sensor. My measurements were taken from various locations around the methane source, so having our instrument on something that can move like a rover (or even a drone!) may be more useful than if the instrument just stands still.
There’s more I could say about this, but I don’t entirely want to spoil my paper (coming soon to an Acta Astronautica near you!). Even though it took nearly three years, it turns out there was quite a bit to learn from a few slap-dash methane measurements in the very distant north.
To read the paper, visit: https://www.sciencedirect.com/science/article/pii/S0094576525003212
No comments:
Post a Comment