The Mars Global Dust Storm of 2018: how dusty was it really?

This past year we've been treated to a once-in-five-years event, a Global Dust Storm on Mars. Curiosity has had a front row seat and PVL PDF Christina has been right there, through it all: designing observations, acquiring images and analyzing them to see just how dusty everything gets in Gale. The TLDR? very very very dusty.

by Dr. Christina L. Smith

Some of the members of the Planetary Volatiles Laboratory are also members of the Science Operations Team via Prof. Moores’ Participating Scientist proposal (thanks NASA and the Canadian Space Agency!). As part of that we get to participate in operations roles (aka be part of the team that plans what the rover does on any given day – which is super cool in case you were wondering :) ). But also we get to use the data that comes back and, if the science case warrants it, propose new observations.

For me, that means monitoring the dust using images that we take using two different cameras. One is Curiosity’s Navigation Cameras (we call it “Navcam”) - not technically one of the science instruments as they are primarily for navigational and engineering uses. But, luckily for us, these cameras are also scientifically calibrated so we can happily use them for science! The other camera I use data from is Curiosity’s Mast Camera (we call it “Mastcam” - sense a theme here?) and that one is a colour imager so we get more information about the colour than with Navcam’s images as Navcam’s images are taken only in the reddish region of visible light. But Navcam is more sensitive than Mastcam, so they complement each other really nicely.

The observation we at PVL use to monitor dust within Gale Crater (where Curiosity is exploring) is an image of the crater rim. One thing you have to know is that Gale Crater is massive – it’s 154km wide and the northern crater rim is roughly 30 km away and is a couple of km high so really it looks like a chain of mountains in the pictures. Using these pictures we can use some pretty cool relations (and if you want to delve into the science behind why we can do it, I refer you here: https://www.sciencedirect.com/science/article/pii/S0019103514004825) between how bright the sky just above the crater rim, the crater rim itself, and the ground near to the rover are, to calculate how much dust sits between Curiosity and the edge of the crater rim.

Why do we want to know that? Well, finding out how much dust there is in Gale is really relevant to the atmospheric circulation and mixing within and around Gale Crater. This crater (see the topographical map – red and white are really high, and blue is really low) has some really interesting topography, including a 5 km high mountain in the middle, which really changes the way that the atmosphere circulates and mixes and, as atmospheric scientists, that’s something that is really of interest to us! And by comparing how much dust is in Gale Crater to how much dust is in the atmosphere above the crater, we can actually get some really interesting insights into how the atmosphere behaves.

  

We measure the amount of dust using something called “extinction” which tells you how much light is scattered or absorbed by the dust, and we use the extinction per kilometer rather than total amount between the rover and the crater rim because of course she drives so we need to use something we can compare easily over the course of her travels. These values vary seasonally (yes, Mars gets seasons too!) between about 0.04 and 0.1 per km. 0.1 is pretty dusty, 0.04 is pretty clear.

Now, I’m sure many of you have heard about the gargantuan global dust storm that happened in 2018. But, just in case: roughly 1 in 3 Mars Years (5.5. Earth years-ish), a dust storm happens that is so big that it wraps around the whole planet. It’s not really well understood why these occur in some years and not others, so it’s really important to study them whenever we get the chance. And 2018’s was a doozie. Luckily for us, Curiosity isn’t solar powered so didn’t suffer the same fate that Opportunity did (RIP Oppy!), and we were actually able to take measurements from the surface throughout this massive global dust storm. EXCITING.

So what did it look like during the dust storm? Well, take a look at these pictures: the first was taken before the dust storm began and the second was around the peak. Visibility reduced down to about 2-3 km, which is a massive drop – remember those “mountains” of the crater rim are about 30 km away. And in fact, we actually measured that the extinction inside Gale Crater increased by a factor of ten!

I’ll repeat that because it’s important: TEN TIMES MORE.

That is massive.

Remember normal values are 0.04-0.1. We hit 1.1. One. Point. One.

Similar increases were seen in the dust above Curiosity and Opportunity, but that’s someone else’s tale to tell!

And if you want to know how quickly that dust rolled in, take a look at the animation at the top of  this post and, just so you know, sol is the word for Martian day :)