Just another dusty day on Mars! This week's PVL blogpost discusses what we can learn about the atmosphere of Mars from images like this one.
by Grace Bischof
In 2021, I gave a research update about the project I was working on for my MSc involving modelling water-ice cloud thermal emissions at the Phoenix landing site. At that time, I was just shy of my one-year anniversary at PVL. Now nearly two years into my PhD, and only a few short months away from my 4-year PVL anniversary, I figured it was time to give another research update.
Over the last couple of years, I have moved away from the Martian arctic where Phoenix operated down to the equatorial region of Mars, where the Curiosity Rover is exploring Gale Crater. To shake things up further, instead of studying water-ice clouds like at the Phoenix landing site, I switched gears and am now focussing on the temporal and spatial variability of dust in Gale Crater.
Now, why is it important to constrain the behaviour of dust on Mars? Dust is ubiquitous in the atmosphere of Mars and is largely responsible for giving Mars its nickname of “the Red Planet”. Dust particles are suspended in the atmosphere at all times of year and at all locations on Mars, though the specific amount of dust in the atmosphere varies on a repeatable, yearly cycle. The thermal composition of the atmosphere is strongly driven by radiative interactions with airborne dust – in fact, before including dust in their calculations, scientists were unable to reproduce temperatures seen on Mars because they were missing a vital component of Mars’ heat budget.
To put the importance of studying dust into a broader perspective, we can imagine a human-led mission to Mars attempting to land on the surface. One of the most challenging aspects of landing spacecraft on the surface of Mars is navigating the spacecraft through the atmosphere, where uncertainties in the thermal structure and turbulence pose a threat to spacecraft reaching the surface. Additionally, planet-encircling dust storms, which are still not well understood, can prevent landings. Once landed, statically charged dust can coat electronics, as well as decrease the efficiency of solar panels by reducing sunlight. With all these hazards, having a strong understanding of the behaviour of dust is not just important for understanding the climate and meteorology of the planet, but it is also crucial for humans who will one day land on the surface.
So, back to my research. While my work will not solve every problem posed to human-led missions, this work helps to characterize the behaviour of dust in Gale Crater, which aids in better understanding the dust cycle as a whole. For this project, I have been using Navcam images taken by the Curiosity rover. These images are single-framed, north-pointing images that capture the sky, the ground, and Gale’s northern crater rim. By analyzing the radiances of each of the three sections, we can calculate the amount of dust in the air between the rover and the crater rim, also known as the optical depth. By dividing the optical depth by the distance to the crater (since the distance changes after each time the rover drives), we get a value of dust extinction in the crater along a line-of-sight (LOS).
The LOS extinction has been previously studied by prior members of PVL, Casey Moore and Christina Smith, through sol 2500 of the mission. I have since updated the record of the LOS extinction to sol 3663, which is through the end of Mars Year 37. In contrast to previous LOS studies, we used images captured throughout the entire day to discover a diurnal cycle in the dust extinction at Gale. In the past, only images taken between 10 am and 2 pm were used because the analytical method to calculate the extinction made assumptions that did not hold outside of that timeframe. In my work, we updated our analysis method to factor in the location of the sun in the sky throughout the day, allowing us to use all of the images which contain the ground, the crater rim and the sky (typically captured between 6 am and 6 pm).
The diurnal trend in the LOS extinction shows that the amount of dust in Gale Crater changes throughout the day. The dustiness is lower in the morning and increases into the midday. At noon, the dustiness reaches a maximum, and then decreases into the evening. The diurnal variation also includes a seasonal component – that is, in the winter, the change over the day is smaller than in the summer, when there is a large difference in dust loading between morning/evening and noon. Diurnal and seasonal trends in surface dust-lifting caused by wind-stress and convective vortices aligns with what is seen in the LOS extinction, suggesting that much of the change in dust loading in Gale Crater is due to dust-lifting from the surface.
I wrote up these findings and submitted to a journal. Currently that manuscript is in the review process. I presented this work at LPSC in March and will soon be presenting it at the 10th International Conference on Mars in Pasadena in July. In addition to the pattern in dust extinction, there are a couple other interesting things that we found and we will have things to say regarding geographic variability in the dust extinction. But I don’t want to give too many spoilers, so you can read all about it once the paper is published, or come to my talk in Pasadena!
