Researching planetary atmospheres

Researching planetary atmospheres
© Johns Hopkins APL

Scot Rafkin, program director in space science & engineering at Southwest Research Institute, talks about some of the latest development in planetary atmospheres research.

Speaking with Dr Rafkin, we learn more about his research regarding planetary atmospheres with a specialisation in numerical weather prediction, mesoscale modelling, atmospheric dynamics, and cloud microphysics. He has extensive experience developing weather and research modelling codes and has applied these to the study of weather, clouds, and aerosols for Earth, and to other terrestrial planetary atmospheres including Mars, Titan, Venus, and Pluto. NASA has selected Rafkin and his team to provide atmospheric predictions for every Mars rover and lander since 2003. He also develops and leads proposals for space instrumentation and missions for remote and in situ atmospheric measurements, currently serves as the MSL-RAD Instrument Project Scientist, and is a science Co-Investigator on the MSL Rover Curiosity Meteorology team and the NASA Dragonfly mission to Titan.

What would you say has been your main inspiration behind researching planetary atmospheres?

I have always been fascinated by the weather. Some of my earliest memories are pressing my face up against the window staring at snow storms, thunderstorms, and clouds (or running outside to be in them). I always excelled at the physical sciences in primary and secondary school, and when the time came to pick a major in college, I flipped to the appendix of the university catalogue and began to go down the alphabetical list looking for just the right scientific study program. Anthropology: Interesting, but no. Astronomy: More interesting, but no. Atmospheric Science: That’s it! I closed the catalog and never looked back.

I received my B.S. in Atmospheric Science from UCLA and went to Colorado State University for my M.S. and Ph.D. After a short postdoctoral fellowship at the University Corporation for Atmospheric Research, I took a tenure-track position in the Meteorology Department at San José State University in California. At that point, all my education and research endeavors were focused on Earth weather.

The NASA Mars Pathfinder mission arrived at Mars in 1997, shortly after I began my faculty position. That mission carried a small meteorological station, and it caught my attention. I was looking to establish an atmospheric research niche, and Mars atmospheric research struck me as an opportunity to break out from the crowded terrestrial research environment. There were still lots of ‘big’ questions about Mars to sink my research teeth into.

As luck would have it, the NASA Ames Research Center was just a short drive up the road from the University, and the research group at the Center led by Dr Robert Haberle was comprised of numerous experts in Mars weather and climate. After some discussions about possible research collaborations, he agreed to provide a small amount of funding to explore the modification and application of the numerical weather research model that I was using for the Earth to study Mars.

That seed project was a success and the rest is history. After demonstrating the viability of my Mars model, I submitted a proposal to NASA’s Planetary Atmosphere Research Program.

That proposal was selected and sent me down the road of additional proposals and successes. Within a few years I had mostly abandoned my Earth research and was fully engaged in Mars. Since then, I have expanded my research to other so-called terrestrial atmospheres (those planetary bodies with rocky surfaces and atmospheres such as Venus, Titan, and Pluto).

Can you give us a brief introduction into the investigation of planetary atmospheres, and what this involves?

The study of any atmosphere, whether Earth or extraterrestrial, involves the application of fundamental physical principles in an attempt to understand the observed weather and climate. Atmospheres are all governed by the same physics, but the importance of the various processes can be dramatically different between different planets.

For example, the condensation and freezing of water is a key process on Earth that results in clouds, rain, and snow, but on Saturn’s moon Titan, it’s so cold that water is frozen as hard as the rocks on Earth. Instead, methane can condense on Titan to form clouds and rain. Thus, the same basic cloud processes apply to Titan as they do for Earth, but the details are different. I solve the complex mathematical equations that describe the physics of atmospheres in an attempt to understand the underlying mechanisms behind the phenomena observed by telescopes and spacecraft.

In terms of the development of weather and research modelling codes, can you give us an example of how you have applied these to terrestrial planetary atmospheres?

Adapting modelling codes originally developed for Earth to other planetary atmospheres necessitates modifications on a number of levels. The simplest changes require changing the value of parameters appropriate for Earth to those appropriate for the planet in question. For example, those parameters associated with gravitational acceleration and atmospheric composition require only simple code changes.

Also, different physical characteristics of the planet such as topography, albedo (the reflectivity of the planet’s surface), and physical properties of the surface need to be appropriately modified. Basic calculations, such as the planet’s location in space with respect to the Sun and the amount of sunlight reaching the top of the atmosphere as a function of orbital position, need to be adjusted. Up to this point, most of these modifications are relatively straightforward.

Some planets (e.g. Mars), involve the development and incorporation of physical processes that are not found on Earth. For example, over 25% of Mars’ carbon dioxide atmosphere condenses onto its poles during the winter. Nothing like this happens on Earth. This process must be included for Mars whereas it’s of no concern for Earth. Some processes that occur on Earth must be modified for different planets. For example, water condenses and freezes to form clouds on Earth. Methane on Titan can also condense and freeze, but the details of those processes are slightly different, and those differences must be incorporated into the equations representing those processes in the model.

In some Earth models, particularly older codes, various assumptions or simplifications were made in order to make the models computationally feasible. Running weather and climate models is extremely computer intensive, and the speed at which the model runs is an important consideration. What may be perfectly valid and acceptable for Earth may not necessarily be appropriate for other planets. In these cases, removing the simplifying assumptions and incorporating a more complex representation of a process is required.

These changes can be the most difficult; for example, the heating of the dusty Mars atmosphere by the sun can induce daily pressure changes that are equivalent to the daily passage of a strong hurricane. The heating of the atmosphere by the sun on Earth does no such thing, and the equations that describe the motion of the atmosphere of Earth can use this fact to ignore or approximate the relatively small effect. Ignoring or even simplifying the representation on Mars results in grave errors. Sometimes it is not even possible to incorporate the needed changes in a given model. In this case, it is necessary to choose a different model as a starting point.

Can you tell us about some of the recent projects you are working on and why they are important?

I have a diverse portfolio of projects that range from modeling studies of Mars and Titan, to the development of instrumentation to measure and study atmospheres, to full mission proposals for Earth, Mars, and beyond. Current Mars projects involve the study of dust storms in order to better understand how they form, what their properties are, and how they might affect future robotic and especially human missions.

I am also looking at how contaminants are transported by the atmosphere on Mars. This is important for understanding how Mars missions might contaminate Mars with Earth organisms that have hitched a ride on spacecraft, but also how any Mars contaminants might find their way to future human martian outposts or even arrive to Earth as part of a Mars sample return mission.

My recent work on Titan is focused on the dynamics and evolution of methane storms and the interaction of the atmosphere with Titan’s liquid methane lakes. Titan’s methane storms are similar in many ways to Earth, but they are three to five times taller, and can regularly drop over a meter of methane rain over a very broad area. Titan’s lakes appear to drive sea breeze circulations similar to those on Earth, and the processes controlling evaporation of the lakes is an important component of the overall global methane cycle.

My most recent instrument project is a laser spectrometer that can take a sample of a planetary surface or planetary ice and quickly measure the volatile gases (e.g. water, methane, carbon dioxide, etc.) that it contains. The composition of surfaces has important science implications, but it is also crucial for future human exploration of the moon, Mars, and asteroids. These volatiles will need to be used as resources to support long-term human missions.

During your time at SwRI, what would you say has been the most interesting discovery so far?

The most interesting finding has not been a single eureka moment, but rather the accumulation of multiple findings that all point to the importance of small and so-called mesoscale (middle-scale) processes to the global circulation and energy cycles of every planet that I’ve studied. The importance of these smaller scale processes has been recognised for Earth for more than half a decade, but the realisation that they are also important on other planets has been a slow paradigm shift that began in earnest only ten or so years ago.

Prior to this shift in understanding, the atmospheres of Mars, Titan, Venus, and even Pluto was limited to how the atmospheres behaved on large, global scales. Thanks to a combination of missions making detailed atmospheric measurements and improved modeling of the atmospheres, we now know that taking only a global view of the atmosphere provides an incomplete picture.

For example, the dust distribution in the atmosphere of Mars was found to exhibit a persistent maximum in concentration at 20 to 40 km above the surface. This dust distribution cannot be explained by the global circulations of the planet. It can, however, be explained by the local injection of dust high into the atmosphere by local dust storms and the strong daytime upslope circulations along Mars’ steep topography that transport the dust high into the atmosphere.

Because Mars’ climate is so strongly controlled by the solar heating of dust, being able to explain the distribution of dust is essential to fully understanding the Mars climate. It is small and mesoscale process that feed into the dust distribution that ultimately controls the global climate.

One of the greatest challenges to continued scientific advancement and discovery is not the science itself, but the federal research structure that supports it. Research budgets are set annually subject to the whims of the executive administration and congress. Budgets and priorities that shift dramatically and to some degree, randomly, play havoc with research programs that best thrive in a long-term stable and more predictable funding environment.

Many key findings are years in the making and this work is discordant with a funding environment that changes on short time scales. Planetary missions can take two or more decades to go from concept to launch, and these are particularly vulnerable to budgetary uncertainty.

Scot C R Rafkin
Program Director
Southwest Research Institute
+1 210 684 5111
Tweet @swri

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