Mars is made of Swiss cheese

  

If the Moon is made of Green Cheese, then what cultured dairy confection makes up Mars? Why Swiss Cheese, of course! This week, Alex takes us on a tour of the pitted south polar terrain of Mars whose interplay of sunlight, water and carbon dioxide ices result in something that looks visibly similar to Swiss Cheese. Naming planetary terrains after food is not new, nor is it limited to the inner solar system. If you were putting together a platter of hors d'oeuvres, Cantaloupe makes an excellent accompaniment to Swiss Cheese. Perhaps we will have to take a closer look at Neptune's moon Triton in the future...

By Alex Innanen

Long-time PVL blog enthusiasts may recall that my planetary journey began at the Martian north pole looking at many, many HiRISE images. Over the past year I’ve returned to the Martian poles – the south pole this time.

Both poles have layered deposits of mostly water ice and dust, and residual water ice caps left behind when the winter layer of CO
2 ice sublimates in the summer. The south polar residual cap (or SPRC for the acronym fans) is mostly made up of carbon dioxide ice as well, overlying water-ice. The terrain of the SPRC is as varied as the North pole, but has some features that are unique to it. One of these are circular or circular-ish pits with steep sides and flat bottoms. The terrain they carve out is similar to a piece of Swiss cheese, giving the features their nickname. 

The distinctive pits of Swiss cheese terrain, from the HiRISE instrument.
[NASA/JPL/University of Arizona]

In Swiss cheese – the kind you can eat – the distinctive holes are formed by carbon dioxide bubbles that are released by the cheese-making bacteria. The Swiss cheese features of the SPRC are much larger than the ‘eyes’ in a piece of cheese – on the order of tens to a few hundreds of metres in diameter. No bacteria are forming these holes, instead they’re likely formed from fractures in the residual cap, which are widened into pits through sublimation from their walls. In the southern spring and summer, the steep, dark sides of the pits get more sunlight than the flat floors, causing the walls to sublimate and grow outwards by a few metres per year.

If the pits grow large enough, they can even grow into each other, creating intricate, branching features that can cover large swaths of the residual cap, like you can see in the HiRISE image here. It’s been suggested that based on this rate of growth, every century or so the entire SPRC could be entirely carved out by Swiss cheese features, causing a total resurfacing. 

[NASA/JPL/University of Arizona]

The Swiss cheese features occasionally show more ephemeral features such as bright, surrounding halos or dark fans emanating from higher standing areas. There’s a fairly clear halo around the feature shown at the top of this post – sometimes nicknamed the ‘Happy Face’. It looks almost like the feature is glowing, but what we’re really seeing is a localized region of higher albedo (i.e. more white) surrounding the Swiss cheese feature. These halos have only been observed during the Southern summer of Mars year 28 (2007, for Earthlings), and their appearance happened to follow a global dust storm. It’s likely, though, that these halos aren’t actually a ring of material getting lighter, but rather the SPRC as a whole getting darker from settling dust, except in the areas close to the pit walls. The mechanism that was proposed to explain this in a 2014 paper, is that the sublimation from the pit walls that I discussed above raises the amount of CO2 in the atmosphere and pushes the settling dust from the storm away from the edges of the pits. Lower rates of sublimation on flat areas allow the dust to settle normally.

The dark fans are much smaller and harder to pick out of even HiRISE images – on the scale of 1-10 m². They tend to appear at the edges of high-standing areas, ‘fanning’ into the lower areas. They appear in the southern spring, and unlike the halos they have been seen over multiple Mars years. Moving into the summer, as CO
2 ice sublimates, the terrain around the fans darkens until the fans disappear. Their formation is also much more exciting – they’re formed when jets of gas rupture through the CO
2 ice layer, lifting dust and depositing it outward in the fan shape. Dust can then get trapped in layers of ice, making it darker, absorbing more sunlight, and leading to more sublimation, creating more trapped gas to explode out and create more fans.

Until now I’ve been talking about CO
2 ice which makes up the majority of the SPRC. But what about water ice? The polar layered deposits are composed mostly of water ice and dust, and in the Southern summer the SPRC shrinks and exposes some of the water ice of the south polar layered deposits. It is possible that the flat floors of Swiss cheese pits also expose water ice in the summer. There have been detections of water vapour associated with the pits, but this could also be from their walls, which could be layers of CO
2 and water ice. In either event, the work I’ve been doing looks as if it is possible for the water ice in the Swiss cheese pits to have any appreciable contribution to atmospheric water vapour. The polar caps are the major source of surface water ice, and the yearly formation and retreat of overlying CO
2 ice, exposing water ice, drives Mars’ water cycle. I’m interested in finding out how much, if any, water vapour could be released from the Swiss cheese pits, and in the event of most or all of the SPRC being removed by Swiss cheese pits, whether this could have a significant impact on the amount of atmospheric water vapour.

Modelling the atmosphere of K2-141b: June update

 

A model of a planetary environment doesn't spring forth in all of its detail. Typically we start with the simplest model that captures the essential physics, but which also leaves out important details. Sometimes the description of such a model even fits on the back of an envelope! We then build in the complexity piece by piece. This is a process that PhD student Giang has been pursuing over the past couple of years as his models of K2-141b becomes ever more sophisticated. At each stage, we learn something new as we proceed from a solution accurate to a particular order of magnitude, to a 10% level solution to a 1% level solution. There is benefit in the complexity - but it's important not to outrun the data by too much. If we make a prediction or add a minor process that cannot be verified through the data, we run the risk of inventing stories about these worlds that are mere delusions.

By Giang Nguyen

In my previous post, I showed what happened when I introduce UV radiation absorption to K2-141b’s atmosphere. The results from the model went bizarre as the atmosphere kept heating up to essentially become plasma. Although numerically sound within our mathematical construct, this ultra-hot atmosphere simply isn’t realistic as that would make the atmosphere on the planet even hotter than its star.

As I suspected, there was an issue with how I dealt with radiative cooling. The original way for the atmosphere to cool would be exclusively through infrared emissions. Although most of the energy does radiate in the infrared wavelengths, the emissivity of silicon monoxide in that spectral range is very small compared to UV light. Therefore, there is some UV emission that is unaccounted for that would significantly cools the atmosphere.

The solution to this problem is to separately calculate the blackbody radiation of the atmosphere in both infrared and UV. This is done by integrating the Planck function over the desired wavelength range and multiply it with the corresponding emissivity. Here’s the thing with blackbody radiation, especially for hot temperatures of thousands of kelvins. Most of the radiance comes from a very small sliver of wavelengths, and it is pretty much negligible in comparison at every other wavelength. Therefore, when you have low spectral resolution, the estimate of the radiance becomes very inaccurate once you do your integration.

My next step was to do the Planck integration separately solely as a function of temperature with adequate spectral resolution and then to fit that integration to a polynomial. As the integration process now becomes a single line of calculations instead of a bunch of for loops, we’re back to our old speedy model. However, we are at the mercy of our fit coefficients and it seems that our temperature range is too large for a polynomial fit to be accurate; note that our temperature can range from 0 – 3000 K.

All hope seemed to be lost. I was going to have to run the slow model which I estimate will take weeks to pump out a solution, which might not even be correct solution. Thankfully, some scientists in the 1970s ran into the same problem and were able to solve it themselves. When you integrate the Planck function by parts, you end up with an infinite sum (a little bit of math identities is needed here as well). Computing this infinite sum is much faster than the classic way as this sum converges much faster. Finally, with the Planck finite integral taken care of, we can deal with radiative cooling.

As expected, UV emissions capped the temperature of the atmosphere – but it was still hot. The temperature hovers around 2900K across the dayside almost uniformly. Because UV emission only becomes significant when the atmosphere is hot, it never forces the temperature to drop further at low temperatures. When UV absorption and emission cancel each other out at a specific temperature, a very stable sort of radiative balance occurs. This turns out to be important as the atmosphere becomes too thin for IR radiation to take effect.

A warm SiO atmosphere is expected, but for it to be so horizontally consistent and warmer than the surface is a surprise. A welcoming surprise. For emission spectra, a warmer atmosphere means a brighter signal. Using SiO spectral features, we could ultimately see K2-141b’s atmosphere instead of the ground beneath it. Also, the scale height is thicker, even near the terminator (where on the planet you would see the star on the horizon). This means that during a transit, the planet’s atmosphere is optically thick enough to absorb the star’s light that travels through that atmosphere on the way to Earth. With supersonic winds, this might induce an observable Doppler shift when measuring K2-141b’s transmission spectra.

Ultimately, when considering UV absorption and emission, the atmosphere on K2-141b is easier to detect, for either low-resolution and high-resolution spectral instruments. This is very good news as K2-141b is slotted for observation time with the James Webb Space Telescope (JWST). Along with possible future observations from ground-based telescopes, we may definitively detect and characterize K2-141b’s atmosphere - a first for terrestrial exoplanets.

This concludes my update for my current research project. Using a convenient numerical method to evaluate definite Planck integrals, we solved the problem of dealing with K2-141b’s atmospheric radiative cooling. The resultant atmosphere with the full radiative transfer is almost uniformly hot across the planet’s dayside. This suggests that K2-141b’s atmosphere is a lot easier to detect than anticipated. This is exciting as K2-141b is a high valued target for observation, and it might be the first terrestrial exoplanet where we have observed an atmosphere. Although a small step, it is still a step towards finding habitable worlds and life beyond the solar system.