Curiosity Blog, Sols 4661-4667: Peaking Into the Hollows

By Susanne P. Schwenzer, Professor of Planetary Mineralogy at The Open University, UK

Earth planning date: Friday, Sept. 19, 2025

Curiosity is currently driving along the ridges of a very uneven terrain. One of the bigger ridges we nicknamed “Autobahn,” which is the German word for a highway. But the rover didn’t stay on that autobahn, now more officially named “Arare,” for very long. Instead it went on a trip along several of the smaller ridges and even into some hollows. You can get a good impression of the landscape in the image above, or view a wider panorama image here.

Today, I was science operations working group (SOWG) chair, the one responsible for coordinating all the science planning and making sure we stay within power and data budgets. As we have so much to do with imaging ridges and hollows, and the team members are also keeping APXS and LIBS busy planning to investigate the chemistry of the ridge tops, the sides of the ridges, and of course the rocks within the hollows, the demands on power and data volume are high. Alongside the “geo” observations, we are still in aphelion cloud season and want to make sure we capture enough atmospheric and environmental observations, too. In each plan, the DAN instrument and MARDI camera are regularly looking down. DAN informs us about hydrogen in the subsurface underneath the rover, which is most likely associated with water-bearing minerals. MARDI is documenting the rocks underneath the rover, more precisely underneath the left-front wheel. 

With so many demands, and the fact that we are just coming out of Martian winter, where cold temperatures demand more heating to keep the rover safe, there was lots of demand on the power budgets all of this week. Thus, myself and my SOWG chair colleagues had many discussions to facilitate. What amazes me each time about our team, though, is how smoothly those discussions go and how deep an understanding we all have developed about the seasonality and cadence of each other’s investigations. It is so nice to see how smooth it has become to — as a team — figure out what has the highest priority on a given planning day.

After a range of good discussions, and luck that the rover was parked in a stable position for each planning cycle, we had many arm activities. APXS and MAHLI focused on measuring and imaging the ridge tops — we call them bedrock — and those bedrocks look very smooth on top of the ridges. Targets “Turbio,” “Río Aguas Blancas,” and “Isiboro” were measured and imaged earlier in the week, and today it was “Colonia Santa Rosa” and “Le Lentias.” (I am learning Spanish as we go; all those names are from the Uyuni region in South America.) We entered the Uyuni quadrangle on sol 4573; you can read all about it in the blog post, ‘Welcome to the Uyuni Quad.’ More chemistry investigations were added by ChemCam using the LIBS instrument on a wide range of smoother bedrock, complementing APXS observations in many places, and then adding chemical information from locations that have more variable features such as veins, nodules and fractures.

Mastcam and ChemCam, through its remote imager, are taking images of many different features in the landscape. You can see its variation in the image at the top of the blog. What we are interested in is the variability of all those features, but also how they relate to each other. Are some features always on top of others, or are the veins cutting across the fractures? Those are the questions that we can answer with the images taken from a distance for the wider context, and then close-up to see all the details. We have taken overview images such as the one in the image above, and we have taken close-up images with the remote micro-imager and, of course, MAHLI. Many of those images come from the sides of the ridges as this allows us to see “into” the rock record, and how the ridges are constructed. If you look at the image above closely, you can see some of this yourself. You can spot some patterns, too. The ridge tops are more smooth, mostly at least. And that’s how the “Autobahn” was nicknamed in the first place! The hollows look more rough and a little more chaotic, too.

With all the excitement about the rocks, we didn’t forget the environmental observations. Those include temperature and wind, but also imaging of the atmosphere for its opacity and looking for dust devils. We are just coming out of the season with the least dust in the atmosphere. That allowed us to do a first for the mission: image rocks outside the crater rim, 90 kilometers away (about 56 miles)! We are very excited about those images taken with the remote micro-imager of ChemCam and with added Mastcam context. They show what’s beyond the crater rim, and what will have been the source region for some of the sediments we saw very early in the mission, when we explored the Peace Vallis Fan! Have a look at the wide overview image, and this close-up of rocks, 90 kilometers away, with the remote micro-imager.

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NASA’s 2025 astronaut candidate class greets the crowd in this Sept. 22, 2025, image. The group was introduced Monday following a competitive selection process of more than 8,000 applicants from across the United States. The class now will complete nearly two years of training before becoming eligible for flight assignments supporting future science and exploration missions to low Earth orbit, the Moon, and Mars.

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The Need to Bake Out Silicone Based Thermal Control Coatings

Background
The NASA Engineering and Safety Center (NESC) has reviewed flight, ground test, and published data on ultraviolet-induced degradation of silicone based thermal control coatings. Analysis has shown, for at least one silicone coating, that bake-out plays an important role in ultraviolet (UV) degradation, indicating that UV interaction with paint volatiles, and not the structural material, is the primary source of coating discoloration.

Discussion
Spacecraft temperature is primarily determined by the absorptivity and emissivity of the vehicle’s coating. Absorptivity is the fraction of the sun’s irradiance that is absorbed, and emissivity determines the amount of infrared power that is emitted. The combination of these properties, along with additional heat from internal sources and other external radiation sources, determines the spacecraft’s thermal environment. Choosing an appropriate coating, referred to as a thermal control coating, is key to keeping the vehicle within a desired temperature range. However, these coatings can degrade, (i.e., darken), in low earth orbit (LEO), primarily due to solar UV exposure, complicating the choice of coating. Zinc-oxide (ZnO) scatterers in a silicate binder are among the most stable white coatings but suffer from poor adherence. Replacing the silicate with organic silicone improves paint mechanical properties, but optical property measurements of UV exposure stability for ZnO-silicone coatings have been widely divergent. This led to a request that the NESC resolve the variations to better predict the stability of specific ZnO-silicone coatings in LEO. Testing of coupons began in FY25 and will complete in FY26.

Many silicone-based thermal control coatings have been evaluated in ground simulation chambers and tested in space since the mid-1960s [ref 1, 2], demonstrating a wide range of UV degradation rates, sometimes for the same formulation. Ground testing a particular ZnO-silicone coating in two different facilities yielded degradation rates that differed by more than a factor of 6. This is similar to variations seen in a round-robin test of ground UV exposure facilities in the 1960s [ref. 2] and casts doubt as to the usefulness of ground testing to predict flight performance. In this case, consideration of the differences between the two ground tests along with partial retesting, pointed to the presence of volatiles as the source of the difference. In one facility, the samples were baked out prior to testing, removing most of the volatiles in the paint, but in the other facility the samples were not baked out. This indicated that the primary source of absorptivity change was UV interaction, not with the silicone substrate material, nor with the ZnO scatterers, but with the volatiles. In addition, the two facilities had different UV irradiance spectra, which may have contributed to the large degradation variation [ref.3].

A literature search was conducted and, surprisingly, only one paper was found that tested ZnO-silicone paint degradation with and without a prebake [ref. 1]. In this publication, paint S-31 without a bake-out was exposed to 1780 equivalent solar hours (ESH) of UV and saw a change in absorptivity of 0.02, but a sample that was baked at 260°C (500°F) for 1 hour and then exposed to 1780 ESH saw only a change of 0.006. In a second case, two S-33 samples were exposed to 4170 ESH, both with a one hour 150°C prebake out and one with an additional one hour 260°C prebake. The one with the single bake-out saw an absorptivity change of 0.02 and the one with the additional bake-out saw a change of only 0.011, comparable to the “best zinc oxide…silicate paint.”

Testing of ZnO-silicone paints has been conducted on the Materials International Space Station experiment (MISSE), [ref. 4], showing a similar reduction in UV degradation for samples that were baked out prior to flight and those that were not. In MISSE-19, a sample of a ZnO-silicone paint that was baked out showed a net change in absorptivity of 0.011 (Wake position) versus 0.27 for a sample of the same paint in the Zenith position that was not baked out. There is positional variation that may have contributed to this difference, but the removal of volatiles is a likely contributor.

Finally, spacecraft testing of the same ZnO-silicone paint has shown very low UV degradation over extended periods in LEO which is interesting given that the paint on the spacecraft is not baked out. Aerodynamic heating on ascent is insufficient to remove the volatiles, however, surface temperatures while in orbit are sufficient. On the spacecraft, the paint covers an insulative, micrometeor protective layer allowing the paint to heat in sunlight (unlike the MISSE samples that are painted on aluminum disks mounted to an aluminum tray). This heating in orbit provides a nearly continuous bake-out, removing not only residual volatiles, but newly formed volatiles created by UV induced decomposition of longer chain molecules. Comparing outgassing data to the bake-out conditions further supports the proposition that volatiles within the paint, and not the binder or scatterers, discolor under solar UV exposure. Indicating that prebake or, in-flight continuous baking, is a key requirement for long duration performance of a specific family of ZnO-silicone based thermal control coatings.

References

1. Ref. Zerlaut, G. A., Y. Harada, and E. H. Tompkins. “41. Ultraviolet Irradiation of White Spacecraft Coatings.” In Symposium on Thermal Radiation of Solids, vol. 55, p. 391. Scientific and Technical Information Division, National Aeronautics andSpace Administration, 1965.

2. Arvesen, J. C., C. B. Neel, and C. C. Shaw. “44. Preliminary Results From a Round- Robin Study of Ultraviolet Degradation of Spacecraft.” In Symposium on Thermal Radiation of Solids, vol. 55, p. 443. Scientific and Technical Information Division, National Aeronautics and Space Administration, 1965.

3. ARVESEN, J. “Spectral dependence of ultraviolet-induced degradation of coatings for spacecraft thermal control.” In 2nd Thermophysics Specialist Conference, p. 340. 1967.

4. Kenny, Mike, Robert McNulty, and Miria Finckenor. “Further Analysis of Thermal Control Coatings on MISSE for Aerospace Applications.” In National Space and Missile Materials Symposium, no. M09-0535. 2009.

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You can join the OSDR-Analysis Working Groups yourself and help plan the future of human space exploration. Dozens of project groups are active at any time. Learn more about the AWGs.

B Barbero Barcenilla, R Rivero, A Lynch, W Cromer, J Gong, B Harandi, M Stegmann, H Le, D Lundine, M Chung, J Puig, K Mikhailova, H Coker, A Marks, Rachel R. Gilbert, Ryan T. Scott, R Barker, P Glowe, EG Overbey, CE Mason. (2025) Feeding the cosmos: tackling personalized space nutrition and the leaky gut challenge. npj Microgravity 11, 45. https://doi.org/10.1038/s41526-025-00490-z

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