Readers not interested in cultural musings used as introduction to today’s post, are kindly requested to skip the part written in italic below and proceed directly to the next paragraph.
With the Epiphany behind us, we can say that the period of Christmas festivities is now over. It’s interesting to note that in my part of the world, each year we revert more from using Christian symbolic of, well, Christmas, to pagan Germanic tradition of Yule. (Or, to be more specific, we seem to be doing away with the Christian part of the symbolics, so only the pagan part remains). Now, I am not going to get into the Yule-vs-Christmas discussion, because first, the subject has been widely covered already, and second, this is, after all, a blog about astronomy, and not culture or religion. But I can’t help noting one very ironic twist. Although one can say that Christmas is not a native holiday in my country, so shifting focus back to Yule symbolics is coming back to one’s cultural roots, the joke is that Yule isn’t native here either. Depending on the exact location, actual Slavic holiday celebrated around winter solstice would be Święto Godowe (here’s a Wikipedia article on it, readable although mutilated by Google Translate), Kračún or Koleda. That’s not like these customs were completly lost, though, as some parts of it were simply incorporated into the Christian rituals. But as we are moving away from Christian character of midwinter celebrations, and the major deity nowadays is an overweight, bearded dude in a red coat, affiliated with the Coca-Cola company, there’s a good chance that this part of my cultural legacy will die out completely in my lifetime. So if you want to know what the world will be missing by the time humans return to the Moon, you can have a look here and here.
Regardless of what particular religious (or commercial) holiday someone celebrates at this point of the year (or spends at work, like me), one must remember that midwinter festivities are in fact astronomical in nature. After the winter solstice, northern hemisphere days become longer and so the world is again moving from darkness into light. And that transition from darkness to light (and back) brings me to the topic of today’s post.
Illumination data are very important from the perspective of future surface operations. Since the Moon has no atmosphere, there is no ambient (scattered) light. A given point on the surface is either directly illuminated by Sun (i.e. you can see the Sun if you are there), or it is not. An illuminated region will be hot (+107°C); a dark region will be cold (-153°C). In the equatorial regions (where Apollo landed) the Sun is high above the horizon during the day, so everything is illuminated (and hot); at night, nothing is illuminated (the Sun is below the horizon), so the temperature drops.
Polar regions are different. There, the Sun grazes the horizon all the time. (Since the tilt of the Moon’s axis is minuscule, there is no polar day / polar night effect we have on Earth). So, if you stand on a mountain peak you can see the Sun all the time, going around you. This situation is known as a peak of eternal light. On the other hand, if you dive into a deep crater, you never see the light. Of course, the whole thing is a bit tricky: remember, there is no ambient light. So if you happen to be high up, but a mountain obstructs your view of the horizon, you will be in darkness when the Sun happens to be behind the mountain.
Now, assume that you want to build a base. What would be a good spot to do so? The equatorial regions are not a good choice; the 250°C temperature difference between day and night makes engineers nervous. On the other hand, a peak of eternal light would be a nice place. Permanent illumination ensures that the thermal environment would be stable — around -50°C. Also, it provides access to abundant solar energy. (Although in practice you have to erect your solar panels vertically and rotate them following the Sun, which complicates matters a bit).
But wait a moment. We also know that the permanently shadowed regions harbor water and other interesting volatiles. Wouldn’t it be better to set up a base there? Not really, because at ca. -200°C the conditions are not really likable. So what does one need? Well, of course: a well illuminated region near a permanently shadowed region!
Meet the Shackleton crater:
(for a larger view, click the image or download a full resolution 300dpi PDF with description).
The above map combines the illumination data from LRO (linked above), expressed as shades of gray with the Kaguya laser altimetry data (red isolines) and some annotations. I have drawn isolines only every 250m, to avoid too much clutter inside the crater.
Letters A, B and D mark the areas which are illuminated at least 80% of time. These locations have been identified by Bussey et. al. (see the paper here) by creating a relief model of the terrain and performing a computer simulation of solar illumination; see the paper for details. Also, Paul Spudis’ site contains an iconic image of these interesting locations marked over the Kaguya “Earthset” photography. (The “C” location is too far from Shackleton to be included on my map.)
I must note that a major discrepancy exists between the LRO image and the Bussey et.al. paper. The LRO image is 8-bit grayscale image (i.e. values between 0 and 255) with actual pixel values between 2 and 254. So, logically,the value of 254 would correspond to 100% illumination (peak of eternal light) and 2 to 0% illumination (eternal darkness). At the same time, Bussey et.al. say that the best illuminated point (D) receives around 86% of illumination on average. Since there is no additional information to resolve this, I have chosen to render the LRO data in 0-100% range anyway.
One can now easily see the attractiveness of the “A” spot for mission planners. The following image, taken from a BBC article about the ill-fated Constellation program, confirms this expectation:
As for getting the volatiles from the crater floor, the matter is a bit tricky. A quick look at my map tells you that the crater floor has the elevation around -2500m and the “A” spot is at +1500m. This produces a 4000m difference in elevation over a horizontal distance of about 8km. That means an average slope of 26 degrees or 46%. The upside is, we have engineered such systems on Earth already. This is Pilatus railway in Switzerland, world’s steepest cog railway, climbing a track with 38% average and 48% maximum slope.
The downside is, building something like that on the Moon is pure madness. It would operate in hard vacuum, low gravity and have to be able to survive 150K temperature difference between the crater bottom and its rim. And there will be no industry around to supply the materials.
I have no idea what will be used to transport the water out of the lunar cold traps, but I am sure that it will require some brilliant engineering.
- Spudis et.al., Geology of the South Pole of the Moon and the age of the Shackleton crater
- Spudis et.al., Geology of Shackleton Crater and the south pole of the Moon
- Shackleton area imaged by SMART-1 (includes potential landing sites)