With this week's waxing Moon, we set off to explore its volcanic past with a look at a dozen intriguing lunar domes.
In the coming week, the Moon will wax from a thick crescent to nearly full. Most of us will put deep-sky observing on hold as lunar glare intrudes on dark skies. Instead of capping your scope, why not make the Moon your focus? If you haven't already, it's a good time to get acquainted with one of our satellite's most evocative features: domes.
Many of the Moon's characteristic landscapes were created by impact. Craters, rays, mountain ranges, maria, and basins abound. Lunar domes are different. They formed as a result of the Moon's own internal volcanism. Similar to shield volcanoes in Iceland, Hawaiʻi (including Mauna Kea on the Big Island), and Olympus Mons on Mars, they form when highly fluid lavas erupt through a central caldera onto the surface. They're almost all of low-explosivity, unlike their cousins, the more violent stratovolcanoes that grab the headlines.
As sheet after sheet after sheet of lava oozes up from beneath the crust, a dome slowly builds up over time into a broad, gently-sloped mound shaped like a warrior's shield with a raised center and lower edge. Shield volcanoes can be small like the Icelandic and lunar varieties, or broad and massive like Olympus Mons. A typical lunar dome measures between 5 and 7.5 miles (8-12 km) in diameter with a peak or caldera ~900 feet (~300 meters) high. Slopes are very gentle — only a few degrees at most — making for very easy walking should astronauts ever get the chance to explore one.
Over 300 lunar domes are known, with many visible in amateur telescopes with apertures from 3-inches on up. We'll look at the historically most interesting and easiest examples along with a few oddballs tossed in to keep things lively. There are two key requirements for happy dome watching — steady atmospheric seeing and observing the dome near the terminator shortly after lunar sunrise or before sunset.
Most domes are subtle, low contrast features that turn mushy in poor seeing. Low light, the kind that produces long shadows from peaks and crater rims, brings out their gently sloping forms and provides the best contrast. Don't bother dome hunting when the Sun rides high in the lunar sky. No shadows, no domes!
Under good light and excellent seeing, domes look like swellings or blisters on the lunar surface. Many appear almost smooth, though a few have rougher areas that again show up best in low, slanted sunlight. I get most excited when I can tease out a caldera. Seeing the summit blow-hole, you really see a dome for what it is: a formerly active volcano back in the Moon's rough and tumble days.
The full-disk map above plots the general location of each of the featured domes followed by individual descriptions. If you're thwarted by poor seeing or discover that the dome you hope to view is too far from the terminator to make out this lunation, save it for the next. Persistence will eventually reward you with a thorough working knowledge of the Moon's bumps and lumps. You may even discover a newfound eagerness for the Moon's return after dark nights!
Use the list below as a general guide to know when lighting is best for viewing a particular dome or dome complex. If the day falls within the current viewing period, a calendar date is included:
* Cauchy domes: 4 days past new
* Gardner Megadome: 5 days
* Arago domes: 6 days (Sept. 7th)
* Valentine dome: 7 days (Sept. 8th)
* Birt domes: 8 days (Sept. 9th)
* Kies Pi and Capuanus crater: 9, 10 days (Sept. 10-11)
* Hortenius and Milichius domes: 10 days (Sept. 11th)
* Gruithuisen domes: 11 days (Sept. 12th)
* Marius Hills: 12 days (Sept. 13th)
* Mons Rümker: 12, 13 days (Sept. 13-14)
To make the most of your observations, consider the following at each feature:
- Note the surroundings, whether highlands or sea (mare). You'll soon discover that more domes are found within and near the maria and are undoubtedly related to mare volcanism.
- Size and shape. Is it small, large? Elliptical – circular – polygonal – irregular?
- Steepness of the incline. Is the dome's gentle, steeper?
- Study the shape of the summit. Is it flat – multiple – complex?
- Can you discern a central depression (caldera) or does the top appear smooth? Are there any rills cutting through or near the dome? Sinuous rills once channeled lava.
The above list of descriptors is based on a more detailed one compiled by the American Lunar Society Lunar Dome Section. Take a read when you get a moment. Now I'll get out of the way, so you can begin your work as an amateur lunar volcanologist.
ACT-REACT-Quickmap. A zoomable, highly detailed Moon atlas based on photos taken by NASA's Lunar Orbiter.
Chuck Wood's Moon. Lots of photo links and great information.
Chameleon Observatory Photographic Moon Atlas. Superb photographic Moon atlas divided into sections. Many domes are featured!
Virtual Moon Atlas. A free program that gives a realistic view of the Moon with librations at anytime you chose. Great for planning your observing sessions.