The Moon: South Polar Highlands

In the couple of months since I presented The Sky This Month NASA’s Exploration Systems Architecture Study has outlined a plan to put a manned mission on the moon sometime in the next twenty years.   The most likely landing spot for the mission is the Moon’s South Pole.   Let’s look at reasons why.   If this region is going to be in the news someday, it might be worthwhile to find out more about it.

Compared to the dark mare areas we’ve looked at in past months, the Moon’s southern highlands are covered with bright craters.   There is almost no dark mare fill leaking through the bedrock in this area, even underneath large craters and impact basins.   The best example is the huge Aitken Basin just on the far side.   It is understood that most of this highland region, though covered in craters, represents the oldest part of the Moon’s surface that has not been repaved by lava.   The crust here must also be quite thick to not have been punctured by outflowing lavas into the basins.   Perhaps this is connected to the deficiency of mare lavas on the Moon’s far side.

Geologists are very interested in the rocks in the southern highlands.   Apart from Apollo 16, manned missions never landed in any highland areas of the Moon’s surface…   and Apollo 16’s experience in the Descartes Highlands is a long story.   Apollo missions could only collect highland material in fragmented, brecciated rocks that had been flung hundreds or thousands of kilometers from out of impact basins to land on mare pavement, and it is next to impossible to tell where they were excavated.   But the highlands are the oldest part of the surface, and they can tell us a great deal about how the Moon was formed and, by extrapolation, the history of other parts of the solar system as well.   The southern highlands is a priority area for lunar explorers who want to “go pick up rocks”.

The Aitken Basin is the largest impact basin in the entire solar system.   The closest thing to viewing the Aitken Basin from Earth is to see the Leibnitz Mountains at the Moon’s South Pole.   These mountains are part of the outer rim of the basin. The Leibnitz Mountains are not officially named as such, and according to Charles Wood “These peaks used to be called the Leibnitz Mts, but the name was decommissioned by an IAU committee ignorant of the real Moon.” I wonder what happened there!  

For those who picked up the hand-out for The Sky This Month, one of the South Polar Mountains labeled “M5” is also known as Mount Clementine.   It was so named by the mission team of the Clementine lunar orbiter, the first of two notable 1990s lunar orbiters.

Clementine searched for ice locked up in the Moon’s north and south pole, and what evidence it found was backed up by the Lunar Prospector orbiter a couple of years later.

Lunar Prospector was deliberately crashed into the south polar crater Shoemaker (named after the lunar geologist Eugene Shoemaker, labeled “R4” on the handout) at the end of its mission in an attempt to excavate some of this ice.   Although none was detected afterwards, the combined results from the two probes suggest that there are billions of metric tons of ice locked up at the Moon’s north and south poles.

Obviously H₂0 on the moon would be a useful resource for a sustainable presence there, as drinking water, insulation, electricity, and a source of hydrogen for rocket fuel and oxygen for oxidizer and air for breathing.   This is another reason why NASA is interested in both of the Moon’s polar areas.

Another reason for interest in a manned presence at the Moon’s south pole dovetails nicely with amateur observations from Earth.   The poles are the only part of the Moon where sunlight is available all the time.   With a night that lasts two weeks, the Moon is a punishing environment for any solar-powered battery.   Imagine landing on the Moon in an Apollo capsule in the middle of the night, and trying to explore the surface in extreme cold with only the light of the Earth to see by.   Some of the peaks at the south pole are in constant sunlight, and the long night is no longer a problem.   Sunlight is not an issue for Earthbound observers looking at some of the south pole region, most of the time.

Even outside of the Full Moon, either the east or west sides of the region are always visible depending on the phase of the moon.   The only other variable is the tilt of the pole toward or away from the Earth.   The Moon’s rotational axis is tilted with regards to its orbital path around the Earth, and the orbital path around the Earth is tilted a further six degrees with regards to the plane in which the Sun and the planets orbit.   Depending on how the variables add up, sometimes we can see an extra six or so degrees beyond the polar horizon… and sometimes the south pole isn’t visible at all!

According to Westfall’s “Atlas of the Lunar Terminator” the best librations of the south pole past 6 degrees in late 2005 are:

October 23^rd & 24^th Waning Gibbous
November 20^th Just past Full
December 17^th Full Moon
January 13^th Just before Full

<!--[endif]-->The best image available from the Lunar Picture of the Day is not from as favourable a libration as the handout.   Mount Clementine is not visible above the horizon, and Shoemaker Crater is too far to the east.   The image was taken on September 4^th of last year, and checking the RASC Observer’s Handbook it looks like the south pole was not well tilted towards us at that time.   The good news is that this interesting area of the Moon is better placed for observing over the next little while.

Lunar Picture of the Day September and December 29^th, 2004

Sky and Telescope Lunar 100
#94 Drygalski Crater
#96 Leibnitz Mountains

Sky and Telescope vol. 110 #1 July 2005 p. 62-3 “Polar Pleasures”

‘The Modern Moon: A Personal View’ by Charles Wood, Chapter 13
(specifically the south pole chart on p. 127, copied from Ewen Whitaker 1954)

Rukl Charts

64 Tycho
65 Walter
72 Clavius
73 Moretus
V Hale
VI Hausen

RASC I.K. Williamson Lunar Observing Certificate

58 Stofler & Faraday
69 Walter
70 Huggins, Nasireddin & Miller
81 Deslandres
82 Gauricus
83 Tycho
84 Maginus
85 Clavius
91 Wilhelm & Heinsius
92 Longomontanus
93 Blancanus & Steiner
101 Klaproth & Casatus

In connection with August's The Sky This Month, the Hubble Space Telescope has recently been used to scan the Moon's Aristarchus Plateau.  As part of the initiative to get back to the Moon Aristarchus was studied for its mineral resources and possible oxygen locked up in the surface rocks.

Extreme close-up images of Aristarchus from Hubble's Advanced Camera for Surveys

Dust Storm on Mars

Sky and Telescope's website carries a good article about the current storm.  The storm started about October 18th in Chryse (check the map included in August's presentation).  Since then it passed south and west through Valles Marineris, Solis Lacus, Mare Erythraeum and is now around Noachis.

Dust storms can blot out Mars' already difficult-to-see albedo markings, so the occurrence of one just before opposition can be very inconvenient.  Fortunately, this one is covering a part of Martian landscape that will not be turned towards us at the Halloween Star Party at the Science Centre on Saturday, October 29th.

Surface winds inside a dust storm can go above 100 km/h, and winds at altitude can be higher than 500 km/h.  Because the Martian atmosphere is less than 1% as dense as Earth's, these winds don't have the same strength as hurricane-force winds on Earth, but they are strong enough to whip up large clouds of micron-sized fines and dust high into the atmosphere that can take weeks to settle out.

You can follow the storm's progress at this link, the Communications in Mars Observations website.