Basins and Volcanism

The largest of the Moon's great basalt-filled basins were all formed during the Late Heavy Bombardment. These and some lesser basins are listed in the table on page 142.These large ancient impact craters are traditionally called mare, meaning sea, since they have the appearance of dark oceans when viewed from Earth. The dark material is basalt, formed by melting in the Moon's interior that erupted into the low areas excavated by the large impacts of the Late Heavy Bombardment. The low elevations of the craters along with their

The lunar basins Imbrium (left), Serenitatis (center), and Crisium (right) are all filled with dark mare basalts.

(NASA/Galileo)

large size and basalt fill has also led them to be called basins. Small basins were named palus, meaning swamps, though of course they are perfectly dry areas with pools of frozen basalt. Some examples are the Palus Epidemiarum (the marsh of epidemics), the Palus Putredinis (the marsh of decay) and the Palus Somni (the marsh of sleep). The Apollo 15 mission landed near the Palus Putredinis.

This image from the Galileo mission in 1992 clearly shows a number of the larger basins.The North Polar region is near the top part of the photomosaic. Mare Imbrium is the dark area on the left, Mare Serenitatis is at center, and Mare Crisium is the circular dark area to the right. Bright crater rim and ray deposits are from Copernicus, an impact crater about 60 miles (95 km) in diameter.

In the table on page 142 the mare are listed in approximate order of size, though measuring the size of an ancient crater is an inexact science. Similarly, estimating the ages of the basins is a difficult exercise. In the best case, melt caused by the heat and shock of impact is

The lunar basins Imbrium (left), Serenitatis (center), and Crisium (right) are all filled with dark mare basalts.

(NASA/Galileo)

IMPACT BASINS ON THE MOON

Latitude

Longitude

Diameter

Approx. age

Latin name

English name

(degrees)

(degrees)

(miles [km])

(billion years)

Oceanus

Ocean of Storms

18.4N

57.4W

1,605 (2,568)

Procellarum

Mare Frigoris

Sea of Cold

56.0N

1.4E

998 (1,596)

Mare Imbrium

Sea of Showers

32.8N

15.6W

702 (1,123)

3.85

Mare

Sea of Fecundity

7.8S

51.3E

568 (909)

4.0

Fecunditatis

Mare

Sea of Tranquility

8.5N

31.4E

546 (873)

3.99

Tranquillitatis

Mare Nubium

Sea of Clouds

21.3S

16.6W

447 (715)

3.98

Mare Serenitatis

Sea of Serenity

28.0N

17.5E

442 (707)

3.87

Mare Australe

Southern Sea

38.9S

93.0E

377 (603)

4.05

Mare Insularum

Sea of Islands

7.5N

30.9W

321 (513)

Mare Marginis

Sea of the Edge

13.3N

86.1E

263 (420)

Mare Crisium

Sea of Crises

17.0N

59.1E

261 (418)

3.84

Mare Humorum

Sea of Moisture

24.4S

38.6W

243 (389)

3.87

Mare Cognitum

Known Sea

10.0S

23.1W

235 (376)

Mare Smythii

Sea of William

1.3N

87.5E

233 (373)

3.97

Henry Smyth

Mare Nectaris

Sea of Nectar

15.2S

35.5E

208 (333)

3.91

Mare Orientale

Eastern sea

19.4S

92.8W

204 (327)

3.80

Mare Ingenii

Sea of Cleverness

33.7S

163.5E

199 (318)

Mare

Sea of Muscovy

27.3N

147.9E

173 (277)

3.91

Moscoviense

Mare

Sea of Alexander

56.8N

81.5E

171 (273)

3.89

Humboldtianum

von Humboldt

Mare Vaporum

Sea of Vapors

13.3N

3.6E

153 (245)

Mare Anguis

Serpent Sea

22.6N

67.7E

94 (150)

Mare Spumans

Foaming Sea

1.1N

65.1E

87 (139)

found in some rock near the crater and dated using radioisotopes.This kind of high-precision field geology is almost completely unavailable for the Moon, and so crater ages are estimated by the relationships among craters (often judged by their overlapping ejecta), combined with whatever radiodates that can be obtained. This table also lists some age estimates for the craters. The ages listed all have possible errors of at least plus or minus 25 million years (0.02 billion years). Some of the ages come from radiometric dating of crater ejecta returned by Apollo missions, and other ages come from relationships among craters judged from overlapping crater ejecta. The ages have been compiled and analyzed by Greg Neumann and Maria Zuber at the Massachusetts Institute of Technology with David Smith and Frank Lemoine at the Laboratory for Terrestrial Physics at the NASA/ Goddard Space Flight Center, using the extensive mapping and analysis of the Moon done by Don Wilhelms, an astrogeologist with the United States Geological Survey.

Mare Orientale is the youngest of the large impact basins, and makes a striking image. In the left image of this pair from the Galileo mission Orientale is centered on the Moon. Its complex rings can be seen but its center contains only a small pool of mare basalt. By

These two views of Mare Orientale, taken by the Galileo spacecraft, also show large regions of dark mare basalt. (NASA/JPL/Galileo)

contrast, the older Oceanus Procellarum covers the upper right of the Moon in this image, with the Mare Imbrium above it and the small Mare Humorum beneath. In the right image Imbrium lies on the extreme left limb of the Moon.

The Apollo samples returned from the Moon include pieces of the dark mare basalts sufficient to analyze and learn a significant amount about their origins and age.The majority of the basalts appear to date from the time of the Late Heavy Bombardment, corresponding to the times of formation of the great basins that they fill. Other basalts are younger, with ages that stretch through the period from 4.3 to around 3 billion years ago (a collection of age data for basalts is shown in the figure titled "Time Line of Lunar Events" on page 134). Lunar volcanic eruptions peaked in the Imbrian Epoch (3.85 to 3.2 billion years ago), the majority of the basalts lie on the near side, and the total volume of mare basalt is about 2.4 million cubic miles (10 million km3). Though most scientists believe that the mare basalts finished erupting by about 3 billion years ago, some others say they have found evidence for eruptions as recent as 2 billion years ago, through comparisons of crater ejecta relationships and cratering intensities (the fewer craters on a surface, the younger it is).

Some of these basalts closely resemble Earth basalts, but others have exceptionally high titanium contents. This high titanium is again thought to tie the basalts back to the fundamental processes of magma ocean crystallization: As the magma ocean liquids evolved further and further toward the end of crystallization, they were progressively enriched in titanium because it does not fit well into the common mantle forming minerals. After the magma ocean was about 94 percent crystalline the remaining liquids were sufficiently enriched in titanium to start crystallizing ilmenite, a titanium oxide mineral.This process produced a region enriched in titanium.

Planetary scientists immediately saw the possibility that the high-titanium mare basalts could have formed by melting this titanium-rich region. This gave a simple explanation for the existence of such strangely titanium-rich magmas, while tying them to magma ocean processes. Experimental petrologists started conducting high-pressure experiments using synthetic rock compositions identical to the high-titanium basalts to see if they could form from an ilmenite-rich source (see the sidebar "High-Pressure Experiments" on page 36). Much to their surprise, they found that the high-titanium basalts

The dark beads are volcanic glasses from an Apollo 15 lunar soil sample, and the central bead is mantled by an agglutinate, a mix of melted soil components caused by heat from the bead as it landed, or possibly by a separate small impact. (Linda T. Elkins-Tanton/NASA/JSC)

lülium cannot be made by simply melting an ilmenite-rich source: All the resulting magmas were much to high in calcium. While most lunar scientists remain convinced that the titanium-rich basalts must be related to the titanium-rich layer produced in magma ocean crystallization, the exact process that made the basalts remains unknown.

Along with the pieces of basalt returned by the Apollo missions came samples of lunar soil. The lunar soil is properly called regolith since it consists of many tiny rock fragments without any organic material. By examining the regolith with microscopes and analyzing the pieces with an instrument called an electron microprobe, which can measure the compositions of rocks as small as 10 microns across, a whole new world of lunar data was discovered. One of the most exciting discoveries was a large assortment of beads of glass, each just a few tens of microns across. Some are bright green, and others yellow, red, or black.

These beads, it appeared after they were analyzed with an electron microprobe, are volcanic in origin. They are thought to have been erupted in fire fountains like those that sometimes spew from the

The dark beads are volcanic glasses from an Apollo 15 lunar soil sample, and the central bead is mantled by an agglutinate, a mix of melted soil components caused by heat from the bead as it landed, or possibly by a separate small impact. (Linda T. Elkins-Tanton/NASA/JSC)

volcanic vents in Hawaii.When the magma in the fire fountains hit the frozen near-vacuum at the lunar surface they chilled so quickly ("quenched") that there was no time for crystals to form, and the beads froze as glass. The image of glass beads from an Apollo 15 soil sample

Picritic Glasses and Conditions in the Lunar Interior

n—i—i—i—i—i—i—i—i—i—i—i—r s.

-Luna 24

H 15065

1.01- 14072

- Mare basalts

15 Red n—i—i—i—i—i—i—i—i—i—i—i—r

H 15065

-Luna 24

15 Red

3.0_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_Il_I_I_I_I_I_I_I_I_372 (600)

3.0_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_Il_I_I_I_I_I_I_I_I_372 (600)

Experimental work on the lunar picritic glasses has created an image of pressure, temperature, and compositional conditions in the lunar interior around 3.5 billion years ago.

(see page 145) was taken by the author using an electron microprobe in the laboratory of Tim Grove at the Massachusetts Institute of Technology.The large, round, gray objects are glass beads that would appear bright green in visible light, though in this image their color indicates their density (the image is created by bouncing beams of electrons off the small samples, and recording how many of the electrons bounce back; the denser the material, the more electrons bounce back). The central glass bead in the image is cloaked with a mantle of brighter material containing spots and swirls.This is termed an agglutinate, material created by the melting and mixing soil components.

These beads have become perhaps the most important window into the lunar interior. Many of them are near-pristine melts of mantle material and have experienced almost no later processing (magmas can cool on their way to the surface and precipitate crystals, or they can heat, melt, and assimilate rocks that they pass through; these processes cloud the clear record of mantle melting that the original magma carried). Pure mantle melts are rich in magnesium, as are picritic glasses, picritic being the geologic term for high magnesium.

Doing high-pressure experiments on synthetic versions of these picritic beads provides information about the pressure and temperature of melting that produced the magmas, as well as the material that melted. The experiments are time-consuming: The time needed to obtain this information for just one picritic bead takes months of work and between 20 and 40 separate experiments. Over the years since the Apollo missions a number of scientists in different laboratories have conducted studies on these beads and have thus produced a kind of picture of the interior of the Moon at about 3.5 billion years ago, shown in the figure on page 146.

This figure also shows the same information for the mare basalts. The mare basalts originated at shallower depths, and the picritic glasses came from deep in the Moon. All of the lunar interior was hot at that time, almost hot enough to melt. Just a little addition of heat, for example from radiogenic elements, or a little depressurization in a convective upwelling, would be enough to allow the mantle to melt.

An important result of these studies is also the knowledge that basalts and picritic glasses with different compositions can erupt from about the same depth in the Moon. Jeffrey Gillis, a professor at the Washington University in St. Louis, used Clementine and Prospector mission orbital data to investigate the nature of the lunar maria. He reported that there are basaltic rocks in Mare Australe that are distinctly different from other maria.They are lower in iron and contain significant amounts of low-calcium pyroxene. This implies that they formed by melting of compositionally and mineralogically distinctive regions in the lunar mantle. Harry Hiesinger, a researcher at Brown University, and his colleagues have also been making detailed studies of the lunar maria, including determining their ages by counting craters. They find no correlation between age and composition. In fact, high-titanium and low-titanium lava flows can occur almost simultaneously. Age is, however, correlated with location: The flows in the western maria are youngest. According to Paul Spudis, then at the Lunar and Planetary Institute and now at the Applied Physics Laboratory at Johns Hopkins University, the eastern maria, such as Mare Smythii, are also relatively young.

Models of magma ocean crystallization produced by the author and her colleagues Marc Parmentier and Sarah Zaranek at Brown University indicate that during magma ocean cumulate overturn mantle parcels with different compositions but the same buoyancy end up at the same depths in the shallow Moon, making a mantle with distinct compositional regions. This theoretical model is consistent with the differing compositions of the mare basalts and picritic glasses.

Both the mare basalts and the picritic glasses may have had a difficult time erupting to the surface. Remember that the lunar crust is largely made of plagioclase, which is an unusually buoyant mineral. On Earth, mantle melts are generally more buoyant than the crust they are attempting to erupt through, so magma moves onto the surface relatively easily. On the Moon in many cases the thick plagioclase crust is actually more buoyant the almost all the mantle melts, meaning that additional force is needed to move the melts onto the surface. The picritic glasses were clearly blasted onto the surface by some sort of propellant that sprayed them into fountains above the lunar surface. Tiny films on the surface of the beads provide evidence for the sort of propellant that caused the eruptions. Mac Rutherford, a professor at Brown University, and his colleague Michael Nicholis studied the orange glass beads, and believe their strong eruption was driven by carbon. The carbon started as graphite in the magma at depth, and then as pressure decreased, the graphite converted to a CO and CO2 gas mix, driving the eruption much the way bubbles make soda bottles squirt.

The author and her colleagues at the Massachusetts Institute of Technology studied green glass beads from the Apollo 15 landing site and suggested that fluorine was partly the driving force, possibly in addition to carbon.These traces of volatile gases expand catastrophi-cally as they leave the pressure of the lunar interior and reach the near-vacuum of the surface. Their violent expansion raises the pressure in the liquid magma catastrophically, driving it out onto the surface and breaking it into tiny drops. Newer work by Michael Nicholis and Mac Rutherford at Brown University shows that a small proportion of carbon in the magma is a highly effective way to drive fire-fountaining eruptions, because at an appropriate depth the carbon breaks down to carbon dioxide and other volatile compounds, the pressure of which drives eruption.

Without additional driving force, the mare basalts had to find a way to move onto the surface through buoyancy, with little volatile driving force.The giant impact basins made the eruption of the mare basalts possible. When these huge impactors struck the Moon, they removed a large fraction of the anorthositic crust from the site of impact and threw it elsewhere on the Moon as ejecta. The mare basalts had just enough buoyancy to seep into these holes, when they could not erupt onto the higher anorthosite highlands. This process for eruption and explanation for the basalt fill in the basins was proposed early on by Sean Solomon, now director of the Carnegie Institute in Washington, D.C. Marc Wieczorek, at the time a researcher at the Massachusetts Institute of Technology, made a series of calculations of magma buoyancy and proved that the magmas could just seep into the basins, while they could not rise onto the highlands.

The early history of the Moon contains almost all its dynamic action. The Moon was probably formed in a catastrophic giant impact that partly destroyed the early Earth and heated the early Moon until it was a molten magma ocean. This magma ocean crystallized over 10 or 20 million years, producing a mantle made of cumulate minerals layered by density and composition. Magma ocean crystallization also formed the anorthositic highlands, probably by flotation of plagioclase in the magma ocean, though possibly through secondary melting when the gravitationally unstable mantle overturned to a stable stratification. Between about 4 and 3.8 billion years ago, in the Pre-Nectarian and Nectarian, at least 400

million years after the magma ocean crystallized, came the peak of the Late Heavy Bombardment. The basins formed by these catastrophic impacts filled with mare basalts between then and about 3 billion years ago, after which magmatism on the Moon tailed off. Today the Moon is largely inactive.There is no more volcanic activity and cratering has tailed off as debris becomes less and less common in the inner solar system.

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