[meteorite-list] Mound Near Lunar South Pole Formed by Unique Volcanic Process

[Ron Baalke]

https://news.brown.edu/articles/2015/10/mound

Mound near lunar south pole formed by unique volcanic process
Brown University
Contact: Kevin Stacey   401-863-3766
October 15, 2015   

Within a giant impact basin near the Moon's south pole, there sits a large 
mound of mysterious origin. Research by Brown University geologists suggests 
that the mound was formed by unique volcanic processes set in motion by 
the impact that formed the basin.

PROVIDENCE, R.I. [Brown University] - A giant mound near the Moon's south 
pole appears to be a volcanic structure unlike any other found on the 
lunar surface, according to new research by Brown University geologists.

The formation, known as Mafic Mound, stands about 800 meters tall and 
75 kilometers across, smack in the middle of a giant impact crater known 
as the South Pole-Aitken Basin. This new study suggests that the mound 
is the result of a unique kind of volcanic activity set in motion by the 
colossal impact that formed the basin.

"If the scenarios that we lay out for its formation are correct, it could 
represent a totally new volcanic process that's never been seen before," 
said Daniel Moriarty, a Ph.D. student in Brown's Department of Earth, 
Environmental and Planetary Sciences and the study's lead author.

The research has been accepted for publication in Geophysical Research 
Letters, a publication of the American Geophysical Union, and is available 
online.

Mafic Mound (mafic is a term for rocks rich in minerals such as pyroxene 
and olivine) was first discovered in the 1990s by Carle Pieters, a planetary 
geologist at Brown and Moriarty's adviser. What makes it curious, other 
than its substantial size, is the fact that it has a different mineralogical 
composition than the surrounding rock. The mound is rich in high-calcium 
pyroxene, whereas the surrounding rock is low-calcium.


A volcanic structure

"This unusual structure at the very center of the basin begs the question: 
What is this thing, and might it be related to the basin formation process?" 
Moriarty said.

To investigate that, Moriarty and Pieters looked at a rich suite of data 
from multiple lunar exploration missions. They used detailed mineralogical 
data from the Moon Mineralogy Mapper, which flew aboard India's Chandrayaan-1 
spacecraft. NASA's Lunar Orbiter Laser Altimeter provided precise topographic 
data, and data from the GRAIL mission characterized gravitational anomalies 
in the region.

Those combined datasets suggested that Mafic Mound was created by one 
of two unique volcanic processes set in motion by the giant South Pole-Aitken 
impact. An impact of that size would have created a cauldron of melted 
rock as much as 50 kilometers deep, some researchers think. As that sheet 
of impact melt cooled and crystalized, it would have shrunk. As it did, 
still-molten material in the middle of the melt sheet may have been squeezed 
out the top like toothpaste from a tube. Eventually, that erupted material 
may have formed the mound.

Such a process could explain the mound's strange mineralogy. Models of 
how the South Pole-Aitken melt sheet may have crystalized suggest that 
the erupting material should be rich in high-calcium pyroxene, which is 
consistent with the observed mineralogy of the mound.

Another scenario that fits the data involves possible melting of the Moon's 
mantle shortly after the South Pole-Aitken impact. The impact would have 
blasted tons of rock out of the basin, creating a low-gravity region. 
The lower gravity condition could have enabled the center of the basin 
to rebound upward. Such upward movement would have caused partial melting 
of mantle material, which could have erupted to form the mound.

These scenarios make for a strong fit to those very detailed datasets, 
Moriarty said. And if either is true, it would represent a unique process 
on lunar surface. Moriarty said a sample return mission to the South Pole 
Aitken Basin would be a great way to try to verify the results. The basin 
has long been an interesting mission target for lunar scientists.

"It's the largest confirmed impact structure in the solar system and has 
shaped many aspects of the evolution of the Moon,' Moriarty said. "So 
a big topic in lunar science is studying this basin and the effects it 
had on the geology of the Moon through time."

A sample return mission to the basin could bring back bits of lunar mantle, 
the composition of which is still not fully understood. A returned sample 
could also put a firm date on when the impact occurred, which could be 
used as a standard to date other features on the surface.

And in light of this work, a sample could also help to shed light on a 
unique lunar volcanic process.