October 8, 2011
It’s a gas, man!
The lunar feature Ina, an extremely young, unusual depression that may represent a gas eruption site on the Moon. LROC narrow angle camera images.
There are times when seemingly unrelated discoveries about other planets come forward to enlighten us about the history and processes of the Moon. A recent paper, using data from the orbiting MESSENGER mission mapping Mercury, describes a number of newly discovered rimless pits and depressions. These pits (called hollows by the mission team) are difficult to explain by impact processes and are hypothesized to be the products of outgassing from the planet’s interior. They are often associated with color anomalies (which implies compositional differences from the surrounding terrain) and frequently found on the floors of impact craters and basins.
Impact craters come in a wide variety of sizes, but within selected size ranges, they all appear more or less similar. Small craters are nearly perfectly round and bowl-shaped with smooth rims that are raised above the surrounding terrain. Craters with irregular shapes and no raised rims suggest that processes other than impact might be at work. It has been suggested that on Mercury, these “hollows” were created by the violent release of volatile substances. Such a release of gas under pressure accompanies volcanic eruptions called pyroclastic, meaning “fire-broken” (fine liquid rock (magma) fragments spewed into space and cooled during flight).
We’ve known about pyroclastic eruptions on the Moon for many years, evidenced by the green glass of the Apollo 15 site and the orange-black glass from Apollo 17. Careful search of the images taken from lunar orbit reveal the rimless pits that served as vents for the pyroclastic eruptions that produced these Apollo glasses. They are distinct from impact craters and often are found on the floors of craters and basins along fractures, the conduit by which volcanic magma travels to the lunar surface.
Sometimes pit craters or “hollows,” found across the surface of the Moon, take unusual form. The kidney-shaped feature shown above is named Ina; after its discovery in one of the Apollo orbital images, it was informally named the “D-caldera” after its shape and the interpretation that it represented a volcanic collapse feature. Ina is about 3 km across and consists of a series of small platforms, mounds and holes within a larger irregular depression. Other similar pits and hollows occur elsewhere on the Moon (e.g., on the floor of Rima Hyginis). And while not major features, they have been found often enough to bother many lunar scientists, who had no good explanation for their origin.
About five years ago, we got a clue as to the possible origins of these features. Pete Schultz and associates from Brown University published a paper showing Ina displayed unusual spectral reflectance characteristics. The slow micrometeorite bombardment of the Moon adds craters to the surface and also makes small iron-rich glass particles that darken and redden the surface. As these glass particles build up in the soil, a soil is said to “mature.” Fresh surfaces are more “blue” in color (actually, less red) and become redder with time as the soil matures. Most lunar features show age or “become mature” on timescales of millions of years. Ina shows very few impact craters on top of it, meaning that geologically, it is very young. Moreover, the soils associated with Ina are much bluer than surrounding areas. Both of these observations suggest that Ina is young with immature surfaces.
How are these features created? Significant volcanism on the Moon largely stopped at least a couple of billion years ago. The Brown team thought that the combination of young age, low maturity and unusual morphology suggested a relatively uncommon pit-forming process. They proposed that the explosive release of volatile substances from the lunar interior would have disrupted the surface, created a chaotic mixture of rock and soil, exposed fresh surfaces (creating the immature spectral signature), and formed a collapse depression caused by the instantaneous removal of mass from below.
Now we can see that the new Mercurian hollows have morphologies displaying spectral anomalies similar to the lunar collapse pits such as Ina. The new data suggest that Mercury contains significant volatile substances. These volatiles must be present at some depth, accumulated under high pressure until crustal failure ensues and a massive gas release results in an “eruption.” This explosive event leaves behind a chaotic, disrupted surface (“immature,” with fresh bedrock and deep regolith “newly” exposed to space).
In the case of Ina on the Moon, its extreme youth is suggested both by the lack of overlying impact craters of almost any size, as well as the sharp preservation of topography in its cliff and pit interior morphology. This extreme youth may be on the order of thousands to hundreds of thousands of years, not the millions and billions of years that typify most lunar landforms. Such youth and the widespread distribution of Ina-like collapse pits across the lunar surface implies that outgassing events are occurring on the Moon now; it is highly unlikely that we were just lucky enough to find a singular or unique occurrence.
What might these volatile substances be? Before the recent lunar missions flew, it was common to declare that water was not a possibility. However, we recently discovered from study of the lunar samples that water was present in the deep interior of the Moon during the epoch of mare volcanism three billion years ago; water could still be present in the subsurface. There are many other volatile substances that could be responsible as well, including carbon monoxide, hydrogen sulfide, gaseous sulfur, as well as other more exotic gases. Because the compositions on Mercury are poorly known, the possibilities for exotic materials there are even more extensive.
The explosive release of gas from the deep interior (without the eruption of magma) appears to be an ongoing lunar process. This gas release could provide at least a partial answer to two vexing lunar problems: the accumulation of volatiles at the poles of the Moon (discussed in my blogging many times, most recently HERE) and the infamous phenomena of Lunar Transient Phenomena (LTP), described as glowing reddish “clouds” hovering over the lunar surface that mysteriously appear and disappear. Telescopic observers have reported seeing LTP for many years. Unfortunately, we have not been able to verify and document these events, largely because they are transient. Now we have direct morphological evidence for the venting of gas from both planets, making it possible that at least some LTP might be related to gas release from inside the Moon. Stay tuned – the book of the Moon continues to be rewritten and expanded with new and interesting discoveries.
NOTE: The latest version of the paper Tony Lavoie and I wrote on using lunar resources to create a cislunar space faring system has been published in the Proceedings of the AIAA Space 2011 Conference. A copy is available for download HERE.
20 Comments
RSS feed for comments on this post.
The web editors have closed comments for this blog.
Rather than volcanic, couldn’t these gases be from radioactive decay?
Radon gas leaks from earth, uranium is fairly abundant on earth, it may less abundant of the Moon, but even as trace substance there has a lot of it in the large mass of the Moon.
Whereas I have never heard of a significant eruption of Radon, perhaps the active geologic nature of Earth prevents or better, obscure this with so many other types of out gassing and/or hydrological activity.
Or said differently radon could take millions year to accumulate is a significant quantity to erupt in a significant way,
Comment by gbaikie — October 8, 2011 @ 4:38 pm
I didn’t say they were volcanic. I said that gas “eruptions” appear to have created these features by accumulating and building up pressure, then catastrophically rupturing the crust and venting into space.
There is lunar radon and it has been detected around the KREEP-rich (U-rich) areas of Oceanus Procellarum but it appears to be more or less constantly emitted, not catastrophically erupted.
Comment by Paul D. Spudis — October 8, 2011 @ 4:55 pm
I just read your linked paper.
Nice!
I would suggest a version which was more on what rather than how and reducing the wordage by about 90% and more focus
where such a program gets the nation.
And a video based on it and more emphasis on future this will bring.
So a version of what rather than how would a executive summary- something people not very informed about space could get fairly quickly.
It took me quite while to read thru it, and probably I want to look at it again for more detailed understanding.
It would great if most members of congress could read this.
Comment by gbaikie — October 8, 2011 @ 6:22 pm
I would suggest a version which was more on what rather than how and reducing the wordage by about 90% and more focus where such a program gets the nation.
Thanks for the comment. The reason it is long is because this version is the full, technical paper. I wrote up a blog post on this work last December:
http://blogs.airspacemag.com/moon/2010/12/can-we-afford-to-return-to-the-moon/
I have a shortened, “bulletized elevator speech” version posted here:
http://www.spudislunarresources.com/Rationale.htm
I also have a slideshow version of the story here:
http://www.spudislunarresources.com/Papers/Spudis_ISDC_2011.pdf
Comment by Paul D. Spudis — October 9, 2011 @ 4:46 am
What I mean by what rather than how, is such as
Save NASA- from endless powerpoints
Save science education
Invite global participation.
Head off possible confrontation by involving international
effort. [ref:
Enhance security and commercial satellites.
Create new jobs and remain on cutting edge of technology- and teleoperation is increasing used in many private sector activities.
Create future in which the near limitless resources.
People are concerned about your direction in space program.
Or one could just focus most important and pertinent.
Saving NASA:
If NASA doesn't have defined and immediate goals it tends
to flounder.
That such a plan Aligns with Augustine report
Providing direction for the space program which it can achieve within it's budget constraints, should conserve the NASA workforce and inspire the younger generation which may work for NASA or other science careers.
Etc.
And/or
Robotic programs had drawn a lot interest from the public.
The Lunar robotic program could engage the public more than past programs because one could greater bandwidth and transmitted images can closer to real time- e.g. one could drill a few rocks in few hours instead of drilling one rock per week.
{I assume this is true}
Or said differently, your elevator speech would work if speaking someone with technical knowledge, which no doubt some congress member have, but I think need something which more political- something that most of congress or general public can get in terms such a plan being necessary and having easily seen benefits.
NASA doing something new in a short time period instead decades. Something people can be involved with. Something to look forward to. You point to the moon and say we doing something up there. The lunar polar region is mysterious, strange.
One should counter the notion that we have already gone to the Moon.
Don't you think that polar region look different than you or anyone expects. Simply having a color camera would make look different. Being on mountain on the Moon is different. One could Earthshine in shadows [maybe].
The polar region on the Moon hasn’t been explored, it’s something the size of california in shadows.
Comment by gbaikie — October 9, 2011 @ 8:34 am
missed putting in ref above:
http://www.youtube.com/watch?v=7lcV6fhaX-E
where Robert Bigelow talks of possibility that
China “could” in future claim the moon- gives
a example in clip. So avoiding that and maintaining the idea
that space is for all humankind is good idea, IMO.
Comment by gbaikie — October 9, 2011 @ 8:39 am
So relatively recently, there may have been volatiles beneath Ina? And the LTPs suggest outgassing may still be going on?
If there are subsurface volatile deposits, are they a potential resource?
Comment by Hop David — October 11, 2011 @ 12:13 am
In your “bulletized elevator speech” you point to a map of cislunar space. The metric used is kilometers.
Using this metric, LEO and GEO are much closer to the earth than the moon. This is somewhat misleading. A better metric for assessing difficulty of reaching a destination is kilometers/second.
Here is a map of cislunar space I have been using:
http://clowder.net/hop/TMI/FuelDepotGEO.jpg
Comment by Hop David — October 11, 2011 @ 12:24 am
So relatively recently, there may have been volatiles beneath Ina? And the LTPs suggest outgassing may still be going on? If there are subsurface volatile deposits, are they a potential resource?
Yes. It would be unlikely that we have found features active a million years ago but dead now. Of course, the rates of gas eruption may be extremely slow, so the likelihood of this becoming a usable resource is low. For resource planning, I would simply assume the presence of what we can directly observe in the polar soils.
In your “bulletized elevator speech” you point to a map of cislunar space. The metric used is kilometers. A better metric for assessing difficulty of reaching a destination is kilometers/second
I like your diagram and with your permission, will incorporate it into my spiel. However, a delta-v chart is useful but inadequate by itself. It does not convey the key dimension of transit time — this is not important in cislunar space but critical in discussing comparative NEO and Mars moon missions.
Comment by Paul D. Spudis — October 11, 2011 @ 4:42 am
I’d be delighted if you used that graphic. If you want something nicer than a low res jpeg, here is a vector graphic in pdf form:
http://clowder.net/hop/TMI/CislunarMap.pdf
I could export it in other formats as well.
Yes, you are quite correct that transit times become an important metric when considering NEOs and Mars.
Another very important metric is frequency of launch windows. An oft cited paper for NEO delta Vs is Shoemaker and Helin’s 1978 “Earth-approaching asteroids as targets for exploration, NASA CP-2053, pp. 245-256.” The figure they use is based on a Hohmann like path to the asteroid’s aphelion. Launch windows to the aphelion rendezvous Shoemaker and Helin imagined are vanishingly rare.
There are other transfer orbits besides a Hohmann like orbit to the NEO’s aphelion, but these take more delta V.
Comment by Hop David — October 11, 2011 @ 8:57 am
Comment by Hop David — October 11, 2011 @ 8:57 am
“There are other transfer orbits besides a Hohmann like orbit to the NEO’s aphelion, but these take more delta V.”
Exactly, which is why when we do start to explore/utilize the NEO’s we will want the capabilities made possible by Lunar propellants to make those transfer orbits practical.
By the way, for what it is worth, I also like the delta-V graphic.
Comment by Joe — October 11, 2011 @ 1:40 pm
Joe, believe it or not, I’m an asteroid guy. One of my most treasured possessions is an autographed copy of “Mining The Sky” by J.S. Lewis.
I am chiefly interested in the moon because lunar supplied propellant depots at EML1 and EML2 would make NEOs far more accessible.
Comment by Hop David — October 11, 2011 @ 2:50 pm
Comment by Hop David — October 11, 2011 @ 2:50 pm
“Joe, believe it or not, I’m an asteroid guy. One of my most treasured possessions is an autographed copy of “Mining The Sky” by J.S. Lewis.
I am chiefly interested in the moon because lunar supplied propellant depots at EML1 and EML2 would make NEOs far more accessible.”
Believe it or not I am (at heart) an asteroid guy as well. My first exposure to taking space development seriously was an even older book “Islands in Space” by Cole/Cox.
When I got into the actual program (working in EVA Operations) and then worked on how the first President Bush’s Space Exploration Initiative, I began to realize the challenges (in terms of both onsite operations and orbital mechanics) that would have to be overcome to utilize asteroid resources. At that point I became a ‘Moon First’ advocate not as a replacement for the asteroids, but as a path to eventually to get to them in a practical way. Since then I have also looked at the approach of Krafft Ehricke which combines the two concepts.
Nice to talk to someone of similar views.
Comment by Joe — October 11, 2011 @ 5:41 pm
Not being a lunar scientist, looking at the Ina feature seems like it would be more likely a blob of something (loose collection of some material, volatiles/ice?) hitting the lunar surface rather than a collapse feature. Is there some reason I can rule out my lay interpretation?
Comment by James Fincannon — October 12, 2011 @ 11:48 am
There are lots of features caused by blobs of material hitting the Moon — secondary impact craters. They do not possess this morphology. Ina is something very different. The rimless pit morphology suggests collapse rather than ejecta.
Comment by Paul D. Spudis — October 12, 2011 @ 12:35 pm
If I’m not mistaken, Ina also appears to be sitting on top of a large, low-relief shield-like structure that some interpret as a shield volcano, which further suggests it is a collapsed caldera-like pit associated with volcanism.
Comment by JohnG — October 13, 2011 @ 10:32 am
JohnG,
You are quite correct — Ina lies on the summit of a topographic swell that looks like a small shield volcano. However, that shield is considerably older than the surface of the extremely immature floor surface of Ina, so the gas eruption must post-date the shield building by a considerable length of time.
Comment by Paul D. Spudis — October 13, 2011 @ 12:35 pm
I just saw this. Excellent article Paul! The Moon really is a dynamic place. I was interested in your discussion of how “fresh” material matures once exposed to space after excavation. I was aware of several such processes that can expose “new” rocks and soil–impact events, basaltic floods, volcanos, lava tube collapses, ridge collapses–but I hadn’t thought of catastrophic gas eruptions.
Thus, I have a question that’s not too far off topic: if you had to hazard a guess, What is the typical turnover time for Lunar soil (beyond the ordinary gardening depth of a meter or two)?
Another way to put the question is, For a given square meter of Lunar surface,on average, how many times in the course of the entire history of the Moon was that square meter blanketed by or eroded by whatever process (from impacts to gas explosions) such that result is fresh, pristine material exposed to the vacuum of space?
10 million years? 100 million years? A billion years?
Perhaps there’s a classic reference on this topic?
Comment by Warren Platts — October 16, 2011 @ 11:22 pm
Warren,
The regolith is “gardened” by impact mixing but at very low rates, typically millions of years. The rates of “turnover” decrease with increasing depth, so the most active layer is the upper few cm of the soil. Average erosion rates are on the order of 1-2 millimeters per million years.
Cratering and regolith formation is discussed at length in several texts, the most thorough one (and written for the non-specialist) is The Lunar Sourcebook:
http://www.lpi.usra.edu/lunar_sourcebook/
This treatise was written by over 50 active lunar scientists and includes hard data on lunar materials and processes. It is a bit out of date (written in the late 1980′s-early 1990′s) and so pre-dates all the new information about the poles and lunar water. But as a general text for surface processes, it is unsurpassed and quite current.
Comment by Paul D. Spudis — October 17, 2011 @ 4:47 am
Thanks for the reference–the book’s on order!
Comment by Warren Platts — October 17, 2011 @ 9:32 am