Recently I watched the presentation that Brian Day did on Exploring the lunar atmosphere. I had some questions. I don’t want to sound like a jerk, but Brian Day said that your organization THE LUNAR SCIENCE INSTITTUTE did not know what the lunar atmosphere consisted of, he also said that your organization did not know what causes the 10 minute 5.5 Richter scale shallow moon quakes. I am sure this can’t be true is it that this information is not being released to the public yet.

my questions are

1. WHAT DOES THE LUNAR ATMOSPHERE CONSIST OF?

2. WHAT CAUSES THESE MOON QUAKES?

We have had Spectrographs for a long time, and NASA publishes what other atmospheres consist of that are much farther away and less studied than our moon.

P.S. Is there anywhere I could go to have a conversation about the moon I would pay money or travel

DAVID

Hi David,

Thanks for your questions. Let me see if I can provide some answers for you.

You may recall from our discussion that the Moon’s atmosphere is actually a surface boundary exosphere. This is both qualitatively and quantitatively different from the thick collision-dominated atmospheres exhibited by planets such as the Earth, Venus, Mars, Jupiter, etc. Here on Earth’s surface, we experience an atmosphere with approximately ten to the nineteenth molecules per cubic centimeter. The atmosphere at the Moon’s surface contains more like 100,000 molecules per cubic centimeter. This is what we would consider to be an excellent vacuum here on Earth and is similar to what our astronauts experience in the outer fringes of the Earth’s atmosphere where the International Space Station orbits. It is true that spectroscopes here on Earth can retrieve excellent data from the thick atmospheres of other planetary bodies in our solar system and even from tenuous nebulae in deep space. But those planetary atmospheres, while relatively small, are very dense, and the nebulae, while very tenuous, are very large. In both of those cases, we are observing through a large column density of gas. A photon passing through that gas on its way to our spectroscope will encounter lots of atoms and have lots of opportunities to help create an observable spectral line. In the case of the lunar exosphere, we are dealing with a subject that is both very tenuous and very small, resulting in a very low column density. A photon passing through the lunar exosphere on its way to our spectroscope has a good chance of making it through without interacting with the few atoms and molecules along its path. Therefore, the spectral signature that we can gather from the lunar exosphere is very weak compared to what we can gather from dense atmospheres. We can indeed detect some lunar exosphere spectral lines from here on Earth. You may remember me speaking specifically about sodium. However, we can only detect the strongest lines from here on Earth. It also is very important to remember that the relative strength of spectral lines produced by two different types of atoms does not necessarily indicate the relative abundances of those atoms. Certain atoms, by virtue of the way their electrons are configured, will produce brighter lines. Thus sodium dominates the visible spectrum of the lunar atmosphere even though it is probably a minor constituent. Many of the atoms and molecules in the very thin lunar exosphere produce signals that are just too weak to be measured from here on Earth. That is why we want to get our instruments directly into the lunar exosphere, as we will do with the LADEE mission. For more information, see http://sirius.bu.edu/planetary/moon.html, http://science.nasa.gov/science-news/science-at-nasa/2009/23oct_ladee/, and http://science.nasa.gov/science-news/science-at-nasa/2011/14nov_lunarionosphere/.

In our discussion, we mentioned that we expected and found moonquakes from a variety of sources. These included those generated from tidal interactions with the Earth, meteoroid impacts on the Moon, and fracturing resulting from repeated thermal expansion and contraction of rock. However, not all moonquakes are nice enough to conform to our expectations. These include relatively shallow moonquakes that can reach magnitude 5.5 and last over 10 minutes. See (http://science.nasa.gov/science-news/science-at-nasa/2006/15mar_moonquakes/) for more information. It is important to remember that while the Earth has had an extensive network of seismometers studying its interior for many years, only a few seismometers were placed on the Moon’s surface, and they were operational for only a few years. It is therefore not at all surprising that our understanding of moonquakes falls far short of our understanding of earthquakes. We hope to improve our understanding of the lunar interior and its behavior through observations from the GRAIL mission (http://www.nasa.gov/mission_pages/grail/overview/index.html).

There are several outstanding events that you can attend that will allow you to interact with leading lunar researchers from NASA and a wide variety of institutions from around the world. The Lunar Science Forum (http://lunarscience.nasa.gov/event/lunar-science-forum-2012/) is sponsored by the NASA Lunar Science Institute and is held each year at NASA Ames Research Center. The annual Lunar and Planetary Science Conference is held in the Woodlands outside of Houston Texas and is hosted by the Lunar and Planetary Institute (http://www.lpi.usra.edu/). Also consider attending the annual Fall Meeting of the American Geophysical Union in San Francisco (http://www.agu.org/meetings/).

I hope this helps,
Brian

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