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The Lunar Environment
Wendell Mendell
Astromaterials Research & Exploration
Science Directorate
Environment: Science or Engineering?
• Forty-five years ago, NASA engineers were very interested in the lunar
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environment; but no one knew very much about it.
Over the past 35 years, engineers have had little interest in the Moon,
but scientists have been studying various aspects of it for their own
reasons.
– The scientific findings reside in specialized papers, in a few textbooks
without much detail, and in one or two reference books written by
scientists.
– NASA engineers now need access to that information but have no time or
patience to dig it out.
– In addition, the information is presented by scientists for scientists and, in
some cases, can be misinterpreted by engineers.
– Some types of information needed by engineers are of no interest to
scientists and has never been systematically collected.
Today, Constellation needs lunar environment documentation.
– The very few lunar scientists who still exist are often unsuited for such
work.
– Cx has assigned engineers with little science background to write lunar
environment tracts, and they struggle to find all of the information.
Presenting the Lunar Environment
• For today’s talk, I want to avoid lists and tables of facts and
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figures.
I have chosen to organize the material in an unusual way,
incorporating some explanatory text for parts of it.
At the end of the talk, I will briefly summarize what you should
have heard if you stayed awake.
As an aside, I have found over the last two years that:
– Some engineers believe we know nothing about the Moon and
must fund large efforts to model the environment and to perform
experiments in simulations.
– (If we really knew nothing, the models would be no more than
educated guesses, and the simulations would have little credibility.)
– Some engineers decide that the Moon is like some particular region
of the Earth and use that environment for their designs.
– A few engineers believe we have all information for the Moon and
demand a specific piece of data by COB tomorrow.
This is the Moon. Where is it?
• In the Universe and, more specifically, in the Milky Way Galaxy.
• In the Solar System, i.e., near the Sun
• Near the Earth
This is a Lunar Explorer. Where is she?
• On a planetary surface.
• In an operational context.
• In a thermal environment.
• In a radiation environment.
• In a plasma environment.
In the Universe
• The Universe is generally dark, except for starlight.
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– The Japanese Kaguya lunar orbiter has taken a picture of the
interior of a large, permanently shadowed lunar crater using
starlight for illumination.
– We will talk later about sunlight (insolation) and Earthshine.
The Universe is generally cold.
– The Cosmic Microwave Background has a temperature of 2.7K. It
is a thermal radiation heat sink.
– We will talk later about the Sun in the sky (insolation).
In the Milky Way
• The Milky Way immerses the Moon in a constant flux of Galactic
Cosmic Rays (GCR).
– The Moon has no magnetic field to deflect the GCR.
– The lunar atmosphere is too thin to materially affect the incoming
GCR.
Galactic Cosmic Rays
• Galactic Cosmic Rays (GCR) are high-energy charged particles that
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enter the solar system from the galaxy. They are composed of
protons, electrons, and fully ionized nuclei of light elements.
Most galactic cosmic rays have energies too low to penetrate the
Earth's atmosphere, but all strike the Moon.
Like other ionizing radiation, GCR can damage DNA, increasing the
risk of cancer, cataracts, neurological disorders, and non-cancer
mortality risks.
Material shielding may be partially effective against GCR in certain
energy ranges, but may actually make the problem worse for some of
the higher energy rays because more shielding causes an increased
amount of secondary radiation.
The best shielding strategy appears to be structural shielding
combined with highly hydrogenated materials such as water or
plastics.
In the Solar System, Orbiting the Sun
• The Solar System environment
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– Inside the heliosphere
– Within the collisional environment of objects orbiting the Sun
Direct solar influence from electromagnetic and particle flux
– Electromagnetic, including insolation
– The solar wind plasma
– Solar Particle Events (SPE)
The Solar System Environment: Solar Wind
• The Sun continually emits streams of plasma (a mixture of ions and
magnetic fields) at high speed into the solar system.
– The flow is called the solar wind and streams through the solar system,
creating a bubble in the interstellar medium (also a plasma but less
dense).
– The interaction between the two mediums is a shock front called the
heliopause, which forms a partial barrier for GCR.
Solar Wind
• Steady streams of hot ions entrained along magnetic field lines
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flow outward from the solar corona whose effective temperature
is ~106 K. This combination of ionized gas and magnetic fields
is called a plasma.
The Moon, having no global magnetic field, poses a solid
dielectric obstacle to the solar wind. The magnetic fields pass
through the Moon, but the particles impact the surface, burying
themselves into surface grains.
A void in the solar wind is created behind the Moon.
The low-speed solar wind travels outward from the Sun at a
speed of about 400 km/sec, but a high-speed solar wind flows
out of solar coronal holes a factor of two faster.
The ions are overwhelmingly protons and electrons, but ionized
nuclei of various elements are included and are found in the
solar wind gases in the surface layers of lunar grains.
Solar Particle Events
• Gigantic explosions on the Sun can generate large fluxes of very high-energy
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ions. These Solar Particle Events (SPE) can be deadly to humans.
A particularly dangerous SPE occurred between the Apollo 16 and Apollo 17
missions. Had this event taken place during one of those missions, loss of life
would have been likely.
Under worst case conditions, a deadly flux of ions could arrive less than 30
minutes after the event is sighted on the Sun. (Light travels from the Sun to the
Earth in 8 minutes.)
Astronauts on the Moon must seek shelter from such an event, but a warning
system using satellite detectors may not be able to provide enough time to
react.
Heliophysicists are working to learn enough about the Sun to be able to predict
the eruptions on the Sun before they actually occur.
The Solar System Environment: Meteoroids
• In addition to the major planets (and the plutoids), a large number of
bodies orbit the Sun, ranging from comets and asteroids to
submicroscopic dust particles.
– The flux meteoroids is similar to that in Earth orbit and is well understood.
– A solid body as small as submicron can impact the surface of the Moon
and affect the surface in some way.
• Most meteoroids, even large ones, burn up in the Earth’s atmosphere.
• On the Moon, all impactors make some kind of crater and spray ejecta.
Meteoroid Impacts
• The lunar surface layer, the regolith, has been formed by impacts over
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geologic time. Pieces of the regolith have been eroded and fractured
and smashed by impacts. The regolith has been mixed and gardened
by impacts. Impacts Rule!
Micrometeoroid impacts, by far the most common, melt or vaporize
individual grains and deposit the residue on nearby grains. Or they can
weld individual grains into fragile glassy structures called agglutinates.
At MSFC the unilluminated Moon is routinely monitored with a video
camera on a telescope and 100 flashes from impacts have been
recorded over 30 months. No one knows how large the impacts are,
but they are clearly larger than micrometeoroids.
Risk to astronauts from a direct hit is small. Risk of damage to a
habitat can be calculated and appropriate shielding designed.
Insolation
• The environment attributed to the Sun is already familiar to
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engineers designing hardware or conducting operations in
Earth orbit or in deep space.
The Sun radiates as a black body at an effective temperature of
5777 K. The solar constant (at 1 AU) is 1367 W/m2, with most of
its radiant energy at visible and near-infrared wavelengths.
The unfiltered solar ultraviolet can be harmful to living tissue
and to various materials. Aerospace engineers already know
these environmental hazards.
The Sun is also a source of x-rays and gamma rays.
Near the Earth
• The dominant mutual attraction between the Moon and the Earth
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causes them to orbit about their mutual barycenter, defined as
the center of mass (CM) of the system.
– If one body is very much smaller, the barycenter is very close to the
CM of the larger body. We say the smaller is a satellite of the
larger.
– If the two bodies are close enough in mass such that the
barycenter lies in space between them, we talk about a binary
system, e.g., binary stars, binary asteroids.
– The Moon is large enough that the barycenter is significantly
removed from the center of the Earth, giving us some justification
for calling them a binary planet.
– From the perspective of a planetary scientist, the Moon is an
example of a small terrestrial planet, which is the reason we
scientists are so interested in studying it.
The propinquity of the Moon to the Earth also makes it a good
target for the initial phases of human exploration of the solar
system.
The Lunar Coordinate System
• If one imagines a thin wire connecting the CM of the Earth to the
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CM of the Moon, the point where the wire crosses the surface of
the Moon is called the subEarth point. A person standing on that
point at that time would see the Earth directly overhead.
The Moon is tidally locked to the Earth, meaning that it rotates
exactly once as it completes one orbit.
The rotation speed is constant, but the orbital speed varies along
the elliptical orbital path, being a little higher at perigee and
lower at apogee.
Thus, the subEarth point varies a bit during the month (orbital
period). A person living on the nearside of the Moon will see the
Earth hang permanently in the sky, moving to and fro.
The average position of the subEarth point is defined as the (0, 0)
point for selenographic longitude and latitude. Modern maps of
the Moon are drawn with north up and east to the right.
To an observer on Earth, the Moon looks like a disk.
Astronomers call the edge of the disk the limb. The average limb
is just the 90E and 90W lines of selenographic longitude.
Day, Night, & Seasons on the Moon
• The length of a lunation (time for successive zenith passage of
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the Sun, i.e., ‘noon’) is 29.53 Earth days or 708 hrs, 44 min.
The obliquity of the Moon (tilt of its spin axis from the ecliptic
pole) is 1.5˚. A person living near either lunar pole, at a latitude
higher than 88.5˚, will have 6 months of daylight and 6 months
of night, just as you do on Earth above the Arctic Circle.
I hope everyone knows that full moon is when the Sun & Moon
are on opposite sides of the Earth; new moon occurs when the
Moon is on the same side of the Earth as the Sun.
A nearside Moon dweller will see a full Earth at our new Moon
and a new Earth at our full Moon.
Earthshine
• The disk of the Earth in the sky as seen from the Moon is about 4
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times the solid angle of the disk of the Moon as seen from the Earth.
In addition, the Earth is a much more reflective object than the
Moon, particularly when covered with clouds.
Thus, the light from Earthshine is much brighter than light from the
Moon on a clear night.
When the sky is clear (on the Earth) at new Moon, you can see the
Old Moon in the arms of the New. You are seeing the unlit portion
of the Moon illuminated by Earthshine!
Astronauts may well be able to work on the lunar surface at night
under the illumination by the Earth.
Magnetotail
• When the solar wind encounters the Earth’s extended magnetic
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field, it tries to push through the magnetic field, compressing it
and locally increasing the magnetic pressure.
The solar plasma flows around the magnetosphere, forming a
cavity behind the Earth, which fills in at some great distance.
The cavity is called the magnetotail, and the shock formed in the
interaction between the two fields is called the bow shock.
The Moon, at a distance of 60 Earth radii, passes through the
bow shock, magnetopause, and magnetotail, altering the
charging environment that it experiences in the solar wind.
How does the Moon itself contribute?
• Most of the contributions to the engineering environment by the
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Moon itself come from effects generated by the outside
influences.
– Examples include reflection of insolation, thermal emission from
solar heating, ejecta from impacts, neutrons generated in the
surface layer by GCR, charging of the surface through interaction
with the plasma environment.
Some effects will manifest themselves as products of human
activity.
– Examples include kicking up dust particles on the regolith,
generation of a temporary atmosphere from rocket exhausts or
human life support systems, particles entrained in rocket exhausts
and eroding structures
Although the Moon is generally regarded as a dead planet, some
evidence exists for current or for geologically recent activity.
– Examples include emanations of certain gases observed from orbit,
seismic activity detected by Apollo seismometers, certain features
that appear to represent geologically young activity
Low Gravity
• Everyone is familiar with 1/6 g on the lunar surface. For the
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record, the acceleration of gravity is 1.62 m/sec2 and the escape
velocity is 2.37 km/sec.
Astronauts report that working in lunar gravity is easier than
working in weightlessness.
However, low gravity is a design environment for which we have
limited experience.
– How will people move about in a habitat?
– How high should ceilings be?
– How do bubbles move in fluids?
– How do earthmovers work when their weight is reduced?
Lunar Gravitational Field
• The lunar gravitational field is unusually ‘lumpy’ because the
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Moon has large, near-surface mass concentrations.
Low orbits are unstable and require extra fuel for orbit
maintenance.
Tenuous Lunar Atmosphere
• Technically, the lunar atmosphere is surface-bounded
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exosphere, meaning the individual molecules are so sparse that
they rarely collide.
– The number density is one-thousandth the number density at ISS
altitude at the Earth.
– The entire lunar atmosphere would occupy a cube 64m on a side at
Earth pressure at sea level.
– Each Apollo landing increased the volume of the atmosphere by
30%.
The composition of the lunar atmosphere is poorly known.
– Various noble gases, either of solar wind origin or outgassing from
the Moon itself.
– Sodium and potassium have been detected (discovery by JSC
astronomers).
Visual observations by Apollo 17 astronaut has been interpreted
as a population of very small particles streaming from the Moon,
presumably driven by electric fields.
Reflected Sunlight
(Visible & IR)
• Lunar albedo (solar reflectance) is generally bimodal:
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– Dark in mare rocks
– Not so dark in highland terrain
Lunar soils show increasing reflectance from the blue to the red,
further increasing in the near-infrared.
The upper millimeter or so of the lunar surface has a very porous
structure (described as a ‘fairy castle’) that causes high
retroreflection of light (like highway reflectors).
– Looking downsun, one sees a loss of detail, like a white-out in snow.
– The lunar photometric function is well documented and modeled.
This kind of reflectance function is characteristic of bodies
without substantial atmosphere and a regolith.
Lunar Surface Thermal Regime
• The lunar surface temperature is periodic with the period of the lunar
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‘day’.
Classical heat conduction theory shows that a periodically varying
surface temperature will generate exponentially damped thermal waves
in the subsurface with the same frequencies seen in the surface
variation.
– Higher frequencies are damped more quickly with depth.
– The lower the thermal inertia, the greater the damping with depth.
The lunar surface has an unusually low thermal inertia, and the large
surface variation in temperature is almost completely damped within
the first meter of the subsurface.
The low thermal inertia means that the upper surface temperature
quickly comes into radiative equilibrium with incident insolation and
cools dramatically when it is removed.
– Topography will exhibit a wide range of surface temperatures, depending
on whether a surface element is in the sunlight or not and on what the
incident angle of the sunlight might be.
Thermal reradiation is directional (not Lambertian), directed back
toward the source of insolation.
Rocks have a much higher thermal inertia than the soil and will remain
warmer at night and cooler during the day.
Lunar Neutron Albedo
• GCR and SPE protons strike the lunar surface with high energy,
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causing nuclear reactions in atoms of the surface material.
Free neutrons are created by this process and were detected by
an experiment aboard Lunar Prospector and by direct
measurement on the surface during Apollo.
Thermalized neutrons are absorbed by hydrogenated materials,
and this must be taken into account in shielding design.
You also live in a neutron sea on Earth.
– At the 2006 AGU meeting in San Francisco, a neutron detector was
used to estimate the relative number of people on the floor in the
Exhibits Area at any one time.
Surface Charging
• The Moon is immersed in the plasma of the solar wind and bombarded
by solar UV radiation, so the charge state varies across the surface
depending upon whether the surface is illuminated or not.
– On the day side, the photon flux dominates, causing the ejection of
photoelectrons; the surface should acquire a net positive charge.
– The surface potential grows more negative toward the terminator.
– At the terminator & on the night side, the charge becomes negative with a
positive sheath.
The charge on the day side is ~10V to
18V positive. On the night side, about
10V negative, directed outward.
At the poles, the constant low sun
angle and complex topography
creates a patchwork of light &
shadow, making prediction of
charging difficult.
Dust
• Dust is of great interest to engineers and of little interest to scientists.
• The regolith is a collection of particles ranging in size from as-big-as-
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you-want to submicroscopic. Designers and operations planners worry
about mobilization of small particles that might:
– Be inhaled by astronauts in a habitat;
– Coat solar arrays and thermal control surfaces;
– Work their way into mechanisms and electronics.
Apollo designers did not spend much effort on dust mitigation.
– No failures occurred due to the dust, but evidence suggests that problems
would have arisen if the mission were longer.
– Dust does not hang in the air because there is no air.
– Dust was mobilized by rover operations.
– Astronauts fell down (sometimes on purpose), coating their suits with dust
that then was transported into the habitat.
Some engineers are concerned that small particles will be mobilized by
naturally occurring electric fields on the Moon even if operations are
carefully designed to minimize dust generation.
– The subject is controversial due to lack of good data.
Subsequent lectures in this series will be devoted to the regolith and to
small particles.
Ejecta
• Impactors large enough to make visible craters will eject
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material during the impact.
No one has ever seen formation of a lunar crater, so models of
the process are inferred from orbital photographs of very large
craters (km-sized or bigger) and laboratory data for much, much
smaller impacts and some nuclear explosion craters on Earth.
– Some ejecta from the biggest craters traveled around the Moon.
– Some material could be ejected from large craters with velocities of
hundreds of meters per second.
– Large crater events are very rare.
– No one can reliably predict ejecta velocities and fluxes from smaller
craters that might possibly form while astronauts are on the Moon.
– The size of the events causing observed flashes are not known.
We need more data from the Moon on craters before we can
confidently evaluate the risk to crews from ejecta.
Seismic Events
• Four seismometers emplaced by the Apollo astronauts operated for up
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to 7 years on the lunar surface.
– Lunar seismograms were entirely unlike terrestrial seismograms.
The seismic energy of the Moon is many orders of magnitude less than
that of the Earth.
– The ground movement associated with the largest lunar signals was a few
tens of nanometers.
Types of events that could be categorized are:
– Deep Moonquakes - most abundant type; small magnitude (typically ~1);
occur at depths of 800-1000 km; occurrence is strongly linked to tides
raised by the Earth and the Sun.
– Shallow Moonquakes - highest energies, but rare (~4 events/yr); two had
magnitudes close to 5; focal depths not determined.
– Thermal Moonquakes - 1000’s of very small events; caused by
temperature variations near or at the surface.
– Impact events, both artificial (rocket bodies) and natural; largest natural
impactor estimated at 5 tons.
– Unclassified - majority of events.
In general, the Moon is extremely stable against moonquakes.
Emanations
• All of the features and samples we studied from the Moon were billions
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of years old.
– A few small craters sampled at Apollo sites were only a few hundred
million years old
Models of the thermal history of the Moon indicate that it is now
inactive, unlike the Earth. However,…
– The alpha-particle spectrometer on Apollos 15 and 16 detected radon and
polonium (both radioactive) emanating from certain parts of the Moon,
most notably the edges of maria.
– The Apollo Suprathermal Ion Detector (SIDE), part of ALSEP, saw bursts
of gas, possibly correlated with seismic signals interpreted as impacts.
– At least one class of seismic events are not well understood and may
associated with tectonic activity.
– Certain unusual features seen in orbital photography appear to
geologically young (low numbers of impact craters on the surface).
Should evidence of indigenous lunar activity be confirmed, our picture
of the Moon would have to be revised.
Long-lived surface monitoring stations would address this issue,
although human activity might mask the weak signals.
Résumé
• We have discussed characteristics of the environment on the surface of
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the Moon that are for people who will be designing equipment and
planning operations and people who will be working there.
We have seen that many of the unfamiliar characteristics of the
environment trace back to processes in our galaxy or on our Sun.
– These phenomena are unfamiliar to Earthlings because our geomagnetic
field and our dense atmosphere protect us.
Many of the ‘exotic’ phenomena should be familiar to aerospace
engineers who design spacecraft or plan astronaut activities in orbit.
Unlike the Earth, the Moon has no weather or geologic activity; and the
state of the surface materials may provide unexpected challenges.
– With experience, we will prevail. Unfortunately, we have to get it right the
first time.
Please remember the laws of nature still apply on the Moon, and the
materials there are not inherently malicious.