Lunar Environment

From TeamFrednetWiki

This document intends to cover all general characteristics of the lunar environment that are relevant to the mission.

For in-depth analysis of potential landing sites, see Landing Sites.

Contents 1 Gravity field 2 Magnetic field 3 Atmosphere 4 Temperatures 5 Radiation 5.1 Cosmic 5.2 Sun 5.3 Earth 5.4 Moon 6 Micrometeroids 7 Surface 7.1 Topography 7.2 Seismology 7.3 Regolith 7.3.1 Composition 7.3.2 Physical properties 7.3.3 Chemical properties 7.3.4 Thermal properties 7.3.5 Electrical properties 7.3.6 Mechanical properties 7.4 Electrostatic potential 7.5 Aerosols 7.6 Illumination 8 Electromagnetic environment 9 References 10 Acknowledgments

Gravity field

The major characteristic of the Moon's gravitational field is the presence of mascons, which are large positive gravitational anomalies associated with some of the giant impact basins. These anomalies greatly influence the orbit of spacecraft about the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions.^[1]

Magnetic field

The Moon has an external magnetic field of the order of one to a hundred nanotesla—less than one hundredth that of the Earth, which is 30–60 microtesla. Other major differences are that the Moon does not currently have a dipolar magnetic field (as would be generated by a geodynamo in its core) and the varying magnetization that is present is almost entirely crustal in origin.

Roughly once every Lunar orbit, the Moon passes through Earth's magnetotail for approximately 6 days, starting 3 days before lunar noon (full moon) and ending 3 days after.

Atmosphere

The atmosphere of the Moon is very tenuous and insignificant in comparison with that of the Earth. One source of the lunar atmosphere is outgassing: the release of gases such as radon that originate from radioactive decay within the crust and mantle. Another important source is the bombardment of the lunar surface by micrometeorites, the solar wind, and sunlight, in a process known as sputtering.^[2] Gases that are released by sputtering can either be reimplanted into the regolith as a result of the Moon's gravity, be lost to space either by solar radiation pressure or, if the gases are ionized, by being swept away in the solar wind's magnetic field.

The elements sodium (Na) and potassium (K) have been detected using Earth-based spectroscopic methods, whereas the element radon-222 and polonium-210 have been inferred from data obtained by the Lunar Prospector alpha particle spectrometer.^[3] Argon-40, helium-4, oxygen and/or methane (CH4), nitrogen gas (N2) and/or carbon monoxide (CO), and carbon dioxide (CO2) were detected by in-suit detectors placed by the Apollo astronauts.^[4]

The average daytime abundances of the elements known to be present in the lunar atmosphere, in atoms per cubic centimeter, are as follows: H <17, He 2-40x10³, Na 70,K 17, Ar 4x10⁴, yielding ~8x10⁴ total atoms per cubic centimeter, marginally higher than the quantity posited to exist in the atmosphere of Mercury^[5]. It is worth noting however, that while this greatly exceeds the density of the solar wind (a component of the lunar atmosphere), which is usually on the order of just a few protons per cubic centimeter, the lunar atmosphere is less than one trillionth the density of the Earth's atmosphere at sea level. The Moon is usually considered to not have an atmosphere, as it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment given the high rate at which the atmosphere is lost to space (solar wind and outgasing are not primary components of the Earth's, or any stable atmosphere yet known).

The Moon may also have a tenuous "atmosphere" of electrostatically-levitated dust. See Aerosols for more details.

The atmospheric pressure on the lunar surface is approximately 1 nPa, making the surface environment a hard vacuum. The vacuous environment pose a challenge to all material components of a lunar mission as it increase the rate of outgassing:

The vacuum environment will result in material outgassing and will promote degradation of materials and optical components from exposure to ultraviolet radiation. The hard vacuum precludes the use of many common plastics and rubbers whose strength and pliability become reduced by out gassing of their volatile components. Out gassed materials can also collect on optical and sensing surfaces, which can reduce their effectiveness. Organic, organo-metallic, and organo-silane polymers (and copolymers) which are fully reacted, and consequently have low vapor pressures, may be used if their optical and/or mechanical properties are stable over the expected influences of solar radiation and their temperatures are maintained above “glass” phase transitions.^[6]

Temperatures

The mean surface temperatures at the equator of the Moon is approximately 220 K (-53.15 C, -63.67 °F). The minimum and maximum temperatures at the equator is approximately 100 K (-173.15 °C, -279.67 °F) and 390 K (116.85 °C, 242.33 °F) respectively.

The thermal environment resulting from the long day/night cycle (~ 2 weeks each) will mean a long period of intense heating followed by a similar period of intense cold. The thermal environment around a rover will consist of direct solar flux from the sun, reflected lunar albedo flux, and infrared radiation directly from the lunar surface.

The solar flux is the amount of power that passes through a given area at a given distance from the sun. The nominal value at the Earth’s (or Moon’s) distance from the sun is called the Solar Constant, and the average value is 1358W/m2.

The Moon’s albedo (reflectivity) is less than 10 percent. This means that 90 percent of incipient solar radiation heats the surface. The amount of reflected energy that impinges on the rover is dependant on the orientation of the rover and is much smaller in magnitude than direct solar radiation and IR radiation from the lunar surface. The lunar surface acts as a grey body source at the temperature of the surface. This surface temperature varies according to latitude and the time in the lunar day/night cycle. The extremes that the rover expects to see are +120 to –150 °C. These extremes are similar to going from super heated steam to liquid nitrogen temperatures. Areas near the poles can be more benign, and vary from approximately –63 to –43 °C, assuming a mission profile that avoids lunar night. Of particular concern, however, is prolonged rover operations in permanently shadowed craters at the lunar South poles, where water ice is most likely to be found. Here ambient temperatures have been estimated at < –260 °C. Operation at this extreme cold environment will require additional power for many heaters, or advanced technologies to withstand the cold, such as ultra-low temperature lubrication and materials.^[6]

A discussion about the infrared radiation from the regolith that is incident upon rover/lander structures is going on here. Tasks:IR-radiation calculation on lunar surface

Radiation

In physics, radiation is energy in the form of waves or moving subatomic particles emitted by an atom or other body as it changes from a higher energy state to a lower energy state. Radiation can be classified as ionizing or non-ionizing radiation, depending on its effect on atomic matter. Ionizing radiation has enough energy to ionize atoms or molecules while non-ionizing radiation does not. Radioactive material is a physical material that emits ionizing radiation.

[On the lunar surface, the] radiation exposure, with no appreciable atmosphere or magnetic field for protection, can be as high as that of interplanetary space in the solar system. Solar and cosmic radiation concerns will dominate human protection as well as electrical component protection from single event upsets and hard failures. Degradation of optical components will also be a factor. The rovers will encounter the harsh space ionizing radiation environment: large fluxes of low-energy solar wind particles, smaller fluxes of high-energy galactic cosmic rays, and occasional intense particle fluxes emitted by solar flares. In addition to the ionizing radiation that reaches the lunar surface, soft x-rays and ultraviolet light are also present in significant quantities.^[6]

Cosmic

Main article: Cosmic radiation

Stars and other celestial phenomena are technically visible in the lunar sky throughout the lunar day and night due to the absence of atmospheric filtering. In the daytime, stars are not visible to the naked human eye because of the intensity of direct or diffusely reflected rays of sunshine. Stars did not appear in photographs from the Apollo missions either, due to underexposure.

Cosmic rays occur very infrequently (~4 protons/cm2-sec), but are very high energy. While the number of particles is not an issue, their high energy can cause damage to electrical components. A single particle can damage an electrical component and cause its failure through energy loss and elastic and inelastic scattering processes.

Soft x-rays and ultraviolet light affect surface coatings and optics, due to their energy levels in the solar electromagnetic spectrum. Solar ultraviolet and soft x-ray photons are sufficiently energetic to induce defect centers in optical materials, and can cause darkening throughout shallow depths.^[6]

Sun

The solar wind particles are the most numerous particles striking the rovers, but due to their comparative low-energy, are of less concern than galactic cosmic rays and solar flare events. Solar flares can occur several times a year, and emit a large number of particles at relatively high energies (1–100MeV). These flares can last from several hours to many days, and have the potential to bombard the rovers with high energy particles that can damage the rover's surface and structural integrity and electronic components. ... These energetic protons ionize optical materials and since they are massive they create defects throughout the bulk of those materials. This radiation must be considered when choosing structural materials and component placement within the rover.^[6]

Earth

The Earth is nearly fixed in the lunar sky and reflects a certain amount of sunlight, known as earthshine, to the lunar surface. The albedo of the Earth is 0.36 (+/- .06, depending on cloud cover). Even when full, the Earth is 10,000 times less bright than the sun or 76 times brighter than the Moon seen from Earth^[7]. At half phase, which is the worst case for a base located at the center of the near side of the moon, the Earth is 20,000 times less bright than the sun.

Jörg Schnyder has calculated the power density of earthshine on the lunar surface as being approximately 91 mW/m2.^[8]

Moon

Apart from being bombarded by celestial sources of radiation, the lunar regolith is itself radioactive.

Micrometeroids

Due to the lack of atmosphere on the Moon, there is a risk of micrometeoroids striking the lunar surface.

On the basis of the lunar meteoroid flux, the cratering rate on an hypothetical flat aluminum alloy target exposed on the lunar surface for one year has been predicted. The cumulative density of craters, as a function of diameter is about 30 microcraters with size larger than 0.1 mm are produced per m² per year.

As an example, a surface of about 150 m² located on the Moon is hit, on average, by one micrometeoroid larger than 0.5 mm in diameter per year: a projectile of that size, impacting with an average velocity of about 13 km s^–1, excavates in aluminum alloy material of an hypothetical lunar basis structure a crater with diameter larger than about 1.8 mm and depth greater than about 1 mm. Micrometeoroids of about 0.1 mm in size can produce craters of 350 μm in diameter and of comparable depth in metal targets.^[9]

This model seems to indicate that the micrometeoroid hazard is negligible for short missions, but increasingly more significant as the mission duration increase. In particular functionally very dense structures and systems such as unshielded microelectronics would be at noticeable risk.

It is suggested that two to three millimeters of a tough composite material can provide effective shielding from micrometeoroids in the milligram mass range traveling at 13 to 18 km/sec. The lunar environment possesses a hard vacuum with 2 orders of magnitude fewer particles per unit volume than low earth orbit.^[6]

Surface

Topography

The topography of the Moon has been measured by the methods of laser altimetry and stereo image analysis, most recently from data obtained during the Clementine mission. The most visible topographic feature is the giant far side South Pole-Aitken basin, which possesses the lowest elevations of the Moon. The highest elevations are found just to the north-east of this basin, and it has been suggested that this area might represent thick ejecta deposits that were emplaced during an oblique South Pole-Aitken basin impact event. Other large impact basins, such as the maria Imbrium, Serenitatis, Crisium, Smythii, and Orientale, also possess regionally low elevations and elevated rims.

The terrain of the lunar surface has been defined by meteor strikes. Continual impacts of micrometeoroids have resulted in an extremely fine, loosely-compacted soil. Many of the large-scale features, such as steep crater walls and large boulders, are insurmountable obstacles to the rover. ... The locomotion limit for typical lunar rovers on friable slopes is in the range of 30° to 35°^[6]

New topographic data from the Moon can be found from current lunar orbiters such as JAXA's Kaguya mission here:

http://wms.selene.jaxa.jp/selene_viewer/en/observation_mission/lalt/lalt_004.html

http://wms.selene.jaxa.jp/selene_viewer/en/observation_mission/lalt/lalt_005.html

See also Forum post: Lunar Maps

See also LOS Luna: Ideal Line Of Sight Sheet

See also inbound LRO data from USGS ISIS

Seismology

Moonquakes

Regolith

Blanketed atop the Moon's crust is a highly comminuted (broken into ever smaller particles) and "impact gardened" surface layer called regolith. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3–5 m in the maria, and by about 10–20 m in the highlands.^[10] Beneath the finely comminuted regolith layer is what is generally referred to as the megaregolith. This layer is much thicker (on the order of tens of kilometers) and comprises highly fractured bedrock.^[11]

On average, 95% of the soil is finer than 1.37 mm by weight; and 5% is finer than 0.0033 mm. The average or median particle size, D₅₀ (where 50 refers to 50% passing) is approximately 0.072 mm. This size is very close to the boundary between sand and silt (0.074 mm), and the lunar soil is usually described as either silty sand or sandy silt. The sand-silt boundary is the limit of size that can be resolved with the unaided human eye; thus, about half of the soil, by weight, is too fine to be seen.^[12]

The dusty fraction of regolith pose a challenge to many components of a vehicular system:

Dust will coat mechanical components, causing abrasion of surfaces and wear of moving parts. The dust also forms a thermal insulator that makes heat removal difficult. The particles of the lunar regolith are very fine ..., sharp, and highly abrasive. These particles will erode bearings, gears, and other mechanical mechanisms not properly sealed. The dust will also abrade seals.^[6]

Composition

Physical properties

The bulk density of regolith samples returned by Apollo 15 and 16 varied between 1.92 g/cm³ and 2.84 g/cm³. ^[13]

The recommended typical average values of regolith bulk density in intercrater areas are^[14]:

Depth range (cm)  Bulk density (g/cm³)
     0 - 15           1.45 - 1.55
     0 - 30           1.53 - 1.63
    30 - 60           1.69 - 1.79
     0 - 60           1.61 - 1.71

Chemical properties

Thermal properties

The lunar soil has low thermal conductivity.

Electrical properties

Lunar regolith has extremely low conductivity.

Various effects, combined with its low conductivity, cause the lunar dust to become electrostatically charged:

Lunar dust carries an electrostatic charge which enables it to cling to nongrounded conductive and nonconductive surfaces. Astronauts from manned landings reported that removing dust from their equipment was difficult. The accumulation of dust on optics and radiators is also of concern. Even small quantities on the front surfaces of refractive optics will severely increase stray light scattering. Conversely, thin layering on thermal radiators is not likely to cause problems. Thicker accumulations will degrade radiator system performance and hence must be kept acceptably low.^[6]

See Electrostatic potential for details on the electrostatics of lunar regolith.

Mechanical properties

The rolling resistance coefficient of lunar regolith is 0.18.^[15]

Trafficability on disturbed [lunar soils] due to grading, backfilling, or other soil excavation, is also not well understood. These soils may not be able to support loads well, possibly resulting in soil behavior similar to quick-sand or snow. Compact soil can carry loads well because of a rigid network of connections between grains. It is possible that disturbed planetary soil may expand too much and reduce its load carrying capacity. The maximum contact pressure for undisturbed lunar soil is proposed to be 7 kN/m2 (the lunar rover averaged about 4.2 kN/m2). This is an important factor when designing wheels or other surface traction device for rovers.^[6]

Electrostatic potential

Due to the extremely low conductivity of lunar regolith, the lunar surface builds up electrostatic potential by photoelectric effects in the daytime and by solar wind and Earth magnetotail crossing effects in the nighttime.

Roughly once every Lunar orbit, the Moon passes through Earth's magnetotail for approximately 6 days, starting 3 days before full moon and ending 3 days after. Interaction with the plasma sheet causes the Moon's surface to become negatively charged. On the Moon's dayside this effect is counteracted to a degree by sunlight: ultraviolet photons knock electrons back off the surface, keeping the build-up of charge at relatively low levels. But on the nightside electrons accumulate and surface voltages can climb to hundreds or thousands of volts.

The Lunar Prospector spacecraft detected changes in the lunar nightside voltage during magnetotail crossings, jumping from -200 V to -1000 V. The plasma sheet is a very dynamic structure, in a constant state of motion, so as the Moon orbits through the magnetotail the plasma sheet can sweep across it many times with encounters lasting anywhere from minutes to hours or even days.^[16]

The risk of electrostatic discharge of surface potentials may constitute a hazard to electronic and electrical equipment.

Aerosols

The Moon appears to have a tenuous atmosphere of moving dust particles constantly leaping up from and falling back to the Moon's surface, giving rise to a "dust atmosphere" that looks static but is composed of dust particles in constant motion, almost with a whole meteorology of its own. The term "Moon fountain" has been used to describe this effect by analogy with the stream of molecules of water in a fountain following a ballistic trajectory but appearing static due to the constancy of the stream.

According to the model recently proposed by Timothy J. Stubbs, Richard R. Vondrak, and William M. Farrell of the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center,^[17] this is caused by electrostatic levitation. On the daylit side of the Moon, solar ultraviolet and X-ray radiation is so energetic that it knocks electrons out of atoms and molecules in the lunar soil. Positive charges build up until the tiniest particles of lunar dust (measuring 1 micrometre and smaller) are repelled from the surface and lofted anywhere from meters to kilometers high, with the smallest particles reaching the highest altitudes. Eventually they fall back toward the surface where the process is repeated over and over again. On the night side the dust is negatively charged by electrons in the solar wind. Indeed, the fountain model suggests that the night side would charge up to higher voltages than the day side, possibly launching dust particles to higher velocities and altitudes.^[18] This effect could be further enhanced during the portion of the Moon's orbit where it passes through Earth's magnetotail; see Magnetic field for more details.^[16]

On the terminator there could be significant horizontal electric fields forming between the positively charged day and negatively charged night areas, resulting in horizontal dust transport - a form of "moon storm".[8]^[19]

Due to the highly abrasive nature of moon dust, these 'winds' constitute a hazard to equipment as dust particles may rub and wear down surfaces through friction.

The dominant source of suspended dust will be the rover interaction with the soil. As seen in video footage of the Apollo 17 LRV, the amount of dust sprayed from the wheels was large and reached heights of over two meters. ... Since the lunar atmosphere is essentially a vacuum, the lifted particles do not remain suspended, but quickly return to the surface, with each particle following a ballistic trajectory.^[6]

Illumination

During the two-week lunar day, the dominant source of illumination of the lunar surface is the sun and unfiltered sunlight is available full time, without diminution by solar angle or obscuration by clouds, at 128,770 lux. Illumination by sunlight reflected by the Earth (earthshine) varies between 0 lux and 13,5 lux, depending on phase angle.

During lunar night, earthshine is the dominant source of illumination, again varying between 0 lux and 13,5 lux, depending on phase angle.^[20]

Electromagnetic environment

Turns out the moon dust stickiness depends on time-of-day (ie. sun angle)! Sticky Dust

References

1. ↑ Gravity of the Moon. Wikipedia. (http://en.wikipedia.org/wiki/Gravity_of_the_Moon)
2. ↑ P. Lucey and 17 coauthors (2006). "Understanding the lunar surface and space-Moon interactions". Reviews in Mineralogy and Geochemistry 60 (1): 83–219. doi:10.2138/rmg.2006.60.2.
3. ↑ S. Lawson, W. Feldman, D. Lawrence, K. Moore, R. Elphic, and R. Belian (2005). "Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer". J. Geophys. Res. 110 (E9): E9009. doi:10.1029/2005JE002433.
4. ↑ S. Alan Stern (1999). "The Lunar atmosphere: History, status, current problems, and context". Rev. Geophys. 37 (4): 453–491. doi:10.1029/1999RG900005.
5. ↑ Adapted from Stern, S.A. (1999) Rev. Geophys. 37, 453
6. ↑ ^{6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10} Exploration Rover Concepts and Development Challenges. J.J. Zakrajsek, D.B. McKissock, J.M. Woytach et al. March 2005. (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050175879_2005173639.pdf)
7. ↑ Lighting constraints on lunar surface operations. NASA Technical Memorandum 4271, Dean B. Eppler. 1991. (http://www.lpi.usra.edu/lunar/strategies/human_ex/lighting_constraints.pdf)
8. ↑ Jörg Schnyder. Power from Earth to survive lunar night. October 29, 2008. Team FREDNET forums. (http://forum.xprize.frednet.com/viewtopic.php?p=1861#1861)
9. ↑ Micrometeroid Impacts on the Lunar Surface. V. Vanzani, F. Marzari, and E. Dotto, Dipartimento di Fisica “G. Galilei” and Centro Interdipartimentale Studi e Attività Spaziali “G. Colombo”, Università, 35131 Padova, Italy. (http://www.lpi.usra.edu/meetings/lpsc97/pdf/1025.PDF)
10. ↑ Heiken, G.; Vaniman, D.; French, B. (eds.) (1991). Lunar Sourcebook, a user's guide to the Moon. New York: Cambridge University Press, 736.
11. ↑ Rasmussen, K.L.; Warren, P.H. (1985). "Megaregolith thickness, heat flow, and the bulk composition of the moon". Nature 313: 121–124. doi:10.1038/313121a0. Retrieved on 2007-04-12.
12. ↑ The four things you need to know about the GEOTECHNICAL PROPERTIES OF LUNAR SOIL. W. David Carrier, III. Lunar Geotechnical Institute. September 2005. (http://www.lpi.usra.edu/lunar/surface/carrier_lunar_soils.pdf)
13. ↑ Density And Porosity Calculations For Apollo 15 And 16 Regolith Breccias. S. J. Wentwoth, et al., NASA Johnson Space Center, Houston (1984) (http://adsabs.harvard.edu//abs/1984LPI....15..906W)
14. ↑ Heiken, G.; Vaniman, D.; French, B. (eds.) (1991). Lunar Sourcebook, a user's guide to the Moon. New York: Cambridge University Press, 484.
15. ↑ Frictional properties of lunar regolith. November 12, 2008. Team FREDNET Survey Working Group.
16. ↑ ^{16.0 16.1} The Moon and the Magnetotail. NASA. (http://www.nasa.gov/topics/moonmars/features/magnetotail_080416.html)
17. ↑ Stubbs, Timothy J.; Richard R. Vondrak and William M. Farrell (2005). "A Dynamic Fountain Model for Lunar Dust". Lunar and Planetary Science XXXVI.
18. ↑ Moon fountains. NASA. (http://science.nasa.gov/headlines/y2005/30mar_moonfountains.htm)
19. ↑ Moon Storms. NASA. (http://science.nasa.gov/headlines/y2005/07dec_moonstorms.htm)
20. ↑ Illumination on the lunar surface. November 13, 2008. Team FREDNET Survey Working Group.

Acknowledgments

Passages of this document has been copied directly from Wikipedia and are licensed under the "GNU Free Documentation License".

Maps of the Moon produced by Mark A. Wieczorek and licensed under the "GNU Free Documentation License".