Measuring almost nothing, looking for the almost invisible

NASA's LADEE spacecraft entered it's 250 km Commissioning phase orbit October 12 [NASA/JAXA].

Paul D. Spudis

The Once and Future Moon
Smithsonian Air & Space

Launched last month from the Wallops Island site, LADEE (for Lunar Atmosphere and Dust Environment Explorer) will spend the next few months orbiting the Moon.  This small spacecraft will attempt to characterize and measure the lunar “atmosphere,” while also looking for dust that might be electrostatically levitated above the surface or thrown into ballistic flight by impacts.

Wait a minute.  Did I say “atmosphere?”  Isn’t the Moon renowned for its lack of an atmosphere?  Indeed it is.  In fact, the 10-12 torr surface pressure of the Moon is a better vacuum than we can achieve with even the most advanced equipment in Earth laboratories.  (For comparison, sea level pressure on the Earth is about 760 torr, making the lunar surface pressure over one hundred trillion times less dense.)  A better term for the tenuous gas near the Moon is “exosphere,” meaning free flying gas molecules that may or may not be gravitationally bound to the Moon.  In such an “atmosphere,” there may be only a few thousand molecules in a cubic centimeter of space. This is very tenuous indeed.

  

After the Commissioning phase of its mission is complete, the spacecraft's current 250 km circular orbit will be reduced further down to within 50 km to begin its 100 day Science Mission [NASA/GSFC].

LADEE is designed to investigate from where these atoms and molecules come.  Presently, we think the lunar exosphere consists mostly of helium, sodium and perhaps argon atoms, each coming from a completely different source.  Helium likely comes from the Sun, as the solar wind continually “breathes” onto the surface of the Moon.  Some atoms stick to surface dust grains but many simply bounce off, randomly moving in the space above the lunar surface.  Easy to detect, lunar sodium has been observed from Earth-based telescopes.  It most likely comes from rocks vaporized by the continual rain of micrometeorites.  At least some fraction of this vaporous sodium must hang around the surface, unable to escape the Moon.  Argon might have a solar wind origin, but at least some of it comes from the natural decay of radioactive potassium in the lunar interior (potassium-40 (40K) decays to argon-40 (40Ar) with a half-life of a bit more than one billion years).  Gases like argon, venting from the interior of the Moon, were observed by subsatellites left in lunar orbit by the departing Apollo spacecraft over 40 years ago (these small spacecraft have long since crashed into the Moon).

Although helium, sodium and argon are the principal expected components of the lunar exosphere, the LADEE team will search for other species.  An interesting possibility is water (H2O) or its related species, hydroxyl (OH).  One of the most surprising results of recent lunar exploration was the discovery of adsorbed (surface) water and hydroxyl on the dust grains of the lunar surface (observed by the Moon Mineralogy Mapper (M3) aboard the Indian Chandrayaan-1 lunar orbiter in 2009).  Occurring in the form of a monolayer of molecules on dust grains in the cooler portions of the Moon, a clear water signal is best seen above latitudes of 65° and increasing in strength (i.e., increasing water abundance) toward each pole.

The surprise from M3 was not only the presence of water but observing that its abundance increases with decreasing surface temperatures.  This means that water being made or deposited on the surface is in motion, with a net movement toward the poles.  Chandrayaan-1 also carried an impact probe with a mass spectrometer.  During the probe’s half-hour descent to the South Pole, it passed through a cloud of water in space, just above the lunar surface.  The water cloud at this high latitude had a density a hundred times higher than at the equator, providing additional evidence that exospheric water is in motion, moving from lower, hotter latitudes towards higher, cooler ones.

LADEE cannot directly measure this water in a neutral state, but if some process ionizes it (e.g., if a water molecule breaks apart into a proton and a hydroxyl by UV radiation from the Sun), it will be visible to the ultraviolet spectrometer aboard the spacecraft.  If the process of water migration on the lunar surface is correct, we should be able to observe exospheric water and by measuring its density with time, track the water migration to higher latitudes.

Lunar Horizon Glow (LHC) observed for several hours following local sunset from Surveyor 7 and its landing site just north of Tycho crater. [NASA].

LADEE will also tackle another controversial issue – the amounts and mechanisms of dust movement on and around the Moon.  During the unmanned Surveyor lander missions over 40 years ago, a strange illumination or glow was observed by television for several hours after local sunset, just above the horizon.  This phenomenon was termed “horizon glow” by surprised Surveyor investigators.  At a loss to explain it, the team postulated that some mechanism was lofting dust up above the surface and this dust was scattering sunlight.  Exactly how the dust was lofted was uncertain; some thought it must be fragments in ballistic flight from distant impacts, while others thought that it might be levitated by electrostatic force, thus “hovering” above the surface.

Schematic of documented species of Lunar Horizon Glow, including mid-lunar night imagery captured by Surveyor 7 (Horanyi, et.al., The Lunar Dust Environment: Expectations for the LADEE Lunar Dust Experiment (LDEX), 43rd Lunar and Planetary Science Conference (2012), #2635.

A few years later, just before his orbiting spacecraft emerged into the daylight side of the Moon, Apollo 17 Commander Gene Cernan observed and sketched an illuminated limb and “streamers” that could be seen extending into space above where the lunar horizon would be.  At the time, this phenomenon was thought to be the same as that seen in the Surveyor pictures, although they have totally different scales (the Surveyor horizon glow must occur within a few meters of the surface, while Cernan’s horizon glow extended many kilometers above the Moon). Dust (probably of lunar provenance) is certainly involved in whatever causes this horizon glow.

Apollo 17 commander Gene Cernan's sketches and description of horizon glow and streamers observed in lunar orbit, December 1972 [NASA].

As the Moon slowly rotates once every 708 hours, the line between the sunlit and dark hemispheres (the terminator) slowly moves across the lunar surface.  The day and night hemispheres have different fluxes of electrons from the solar wind and thus, the presence of the terminator can induce an electrical charge in surface materials.  It is postulated that this charge might levitate smaller dust particles such that they would hover above the surface.  LADEE will attempt to detect and map this dust, both by searching for scattered sunlight with its ultraviolet spectrometer and via the direct detection of dust particles in flight with an instrument on the top of the orbiting spacecraft.

The issue of levitated dust is thought to be relevant to the future habitation of the Moon.  If dust is lofted above the surface by the passage of the terminator, the particles could degrade clean surfaces and create a hazard for inhabitants of the Moon.  Such a process could have major effects near the poles of the Moon, areas that are in the near-constant presence of a day-night terminator.  Although it is unlikely that levitated dust on the Moon is an environmental hazard, we currently are working in near total absence of hard data.  Thus, it makes sense to at least try to make some direct measurements of the dust environment around the Moon to assess the importance of this proposed surface process.

LADEE arrived in lunar orbit last Sunday. We wish it well on its mission to give us fresh (and welcome) data on a poorly understood aspect of lunar processes and history.

Related Posts:

LADEE, in 250 km orbit, begins commissioning phase (October 15, 2013)
LADEE Away! (September 7, 2013)
LADEE legacies (September 7, 2013)
LADEE Prelaunch Mission Briefing (September 6, 2013)
ESA prepares for LADEE (July 31, 2013)
LADEE arrives at Wallops Island (June 5, 2013)
LADEE ready to baseline dusty lunar exosphere (June 5, 2013)
First laser comm system ready for launch on LADEE (March 16, 2013)
LADEE project manager update (February 6, 2013)
The Mona Lisa test for LADEE communications (January 21, 2013)
Toxicity of lunar dust (July 2, 2012)
Expectations for the LADEE LDEX (March 23, 2012)
The Dust Management Project (August 9, 2010)
LADEE architecture and mission design (July 6, 2010)
DesertRatS testing electrodynamic dust shield (July 5, 2010)
Dust transport and its importance in the origin of lunar swirls (February 21, 2010)
Dust accumulation on Apollo laser reflectors may indicate a surprisingly fast and
   more dynamic lunar exosphere (February 16, 2010)
NASA applies low cost lessons to LADEE (January 18, 2010)
Nanotech advances in lunar dust mitigation (August 19, 2009)
Moon dust hazard influenced by Sun's elevation (April 17, 2009)
LADEE launch by Orbital from Wallops Island (April 14, 2009)
Understanding the activation and solution properties of lunar dust
for future lunar habitation (March 2, 2009)
Respiratory toxicity of lunar highland dust (January 19, 2009)
Toxicological effects of moon dust (June 25, 2008)
Moon dust and duct tape (April 22, 2008)

Originally published October 4, 2013 at his Smithsonian Air & Space blog The Once and Future Moon, Dr. Spudis is a senior staff scientist at the Lunar and Planetary Institute. The opinions expressed are those of the author but are better informed than average.  

India hopes to join JPL MoonRise mission to SPA

From LRO Wide Angle Camera Album -

LROC Wide Angle Camera (WAC) monochrome mosaic of the Moon's oldest (known) basin, the 2,100 km-wide, 4 billion year old South Pole-Aitken (SPA) impact basin. Stretching from just south of the equator on the Moon's farside to the ring of massifs spills over to the Near side, 200 km north of the Moon's south pole on the nearside, SPA has not yet been directly sampled [NASA/GSFC/Arizona State University].

Pallava Bagla
Science

India hopes to join the United States on a sample return mission to the moon, according to K. Radhakrishnan, chair of the Indian Space Research Organisation (ISRO). He announced that India's policy group, the Space Commission, decided on 12 February to give the go-ahead for work on a possible contribution to MoonRise, a U.S. effort to land a probe on the moon's surface, scoop up 1 kilogram of material, and return it to Earth for analysis. India would provide an orbiter similar to its observation satellite Chandrayaan-1, which in 2009 helped clinch evidence of water's presence on the moon.

The new Indian instrument would circle the moon for a few years and aid in communications and imaging. ISRO hopes to invest $38 million on developing this 400 to 500 kg instrument, which would travel into space on a U.S. Atlas rocket.

The mission leader, NASA's Jet Propulsion Laboratory in Pasadena, California, is planning for a launch in 2016, but has not yet selected the MoonRise instruments. The goal is to learn more about the geology and origins of the moon. According to NASA, the mission will "focus on the giant South Pole-Aitken (SPA) basin on the far side, which lies between the moon's South Pole and Aitken crater.

Read the write-up HERE.

Photo-mosaic of the Moon's south pole from the Terrain Mapping Camera aboard ISRO's Chandrayaan-1 (2009). An instrument on the Moon Impact Probe (MIP), released from Chandrayaan soon after its arrival in orbit, was the first to directly sample the presence of hydroxyl and water molecules near the lunar surface, just prior to impacting the Moon at Shackleton crater [ISRO].

The Four Flavors of Lunar Water

From Lunar Pioneer

Earth over the watery north polar regions of the Moon, as viewed from NASA/DOD platform Clementine (1994) [USGS].

Paul D. Spudis
The Once and Future Moon
Smithsonian Air & Space Blogs

The Moon is constantly bombarded by the solid debris of the Solar System. Comets, asteroids and interplanetary dust, all containing varying amounts of water, have pounded the lunar surface for billions of years. Yet until recently, the Moon was considered to be barren and bone-dry. Rock and soil samples returned by the Apollo missions lacked any hydrous mineral phases or water-bearing weathering products. Since water is not stable on the Moon under ordinary conditions, what happens to it?

New studies of lunar samples, along with results from several missions in recent years, have given us a revolutionary new picture of water on the Moon. Study of volcanic glass from the Apollo 15 landing site in 2008 demonstrated that tiny amounts of water (about 50 parts per million) are present in the interiors of these glasses, suggesting that the lunar mantle (whence they came) contains about ten times this amount. This was a startling result, considering the extreme dryness of other lunar samples.

Because the Moon’s spin axis is nearly perpendicular (1.5° from vertical) to the ecliptic plane, the Sun is always on the horizon at the poles, keeping the floors of deep craters in permanent shadow. These dark areas only receive heat from the interior of the Moon and are extremely cold; recent measurements by the DIVINER instrument on the Lunar Reconnaissance Orbiter (LRO) spacecraft indicate temperatures as cold as 25-35° C above absolute zero. Water molecules are trapped by the cold as soon as they find their way into these craters. Over the more than 4.5 billion years of lunar history, significant amounts of water could accumulate in many of these crater “cold traps” at the Moon’s poles.

"The Moon is on the critical path to human expansion into the Solar System."

The first hint of water ice in these polar cold traps came from a radio experiment aboard the 1994 Clementine mapping mission orbiting the Moon. The polarization characteristics of echoes from the south pole were consistent with the presence of ice in the crater Shackleton. Four years later, the Lunar Prospector (LP) spacecraft carried an instrument designed to measure the amount and energy of neutrons given off the Moon’s surface. Hydrogen absorbs neutrons, so when LP investigators saw a decrease in the flux of medium-energy neutrons near the lunar poles, they concluded that excess amounts of hydrogen were present there. Although this observation is consistent with the presence of polar ice, neutron data alone do not tell us what form the hydrogen is in, and it was alternatively postulated that this enhancement was caused by excess solar wind hydrogen.

The Moon Mineralogy Mapper (M3) instrument on the 2008-09 Indian Chandrayaan-1 mission collected reflectance spectra for most of the Moon. It found both water (H2O) and hydroxyl (OH) molecules, present either as a monolayer on lunar dust grains or bound into the mineral structures in surface materials, poleward of about 65° latitude at both poles. Moreover, the abundance of this surface water varies with time, being present in greater quantity in both local early morning and late evening and it increases in abundance with increasing latitude. These results were verified by observations from the Cassini and EPOXI spacecraft during separate flybys of the Moon. The new observations indicate significant quantities of water moving towards areas with lower mean surface temperatures and increasing in abundance with latitude. Taken all together, the results mean that water is being deposited (e.g., by comet impact) and/or created (e.g., by reduction of metal oxides in the surface by solar wind protons) and then transported to the poles. By this process, significant quantities of water ice could accumulate at the poles over geological time.

Last October, the companion satellite to LRO, LCROSS, slammed the upper stage of its launch vehicle into the Moon’s south pole and observed the ejected material. Results show that both water vapor and ice particles were ejected from the LCROSS impact crater; initial analyses indicate that water is present at about the 5-10 wt.% level. The LCROSS impact site exhibits no anomalous radar behavior, suggesting that such an amount of water ice cannot be detected by radar. However, the results do indicate that significant amounts of lunar polar water may be present even in the absence of specific radar evidence for it. Spectra from this impact event show evidence for other volatile substances, including ammonia and simple carbon compounds. The presence of such material may indicate a cometary source for these volatile materials.

Both poles were covered by radar images from the Mini-SAR instrument on Chandrayaan-1. Much of the north polar region displays backscattering properties typical for the ordinary Moon, but one group of craters in the region show elevated polarization enhancements in their interiors, but not in deposits exterior to their rims. Almost all of these anomalous craters are in permanent sun shadow and correlate with proposed locations of ice modeled on the basis of the Lunar Prospector neutron data. These relations suggest that the interiors of these craters contain nearly pure water ice, with approximately 600 million metric tonnes of ice present in over 40 small craters within 10 degrees of the pole. The south polar region shows similar relations, except that it has fewer anomalous craters than the north pole. Small areas of polarization enhancement are found in some craters, notably Shoemaker, Haworth and Faustini; these areas might be deposits of water ice.

So water on the Moon is present in large quantity in at least four different “flavors.” Water was in the deep lunar interior 3.3 billion years ago, at concentration levels of a few hundred parts per million. This water would have been released during the eruption of lunar magma and could have made its way into the polar cold traps. Water is either being made or being deposited nearly continuously by impact all over the Moon. Most of this water is subsequently lost to space (e.g., by sputtering, ionization or thermal escape) but some is retained on the Moon. Any water arriving at a cold trap near the pole will be captured. Water, once in the polar areas, is stable as ice in the permanent darkness or where sublimation is prevented when buried by a thin layer of soil. Significant quantities of water may accumulate there; the LCROSS results suggest several to tens of weight percent water ice may exist in the polar soils. Finally, some of this migrating water apparently collects at rates high enough so that significant soil cannot mix with it during normal impact bombardment, as shown by the presence of relatively “pure” water ice deposits in selected lunar craters imaged by radar.

A significant amount of water at the poles of the Moon is present, with many billions of metric tonnes at each pole (detailed estimates of the water reserves are in progress). Such an amount is more than enough to support both permanent, sustainable human presence on the Moon and for export to cislunar space. Water is useful as rocket fuel and energy storage (hydrogen and oxygen are the two most powerful chemical propellants known) and for life support (water and oxygen) in space. These new discoveries fundamentally alter our understanding of the Moon’s processes and history and highlight both it’s scientific value and utilization potential. The Moon is on the critical path to human expansion into the Solar System.

Addendum. In Comments, below Dr. Spudis original post, Pradeep Mohandas reminded the author of the findings of the Moon Impact Probe, released from Chandrayaan-1, which discovered water vapor in very small concentrations in the space just above the Moon during its descent to the south pole. "This exospheric water (i.e., water in extremely small concentrations) may be related to the time-variable water seen in the spectral data from M3, Cassini, and EPOXI — in other words, it may represent water molecules in motion, migrating toward the poles. Work on the nature and processes of the lunar hydrosphere continues, and I will keep you up to date on the latest research results on this new and exciting subtopic of lunar science."