Lunar Pioneer: The lunar forensic files

View of the Moon at gamma-ray wavelengths, as imaged by the Compton Gamma Ray Observatory satellite. These gamma rays are induced by the collision of cosmic rays with the lunar surface, the same process recently found able to synthesize organic molecules in lunar polar ice deposits [Dave Thompson (NASA/GSFC) et al., EGRET, Compton Observatory].

Paul Spudis

The Once and Future Moon

Smithsonian Air & Space

A recent study indicates that water ice and simple molecules of carbon and nitrogen might form the seed material for more complex substances, some of which might ultimately be involved in the origin of life. The work from the University of Hawaii took measurements of the levels of cosmic radiation from the Lunar Reconnaissance Orbiter (LRO) and applied it to a composition similar to that observed by the impacting LCROSS probe at the south pole of the Moon.

As you may recall, this probe found both water vapor and ice particles ejected by the impact in one of the permanently dark regions near the pole; it also observed additional compounds, including methane, ammonia and some other simple organic molecules. These substances are present in cometary ices and thus, it was thought that their presence could indicate a cometary origin for the Moon’s polar ice.

The new work does not negate that interpretation, but adds complexity to the puzzle by showing that it may be possible to manufacture some of the more complex organic molecules from the simple substances found in cosmic ice, whether deposited from the nuclei of impacting comets or made in place within the cold traps of the lunar poles. Once again, we find that the polar regions of the Moon are even more interesting scientifically than we had thought.

The generation of new and more complex organic compounds must be a surficial process since material buried at levels deeper than a couple of meters is shielded from even the most energetic cosmic rays. For this reason, the material observed during the LCROSS impact is likely of cometary origin because most of the ejecta created by that impact comes from depths of a few meters. While material in the lunar surface is overturned by impact gardening, such overturn is extremely slow (rates of overturn below about 1 meter depth occur on timescales of greater than 1 billion years, the same timescale on which this radiation-induced production occurs).

The generation of complex organic molecules is an important topic of research for the origin of life. Most scientific strategies focus on the search for extraterrestrial life in more Earth-like environments, such as a previously warmer and wetter Mars or in the hypothesized deep oceans of Europa. A few studies have focused on the physical processes of organic chemistry, specifically the generation of complex molecules in space, within small bodies such as cometary nuclei and on primitive planetary surfaces, such as the polar deposits of the Moon and Mercury. Findings to date show that complex organic substances are generated in a variety of environments and under a variety of energetic conditions.

Because they date from early in Solar System history and contain the materials needed for living systems (water and organic matter), comets have long been thought to be the seedbeds of life. Comets are remnants of the original solar nebula, the cloud of debris out of which our Solar System formed. At a certain position and beyond in the nebula, water is stable in solid form (the so-called “frost line”); in our Solar System, the frost line is between the orbits of Jupiter and Mars. Water in nebular material inside this line vaporized and was dissipated by the solar wind, some blown outward and some disassociated by ultraviolet radiation. But water outside of the frost line can condense into ice particles, which then may be accreted into planetary objects. The smallest and most water-rich of these objects are the comets, most of which originate far beyond the frost line in the most distant regions of our Solar System (the so-called “Oort cloud”). Larger icy objects in the outer Solar System include the satellites of the Jovian planets, which are predominantly made of water ice with minor amounts of admixed rocky material. The inner (terrestrial) planets such as Earth and Mars are made mostly of rocky material but contain minor amounts of water, a consequence of their incorporation of cometary material during assembly and subsequent impact bombardment.

This last process operates on the Moon as well. Because the Moon represents a stable, unchanging environment over billions of years, it accumulates the evidence and detritus of the impact history of that era. Most of the volatile component of this impacting debris is lost from the Moon, but any of it that becomes trapped in the cold, dark areas near the poles remains there forever. The poles of the Moon are thus a natural laboratory for the study of one of the early processes in Solar System history – the creation of complex organic substances from the more primitive and simple elements and compounds. In this sense, the pre-biotic organic chemistry of the lifeless and barren Moon serves the cause of the study of life’s processes and origin.

As we continue to study the Moon, we find that it offers much more than one might suspect at first glance. The Moon’s early history reveals the secrets of planetary assembly, impact bombardment, global melting and differentiation into core, mantle and crust. Its middle history tells us about the thermal evolution of planets, as internal heat spawned the volcanism that resurfaced part of the Moon and operates on all of the terrestrial planets. The continued impact history recorded in the Moon’s surface layer documents a phase of Earth history missing from our terrestrial geological record, including the possibility of episodic waves of impacts that are at least partly responsible for extinctions of life recorded in the fossil record. This same surficial layer also records the history and output of our Sun, the provider of energy to the planets and the principal driver of climate change on Earth. The interconnections between the various branches of lunar science with the other sciences grow more evident and more significant over time.

This new research makes the recently renewed interest in the value of the Moon and new lunar missions more comprehensible. Far from being a mere echo of some previous space glory, a return to the Moon to undertake new scientific studies, new exploration and to develop a wholly new set of technologies impacts all of space science and exploration in many different and unexpected ways. Insights into the origins of life can come from detailed examination of lunar polar volatiles. These same materials can also enable travel to more distant destinations and open up Earth-Moon space to economic development. In both cases, lunar return will enable and facilitate our understanding and movement into space.

As my colleague David Lawrence of APL put it, “One of the take-homes is, go back to the moon and look. Dig up samples, see what’s there.” Sound advice.

Proton flux and radiation dose from galactic cosmic rays in the lunar regolith
and implications for organic synthesis at the poles of the Moon and Mercury

Crites, Lucey & Lawrence

Icarus, Vol. 226, No. 2, Nov.–Dec. 2013, pg. 1192–1200

Live CRaTER data from LRO re-broadcast as music (January 14, 2014)

More from CRaTER on radiation health hazards (November 19, 2013)

Cosmic ray threat to manned spaceflight tested on MSL (May 31, 2013)

Earth and Moon share primal water source (May 10, 2013)

"Our view of the Moon has turned upside down" (April 26, 2012)

Cosmic ray flux effects lunar ices (March 19, 2012)

The Moon's metallic water (February 27, 2011)

LRO analysis of LCROSS data proves essential (October 21, 2010)

Managing Space Radiation Risk in the New Era of Space Exploration (2008)

Committee on the Evaluation of Radiation Shielding for Space Exploration