EPSC Abstracts,
EPSC Abstracts,
Vol. 3, EPSC2008-A-00169, 2008
Vol. 3, EPSC2008­A­xxxx (Abstract number will be completed later on), 2008
European Planetary Science Congress, © Author(s) 2008
European Planetary Science Congress, © Author(s) 2008
Exploring lunar ices for past dynamical activity of the Moon and biosignatures from life on Earth
Joop M. Houtkooper (1), Dirk Schulze­Makuch (2)
(1) Center for Psychobiology and Behavioral Medicine, Justus­Liebig­University of Giessen, Germany. (2) School of Earth and Environmental Sciences, Washington State University, USA.
(joophoutkooper@gmail.com / +31204961303)
The Moon is mostly known as a dead, dry and airless ball of rock. Due to the small (1.5o) inclination of the Moon's rotation axis in respect to the ecliptic, some areas at the lunar poles are permanently without sunlight. So­called "doubly dark spots" are also without sunlight reflected from distant crater walls. These doubly dark areas are therefore very cold, by some estimates as cold as 25 K. This results in deposits of volatiles at the lunar poles which may give insights into the Moon's past and present state. These ices, which may also have been deposited at depth in the Moon's porous mega­regolith, may reveal a history that was not as monotonous as it appears. Volatiles and liquids may have been present on or within the Moon for a limited time short after its formation, from outgassing events, and after major asteroid impacts. These ices may even reveal clues about the origin and evolution of early life on Earth or on other bodies of the solar system. Lunar atmosphere
The present lunar atmosphere consists mainly of He, H2, Ne and Ar with a density of about 2 x 10 5 particles/
cm3. These gases, with the exception of Ar, are non­
condensing at the doubly dark areas. The Moon is relatively inert geologically, but the lunar atmosphere continues to be replenished by lunar outgassing events. Although the bulk of the ices probably originated from relatively recent comet or water­rich asteroid impacts [1], some of the lunar ice may well have been produced at earlier times when the Moon had a temporary atmosphere after the impact of a comet or icy asteroid. Another, continuous source of water molecules are solar wind protons impacting oxide minerals on the lunar surface [2].
Transient lunar atmosphere events
In its history, the Moon has been bombarded by asteroids and comets: a 10 km comet would supply as much as a few times 1010 tons of volatiles and produce a transient lunar atmosphere (TLA) with a density of about 10­5 kg/m3. The impact of comets would be at high speed, >10 km/s, but near­earth asteroids might impact at speeds as low as 3 km/s. The lower the speed of impact, the smaller the immediate loss into space would be, because of high temperatures during the impact. The rare TLA may disappear rather quickly, but not only into space. The night side of the Moon is sufficiently cold to have gases like H2O and CO2 condense temporarily on the surface and in the doubly dark areas, at a temperature as low as 25 K, other gases like N2 and CH4 may also condense.
Mega­regolith
Of particular interest is the porosity of the Moon. The mega­regolith, consisting of fractured rock, may extend down to a depth of 50­100 km, where the lithostatic pressure overcomes the compression strength of the rock. Above that depth, voids have been created by impacts, moonquakes and shrinkage of deeper layers due to differential cooling. The porosity and thermal profile of the mega­regolith has been subject4 to model studies. The insulating properties of the mega­regolith [3] determine how far down the low temperatures at the doubly dark areas extend, which may be down to several kilometers. Therefore, condensation of volatiles may also occur within the cracks in the mega­regolith and an inventory of ices may be found at depth in and around these areas. The composition of these ices will reflect the gases present and the local temperature within the mega­regolith. The present lunar atmosphere may increase a billionfold in density by a TLA event, but still be only 1/100,000th as dense as the Earth's atmosphere at sea level. Ice deposits above and below doubly dark areas
During the presence of a TLA gases will fill the cracks of the mega­regolith to some extent. When these gases come into contact with the cold mega­regolith below the doubly dark areas, condensation and freezing will occur. Therefore, the doubly dark areas will incorporate frozen volatiles like H2O, CO2 and CH4, both on the surface and below in the mega­regolith. The differences between the deposits above and below the doubly dark areas are that the deposits above ground are larger, but more subject to sputtering by the solar wind and subject to impact gardening. Also, the above ground deposits may contain impacted meteorites which should be well­
preserved. The below ground deposits may be fragmented, so that a record of different TLA events may be difficult to reconstruct, but otherwise the deposits are of volatiles which may well have remained undisturbed for a geologically long time.
A temporary biosphere after major impact?
The presence of a biosphere usually implies the presence of liquid water and the availability of nutrients and energy sources. Some kind of dynamic activity that supports recycling is especially beneficial. None of this is apparent on the Moon today. However, there must have been times such as after major asteroid impacts and after the Moon­forming event that in fact liquid water has been present on the Moon or within the Moon. These events would likely be extremely short on geological time scales and would have involved only minor amounts of liquid water. However, when considering the astrobiological potential of the Moon, we need to be aware that it is a special case because of its closeness to a planet that has had an extensive biosphere for at least 3.5 billion years. Any major impact on Earth or the Moon – for example during the late heavy bombardment – would have catapulted large amounts of volatiles into the lunar atmosphere with impacting asteroid material providing large amounts of heat. In addition, a higher amount of radiogenic heating would have occurred in the rocks of the early Moon. Surely, organic molecules, traces of life, and microbial organisms would have been ejected from Earth to the Moon in cataclysmic events. Some of this biogenic material would have likely been preserved on the Moon [4], probably frozen into the regolith and later being covered by lunar dust. Some of its microbial load, dislodged with rocks from Earth, might have survived the transport to the Moon, and would have possibly remained in a viable state if buried quickly under the radiation­reworked surface. Some of these organisms may have even landed in lunar locations, where liquid water was present for a temporary period. These pockets of water would have been small in extent, possibly microscopic, within an excavated impact crater. Perhaps, these impact zones would have provided suitable conditions to support a highly locali­
zed biosphere for a limited period of time. Given the dryness of the Moon, its lack of a substantial atmosphere and lack of dynamic activity, we consider it unlikely that any surviving microbes would still be active on the Moon, but it is not entirely impossible if liquid would be found beneath the lunar ice in some locations. More likely, however, is that these temporarily existing liquid water pools froze and may still hold viable microbial organisms, possible even organisms that extended their life time for a short while on the Moon. Rock­ice interfaces are locations where water remains liquid the longest [5], and thus it might be a location where organisms may have withdrawn to before their demise. Thus, any water ice under the radiation­reworked lunar surface and especially at rock­
ice interfaces should be examined with the highest scrutiny for the presence of biogenic traces, which might range from organic biomarkers to full­fledged microbial organisms.
Conclusions
Temporarily and spatially limited habitable conditions may have existed on or within the Moon short after major asteroid impacts and during major outgassing events. Biogenic material dislodged from Earth may have been preserved in lunar ices on the Moon or even reproduced in highly localized environmental niches on or within the Moon that later froze over. Thus, any lunar ice should be highly scrutinized for its possibly organic content and should deserve highest astrobiological interest. Depositories of volatiles from TLA events and lunar outgassing undisturbed by impact gardening and solar wind erosion by itself are of sufficient scientific interest. A mission to drill to a depth of preferably a few 100 m near one of the lunar poles would answer many questions about temperature profiles and volatile inventory at least.
References
[1] Schulze­Makuch, D. et al. (2005) Astrobiology 5, 778­795.
[2] Crider, D.H. and Vondrak, R.R. (2000) JGR 105, E11:26773­26782.
[3] Ziethe, R. and Hiesinger, H. (2008) GRA 10, 06617.
[4] Armstrong et al. (2002) Icarus 160, 183­196.
[5] Möhlmann, D. (2005) Astrobiology 5, 770­777.