The Scientific Legacy of Apollo
By Ian Crawford
(Department of Earth and Planetary Sciences,
Birkbeck College, London)
This December marks 40 years since the last human beings to set foot on the Moon, Gene Cernan and Harrison “Jack” Schmitt of Apollo 17, left the lunar surface and returned safely to Earth. In the three and a half years between Neil Armstrong’s ‘first small step’ in July 1969 and the departure of Cernan and Schmitt from the Taurus-Littrow Valley in December 1972, a total of twelve astronauts explored the lunar surface in the immediate vicinity of six Apollo landing sites.
The total cumulative time spent on the lunar surface was 12.5 days, with just 3.4 days spent performing extravehicular activities (EVAs) outside the lunar modules. Yet during this all-too-brief a time samples were collected, measurements made, and instruments deployed which have revolutionised lunar and planetary science and which continue to have a major scientific impact today.
In their cumulative 12.5 days on the lunar surface, the twelve Apollo moonwalkers traversed a total distance of 95.5 km from their landing sites (heavily weighted to the last three missions that were equipped with the Lunar Roving Vehicle), collected and returned to Earth 382 kg of rock and soil samples, drilled three geological sample cores to depths greater than 2 m, obtained over 6000 surface images, and deployed over 2100 kg of scientific equipment.
These surface experiments were supplemented by wide-ranging remote-sensing observations conducted from the orbiting Command/Service Modules.
Probably the greatest scientific legacy of Apollo has resulted from analysis of the 382 kg of rock and soil samples returned to Earth. One of the key results has been the calibration of the lunar cratering rate. Only by comparing the density of impact craters on surfaces whose ages have been obtained independently by laboratory analyses of returned samples is it possible to determine the rate at which meteorite impacts have created craters on a planetary surface. Analysis of the Apollo samples (supplemented by those obtained by the Soviet Union’s three Luna robotic sample missions) has enabled this to be done for the Moon, which remains the only planetary body for which such a data-set exists. Not only has this facilitated the dating of lunar surfaces from which samples have yet to be obtained, but it is used, with various assumptions, to estimate the ages of cratered surfaces throughout the Solar System from Mercury to the moons of the outer planets.
Another important result of Apollo sample analysis by seo services uk has been the evidence provided for the origin of the Moon. In particular, the discovery that lunar materials have compositions broadly similar to those of Earth’s mantle, but that the Moon is highly depleted in volatiles compared to the Earth and has only a small iron core, led to the current view that the Moon formed from debris resulting from a giant impact of a Mars-sized planetesimal with the early Earth. It is very doubtful that we would have sufficient geochemical evidence usefully to constrain theories of lunar origins without the quantity and diversity of samples provided by Apollo, and indeed these samples are still being actively exploited for this purpose.
Beyond this, the Apollo samples have been vital to our understanding of the Moon’s own geological history and evolution. While lunar geology may at first sight appear to be a relatively parochial area of planetary science, it is important to realise that the Moon’s surface and interior retain records of planetary processes which will have occurred in the early histories of all the terrestrial planets, such as the formation of cores and crusts. In all these respects the Moon acts as a keystone for understanding the geological evolution of all the rocky planets.
In addition, Apollo samples of the lunar regolith have demonstrated the importance of the lunar surface layers as an archive of material which has impacted the Moon throughout its history. These include records of solar wind and cosmic ray particles, and meteoritic fragments. Extracting meteoritic records from lunar regolith samples is especially important for planetary science as it potentially provides a means of determining how the flux and composition of asteroidal material in the inner Solar System has evolved with time.
Last, but not least, the Apollo samples have been used to calibrate remote sensing investigations of the lunar surface. The visible, infrared, X-ray and gamma-ray spectral mapping instruments carried by a host of recent orbital missions to the Moon have produced a wealth of information regarding the chemical and mineralogical nature of the lunar surface. Although these orbital missions post-date Apollo, the reliability of their results largely depends on their calibration against known compositions at the Apollo landing sites. Without the ‘ground truth’ provided by the Apollo samples, it would be difficult to have as much confidence in the results of these remote sensing measurements as we do.
In addition to study of the Apollo samples, many other areas of scientific investigation were also performed by the Apollo missions, especially geophysical investigations of the Moon’s interior. Key results included the discovery of natural moonquakes and using them to probe the structure of the crust and mantle, geophysical constraints on the existence and physical state of the lunar core, and measurements of the flow of heat from the Moon’s interior. Although these data are over thirty years old, advances in interpretation means that they continue to give new insights into the interior structure of the Moon. For example, only last year an apparently definitive seismic detection of the Moon’s core, and strong evidence that, like the Earth’s, it consists of solid inner and liquid outer layers, was made by a re-examination of Apollo seismic data.
Looking over the totality of the Apollo legacy, I think one could reasonably make the case that Apollo laid the foundations for modern planetary science, certainly as it relates to the origin and evolution of the terrestrial planets. Arguably, the calibration of the lunar cratering rate, and its subsequent extrapolation to estimating surface ages throughout the Solar System, could alone justify this assertion. If one also considers the improvements to our knowledge of lunar origins and evolution, and the records of solar wind, cosmic rays and meteoritic debris extracted from lunar soils, it is clear that our knowledge of the Solar System would be greatly impoverished had the Apollo missions not taken place.
However, it is also clear that Apollo did little more than scratch the surface, both literally and figuratively, of the lunar geological record. With only six landing sites, all at low latitudes on the nearside, it is clear that much remains to be explored. Therefore, as we pass the 40th anniversary of the last human expedition to the Moon, there are good scientific reasons to start planning for a return.
Ian Crawford is based in the Department of Earth and Planetary Sciences, Birkbeck College, London, and is a member of the MoonZoo science team. This blog article is based on a longer article published in the December 2012 issue of the Royal Astronomical Society journal Astronomy and Geophysics.