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Cratering and Planetary Evolution he early solar system was a chaot- of planetary atmospheres was also T ic and violent place. Debris from the formation of the solar system rained down onto the surfaces of the affected by impacts. Impacting objects are believed to have been the source of much of the gases that made up Earthâs forming planets much more frequent- early atmosphere as well as most of the ly than it does in the solar system terrestrial water. Learning when these today. Indeed, an early, key advance impacts occurred will offer tremendous in the understanding of shaping of insight into planetary evolution. planetary environments was the real- Impact research also has implications ization that these impacts are not for the origin of life because living merely catastrophic accidents but organisms could not gain a toehold on instead constitute a fundamental Earth, and perhaps elsewhere in the process in planetary formation and solar system, until the era of planet- Quebecâs 70-km-diameter Manicouagan evolution. For example, a variety of wide, sterilizing impacts was at an end. impact structure is one of Earthâs largest. data strongly supports the idea that a By studying the patterns of visible Mars-size object struck the primordial craters on such bodies as the Moon solar system. In the other, the number Earth, resulting in the formation of and Mercury, researchers have devel- of impacts peaked around 4 billion the Moon and setting Earth on a dis- oped some theories on the timing of years ago, before dropping off to their tinctive evolutionary path. the impacts. There is considerable present level. To test these theories (or develop totally new ones) requires knowing the actual ages of the craters, which can be determined only by dat- ing the surface materialsâan objective whose accomplishment depends in turn on obtaining samples from known locations on a number of bodies. At this time, only lunar samples have been returned to Earth, by the Apollo and Luna missions. As a result, the Moon is the only planetary body whose surface age is known with any confidence, and thus all attempts to date other bodies stem from a compari- son with the Moon. While incredibly valuable, the data set from the Apollo missions is very small. Gathering a wider variety of samples from the lunar surface is an important step in more accurately determining ages for objects throughout the solar system and there- by better understanding the evolution of our celestial neighborhood. After the initial formation of plane- tary bodies and the conclusion of the period of heavy bombardment, the internal structure of the planets shaped their history. Key issues in under- standing a terrestrial bodyâs evolution include the dissipation of internal The region around the Moonâs South Pole as seen by the Clementine orbiter. heat, core formation and the associat- ed magnetic field, distribution of heat- The additional heat resulting from uncertainty in this system of interpreta- producing radioactive elements, and impacts likely caused partial, or even tion, however, and two differing mod- styles and extent of volcanism. For global, melting of Earth and other ter- els have been proposed, though both example, Earthâs crust is the product of restrial planets, leading to the formation have considerable uncertainty. In one, differentiation and a few billion years of magma oceans and the differentia- the rate of impacts has decreased expo- of recycling through the movement of tion of their interiors. The development nentially since the beginning of the continental plates. Based on analysis 14 New Frontiers in Solar System Exploration
South Pole-Aitken Basin Sample Return Profile South Pole-Aitken Basin Sample Return Mission Type: Sample Return Cost Class: Medium Priority Measurements: â¢ Measure elemental and mineralogical surface compositions. â¢ Determine interior (mantle) compositions. â¢ Study compositional variations and the evolution of crusts and mantles. â¢ Quantify the large-impactor flux in the early solar system and calibrate the lunar impact record. â¢ Investigate how major impacts early in a planetâs history can alter its evolution and orbital dynamics. of the Apollo lunar samples, scientists believe that the Moon began hot, with an ocean of magma some 400 kilome- ters deep, and that its crust rose to the Artistâs impression of the South Pole-Aitken Basin Sample Return. The largest lunar surface as the low-density component impact structure, this basin occupies the lower two-thirds of the image of the Moonâs during solidification of the magma southern polar region shown on page 14. ocean. Knowledge of the internal structure of the Moon is constrained Pole-Aitken Basin may have a substan- Another benefit of the South Pole- by the small set of Apollo samples, tial effect on our understanding of the Aitken Basin Sample Return mission is limited geophysical measurements on evolution of planetary interiors. that it gives scientists and engineers the surface, and observations from Absolute dating of returned samples, the opportunity to try out sample- orbit. Remote sensing data show that which will include both soil and return techniques and strategies on a the Moon has a strong hemispheric diverse rock chips, could also change relatively easy target before moving asymmetryâthe side facing Earth and our understanding of the timing and on to more challenging and expensive the farside differ significantly. intensity of the bombardment suffered sample-return missions in the rest of Although its cause is not known, the by both early Earth and the Moon. the solar system. asymmetry likely influenced the amount and location of subsequent Guiding Themes Addressed Important Planetary Science Questions Addressed volcanic activity on the Moon. The goal of the South Pole-Aitken The First Billion What processes marked the initial stages of planet and Basin Sample Return mission is to Years of Solar satellite formation? understand the nature of the Moonâs System History How did the impactor flux decay during the solar systemâs internal structure and tie down the youth, and in what way(s) did this decline influence the history of early impacts by returning timing of lifeâs emergence on Earth? samples from the Moonâs South Pole- Aitken Basin. The largest known basin in the solar system and the oldest and Processes How do the processes that shape the contemporary character of planetary bodies operate and interact? deepest impact structure preserved on How Planetary What kinds of minerals are the inner planets made of, and the Moon, this giant excavation pene- Systems Work does this vary depending on a planetâs distance from the trates the lunar crust and, unlike any Sun? other location in the solar system, What is the internal structure of each planet and how did allows access to materials from the the core, crust, and mantle evolve? upper mantle. Data from the South 15