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he early solar system was a chaot- ic and violent place. Debris from the formation of the solar system rained down onto the surfaces of the forming planets much more frequent- ly than it does in the solar system today. Indeed, an early, key advance in the understanding of shaping of planetary environments was the real- ization that these impacts are not merely catastrophic accidents but instead constitute a fundamental process in planetary formation and evolution. For example, a variety of data strongly supports the idea that a Mars-size object struck the primordial Earth, resulting in the formation of the Moon and setting Earth on a dis- tinctive evolutionary path. of planetary atmospheres was also affected by impacts. Impacting objects are believed to have been the source of much of the gases that made up Earth's early atmosphere as well as most of the terrestrial water. Learning when these impacts occurred will offer tremendous insight into planetary evolution. Impact research also has implications for the origin of life because living organisms could not gain a toehold on Earth, and perhaps elsewhere in the solar system, until the era of planet- wide, sterilizing impacts was at an end. By studying the patterns of visible craters on such bodies as the Moon and Mercury, researchers have devel- oped some theories on the timing of the impacts. There is considerable The region around the Moon's South Pole as seen by the Clementine orbiter. The additional heat resulting from uncertainty in this system of interpreta- impacts likely caused partial, or even tion, however, and two differing mod- global, melting of Earth and other ter- els have been proposed, though both restrial planets, leading to the formation have considerable uncertainty. In one, of magma oceans and the differentia- the rate of impacts has decreased expo- tion of their interiors. The development nentially since the beginning of the Faze {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~ Quebec's 70-km-diameter Manicouagan impact structure is one of Earth's largest. solar system. In the other, the number of impacts peaked around 4 billion years ago, before dropping off to their 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 materials an 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 body's evolution include the dissipation of internal heat, core formation and the associat- ed magnetic field, distribution of heat- producing radioactive elements, and styles and extent of volcanism. For example, Earth's crust is the product of differentiation and a few billion years of recycling through the movement of continental plates. Based on analysis

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- S~ ~ ~ ~ ~ ~ 1 outn Jo e-lIt (en asin amp e ~elurn 11 1. i 3 3 3 3 i i Profile South Po~e-Ailken Basin Sample Return Mission Type: Sample Return Cost Class: Medium Priority Measurements: Measure elemental and mineralogical surface compositions. Determine interior (mantle) compositions. Study compositional variations and the evolution of crusts and mantles. Quantify the large-impactor flux in the early solar system and calibrate the lunar impact record. Investigate how major impacts early in a planet's 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 surface as the low-density component during solidification of the magma ocean. Knowledge of the internal structure of the Moon is constrained by the small set of Apollo samples, limited geophysical measurements on the surface, and observations from orbit. Remote sensing data show that the Moon has a strong hemispheric asymmetry the side facing Earth and the farside differ significantly. Although its cause is not known, the asymmetry likely influenced the amount and location of subsequent volcanic activity on the Moon. The goal of the South Pole-Aitken Basin Sample Return mission is to understand the nature of the Moon's internal structure and tie down the history of early impacts by returning samples from the Moon's South Pole- Aitken Basin. The largest known basin in the solar system and the oldest and deepest impact structure preserved on the Moon, this giant excavation pene- trates the lunar crust and, unlike any other location in the solar system, allows access to materials from the upper mantle. Data from the South Artist's impression of the South Pole-Aithen Basin Sample Return. The largest lunar impact structure, this basin occupies the lower two-thirds of the image of the Moon's southern polar region shown on page 14. Pole-Aitken Basin may have a substan- Another benefit of the South Pole- tial effect on our understanding of the Aitken Basin Sample Return mission is evolution of planetary interiors. Absolute dating of returned samples, which will include both soil and diverse rock chips, could also change our understanding of the timing and that it gives scientists and engineers the opportunity to try out sample- return techniques and strategies on a relatively easy target before moving on to more challenging and expensive intensity of the bombardment suffered sample-return missions in the rest of by both early Earth and the Moon. the solar system. Guiding Themes Addressed Important Planetary Science Questions Addressed