No abstract

The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.

NASA's New Horizons spacecraft has revealed the complex geology of Pluto and Charon. Pluto's encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection, with a crater retention age no greater than ~10 million years. Surrounding terrains show active glacial flow, apparent transport and rotation of large buoyant water-ice crustal blocks, and pitting, the latter likely caused by sublimation erosion and/or collapse. More enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic and ridges with complex bladed textures. Pluto also has ancient cratered terrains up to ~4 billion years old that are extensionally faulted and extensively mantled and perhaps eroded by glacial or other processes. Charon does not appear to be currently active, but experienced major extensional tectonism and resurfacing (probably cryovolcanic) nearly 4 billion years ago. Impact crater populations on Pluto and Charon are not consistent with the steepest impactor size-frequency distributions proposed for the Kuiper belt.

Abstract. After decades of speculation and fruitless searches by observers, the lunar atmosphere was first observed by Apollo surface and orbital instruments beginning in 1971. With the end of Apollo missions in 1972 and the termination of funding for Apollo lunar ground station observations in 1977 the field withered for many years, but it has recently enjoyed a renaissance. This renewal was initiated by the discovery of lunar atmospheric sodium and potassium by ground-based observers and was furthered by the in situ detection of metal ions derived from the Moon in interplanetary space, the possible discoveries of H20 ice at the poles of the Moon and Mercury, and the detection of tenuous atmospheres around other remote sites in the solar system, including Mercury and several Galilean satellites. In this review I attempt to summarize the present state of knowledge about the lunar atmosphere, describe the important physical processes taking place within it, and compare the lunar atmosphere with other tenuous atmospheres in the solar system. OVERVIEWOwing to the lack of optical phenomena associated with the lunar atmosphere, it is usually stated that the Moon has no atmosphere. This statement is not correct. In fact, the Moon is surrounded by a tenuous envelope with a surface number density and pressure not unlike that of a cometary coma. (See note 1 in the notes section following the main text.) Since the lunar atmosphere is in fact an exosphere (see note 2), one can think of its various compositional components as "independent atmospheres" occupying the same space. This review is structured as follows: In section 1 I will describe the history and provide an overview of the current state of knowledge about the lunar atmosphere. In section 2 I discuss the structure and dynamics of the lunar atmosphere. In section 3 1 provide a more detailed look at the production and loss mechanisms of the lunar atmosphere. In section 4 I provide a comparison of the lunar atmosphere to tenuous exospheres around other bodies in the solar system, with particular emphasis on comparison to Mercury. In section 5 I examine some special topics, including some comments on both the ancient lunar atmosphere and human influences that may occur in the future. Finally, in section 6, I summarize the major outstanding issues concerning lunar atmospheric science and briefly describe some important experiments that could shed more light on this tenuous but fascinating aspect of Earth's nearest neighbor.Before beginning, I caution the reader that this review could not possibly cover every topic relating to the lunar atmosphere in the depth it deserves, and tough choices had to be made about both the breadth and depth of the discussions that follow. Any deficiencies in this approach are the responsibility of this author. A Brief Pre-Apollo History of Quantitative Atmospheric SearchesAlthough the lunar atmosphere was not detected until the Apollo era, scientifically based searches for it extend back to telescopic observations by Galileo. Based on the...

The New Horizons spacecraft mapped colors and infrared spectra across the encounter hemispheres of Pluto and Charon. The volatile ices CH 4 , CO, and N 2 , that dominate Pluto's surface, have complicated spatial distributions resulting from sublimation, condensation, and glacial flow acting over seasonal and geological timescales. Pluto's H 2 O ice "bedrock" is also mapped, with isolated outcrops occurring in a variety of settings. Pluto's surface exhibits complex regional color diversity associated with its distinct provinces. Charon's color pattern is simpler, dominated by neutral low latitudes and a reddish northern polar region. Charon near infrared spectra reveal highly localized areas with strong NH 3 absorption tied to small craters with relatively fresh-appearing impact ejecta.