The scientific importance of the lunar environment.

Many plans are in preparation to land robotic missions on the surface of the Moon, which will pave the way to return humans to the lunar surface and set the stage for an ongoing human presence. Artemis is a NASA-led international effort to return humans to the Moon. One of the goals of Artemis is to use innovative technologies to address priority science objectives. At the same time, the European Space Agency (ESA) is exploring designs for a Moon base to enable ongoing human activity on the lunar surface. Furthermore, China plans to land humans on the Moon by 2030. In situ resource utilization (ISRU)-the extraction of useful materials from the Moon itself-will be beneficial for efforts to sustain a human presence. ISRU could provide water that can be used by astronauts or split into hydrogen and oxygen rocket fuel for returning to Earth or moving onward to Mars, along with other vital resources. Although this possibility is exciting, it is critical that we recognize how human and robotic activity will affect the lunar environment. Not having a global strategy to characterize lunar volatile materials before their scientific value is lost will substantially degrade the Moon's scientific potential.

Increasing diversity on spacecraft mission teams reduces risk.

Many of the NASA spacecraft sent into Earth orbit and throughout the Solar System are competitively selected and led by principal investigators (PIs). These senior scientists are responsible for directing research activities, managing the use of funds, and reporting to the funding agency. Additionally, the European Space Agency (ESA) has competitively selected missions that are led by ESA-nominated project scientists and include instruments that are overseen by PIs. Usually, the PIs and project scientists form a science working team that provides scientific leadership of the mission. These NASA and ESA PI roles require a deep understanding of the mission's scientific goals and engineering design aspects. And, importantly, these roles also require skills in team management.

The first dedicated ice giants mission.

The mysteries of the Uranus system can be unlocked through interdisciplinary exploration.

The Plasma Environment of Comet 67P/Churyumov-Gerasimenko.

The environment of a comet is a fascinating and unique laboratory to study plasma processes and the formation of structures such as shocks and discontinuities from electron scales to ion scales and above. The European Space Agency's Rosetta mission collected data for more than two years, from the rendezvous with comet 67P/Churyumov-Gerasimenko in August 2014 until the final touch-down of the spacecraft end of September 2016. This escort phase spanned a large arc of the comet's orbit around the Sun, including its perihelion and corresponding to heliocentric distances between 3.8 AU and 1.24 AU. The length of the active mission together with this span in heliocentric and cometocentric distances make the Rosetta data set unique and much richer than sets obtained with previous cometary probes. Here, we review the results from the Rosetta mission that pertain to the plasma environment. We detail all known sources and losses of the plasma and typical processes within it. The findings from in-situ plasma measurements are complemented by remote observations of emissions from the plasma. Overviews of the methods and instruments used in the study are given as well as a short review of the Rosetta mission. The long duration of the Rosetta mission provides the opportunity to better understand how the importance of these processes changes depending on parameters like the outgassing rate and the solar wind conditions. We discuss how the shape and existence of large scale structures depend on these parameters and how the plasma within different regions of the plasma environment can be characterised. We end with a non-exhaustive list of still open questions, as well as suggestions on how to answer them in the future.

Origin of Molecular Oxygen in Comets: Current Knowledge and Perspectives.

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument onboard the Rosetta spacecraft has measured molecular oxygen (O2) in the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) in surprisingly high abundances. These measurements mark the first unequivocal detection of O2 in a cometary environment. The large relative abundance of O2 in 67P/C-G despite its high reactivity and low interstellar abundance poses a puzzle for its origin in comet 67P/C-G, and potentially other comets. Since its detection, there have been a number of hypotheses put forward to explain the production and origin of O2 in the comet. These hypotheses cover a wide range of possibilities from various in situ production mechanisms to protosolar nebula and primordial origins. Here, we review the O2 formation mechanisms from the literature, and provide a comprehensive summary of the current state of knowledge of the sources and origin of cometary O2.

Photochemistry on Pluto: part II HCN and nitrogen isotope fractionation.

We have converted our Titan one-dimensional photochemical model to simulate the photo- chemistry of Pluto's atmosphere and include condensation and aerosol trapping in the model. We find that condensation and aerosol trapping are important processes in producing the HCN altitude profile observed by the Atacama Large Millimeter Array (ALMA). The nitrogen iso- tope chemistry in Pluto's atmosphere does not appear to significantly fractionate the isotope ratio between N2 and HCN as occurs at Titan. However, our simulations only cover a brief period of time in a Pluto year, and thus only a brief portion of the solar forcing conditions that Pluto's atmosphere experiences. More work is needed to evaluate photochemical fractionation over a Pluto year. Condensation and aerosol trapping appear to have a major impact on the altitude profile of the isotope ratio in HCN. Since ALMA did not detect HC15N in Pluto's atmosphere, we conclude that condensation and aerosol trapping must be much more efficient for HC15N compared to HC14N. The large uncertainty in photochemical fractionation makes it difficult to use any potential current measurement of 14N/15N in N2 to determine the origin of Pluto's nitrogen. More work is needed to understand photochemical fractionation and to evaluate how condensation, sublimation and aerosol trapping will fractionate N2 and HCN.

Photochemistry on Pluto - I. Hydrocarbons and aerosols.

In light of the recent New Horizons flyby measurements, we present a coupled ion-neutral-photochemistry model developed for simulating the atmosphere of Pluto. Our model results closely match the observed density profiles of CH4, N2 and the C2 hydrocarbons in the altitude range where available New Horizons measurements are most accurate (above ~ 100-200 km). We found a high eddy coefficient of 106 cm2 s-1 from the surface to an altitude of 150 km, and 3 × 106 cm2 s-1 above 150 km for Pluto's atmosphere. Our results demonstrate that C2 hydrocarbons must stick to and be removed by aerosol particles in order to reproduce the C2 profiles observed by New Horizons. Incorporation into aerosols in Pluto's atmosphere is a significantly more effective process than condensation, and we found that condensation alone cannot account for the observed shape of the vertical profiles. We empirically determined the sticking efficiency of C2 hydrocarbons to aerosol particles as a function of altitude, and found that the sticking efficiency of C2 hydrocarbons is inversely related to the aerosol surface area. Aerosols must harden and become less sticky as they age in Pluto's atmosphere. Such hardening with ageing is both necessary and sufficient to explain the vertical profiles of C2 hydrocarbons in Pluto's atmosphere. This result is in agreement with the fundamental idea of aerosols hardening as they age, as proposed for Titan's aerosols.

The presence of clathrates in comet 67P/Churyumov-Gerasimenko.

Cometary nuclei are considered to most closely reflect the composition of the building blocks of our solar system. As such, comets carry important information about the prevalent conditions in the solar nebula before and after planet formation. Recent measurements of the time variation of major and minor volatile species in the coma of the Jupiter family comet 67P/Churyumov-Gerasimenko (67P) by the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument onboard Rosetta provide insight into the possible origin of this comet. The observed outgassing pattern indicates that the nucleus of 67P contains crystalline ice, clathrates, and other ices. The observed outgassing is not consistent with gas release from an amorphous ice phase with trapped volatile gases. If the building blocks of 67P were formed from crystalline ices and clathrates, then 67P would have agglomerated from ices that were condensed and altered in the protosolar nebula closer to the Sun instead of more pristine ices originating from the interstellar medium or the outskirts of the disc, where amorphous ice may dominate.

Isotopic constraints on the source of Pluto's nitrogen and the history of atmospheric escape.

The origin and evolution of nitrogen in solar system bodies is an important question for understanding processes that took place during the formation of the planets and solar system bodies. Pluto has an atmosphere that is 99% molecular nitrogen, but it is unclear if this nitrogen is primordial or derived from ammonia in the protosolar nebula. The nitrogen isotope ratio is an important tracer of the origin of nitrogen on solar system bodies, and can be used at Pluto to determine the origin of its nitrogen. After evaluating the potential impact of escape and photochemistry on Pluto's nitrogen isotope ratio (14N/15N), we find that if Pluto's nitrogen originated as N2 the current ratio in Pluto's atmosphere would be greater than 324 while it would be less than 157 if the source of Pluto's nitrogen were NH3. The New Horizons spacecraft successfully visited the Pluto system in July 2015 providing a potential opportunity to measure 14N/15N in N2.

13C and 15N fractionation of CH4/N2 mixtures during photochemical aerosol formation: Relevance to Titan.

The ratios of the stable isotopes that comprise each chemical species in Titan's atmosphere provide critical information towards understanding the processes taking place within its modern and ancient atmosphere. Several stable isotope pairs, including 12C/13C and 14N/15N, have been measured in situ or probed spectroscopically by Cassini-borne instruments, space telescopes, or through ground-based observations. Current attempts to model the observed isotope ratios incorporate fractionation resulting from atmospheric diffusion, hydrodynamic escape, and primary photochemical processes. However, the effect of a potentially critical pathway for isotopic fractionation - organic aerosol formation and subsequent deposition onto the surface of Titan - has not been considered due to insufficient data regarding fractionation during aerosol formation. To better understand the nature of this process, we have conducted a laboratory study to measure the isotopic fractionation associated with the formation of Titan aerosol analogs, commonly referred to as 'tholins', via far-UV irradiation of several methane (CH4) and dinitrogen (N2) mixtures. Analysis of the δ13C and δ15N isotopic signatures of the photochemical aerosol products using an isotope ratio mass spectrometer (IRMS) show that fractionation direction and magnitude are dependent on the initial bulk composition of the gas mixture. In general, the aerosols showed enrichment in 13C and 14N, and the observed fractionation trends can provide insight into the chemical mechanisms controlling photochemical aerosol formation.

Comparative planetology of the history of nitrogen isotopes in the atmospheres of Titan and Mars.

We present here a comparative planetology study of evolution of 14N/15N at Mars and Titan. Studies show that 14N/15N can evolve a great deal as a result of escape in the atmosphere of Mars, but not in Titan's atmosphere. We explain this through the existence of an upper limit to the amount of fractionation allowed to occur due to escape that is a function of the escape flux and the column density of nitrogen.

PROTOSOLAR AMMONIA AS THE UNIQUE SOURCE OF TITAN's NITROGEN.

The origin of Titan's nitrogen-rich atmosphere is thought to be ammonia ice, but this has not yet been confirmed. Furthermore, it is uncertain whether the building blocks of Titan formed within the Saturnian subnebula or in the colder protosolar nebula (PSN). Recent measurements of the nitrogen isotope ratio in cometary ammonia, combined with evolutionary constraints on the nitrogen isotopes in Titan's atmosphere provide firm evidence that the nitrogen in Titan's atmosphere must have originated as ammonia ice formed in the PSN under conditions similar to that of cometary formation. This result has important implications for the projected D/H ratio in cometary methane, nitrogen isotopic fractionation in the PSN and the source of nitrogen for Earth's atmosphere.

A prototype mass spectrometer for in situ analysis of cave atmospheres.

Research in cave environments has many applications: studying local hydrogeologic activity, paleoclimate studies, analyzing white nose syndrome in bat populations, analogs for underground atmospheres in mining facilities, carbon sequestration efforts, and terrestrial analogs for planetary caves. The atmospheres of many caves contain tracers of current geological and biological activity, but up to this point, in situ studies have been limited to sensors that monitor individual components of the cave atmosphere. A prototype cave mass spectrometer system was assembled from commercial off-the-shelf parts to conduct surveys of atmospheric compositions inside four local Texas caves and to perform atmospheric analysis of two aquifer wellheads to a depth of 60 m. We found increased levels of CO(2) in all caves and, surprisingly, increased levels of O(2) in Bracken Bat Cave. Aquifer wellhead measurements showed indications of methane, other hydrocarbons, and other constituents not anticipated.