Solar system

Abstract:

Currently, few thermal infrared measurements exist of fine particulate ( < 63 μm) analogue samples (e.g. minerals, mineral mixtures, rocks, meteorites, and lunar soils) measured under simulated lunar condi- tions. Such measurements are fundamental for interpreting thermal infrared (TIR) observations by the Diviner Lunar Radiometer Experiment (Diviner) onboard NASA’s Lunar Reconnaissance Orbiter as well as future TIR observations of the Moon and other airless bodies. In this work, we present thermal in- frared emissivity measurements of a suite of well-characterized Apollo lunar soils and a fine particu- late ( < 25 μm) San Carlos olivine sample as we systematically vary parameters that control the near- surface environment in our vacuum chamber (atmospheric pressure, incident solar-like radiation, and sample cup temperature). The atmospheric pressure is varied between ambient (1000 mbar) and vacuum ( < 10^−3 mbar) pressures, the incident solar-like radiation is varied between 52 and 146 mW/cm 2 , and the sample cup temperature is varied between 325 and 405 K. Spectral changes are characterized as each parameter is varied, which highlight the sensitivity of thermal infrared emissivity spectra to the atmospheric pressure and the incident solar-like radiation. Finally spectral measurements of Apollo 15 and 16 bulk lunar soils are compared with Diviner thermal infrared observations of the Apollo 15 and 16 sam- pling sites. This comparison allows us to constrain the temperature and pressure conditions that best simulate the near-surface environment of the Moon for future laboratory measurements and to better interpret lunar surface compositions as observed by Diviner.

Abstract:

Multispectral infrared measurements by the Diviner Lunar Radiometer Experiment on the Lunar Renaissance Orbiter enable the characterization of the position of the Christiansen Feature, a thermal infrared spectral feature that laboratory work has shown is proportional to the bulk silica content of lunar surface materials. Diviner measurements show that the position of this feature is also influenced by the changes in optical and physical properties of the lunar surface with exposure to space, the process known as space weathering. Large rayed craters and lunar swirls show corresponding Christiansen Feature anomalies. The space weathering effect is likely due to differences in thermal gradients in the optical surface imposed by the space weathering control of albedo. However, inspected at high resolution, locations with extreme compositions and Christiansen Feature wavelength positions - silica-rich and olivine-rich areas - do not have extreme albedos, and fall off the albedo- Christiansen Feature wavelength position trend occupied by most of the Moon. These areas demonstrate that the Christiansen Feature wavelength position contains compositional information and is not solely dictated by albedo. An optical maturity parameter derived from near-IR measurements is used to partly correct Diviner data for space weathering influences.

Abstract:

Parties to the United Nations Framework Convention on Climate Change (UNFCCC) have requested guidance on common greenhouse gas metrics in accounting for Nationally determined contributions (NDCs) to emission reductions1. Metric choice can affect the relative emphasis placed on reductions of ‘cumulative climate pollutants’ such as carbon dioxide versus ‘short-lived climate pollutants’ (SLCPs), including methane and black carbon2, 3, 4, 5, 6. Here we show that the widely used 100-year global warming potential (GWP100) effectively measures the relative impact of both cumulative pollutants and SLCPs on realized warming 20–40 years after the time of emission. If the overall goal of climate policy is to limit peak warming, GWP100 therefore overstates the importance of current SLCP emissions unless stringent and immediate reductions of all climate pollutants result in temperatures nearing their peak soon after mid-century7, 8, 9, 10, which may be necessary to limit warming to “well below 2 °C” (ref. 1). The GWP100 can be used to approximately equate a one-off pulse emission of a cumulative pollutant and an indefinitely sustained change in the rate of emission of an SLCP11, 12, 13. The climate implications of traditional CO2-equivalent targets are ambiguous unless contributions from cumulative pollutants and SLCPs are specified separately.

Abstract:

The phase function describes the way particles scatter the incoming radiation. This is a fundamental piece of knowledge in order to understand how a planetary atmosphere scatters sunlight and so it has a profound influence in the retrieved atmospheric properties such as cloud height, particle density distribution and radiative forcing by aerosols. In this work we analyze data from the Imaging Science Subsystem (ISS) instrument onboard Cassini spacecraft to determine the particle phase function at blue (451 nm) and near infrared wavelengths (727-890 nm) of particles in the upper troposphere, where most of the incoming visible sunlight is scattered. In order to do so, we use observations taken in later 2010 and 2011 covering a broad range of phase angles from ~10° to ~160° in the blue (BL1) and near infrared filters associated with intermediate and deep methane absorption bands (MT2, CB2, MT3). Particles at all latitudes are found to be strongly forward scattering. The equatorial particles are in good agreement with laboratory measurements of 10 μm ammonia ice crystals, while mid- and sub-polar latitude particles may be similar to the equatorial particles, but they may also be consistent with 1 μm ellipsoids with moderate aspect ratios. Uncertainties due to limited phase coverage and parameter degeneracy prevent strong constraints of the particle shapes and sizes at these locations. Results for the particle phase function are also used to describe the spatial distribution of tropospheric particles both vertically and latitudinally in the Northern hemisphere.