The moons of Uranus have only been visited once by Voyager 2 during its 1986 flyby. Earth-based telescopic observations show a spectral signature of carbon dioxide ice on the Uranian moons Ariel, Umbriel, Titania, and Oberon, with a somewhat higher abundance on their trailing hemispheres. The inner major moon Ariel exhibits the strongest carbon dioxide ice absorption bands, which then decrease in strength with increasing orbital distance from Uranus, with the outer major moon Oberon exhibiting the weakest absorption bands. Previous work has suggested that these hemispherical and radial trends result from radiolytic production of carbon dioxide ice from interactions between the moons' surfaces and charged particles trapped in Uranus' magnetosphere. Here, we use volatile transport modeling to characterize a possible migration cycle of carbon dioxide on Ariel. We find that carbon dioxide is readily mobilized toward Ariel's equator, and that existing topography such as canyons are locations of favorable deposition for carbon dioxide ice. We predict the presence of carbon dioxide ice deposits on the floors of Ariel's canyons. Our work suggests two possible classes of sources of carbon dioxide: an active source, which may be consistent with either radiolytic production from Uranus' magnetosphere or outgassing from Ariel's interior, or an ancient source that produced CO2 that still exists in stable canyon deposits. A future Uranus orbiter could determine which hypothesis is most likely, or if carbon dioxide could be found both in the form of ice deposits on the surface and in a global exosphere. Uranus' moons have only been visited once by the Voyager 2 spacecraft in 1986. Observations from telescopes on Earth show that there is carbon dioxide ice on four of the five largest Uranian moons, but it is mostly concentrated on one hemisphere. Previous work hypothesizes that this carbon dioxide ice could be made by the moons' surfaces interacting with electrons and ions trapped in Uranus' magnetic field. We tested that hypothesis using a model for Ariel, the moon with the strongest evidence for carbon dioxide ice. We find that carbon dioxide ice will move quickly away from Ariel's poles and toward the equator. Because carbon dioxide moves quickly, our results suggest that carbon dioxide could either be actively produced or uncovered on the surface or could be anciently formed from material coming from Ariel's interior; or both mechanisms could source the carbon dioxide. We also find that carbon dioxide ice deposits are likely on the floors of the canyons that were visible in Voyager 2 images of Ariel's surface. The Uranian moons show a spectral signature of carbon dioxide ice (Ariel most strongly) preferentially on their trailing hemispheres We find that carbon dioxide ice is transported toward Ariel's equator, and both an active and ancient source of carbon dioxide is plausible We predict carbon dioxide deposits on the floors of Ariel's canyons, which could be detected by a future Uranus orbiter

The High Energy X-ray spectrometer (HEX) on Chandrayaan-1 was designed to study the photon emission in the range of 30-270 keV from naturally occurring radioactive decay of U-238 and Th-232 series nuclides from the lunar surface. The primary objective of HEX was to study the transport of volatiles on the lunar surface using radon as a tracer and mapping the 46.5 keV line from Pb-210, a decay product of Rn-222. HEX was tested for two days during the commissioning phase of Chandrayaan-1 and performance of all sub systems was found to be as expected. HEX started collecting science data during the first non-prime imaging season (February-April, 2009) of Chandrayaan-1. Certain anomalies persisted in this data set and the early curtailment of Chandrayaan-1 mission in August, 2009, did not allow any further operation of HEX. Despite these issues, HEX provided the first data set for 30-270 keV continuum emission, averaged over a significant portion of the lunar surface, including the polar region. (C) 2013 COSPAR. Published by Elsevier Ltd. All rights reserved.

Chandrayaan-1, India's first planetary exploration mission to Moon carries a suite of payloads including a High Energy X-ray spectrometer (HEX) designed to study low-energy (30-270 keV) natural gamma rays emitted from the lunar surface due to decay of uranium and thorium. The primary science objective of HEX is to study transport of volatiles on the lunar surface through the detection of the 46.5 keV line from Pb-210 decay, which is a decay product of volatile Rn-222, both belonging to the U-238 decay series. HEX is designed to have a spatial resolution of similar to 33 km at energies below 120 keV. The low signal strength of these emissions requires a large area detector with high sensitivity and energy resolution, and a new generation Cd-Zn-Te (CZT) solid state array detector is used in this experiment. Long time integration will be required to detect the emission because of the significant lunar continuum background and weak signal strength. The various sub-systems of the HEX flight payload and test results from ground calibration are described in this article. HEX will be the first experiment aimed at detecting low energy (<300 keV) gamma ray emission from a planetary surface.

The Chandrayaan-1 mission to the Moon scheduled for launch in late 2007 will include a high energy X-ray spectrometer (HEX) for detection of naturally occurring emissions from the lunar surface due to radioactive decay of the U-238 and Th-232 series nuclides in the energy region 20-250 keV. The primary science objective is to study the transport of volatiles on the lunar surface by detection of the 46.5 keV line from radioactive Pb-210, a decay product of the gaseous Rn-222, both of which are members of the U-238 decay series. Mapping of U and Th concentration over the lunar surface, particularly in the polar and U-Th rich regions will also be attempted through detection of prominent lines from the U and Th decay series in the above energy range. The low signal strengths of these emissions require a detector with high sensitivity and good energy resolution. Pixelated Cadmium-Zinc-Telluride (CZT) array detectors having these characteristics will be used in this experiment. Here we describe the science considerations that led to this experiment, anticipated flux and background (lunar continuum), the choice of detectors, the proposed payload configuration and plans for its realization.