Hiromu Nakagawa

Lunar Radar Sounder Observations of Subsurface Layers Under the Nearside Maria of the Moon

Observations of the subsurface geology of the Moon help advance our understanding of lunar origin and evolution. Radar sounding from the Kaguya spacecraft has revealed subsurface layers at an apparent depth of several hundred meters in nearside maria. Comparison with the surface geology in the Serenitatis basin implies that the prominent echoes are probably from buried regolith layers accumulated during the depositional hiatus of mare basalts. The stratification indicates a tectonic quiescence between 3.55 and 2.84 billion years ago; mare ridges were formed subsequently. The basalts that accumulated during this quiet period have a total thickness of only a few hundred meters. These observations suggest that mascon loading did not produce the tectonics in Serenitatis after 3.55 billion years ago. Global cooling probably dominated the tectonics after 2.84 billion years ago.

Comparison of the Martian thermospheric density and temperature from IUVS/MAVEN data and general circulation modeling

IUVS/MAVEN data are archived in the Planetary Atmospheres Node of the Planetary Data System (http://pds-atmospheres.nmsu.edu). Modeling data supporting the figures are available upon request from A.S.M. (medvedev@mps.mpg.de). The work was partially supported by German Science Foundation (DFG) grant ME2752/3-1. E.Y. was partially supported by NASA grant NNX13AO36G.

Synthetic Aperture Radar Processing of Kaguya Lunar Radar Sounder Data for Lunar Subsurface Imaging

Synthetic aperture radar (SAR) processing was applied to the observation data of Lunar Radar Sounder (LRS), which is an HF sounder which was installed onboard a Japanese lunar exploration orbiter, Kaguya, for the purpose of imaging lunar subsurface structure. A two-media model was introduced to the LRS SAR algorithm to define the reference function of the LRS SAR processing. The LRS SAR algorithm has two free parameters, i.e., dielectric constant of the subsurface medium and synthetic aperture. The effect of these free parameters on LRS SAR imaging was studied by simulation and was verified by actual LRS observation data. A practical guideline for LRS SAR processing was drawn. The dielectric constant of the subsurface medium may be ignored in practice so far as the synthetic aperture is smaller than 10 km. For a larger synthetic aperture case, assumption of a moderate dielectric constant (ε = 6 ~ 8) of the subsurface medium is effective in realizing good focusing of deep targets. Finally, taking full advantage of ground processing, advanced processing was attempted. Off-nadir focusing SAR processing proved to be effective in imaging oblique objects whose dominant scattering angle was not the angle toward zenith. Changing the dielectric constant of the two-media model proved to be effective in focusing/defocusing small objects, thus enabling us to localize the objects position as surface or subsurface.

Global Distribution and Parameter Dependences of Gravity Wave Activity in the Martian Upper Thermosphere Derived from MAVEN NGIMS Observations

Wavelike perturbations in the Martian upper thermosphere observed by the Neutral Gas Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft have been analyzed. The amplitudes of small-scale perturbations with apparent wavelengths between ~100 and ~500 km in the Ar density around the exobase show a clear dependence on temperature (T0) of the upper thermosphere. The average amplitude of the perturbations is ~10% on the dayside and ~20% on the nightside, which is about 2 and 10 times larger than those observed in the Venusian upper thermosphere and in the low-latitude region of Earths upper thermosphere, respectively. The amplitudes are inversely proportional to T0, suggesting saturation due to convective instability in the Martian upper thermosphere. After removing the dependence on T0, dependences of the average amplitude on the geographic latitude and longitude and solar wind parameters are found to be not larger than a few percent. These results suggest that the amplitudes of small-scale perturbations are mainly determined by convective breaking/saturation in the upper thermosphere on Mars, unlike those on Venus and Earth.

MAVEN NGIMS observations of atmospheric gravity waves in the Martian thermosphere

Gravity waves have a significant impact on both the dynamics and energy budget of the Martian thermosphere. Strong density variations of spatial scales indicative of gravity waves have previously been identified in this region using in situ observations. Here we use observations from the NGIMS mass spectrometer on MAVEN to identify such waves in the observations of different atmospheric species. The wave signatures seen in CO2 and Ar are almost identical, whereas the wave signature seen in N2, which is lighter and has a larger scale height, are generally smaller in amplitude and slightly out of phase with those seen in CO2 and Ar. Examination of the observed wave properties in these three species suggest that relatively long vertical wavelength atmospheric gravity waves are the likely source of the waves seen by NGIMS in the upper thermosphere. A two-fluid linear model of the wave perturbations in CO2 and N2 has been used to find the best-fit intrinsic wave parameters that match the observed features in these two species. We report the first observationally based estimate of the heating and cooling rates of the Martian thermosphere created by the waves observed in this region. The observed wave density amplitudes are anti-correlated with the background atmospheric temperature. The estimated heating rates show a weak positive correlation with the wave amplitude, whereas the cooling rates show a clearer negative correlation with the wave amplitude. Our estimates support previous model-based findings that atmospheric gravity waves are a significant source of both heating and cooling.

UV optical measurements of the Nozomi spacecraft interpreted with a two-component LIC-flow model

Aims. Following recent reports on spectroscopic observations by SWAN/SOHO suggesting that the flows of neutral interstellar helium and hydrogen in the inner heliosphere are slightly divergent, we tried to verify them on the basis of simultaneous photometric observations of heliospheric hydrogen and helium glows performed by a spacecraft located on an orbit between the Earth and Mars (which differs from the orbit of SWAN/SOHO). The observations were interpreted with the use of various independent models of interstellar hydrogen and helium in the inner heliosphere, evaluated over a mesh of parameters. Methods. The data might suggest that the upwind and downwind directions of interstellar H may differ by less than 180°, which we interpret as due to a side shift of the secondary population of interstellar hydrogen, which might be due to a deformation of the outer heliosheath e.g. because of the action of interstellar magnetic field. The simulations we performed do not support the idea that the secondary population is significantly shifted to the side. Results. The upwind/downwind direction of interstellar hydrogen as derived from our observations agrees within the error bars with the upwind/downwind direction of interstellar helium and the error bars include both the upwind direction of interstellar helium, derived from in-situ observations of GAS/Ulysses, and the upwind direction of interstellar hydrogen, derived from observations of SWAN/SOHO.

Development of infrared Echelle spectrograph and mid-infrared heterodyne spectrometer on a small telescope at Haleakala, Hawaii for planetary observation

We report the development of infrared Echelle spectrograph covering 1 - 4 micron and mid-infrared heterodyne spectrometer around 10 micron installed on the 60-cm telescope at the summit of Haleakala, Hawaii (alt.=3000m). It is essential to carry out continuous measurement of planetary atmosphere, such as the Jovian infrared aurora and the volcanoes on Jovian satellite Io, to understand its time and spatial variations. A compact and easy-to-use high resolution infrared spectrometer provide the good opportunity to investigate these objects continuously. We are developing an Echelle spectrograph called ESPRIT: Echelle Spectrograph for Planetary Research In Tohoku university. The main target of ESPRIT is to measure the Jovian H3+ fundamental line at 3.9 micron, and H2 nu=1 at 2.1 micron. The 256x256 pixel CRC463 InSb array is used. An appropriate Echelle grating is selected to optimize at 3.9 micron and 2.1 micron for the Jovian infrared auroral observations. The pixel scale corresponds to the atmospheric seeing (0.3 arcsec/pixel). This spectrograph is characterized by a long slit field-of-view of ~ 50 arcsec with a spectral resolution is over 20,000. In addition, we recently developed a heterodyne spectrometer called MILAHI on the 60 cm telescope. MILAHI is characterized by super high-resolving power (more than 1,500,000) covering from 7 - 13 microns. Its sensitivity is 2400 K at 9.6 micron with a MCT photo diode detector of which bandwidth of 3000 MHz. ESPRIT and MILAHI is planned to be installed on 60 cm telescope is planned in 2014.

Stringent upper limit of CH4 on Mars based on SOFIA/EXES observations

Discovery of CH4 in the Martian atmosphere has led to much discussion since it could be a signature of biological and/or geological activities on Mars. However, the presence of CH4 and its temporal and spatial variations are still under discussion because of the large uncertainties embedded in the previous observations. We performed sensitive measurements of Martian CH4 by using the Echelon-Cross-Echelle Spectrograph (EXES) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) on 16 March 2016, which corresponds to summer (Ls = 123.2∘) in the northern hemisphere on Mars. The high altitude of SOFIA (~13.7 km) enables us to significantly reduce the effects of terrestrial atmosphere. Thanks to this, SOFIA/EXES improves our chances of detecting Martian CH4 lines because it reduces the impact of telluric CH4 on Martian CH4, and allows us to use CH4 lines in the 7.5 μm band which has less contamination. However, our results show no unambiguous detection of Martian CH4. The Martian disk was spatially resolved into 3 × 3 areas, and the upper limits on the CH4 volume mixing ratio range from 1 to 9 ppb across the Martian atmosphere, which is significantly less than detections in several other studies. These results emphasize that release of CH4 on Mars is sporadic and/or localized if the process is present.

Optical and IR observations of planetary and exoplanetary atmospheres

The spatial and temporal variations of planetary atmospheric phenomena (e.g., auroras on Jupiter and Io, as well as the polarization of expolanetary atmospheres) are extremely complicated. Indeed, the wide range of these phenomena, and the cross-scale coupling processes that exist between them, means that is difficult to understand their underlying mechanisms. To study and understand such planetary atmospheric phenomena it is therefore essential to perform continuous and flexible monitoring with a suitable telescope. From previous studies it is known that Jupiter’s auroras are generated by two separate mechanisms: a rapid 10-hour rotation of the magnetic field (known as the ‘internal source’) and the variation of the solar wind (known as the ‘external source’).1 Although much effort has been made to determine which of these sources plays the major role in causing the spatial and temporal variations of Jupiter’s auroras, the mechanism has still not been resolved.2 In addition, it has been shown that active volcanoes on Io (a moon of Jupiter) intermittently inject large amounts of gases into the magnetosphere and can modulate the auroral activity.3 Past studies have also revealed that the light from exoplanet host stars is polarized with the variation period of the planet’s orbital motion. This result suggests that the polarization is caused by Rayleigh scattering in the exoplanetary atmosphere.4 The examples of exoplanetary atmospheres that have so far been studied, however, have been very limited. Further observations are thus required to fully characterize the atmospheres of different types of exoplanets. Figure 1. The 60cm Cassegrain and Coude telescope (T60) installed at the summit of Mount Haleakala, Hawaii.

High-contrast apodization baffle for instruments onboard solar system exploration missions

We present concept and laboratory demonstration of high-contrast apodization baffle for instruments to be carried on exploration missions of the solar system. The primary science objective of the high-contrast baffle is to reveal escape of atmosphere on Mars, while other faint objects around blight sources are potential targets. We diverted heritages studied for exoplanet science and instrumentation to this work. The apodization in this work is realized by edge with microscopic Gaussian shaped structure. A simulation to confirm the concept and design of the high-contrast apodization baffle was carried out. Then, a baffle which was consisting of transparent flat substrate and thin film of aluminum on it was manufactured. The experiment was executed with He-Ne laser with wavelength of 633 nm. As the result, it was demonstrated that the apodization by the Gaussian edge is significantly working to improve the contrast. Achieved contrast is better than 10-6.5 and 10-8 in θ > 0.5 degree and θ > 1 degree, respectively. These results satisfy the requirement for remote sensing of the atmospheric less on Mars.