Multiwavelength studies of regolith effects in planetary remote sensing

A large proportion of our knowledge about the surfaces of atmosphereless solar-system bodies is obtained through remote-sensing measurements. The measurements can be carried out either as ground-based telescopic observations or space-based observations from orbiting spacecraft. In both cases, the measurement geometry normally varies during the observations due to the orbital motion of the target body, the spacecraft, etc.. As a result, the data are acquired over a variety of viewing and illumination angles. Surfaces of planetary bodies are usually covered with a layer of loose, broken-up rock material called the regolith whose physical properties affect the directional dependence of remote-sensed measurements. It is of utmost importance for correct interpretation of the remote-sensed data to understand the processes behind this alteration. In the thesis, the multi-angular effects that the physical properties of the regolith have on remote-sensing measurements are studied in two regimes of electromagnetic radiation, visible to near infrared and soft X-rays. These effects are here termed generally the regolith effects in remote sensing. Although the physical mechanisms that are important in these regions are largely different, notable similarities arise in the methodology that is used in the study of the regolith effects, including the characterization of the regolith both in experimental studies and in numerical simulations. Several novel experimental setups have been constructed for the thesis. Alongside the experimental work, theoretical modelling has been carried out, and results from both approaches are presented. Modelling of the directional behaviour of light scattered from a regolith is utilized to obtain shape and spin-state information of several asteroids from telescopic observations and to assess the surface roughness and single-scattering properties of lunar maria from spacecraft observations. One of the main conclusions is that the azimuthal direction is an important factor in detailed studies of planetary surfaces. In addition, even a single parameter, such as porosity, can alter the light scattering properties of a regolith significantly. Surface roughness of the regolith is found to alter the elemental fluorescence line ratios of a surface obtained through planetary soft X-ray spectrometry. The results presented in the thesis are among the first to report this phenomenon. Regolith effects need to be taken into account in the analysis of remote-sensed data, providing opportunities for retrieving physical parameters of the surface through inverse methods.Helsingin yliopiston tähtitieteen laitokselta 6.3.2009 väittelevä Jyri Näränen on tutkinut väitöstyössään Aurinkokunnan ilmakehättömien kappaleiden valonheijastusominaisuuksia. Hän on myös kartoittanut laajalti Kuun pinnan fysikaalisia ominaisuuksia sekä saanut uraauurtavia tuloksia planeettojen pintojen ns. röntgenfluoresenssista. Aurinkokunnan kappaleista heijastuneen valon määrä riippuu siitä, missä kulmassa niitä havaitaan suhteessa Aurinkoon. Valtaosa Aurinkokunnan kappaleiden tutkimuksesta tehdään Maasta tai luotaimista käsin. Tällöin sekä havainto- että valaistuskulmat muuttuvat usein havaintojen aikana paljonkin, ja heijastuskulman vaikutukset havaintoihin on tunnettava, jotta tulokset voidaan tulkita oikein. Kiviplaneettoja ja Aurinkokunnan pienkappaleita peittää tavallisesti kerros väljästi pakkautunutta kivimurskaa ja pölyä, jota kutsutaan regoliitiksi. Mm. regoliitin huokoisuudesta ja regoliittihiukkasten muotojen ja kokojen jakaumasta riippuu, miten valo heijastuu siitä takaisin. Jos Aurinko valaisee kappaleen suoraan Maan takaa, kappale on täysin valaistu ja puhutaan nollaheijastuskulmasta. Tällöin kappale on myös kirkkaimmillaan. Kun valonlähteen ja havaitsijan välinen kulma kasvaa, kappale näyttää himmenevän. Himmenemisen määrä riippuu yllämainituista regoliitin ominaisuuksista. Regoliitin heijastuskulmaefektiä voidaan käyttää hyväksi kun halutaan selvittää heijastavan pinnan ominaisuuksia. Tässä nk. inversiomenetelmässä muodostetaan tietokonemalli heijastavasta pinnasta. Mallin fysikaalisia ominaisuuksia muutetaan, kunnes sillä voidaan hyvin mallintaa havaittu heijastuskulmaefekti. Näränen on ollut mukana tutkimassa Kuun tummien merialueiden regoliitin ominaisuuksia ESA:n SMART-1 kuuluotaimen vuosina 2004-2006 keräämän havaintoaineiston perusteella. Tulokset vastasivat melko hyvin Apollo -lennoilta saatuja tuloksia, mutta kattoivat huomattavasti suuremman osan Kuun pintaa kuin mitä miehitetyillä lennoilla pystyttiin tutkimaan. Tutkimuksessa käytetty havaintoaineisto on lajissaan yksi laajimmista maailmassa. Näränen on myös kehittänyt ja käyttänyt tutkimuksessaan uudenlaisia koejärjestelyitä, joilla on tutkittu regoliitin pintakarkeuden vaikutusta mitattuun röntgensäteilyyn mittauskulman muuttuessa. Karkean pinnan huomattiin muuttavan havaittua röntgensäteilyä energisemmäksi kuin sileän pinnan. Tulos on merkittävä, sillä regoliitista lähtevän röntgensäteilyn energiaa mitataan usein, kun tutkitaan sisemmän Aurinkokunnan kappaleiden pinnan alkuainekoostumusta. Koostumuksen avulla pystytään ymmärtämään paremmin pinnan kemiaa ja mineraaleja. Tulokset ovat hyödyksi ensi vuosikymmenellä, kun NASA:n MESSENGER ja ESA:n BepiColombo -luotaimet saapuvat tutkimaan Merkuriusta

Forty-three years of absolute gravity observations of the Fennoscandian postglacial rebound in Finland

Postglacial rebound in Fennoscandia causes striking trends in gravity measurements of the area. We present time series of absolute gravity data collected between 1976 and 2019 on 12 stations in Finland with different types of instruments. First, we determine the trends at each station and analyse the effect of the instrument types. We estimate, for example, an offset of 6.8 μgal for the JILAg-5 instrument with respect to the FG5-type instruments. Applying the offsets in the trend analysis strengthens the trends being in good agreement with the NKG2016LU_gdot model of gravity change. Trends of seven stations were found robust and were used to analyse the stabilization of the trends in time and to determine the relationship between gravity change rates and land uplift rates as measured with global navigation satellite systems (GNSS) as well as from the NKG2016LU_abs land uplift model. Trends calculated from combined and offset-corrected measurements of JILAg-5- and FG5-type instruments stabilized in 15 to 20 years and at some stations even faster. The trends of FG5-type instrument data alone stabilized generally within 10 years. The ratio between gravity change rates and vertical rates from different data sets yields values between − 0.206 ± 0.017 and − 0.227 ± 0.024 µGal/mm and axis intercept values between 0.248 ± 0.089 and 0.335 ± 0.136 µGal/yr. These values are larger than previous estimates for Fennoscandia

Validation of GNSS-based reference point monitoring of the VGOS VLBI telescope at Metsähovi

[EN] VLBI telescope reference point, the closest point in the telescope primary axis from the secondary axis, is typically determined indirectly by observation of points co-rotating with the telescope. We have previously measured telescope reference point indirectly with two GPS-antennas attached on the edge of the dish of the Aalto University Metsähovi radio telescope in 2008-2015. Now we have applied the same technique to the new VGOS-telescope of the FGI Metsähovi geodetic research station. The reference point of the VGOS antenna was estimated using post-processed trajectory coordinates of two GNSS antennas. The antennas are attached on the edge of the radio telescope dish with gimbals where a counterweight with shock absorber act as compensators to ensure zenith pointing at all telescope elevation angles. In addition, spherical prisms are attached to the structure of the telescope for tachymetric reference point determination. One purpose of this study is to evaluate the limit values and uncertainties of the compensator assembly by simulations and precise tachymeter measurements. To ensure that the compensation error is nearly constant or can be modelled, we have measured the residual tilt of the GNSS antennas with different VLBI antenna elevations. The results indicate a need to apply the corrections or to improve the compensator design. We aim to improve the counterweight and dampening so that no extra model corrections to trajectory coordinates are needed. For final assurance of our GNSS-based reference point monitoring performance, we have compared the reference point coordinates determined by simultaneous tachymetric and GNSS data. Our results and simulations showed that, with a small compensation error, the influence on reference point coordinates is marginal but the axis offset will be compromised, provided that the compensating angle bias is nearly constant.  Preliminary reference point estimates show a rather good agreement of simultaneous GNSS-based and tachymetric reference points. The final results will be achieved as part of the 18SIB01 EMPIR GeoMetre project, funded from the EMPIR programme and co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.Kallio, U.; Eskelinen, J.; Jokela, J.; Koivula, H.; Marila, S.; Näränen, J.; Poutanen, M.... (2023). Validation of GNSS-based reference point monitoring of the VGOS VLBI telescope at Metsähovi. Editorial Universitat Politècnica de València. 93-98. https://doi.org/10.4995/JISDM2022.2022.13691939

Contribution française à la gravimétrie polaire

Four campaigns of absolute gravity (AG) measurements were carried out with the FG5 and FG5-X absolute gravimeters in the Ross Sea and Terra Nova Bay region of Antarctica, successively in 2009, 2011, 2015 and 2018. These campaigns are the result of collaboration between French, Finnish, Italian, New Zealand and American agencies and institutes, the 2009, 2011 and 2015 campaigns having been supported by the international POLENET (Polar Earth Observing Network) programme. Absolute gravity was also measured in Terre Adélie in 2000, 2006 and 2019. This poster shows the variations in gravity at different stations, also taking into account previous absolute gravity measurements made in the 1990s. Also presented are the two absolute gravity measurement campaigns carried out in the French southern lands, as well as the one carried out in the Arctic, at Ny-Alesund, Svalbard. This poster was presented at the Résif Scientific and Technical Meetings held in Biarritz in November 2019. Résif is a national research infrastructure dedicated to the observation and understanding of the Earth's internal structure and dynamics. Résif is based on high technology observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout the French territory. The data collected allow the study of ground deformation, surface and deep structures, local and global seismicity and natural hazards, particularly seismic, on the French territory with a high spatio-temporal resolution. Résif is integrated into the European (EPOS - European Plate Observing System) and worldwide instruments that allow to image the Earth's interior in its entirety and to study many natural phenomena.Quatre campagnes de mesures de la gravité absolue (AG) ont été réalisées avec les gravimètres absolus FG5 et FG5-X dans la région de la mer de Ross et de la baie de Terra Nova en Antarctique, successivement en 2009, 2011, 2015 et 2018. Ces campagnes sont le résultat de la collaboration entre des agences et instituts français, finlandais, italiens, néo-zélandais et américains, les campagnes de 2009, 2011 et 2015 ayant été soutenues par le programme international POLENET (Polar Earth Observing Network). Par ailleurs, la gravité absolue a également été mesurée en Terre Adélie en 2000, 2006 et 2019. Ce poster montre les variations de la gravité dans différentes stations, en tenant compte également des mesures de la gravité absolue effectuées précédemment dans les années 1990. Sont également présentées les deux campagnes de mesures de la gravité absolue effectuées dans les terres australes françaises, ainsi que celle effectuée en Arctique, à Ny-Alesund, Svalbard. Ce poster a été présenté lors des Rencontres scientifiques et techniques Résif qui se sont déroulées à Biarritz en novembre 2019. Résif est une infrastructure de recherche nationale dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. Résif se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. Résif s’intègre aux dispositifs européens (EPOS - European Plate Observing System) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels