Poster presentations and abstracts

Abstract

The dosimetric telescope Liulin-MO [1] for measuring the radiation environment onboard the ExoMars TGO is a module of the Fine Resolution Epithermal Neutron Detector (FREND) [2].

Here we present recent results from measurements of the charged particle fluxes, dose rates and estimation of radiation quality factors and dose equivalent rates at ExoMars TGO science orbit (circular orbit with 400 km altitude, 74⁰ inclination, 2 hours orbit period), provided by Liulin-MO dosimeter from May 01, 2018 to June 10, 2020.

Since now the dosimeter has measured the dosimetric parameters of the galactic cosmic rays (GCR). Solar particle events were not registered. The measurements were taken during the declining and minimum of the Solar activity in 24^th Solar cycle.

Compared are the time profiles of particles and neutron detections from Liulin-MO, FREND/TGO and HEND/Odyssey [3] measured for the period May 2018 –December 2019.

Liulin-MO contains two dosimetric telescopes arranged at two perpendicular directions [1]. The parameters, provided by Liulin-MO simultaneously for two perpendicular directions have the following ranges: absorbed dose rate from 10^-7 Gy h^-1 to 0.1 Gy h^-1; particle flux in the range 0 - 10⁴ cm^-2 s^-1; energy deposition spectrum and coincidence energy deposition spectrum in the range 0.08 - 190 MeV.

Similar to FREND, HEND/Odyssey is a neutron spectrometer based on ³He proportional counters but with smaller sensitivity and without collimation as in FREND [3], [4].

The fluxes and dose rates recorded in the perpendicular detectors B(A) and D(C) of Liulin-MO and count rates of Oulu neutron monitor (http://cosmicrays.oulu.fi/) for the period from 1 May 2018 to 10 June 2020 are shown in Figure 1. An increase of all quantities – flux, dose rate and Oulu count rate due to solar cycle modulation is observed. In this period in two perpendicular directions B(A) and D(C) the average values are: dose rate 14.8±1.5/15.4±1.5 microGy h^-1, planar flux 3.11/3.21 cm^-2 s^-1, quality factor Q 3.5±0.26, dose equivalent rate 1.61±0.33/1.66±0.34 mSv d^-1.

In Figure 2 is shown the comparison between the normalised (relatively to the mean daily value for the full period) daily mean Liulin-MO data and HEND data accumulated in energy channels 1 – 8, most sensitive to the charge particles of GCR. These profiles demonstrate good correlation both on a short time scale (days) and long term scale (months). The total amplitude of variations observed during 20 months of TGO mapping is about 8%. Figure 3 presents the comparison between the normalised FREND total counts rate and HEND data accumulated in the energy channels 9 – 16, mostly populated with counts from Martian neutron albedo (which is produced by GCR by interaction with matter in the shallow subsurface). These curves also show high correlation with each other both on short and long term scales.

The obtained data from May 2018 to April 2019 show that 1) An increase of the dose rates and fluxes is observed from May 2018 to March 2020 which corresponds to the increase of GCR intensity during the declining of the solar activity. From March to June 2020 the measured values are practically equal, corresponding to the minimum of the solar activity; 2) The time profiles of GCR daily variations measured by Liulin-MO and HEND demonstrate good correlation. The comparison between the normalised FREND total counts rate and HEND data accumulated in the energy channels mostly populated with counts from Martian neutron albedo also show high correlation with each other. The difference in the long term variations between the profiles of GCR and neutron albedo might be addressed to how the solar modulation changes directly the GCR flux and how it changes emission of neutron albedo by interaction with a matter in the Martian shallow subsurface.

Acknowledgements

The work in Bulgaria is supported by the Contract No. 4000117692/16/NL/NDe funded by the Government of Bulgaria through an ESA Contract under the PECS (Plan for European Cooperating States) and by Project No. 129 for bilateral projects of the National Science Fund of Bulgaria and Russian Foundation for Basic Research. The work in Russia is supported by Grant 19-52-18009 for bilateral projects of the National Science Fund of Bulgaria and Russian Foundation for Basic Research in part of HEND data analysis, and Russian Science Foundation Grant 19-72-10144 in part of FREND data analysis.

References

[1] Semkova, J., et al: Charged particles radiation measurements with Liulin-MO dosimeter of FREND instrument aboard ExoMars Trace Gas Orbiter during the transit and in high elliptic Mars orbit, Icarus 303, 53–66, 2018, https://doi.org/10.1016/j.icarus.2017.12.034

[2] Mitrofanov, I., et al: Fine Resolution Epithermal Neutron Detector (FREND) onboard the Trace Gas Orbiter, Space Sci Rev, August 2018, 214:86, https://doi.org/10.1007/s11214-018-0522-5

[3] Mitrofanov, I., et al: Maps of Subsurface Hydrogen from the High Energy Neutron Detector, Mars Odyssey, Science, 2002, Volume 297, Issue 5578, 78-81

[4] Mitrofanov I.G. et al: Search for water in Martian soil using global neutron mapping by the Russian HEND instrument onboard the US 2001 Mars Odyssey spacecraft, Solar System Research, 2003, Volume 37, Issue 5, 366-377

How to cite: Semkova, J. and the Liulin-MO-FREND: Update of the radiation environment measurement results aboard ExoMars TGO in May 2018-June 2020, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-50, https://doi.org/10.5194/epsc2020-50, 2020.

NOMAD is a suite of three spectrometers on-board the ExoMars Trace Gas Orbiter. The spectrometers operate in solar occultation, nadir and limb observing modes, measuring in the infrared (2.2-4.3um in occultation; 2.2-3.8um in nadir) and UV-visible (0.2-0.65um) spectral regions. The nominal science phase began on 21st April 2018; since then NOMAD has collected over one Martian year of data.

Due to the very high spectral and spatial resolution of NOMAD, an enormous amount of data has already been generated - including tens of millions of solar occultation and nadir spectra - which currently total around four terabytes and are spread across almost half a million files. To serve the scientific community, all calibrated data will eventually be made publicly available in PDS4 format via the ESA Planetary Science Archive at

At the time of writing, the NOMAD data collection has successfully passed peer review, and data from two of the three channels will be available very shortly. This first release will consist of: 1) infrared solar occultation data; 2) UV-visible solar occultation data; and 3) UV-visible nadir data. The infrared nadir and infrared and UV-visible limb data will be released later, once the calibration is finalised. In this presentation I will update the scientific community on the current status of the NOMAD PSA archive, including a description of the data and how to start using it.

How to cite: Thomas, I., Vandaele, A. C., Daerden, F., Ristic, B., Willame, Y., Depiesse, C., Aoki, S., Trompet, L., Erwin, J., Robert, S., Piccialli, A., Neary, L., Viscardy, S., Mason, J., Patel, M., Bellucci, G., and Lopez-Moreno, J. J.: NOMAD on ExoMars TGO: Data processing and public release via the ESA Planetary Science Archive, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-521, https://doi.org/10.5194/epsc2020-521, 2020.

                This project takes advantage of the NOMAD spectrometer observations, on board the 2016 ExoMars Trace Gas Orbiter. These observations will help to determine the Martian surface properties. This work focuses on surface ice detection.

ExoMars is an ESA-Roscosmos joint mission consisting of an orbiter (Trace Gas Orbiter - TGO). The Nadir and Occultation for Mars Discovery (NOMAD) is one of the four instruments on board TGO. The instrument is a suite of three spectrometers designed to observe the atmosphere and the surface of Mars in the UV, visible and IR. For this study, the Limb, Nadir and Occultation (LNO) channel, operating in the IR, is selected [1,3]. Thanks to the nadir geometry, the NOMAD-LNO spectrometer can provide information on Martian surface, in particular on surface ices. Ice deposits exhibit specific IR signatures in the 2.3-3.8 μm range of NOMAD-LNO, which reveals information on their composition, texture and size of the ice grains [4].

We use the NOMAD nadir data to map the presence of ices for MY 34 (Ls = 150°-360°) and MY 35 (Ls = 0°-210°) with a solar zenith angle below 80 degrees. Raw data have been calibrated by using full solar scans to provide the reflectance factor which is the spectral radiance measured by NOMAD divided by the solar spectral irradiance corrected for the incident angle.

We use spectral indices based on reflectance factor similar to G. Bellucci et al., 2019 [2]. An ice index is defined as a ratio between two orders taking into account the albedo variations.  The objective is to remove the atmospheric effects. In this work, we propose different indices to detect the two polar caps and the sublimation process. The orders 194 (2275 - 2293.2 nm), 189 (2335.2 - 2363.9 nm) and 167 (2642.8 - 2663.9 nm) have been selected. We call Index 1, the index defined as 189/167 [5], and Index 2, 194/167. While orders 194 and 189 have wavelengths on the radiance continuum, the order 167 is located on the shoulder of 2.7 µm CO₂/H₂O ices absorption.

All LNO observations considered in this study with the orders given above are organized as Latitude and Solar longitude maps, called seasonal maps. Figure 1 shows the seasonal map for Index 1 during MY 34. The good coverage allows observing the sublimation process for the southern polar cap between the 150°-250° solar longitude.  Due to the lack of observations, the detection of the northern polar cap does not seem very clear. Nevertheless, Index 1 gives high values between Ls = 320°-340° and for latitudes above 60° which should correspond to the polar cap in that region. Figure 2 shows the same index for MY 35. The sublimation process can be observe for the northern polar cap between Ls= 0°-30°. A small detection of the southern polar cap can be see after Ls = 200°.

Figure 3 shows the seasonal map for Index 2. There are observations only for MY35 and many gaps are present due to the lack of observations. Nevertheless, the two polar caps can be observe in the plot. Index 2 gives the two sublimation processes between Ls = 0°-30° for the northern polar cap and around Ls = 200° for the southern polar cap.

Those new results extend the temporal and spatial of the previous work (G. Bellucci et al., 2019 [2] and L. Ruiz Lozano et al., 2019 [5]).

Figure 1: Seasonal map of Index 1 for MY 34 showing the sublimation of the southern polar cap.

Figure 2: Seasonal map of Index 1 for MY 35 showing the sublimation of the northern polar cap.

Figure 3: Seasonal map of Index 2 for MY 35 showing the sublimation of the two polar caps.

Acknowledgements

The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1 and ST/R001405/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency.

References

[1] A.C. Vandaele et al., Optical and radiometric models of the NOMAD instrument part I: the UVIS channel. Optics Express, 23(23):30028–30042,2015.
[2]  G. Bellucci et al., TGO/NOMAD Nadir observations during the 2018 global dust storm event, EPSC-DPS 2019
[3] E. Neefs et al., NOMAD spectrometer on the ExoMars trace gas orbiter mission: part 1—design, manufacturing and testing of the infrared channels. Applied optics, 54(28):8494–8520, 2015.
[4] Y. Langevin et al., Observations of the south seasonal cap of Mars during recession in 2004–2006 by the OMEGA visible/near-infrared imaging spectrometer
[5] L. Ruiz Lozano, Use of NOMAD Observations (Trace Gas Orbiter) for Mars surface depositions, EPSC-DPS 2019

How to cite: Ruiz Lozano, L., Karatekin, Ö., Caldiero, A., Imbreckx, A.-C., Temel, O., Dehant, V., Daerden, F., Thomas, I., Ristic, B., Patel, M., Bellucci, G., López Moreno, J. J., and Vandaele, A. C.: Use of TGO-NOMAD nadir observations for ices detection, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-748, https://doi.org/10.5194/epsc2020-748, 2020.

The NOMAD-SO channel [1, 2] is an infrared spectrometer working in the 2.2 to 4.3 µm spectral range (2325-4545 cm^-1). The instrument is composed of an echelle grating coupled to an Acousto-Optical Tunable Filter for the diffraction order selection [3]. NOMAD started to perform solar occultation measurement on April 21, 2018. As TGO is on a quasi-circular orbit at around 400 km of altitude, it performs one orbit every two hours. During a solar occultation measurement, SO scans six diffraction orders each second. These diffraction orders are recorded on four bins leading to a vertical sampling below 1 km. The calibration of the SO channel is described in [4] and is being refined.

NOMAD-SO regularly scans different diffraction orders containing CO2 lines to allow CO₂ retrievals from low to high altitudes. For each solar occultation measurement, we derive a slant column profile of CO₂ using ASIMUT-ALVL [4]. ASIMUT is a radiative transfer program developed at BIRA-IASB and based on the Optimal Estimation Method [5]. The GEM-Mars GCM provides the a priori profiles of CO₂ local density, pressure and temperature. We then apply Tikhonov linear regularization on the slant column to derive a smoothed local density. We finally apply the hydrostatic equilibrium equation and the ideal gas law to derive the temperature profiles [6-8]. That derived temperature profile serves then in a new loop where we perform again the previous steps until the profiles converge [8]. Several comparisons are ongoing with joint or co-located measurements from MAVEN-EUVM, Maven-NGIMS, and TGO-ACS-NIR as well as with GCM derived profiles from GEM-Mars and LMD-MGCM. We derived the NOMAD-SO CO₂ and temperature profiles for MY34 with solar longitudes (Ls) extending from 298° to 326°. That time range contains the regional dust storm of MY34 that started at Ls 317°. We will present the updated CO2 and temperature profiles from NOMAD-SO measurements

Acknowledgements

ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by the Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grant ST/R005761/1 and Italian Space Agency through grant 2018-2-HH.0. The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant numbers 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and BELSPO BrainBe SCOOP Project. US investigators were supported by the National Aeronautics and Space Administration.

References

[1] Vandaele, A.C., et al.: Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission, Planet. Space Sci., Vol. 119, pp 233-249, 2015.

[2] Neefs, E., et al.: NOMAD spectrometer on the ExoMars trace gas orbiter mission: part1 – design, manufacturing and testing of the infrared channels, Appl. Opt., Vol. 54(28), pp 8494-8520, 2015.

[3] Thomas, I.R., et al.: Optical and radiometric models of the NOMAD instrument part II: the infrared channels – SO and LNO, Opt. Express, Vol. 24(4), pp 3790-3805, 2016.

[4] Liuzzi, G. et al.: Methane on Mars: New insights into the sensitivity of CH4 with the NOMAD/ExoMars spectrometer through its first in-flight calibration. Icarus. 321, 2018.

[4] Vandaele, A.C., Kruglanski, M., De Mazière, M., Modeling and retrieval of atmospheric spectra using ASIMUT. Proceedings of the First Atmospheric Science Conference, ESRIN, Frascati, Italy, 2006.

[5] Rodgers, C.D., Inverse method for atmospheric sounding: theory and practice. Hackensack, N.J. (Ed.), World Scientific University of Oxford, Oxford, 2000.

[6] Gröller, H., et al.: MAVEN/IUVS stellar occultation measurements of Mars atmospheric structure and composition. Journal of Geophysical Research: Planets, 123, 1449–1483, 2018.

[7] Snowden D., et al. :The thermal structure of titan's upper atmosphere, I: temperature profiles from Cassini INMS observations, Icarus, Vol. 226, pp. 552-582, 2013.

[8] Mahieux, A. et al.: Densities and temperatures in the Venus mesosphere and lower thermosphere retrieved from SOIR on board Venus Express: Carbon dioxide measurements at the Venus terminator, J. Geophys. Res., Vol 117, E07001, 2012.

How to cite: Trompet, L., Vandaele, A. C., Aoki, S., Erwin, J., Thomas, I., Villanueva, G., Liuzzi, G., Crismani, M., Lopez-Valverde, M. A., Hill, B., Piccialli, A., Daerden, F., Ristic, B., Lopez-Moreno, J. J., Bellucci, G., and Patel, M.: Update on CO2 and temperature profiles from NOMAD-SO on board ExoMars TGO, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-641, https://doi.org/10.5194/epsc2020-641, 2020.

Abstract
Here we analyse nadir data from both the UVIS and LNO channels of the NOMAD spectrometer [1], onboard ExoMars/TGO, to obtain information about Martian dust densities and grains sizes. The combined dataset is analyzed with the MITRA radiative transfer tool [2,3,4]. The method is validated on a NOMAD orbit registered during the 2018 global dust storm to assess its most critical issues and is currently being extended to the whole available spatially and temporally coincident UVIS and LNO nadir data.
Introduction
The investigation of the presence and distribution of trace gases on Mars is fundamental to understand the atmosphere past evolution and provides insights on the research of biotic activities. Recent studies focused on the 2018 global-scale dust event as observed from the TGO instruments demonstrate that Martian dust can affect the abundance and distribution of atmospheric trace gases [5,6,15], making it a driver for their evolution. Suspended dust on Mars drives the planet’s thermal structure and climate [7], heating the lower atmosphere through absorption in the VIS-NIR spectral range [8] and efficiently radiating heat to space through IR emission [9,10,8,11]. These heating and cooling mechanisms affect the water-ice clouds formation, strengthen the mean meridional circulation and can drive deep localized convection, leading to variations and redistributions of water vapour abundances [6,15]. For the above reasons, the understanding of dust properties is mandatory in order to correctly investigate the vertical distribution of Martian trace gases.
Observations
The NOMAD spectrometer operates with three channels in the ultraviolet/visible spectral range (UVIS channel) and in the infrared (LNO and SO channels) in nadir, limb and solar occultation geometries. Although the instrument has been mainly conceived to study the trace gases in the atmosphere of the red planet, it can also provide valuable information regarding the properties of Martian dust. In this regard, in this work we use the UVIS and LNO channels nadir data to construct a dataset of spatially and temporally coincident data to study the microphysical properties of Martian dust. 
Method
The UVIS spectral range alone (0.20–0.65 μm) does not allow to disentangle with high precision the information related to the dust density from that of dust grains sizes. The use of both the ultraviolet/visible and infrared ranges together is mandatory in order to obtain these properties. Indeed, we show that while the order of magnitude of the dust optical depth can be inferred from the UVIS data alone, dust grains sizes can be retrieved only by studying how the observed spectra bend between visual and near infrared wavelengths. For the purpose of the analysis presented here we use only LNO orders covering the wavelength range 2.20-2.65 μm, which is approximately free from gaseous absorption and, hence, it is suitable to investigate dust properties. We use the MITRA radiative transfer model and inversion algorithm to retrieve dust densities and grain sizes. We take the temperature-pressure profiles from the Mars Climate Database (MCD, [12]) and  use the dust optical constants from [13]. The procedure followed to derive the surface albedo spectra, needed in the forward model, is based on the SAS method [14] applied to the OMEGA dataset.  For this reason, we use UVIS data only down to 0.4 μm to avoid extrapolating the information of the surface albedo at wavelengths shorter than OMEGA lower spectral limit. This method is tested and validated on a NOMAD observation acquired on the 8th of June 2018 during a global dust storm and is now being extended to all spatially and temporally coincident observations of UVIS and LNO channels. If no coincident LNO observations are available for a certain UVIS orbit, we apply the MITRA tool to the UVIS spectral range alone to obtain the dust integrated optical depth.
Summary
The presented method allows to study the properties of Martian dust obtaining information on grains densities and sizes whenever temporally and spatially coincident UVIS and LNO observations are available. When this is not possible, the analysis is performed on the UVIS data alone to obtain the dust integrated optical depth. Through the application of this method to the whole NOMAD dataset we aim to produce a set of dust properties associated to each observation with a precision that is only achievable by exploiting the combined UVIS and LNO spectral ranges.
Acknowledgements
ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). This project acknowledges funding by the Belgian Science Policy Office (BELSPO), with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493), by Spanish Ministry of Science and Innovation (MCIU) and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/R005761/1, ST/P001262/1, ST/R001405/1 and ST/R001405/1 and Italian Space Agency through grant 2018-2-HH.0. This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant number 30442502 (ET_HOME). The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency.
References:[1]Neefs, E., et al, 2015. Appl. Opt. 54, 28, 8494-8520.[2]Oliva, F., et al, 2016. Icarus 278, 215-237.[3]Sindoni, G., et al, 2013. EPSC2013.[4]Oliva, F., et al, 2018. Icarus 300, 1-11.[5]Daerden, F., et al, 2019. Icarus 326, 197-224.[6]Vandaele, A.C., et al, 2019. Nature 568, 521-525.[7]Kahre, M.A., et al, 2008. Icarus 195, 576-597.[8]Korablev, O. ,et al, 2005. Adv. Space Res. 35, 21–30.[9]Gierasch, P.G., Goody, R.M., 1972. J. Atmos. Sci. 29, 400–402.[10]Pollack, J.,et al, 1979. J. Geophys. Res. 84, 2929–2945.[11]Määttänen, A., et al, 2009. Icarus 201, 504-516.[12]Millour, E., et al, 2015. EPSC2015.[13]Wolff, M.J., et al, 2009. J. Geophys. Res., 114, E9.[14]Geminale, A., et al, 2015. Icarus 253, 51-65.[15]Aoki, S., et al. 2019. J. Geophys. Res.: Planets,124, 3482-3497.

How to cite: Oliva, F., D'Aversa, E., Bellucci, G., Altieri, F., Carrozzo, F. G., Amoroso, M., Daerden, F., Thomas, I. R., Ristic, B., Mason, J., Willame, Y., Depiesse, C., Patel, M. R., Lopez-Moreno, J. J., and Vandaele, A. C. and the NOMAD Team: Mars dust properties by means of TGO/NOMAD UVIS and LNO channels nadir data analysis, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-649, https://doi.org/10.5194/epsc2020-649, 2020.

We study the evolution of the size of the polar ice caps on Mars using data collected by the CRISM spectrometer onboard MRO. The presence of H2O and CO2 on the surface is determined with the help of spectral indices. This study represents an extension of previous works to the latest data available, and aims at validating methods which have been applied to observations of similar kind by the NOMAD spectrometer onTGO. Moreover, mesoscale simulations using MarsWRF model will be performed to be compared with the observations.

1. Introduction

The Martian polar caps experience seasonal variability related to mass exchanges with the atmosphere. The evolution of the Martian polar caps has been studied using different satellite observations including gravity field [1], Epithermal Neutron [2] and optical in thermal infrared [3], visible [4], and near-infrared [5]. In particular, in this study we use publicly available datafromtheCompactReconnaissanceImagingSpectrometer for Mars (CRISM) which is a visible to nearinfrared spectrometer (operating in the range of wavelengths between 0.4 and 4.0µm) onboard the Mars Reconnaissance Orbiter (MRO) spacecraft.

2. Data and Methods

The principal data considered in this project are the CRISM pushbroom observations, in particular those obtained in the multispectral (MSP) and hyperspectral (HSP) mapping modes. The dataset analyzed by Brown et al. [6, 7] will be expanded to include the latest observations overlapping with the first science measurements from NOMAD. These measurements are available on the Planetary Data System in units of I/F, and are converted to reflectance factors (R) after dividing by the cosine of the incidence angle. The signature of surface ice in these spectra is detected by studying the strength of absorption bands typical of H2O and CO2 ice. The most prominent ones within the set of wavelengths sampled by CRISM are those centered at 1435 nm for CO2 ice and around 1500 nm for H2O ice. In addition, absorption bands in the region between 2.2 and 2.3 µm are considered, both because they fall in the range of wavelengths detectable by NOMAD and because may prove helpful in the determination of the ice grain sizes [6]. The absorption bands are modeled with simple linear models, so that their band depth (BD) is described by algebraic spectral indices, as provided by Viviano-Beck et al. [8]. The depth of the absorption band (and thus the value of the ice index) for each spatial pixel is not directly related to the abundance of the corresponding substance on the surface, for it depends also on factors like the contamination from dust and the size of the ice grains. These factors are accounted for by comparison with synthetic spectra provided by the Planetary Spectrum Generator (PSG). The data are grouped in bins of solar longitude (LS) and (areocentric) longitude, and for each image the pixels in the cross-track direction are averaged together to increase the signal-to-noise ratio (SNR). The ice indices are evaluated for each of the resulting spectra. A threshold is selected for the values of both indices, below which the corresponding species is assumed to be absent from the surface. Thus, for each LS and longitude bin, the latitude of the pixel furthest away from the pole presenting a value of the ice index higher than the threshold is taken as a point of the polar cap edge. The points are linearly interpolated, to obtain the H2O and the CO2 polar cap edges, also referred to as CROHUS and CROCUS lines, respectively. The results are compared with the other observations as well as Global Circulation Models (MarsWRF) [9].

3. Discussion and Outlook

Figure 1: Evolution of the CO2 ice in the South polar cap from MY30 to MY33. The plot in the lower part displays the corresponding areas. Those obtained by Brown et al. (MY 29) [6] are also shown for comparison.

Figure1 shows results describing the variability of the South polar CO2 ice cap during four Mars years(MY), from MY30 to MY33. In this phase, the values of the thresholds for the H2O and CO2 are set to those employed by Brown et al. [7], which limit the contributions from noisy spectra and clouds absorptions. The plot in the lower part displays the corresponding areas. Those obtained by Brown et al. (MY 29) [6] are also shown for comparison, which exhibits a good agreement with ours. Finally, the interannual differences in cap evolution seem to be of the same order or below the uncertainty of the estimates and therefore, could not be separated from uncertainties of the detection method.

References

[1] O.Karatekin,T.V.Hoolst,andV.Dehant. Martian global-scale CO2 exchange from time-variable gravity measurements. Journal of Geophysical Research: Planets, 111(E6), 2006.

[2] D. Golovin, I. Mitrofanov, A. Anikin, et al. Variations of polar CO2 caps during the first Martian year of FREND onboard TGO. EGU General Assembly 2020, Online, 4–8 May 2020, EGU20209303, 2020.

[3] S.Piqueux,A.Kleinböhl,P.O.Hayne,etal. Variability of the martian seasonal CO2 cap extent over eight Mars Years. Icarus,251:164–180,May 2015.

[4] W.M.Calvin,P.B.James,B.A.Cantor,andE.M. Dixon. Interannual and seasonal changes in the north polar ice deposits of Mars: Observations from MY 29–31 using MARCI. Icarus, 251:181– 190, May 2015.

[5] T. Appéré, B. Schmitt, Y. Langevin, et al. Winter and spring evolution of northern seasonal deposits on Mars from OMEGA on Mars Express. Journal of Geophysical Research: Planets, 116(E5), May 2011.

[6] A. J. Brown, W. M. Calvin, P. C. McGuire, and S. L. Murchie. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) south polar mapping: First Mars year of observations. Journal of Geophysical Research: Planets, 115(E2), 2010.

[7] A. J. Brown, W. M. Calvin, and S. L. Murchie. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) north polar springtime recession mapping: First 3 Mars years of observations. Journal of Geophysical Research: Planets, 117(E12), December 2012.

[8] C. E. Viviano-Beck, F. P. Seelos, S. L. Murchie, et al. Revised CRISM spectral parameters and summary products based on the currently detected  mineral diversity on Mars. Journal of Geophysical Research: Planets, 119(6):1403–1431, 2014.

[9] O. Temel, Ö. Karatekin, E. Gloesener, M. A. Mischna, and J. van Beeck. Atmospheric transport of subsurface, sporadic, time-varying methane releases on mars. Icarus, 325:39–54, 2019.

How to cite: Imbreckx, A.-C., Caldiero, A., Ruiz Lozano, L., Temel, O., and Karatekin, Ö.: Evolution of Mars polar caps extent from CRISM data, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-747, https://doi.org/10.5194/epsc2020-747, 2020.

Introduction:

In an effort to aid the characterisation of Oxia Planum, selected as the Rosalind Franklin rover’s landing site partly due to its extensive Noachian-era clay deposits [1, 2], a comparison of fractured terrains at Oxia and Gale Crater along with an analysis of Colour and Stereo Surface Imaging System (CaSSIS) [3] imagery are currently underway.

An analysis of fractured terrains is a useful tool for determining the history and material properties of Oxia, as the form a fracture network takes varies depending both on the mechanisms which generated it as well as the materials within which the fracturing occurred [4-6]. Comparisons between fractured terrains across Oxia, as well as with those at Gale crater due to the ground truth provided by the Curiosity rover, are being made. This is done in an effort to predict material properties and the fracture’s formation mechanisms, along with determining how fractures across Oxia relate to one another.

CaSSIS is a high-resolution (4.5 m/pixel) 4-band VNIR imager with the ability to take stereo images in a single pass of a target. From a study carried out using CaSSIS, along with co-analysis from CRISM and HiRISE colour imagery, two spectrally and morphologically distinct subunits of the Oxia clay unit [7] were identified. These were a lower member showing metre-scale fracturing and spectral signatures indicative of Fe/Mg-rich clay minerals, and an upper member showing decametre scale fracturing with Fe/Mg-rich clay mineral/olivine signatures. To expand upon the mapping carried out using HiRISE colour and CRISM data, which was limited by data coverage [7], CaSSIS and HiRISE RED i.e. greyscale imagery, was used to identify these sub-units. This was done to aid future planning of rover traverses to high priority surface targets.

Methods:

Fracture Analysis. Our fracture analysis involves tracing out a given fracture network in HiRISE imagery using ArcGIS, then using a tool developed at the Open University (as seen in [8]) to measure metrics of the fractures or the polygons formed by them, such as the angle between intersecting fractures, polygon area, etc. These were then mapped out for comparison using Kernel Density Estimation (KDE) diagrams, and compared statistically via two-sample Kolmogorov-Smirnov tests.

CaSSIS mapping. Radiometrically and geometrically corrected images are initially band ratioed and combined into an RGB image, allowing CaSSIS images to distinguish between ferrous and ferric minerals [9]. This is important given that the lower clay unit has a ferric component potentially due to the presence of hematite/ferric oxides [2], in contrast to the ferrous component of the upper member due to containing olivine [7], making the two members distinguishable with CaSSIS.

The band ratios (BR’s) used were NIR/PAN, PAN/BLU and PAN/NIR, with CaSSIS’s RED channel replacing its NIR depending on which was available from a given CaSSIS image [10]. These are sensitive to ferric and ferrous minerals for the former two and latter one respectively. Dark Subtraction [11] was then applied to minimize the effects of dust-derived atmospheric scatter. These images were used in conjunction with assessment of fracture length using HiRISE imagery to map out the two members. It has previously been identified [7, 12] that high dust opacity at the time of imaging likely skews the apparent ferric content of the image. This has been noted and is addressed via repeat imaging of such affected areas over Oxia Planum.

Results:

Fracture Analysis. The Gale sites which have comparable metric distributions to those at Oxia are the NE section of Yellowknife Bay’s Sheepbed member [13], which has similarities to several sites seen in the centre of the Oxia landing ellipses, and a site at Vera Rubin ridge similar to an area adjacent to Coogoon Vallis in the SE of Oxia Planum and another abutting the capping unit in the centre of the landing ellipses. Both of these are at the 2-sigma level of certainty for the majority of the metrics. See Figure 1 for examples of the KDE graphs used.

CaSSIS mapping. Figure 2 shows the current extent of the mapping within the 1-sigma landing ellipses at Oxia. There are fewer upper member exposures in the area mapped in fig.2, in line with what has been identified previously [7].