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The Heat Flow and Physical Properties Package (HP³) includes an infrared Radiometer attached to the deck of the InSight lander. The radiometer will observe the seasonal temperature variation over the course of the mission. Fitting of diurnal temperature curves and of the response to eclipses provides an estimate of thermophysical properties of the near surface, and possibly constrains atmospheric variables such as the ratio of visible to infrared dust opacity.

The Heat Flow and Physical Properties Package HP3 for the InSight mission will attempt the first measurement of the planetary heat flow of Mars. The data will be taken at the InSight landing site in Elysium planitia (136 ◦E, 5 ◦N) and the uncertainty of the mea- surement aimed for shall be better than ±5 mW m−2 . The package consists of a mechanical hammering device called the “Mole” for penetrating into the regolith, an instrumented tether which the Mole pulls into the ground, a fixed radiometer to determine the surface brightness temperature and an electronic box. The Mole and the tether are housed in a support struc- ture before being deployed. The tether is equipped with 14 platinum resistance temperature sensors to measure temperature differences with a 1-σ uncertainty of 6.5 mK. Depth is de- termined by a tether length measurement device that monitors the amount of tether extracted from the support structure and a tiltmeter that measures the angle of the Mole axis to the local gravity vector. The Mole includes temperature sensors and heaters to measure the re- golith thermal conductivity to better than 3.5% (1-σ ) using the Mole as a modified line heat source. The Mole is planned to advance at least 3 m—sufficiently deep to reduce errors from daily surface temperature forcings—and up to 5 m into the martian regolith. After landing, HP3 will be deployed onto the martian surface by a robotic arm after choosing an instru- ment placement site that minimizes disturbances from shadows caused by the lander and the seismometer. The Mole will then execute hammering cycles, advancing 50 cm into the sub- surface at a time, followed by a cooldown period of at least 48 h to allow heat built up during hammering to dissipate. After an equilibrated thermal state has been reached, a thermal con- ductivity measurement is executed for 24 h. This cycle is repeated until the final depth of 5 m is reached or further progress becomes impossible. The subsequent monitoring phase consists of hourly temperature measurements and lasts until the end of the mission. Model calculations show that the duration of temperature measurement required to sufficiently re- duce the error introduced by annual surface temperature forcings is 0.6 martian years for a final depth of 3 m and 0.1 martian years for the target depth of 5 m.

Thermal brightness on Venus' night side is mainly modulated by the lower cloud layer extending from about 45 to 60 km in altitude, the most recent observations being the outstanding 2.3 µm images recorded by the IR2 camera onboard Akatsuki [Gibney, 2016]. The VEM multispectral imager (P.I.: J. Helbert, DLR) onboard the proposed NASA VERITAS orbiter (P.I.: S. Smrekar, JPL) has the capability to observe these lower clouds. The VEM filter bands at 1.195, 1.310 and 1.510 µm will acquire very accurate images of the clouds: resolution in Phase II orbit after spatial binning will be about 20 km, which is close to the atmospheric blurring limit. This will lead to the acquisition of a large data set that allows for the study of the lower cloud morphology and climatology with good coverage in latitude, planetocentric longitude and local solar time. On the other hand, variations in the ratios of these three bands would help in constraining changes in composition, altitude and/or size distribution of the lower cloud particles [Barstow et al., 2012; Haus et al., 2014, 2015]. Such observations at small horizontal scales would be of great importance to microphysical models of Venus' clouds and haze system [McGouldrick et al., 2007]. Previous (Venus Express) or present (Akatsuki) observations of the lower clouds have proven the validity of these methods, but VEM onboard VERITAS will give an unprecedented coverage of the lower cloud horizontal structure on scales between 20 and 200 km in terms of spatial and temporal sampling, wavelength stability and signal-to-noise ratio. Tracking lower cloud motions as a proxy for wind measurements at a high spatial resolution would also be of great interest to mesoscale and general circulation models. Such a study is made challenging due to the fast zonal super-rotation so that clouds that are visible in the field of view usually cannot be observed 90 min later when VERITAS flies over the same region in its next orbit. However, the super-rotation breaks down for latitudes higher than 80°, so that cloud tracking would be possible in both north and south polar dipoles [Piccioni et al., 2007] well known for their complex and ever-changing dynamics.

The permanent cloud cover of Venus prohibits observations of the surface with traditional imaging techniques over the entire visible spectral range. Fortunately, Venus' atmospheric gases are largely transparent in narrow spectral windows near 1 mm. Ground observers were the first to successfully use these windows, followed by spacecraft observations during the flyby of the Galileo mission on its way to Jupiter and most recently from Venus orbit by ESA's Venus Express with the VMC and VIRTIS instruments. Analyses of VIRTIS measurements have successfully demonstrated that surface information can be extracted from these windows, but the design of the instrument limited its use for more in-depth surface investigations.Based on experience gained from using VIRTIS to observe the surface of Venus and new high temperature laboratory experiments currently performed at the Planetary Spectroscopy Laboratory of DLR, we have designed the multi-spectral Venus Emissivity Mapper (VEM). Observations from VIRTIS have revealed surface emissivity variations correlated with geological features, but existing data sets contain only three spectral channels. VEM is optimized to map the surface composition and texture, and to search for active volcanism using the narrow atmospheric windows, building on lessons from prior instrumentation and methodology. It offers an opportunity to gain important information about surface mineralogy and texture by virtue of having six different channels for surface mapping.VEM is focused mainly on observing the surface, mapping in all near-IR atmospheric windows using filters with spectral characteristics optimized for the wavelengths and widths of those windows. It also observes bands necessary for correcting atmospheric effects; these bands also provide valuable scientific data on composition as well as altitude and size distribution of the cloud particles, and on H2O vapor abundance variations in the lowest 15 km of the atmosphere.In combination with a high-resolution radar mapper that provides accurate topographic data as planned for the NASA VERITAS mission or for the ESA EnVision mission, VEM will provide new insights into current properties and the geologic evolution of Venus.

Even though Venus is in many respects the most Earth-like planet we know today, its surface composition and geology are not well understood yet. The major obstacle is the extremely dense, hot, and opaque atmosphere that complicates both in situ measurements and infrared remote sensing, the wavelength range of the latter often being the range of choice due to its coverage of many spectral properties diagnostic to the surface material’s composition and texture. Thermal emissions of the hot surface depend on surface temperature and on spectral surface emissivity. As this emitted radiation wells upward, it is strongly attenuated through absorption and multiple scattering by the gaseous and particulate components of the dense atmosphere, and it is superimposed by thermal atmospheric emissions. While surface information this way carried to space is completely lost in the scattered sunlight on the dayside, a few narrow atmospheric transparency windows around 1 μm allow the sounding of the surface with nightside measurements. The successfully completed VEX (’Venus Express’) mission, although not dedicated to surface science, en- abled a first glimpse at much of the southern hemisphere’s surface through the nightside spectral transparency windows covered by VIRTIS-M-IR (’Visible and InfraRed Thermal Imaging Spectrometer, Mapping channel in the IR’, 1.0–5.1 μm). Two complementary approaches, a fast semi-empiric technique on the one hand, and a more fundamental but resource-intensive method based on a fully regularized Bayesian multi-spectrum retrieval algorithm in combination with a detailed radiative transfer simulation program on the other hand, were both successfully applied to derive surface emissivity data maps. Both methods suffered from lack of spatial coverage and a small SNR as well as from surface topography maps not sufficiently accurate for the definition of suitable boundary conditions for surface emissivity retrieval. The recently proposed VERITAS mission (’Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy’) comprises two instruments, VEM (’Venus Emissivity Mapper’) and VISAR (’Venus Interferomet- ric Synthetic Aperture Radar’). This mission will yield a vastly improved data basis with respect to both high SNR Venus nightside radiance measurements at all transparency windows around 1 μm as well as topography maps. The new data will enable the derivation of much more complete and reliable global surface emissivity maps that are required to answer fundamental geologic questions. Here, we discuss the selection of the wavelength ranges covered by the spectral filters of VEM as well as improved estimates of expectable emissivity retrieval errors based on this selection. For this purpose, the locations of the relevant spectral transparency windows are studied with detailed line-by-line radiative transfer simulations in dependence on different spectral line databases. Recent work on VIRTIS-M-IR/VEX measurements indicated the presence of interferences due to ever-varying atmospheric parameters that cannot be derived from radiance measurements with limited spectral information content to be a dominant source of surface emissivity retrieval errors. This work is carried over to the configuration of VEM, and the retrieval pipeline is optimized to minimize such errors. A portion of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.

Ein Überblick über die Ziele der InSight Mars Mission.

Überblick über das Radiometer des Heat Flow and Physical Properties Packages (HP3) der InSight Mission.