A. Hagermann

A soft solid surface on Titan as revealed by the Huygens Surface Science Package

The surface of Saturns largest satellite—Titan—is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titans atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov-Gerasimenko

Thermal and mechanical material properties determine comet evolution and even solar system formation because comets are considered remnant volatile-rich planetesimals. Using data from the Multipurpose Sensors for Surface and Sub-Surface Science (MUPUS) instrument package gathered at the Philae landing site Abydos on comet 67P/Churyumov-Gerasimenko, we found the diurnal temperature to vary between 90 and 130 K. The surface emissivity was 0.97, and the local thermal inertia was 85 ± 35 J m−2 K−1s-1/2. The MUPUS thermal probe did not fully penetrate the near-surface layers, suggesting a local resistance of the ground to penetration of >4 megapascals, equivalent to >2 megapascal uniaxial compressive strength. A sintered near-surface microporous dust-ice layer with a porosity of 30 to 65% is consistent with the data.

Planetary heat flow measurements

The year 2005 marks the 35th anniversary of the Apollo 13 mission, probably the most successful failure in the history of manned spaceflight. Naturally, Apollo 13s scientific payload is far less known than the spectacular accident and subsequent rescue of its crew. Among other instruments, it carried the first instrument designed to measure the flux of heat on a planetary body other than Earth. The year 2005 also should have marked the launch of the Japanese LUNAR-A mission, and ESAs Rosetta mission is slowly approaching comet Churyumov-Gerasimenko. Both missions carry penetrators to study the heat flow from their target bodies. What is so interesting about planetary heat flow? What can we learn from it and how do we measure it? Not only the Sun, but all planets in the Solar System are essentially heat engines. Various heat sources or heat reservoirs drive intrinsic and surface processes, causing ‘dead balls of rock, ice or gas’ to evolve dynamically over time, driving convection that powers tectonic processes and spawns magnetic fields. The heat flow constrains models of the thermal evolution of a planet and also its composition because it provides an upper limit for the bulk abundance of radioactive elements. On Earth, the global variation of heat flow also reflects the tectonic activity: heat flow increases towards the young ocean ridges, whereas it is rather low on the old continental shields. It is not surprising that surface heat flow measurements, or even estimates, where performed, contributed greatly to our understanding of what happens inside the planets. In this article, I will review the results and the methods used in past heat flow measurements and speculate on the targets and design of future experiments.

A heat flow and physical properties package for the surface of Mercury

European Space Agencies fifth cornerstone mission BepiColombo includes a ‘Surface Element’ to land a scientific payload on the surface of Mercury. The current strawman payload includes a heat flow and physical properties package (HP3), focussing on key thermal and mechanical properties of the near-surface material (down to a depth of 2–5 m) and the measurement of heat flow from Mercurys interior, an important constraining parameter for models of the planets interior and evolution. We present here an overview of the HP3 experiment package and its possible accommodation in a self-inserting ‘mole’ device. A mole is considered to be the most appropriate deployment method for HP3, at least in the currently-assumed case of an airbag-assisted soft landing architecture for the Mercury Surface Element.

A method to invert MUPUS. Temperature recordings for the subsurface temperature field of P/Wirtanen

Abstract The thermal sensors on the penetrator of the MUPUS experiment package selected for the ESA Rosetta mission will enable us to determine the near-surface energy balance of the nucleus of comet P/Wirtanen by measuring the subsurface temperature profile and the thermal conductivity of the near-surface layers. Model calculations suggest that the penetrator itself will perturb the ambient temperature field such that the temperature profile will be smoothed if the thermal diffusivity of the nucleus is significantly smaller than that of the penetrator tube. It is possible, however, to calculate the undisturbed temperature profile from the data using a method based on a solution of the transient inverse heat conduction problem. Our model calculations show that a satisfactory estimate of the undisturbed temperature field can be obtained in comparatively little computing time by calculating the temperature distribution in the model volume from temperature histories at discrete points representing the penetrator temperature sensors.

The modified Rankine balance: A highly efficient, low-cost method to measure low-temperature magnetic susceptibility in rock samples

In this article, we present a method to measure the relative changes in the magnetic susceptibility of rock samples in the low-temperature range (−200 °C to +20 °C). The method differs from other experimental methods currently used in that it requires—in contrast to ac bridges, the most widely used devices—very little sophisticated laboratory equipment: A high-precision laboratory balance, a Pt-100 thermoelement and a computer with standard input/output interface and analog/digital processing capabilities, as well as a few rare earth magnets, are the only devices needed in addition to standard laboratory equipment. A Dewar container and a few plexiglass panes can either be handcrafted in the workshops of any larger research institute or are commercially available at little cost. The results of our measurements reveal that the temperature-varying magnetic properties of rock samples can be reliably observed.

The Penetration of Solar Radiation Into Carbon Dioxide Ice

Icy surfaces behave differently to rocky or regolith‐covered surfaces in response to irradiation. A key factor is the ability of visible light to penetrate partially into the subsurface. This results in the Solid‐State Greenhouse Effect (SSGE), as ices can be transparent or translucent to visible and shorter wavelengths, whilst opaque in the infrared. This can lead to significant differences in shallow sub‐surface temperature profiles when compared to rocky surfaces. Of particular significance for modelling the SSGE is the e‐folding scale, otherwise known as the absorption scale length, or penetration depth, of the ice. Whilst there have been measurements for water ice and snow, pure and with mixtures, to date there have been no such measurements published for carbon dioxide ice. After an extensive series of measurements we are able to constrain the e‐folding scale of CO2 ice for the cumulative wavelength range 300 nm to 1100 nm, which is a vital parameter in heat transfer models for the Martian surface, enabling us to better understand surface‐atmosphere interactions at Mars’ polar caps.

WatSen: design and testing of a prototype mid-IR spectrometer and microscope package for Mars exploration

We have designed and built a compact breadboard prototype instrument called WatSen: a combined ATR mid-IR spectrometer, fixed-focus microscope, and humidity sensor. The instrument package is enclosed in a rugged cylindrical casing only 26 mm in diameter. The functionality, reliability and performance of the instrument was tested in an environment chamber set up to resemble martian surface conditions. The effective wavelength range of the spectrometer is 6.2–10.3 μm with a resolution Δλ/λ = 0.015. This allows detection of silicates and carbonates, including an indication of the presence of water (ice). Spectra of clusters of grains < 1 mm across were acquired that are comparable with spectra of the same material obtained using a commercial system. The microscope focuses through the diamond ATR crystal. Colour images of the grains being spectroscopically analysed are obtainable with a resolution of ∼20 μm.

The Huygens Surface Science Package sound speed measurements and the methane content of Titan's atmosphere

The Huygens probe descended through Titans atmosphere in January 2005. On board was the Surface Science Package (SSP), a set of nine sensors, which included a speed-of-sound sensor. We present a detailed description of the SSP speed of sound measurements and report constraints on the methane content in Titans lower atmosphere based on these measurements. After extensive instrument calibration and subsequent Bayesian analysis of the data, the most likely result derived from our measurements in Titans lower atmosphere is a methane fraction of approximately 2% at 10 km, increasing to 3.5% at lower altitudes. These estimates are based on a binary composition. Our data show that any large scale variation of methane within the lower 11 km of Titans atmosphere is unlikely. Within experimental and theoretical uncertainties, our estimates are lower than, but compatible with earlier estimates obtained from the mass spectrometry experiment.

Speed of sound in nitrogen as a function of temperature and pressure

Speed of sound measurements in nitrogen by Younglove and McCarty [J. Chem. Thermodynam. 12, 1121–1128 (1980)] are revisited and an empirical polynomial equation for the speed of sound is derived. The polynomial coefficients differ from those given by Wong and Wu [J. Acoust. Soc. Am. 102, 650–651 (1997)] with the result that discrepancies between predicted and measured values at low temperatures are reduced. The maximal error over the complete temperature and pressure range from 80 to 350 K and 0.031 to 0.709 MPa is reduced from 5.38% to 0.78%.