Planetary geoscience was effectively born when Christiaan Huygens took his first look at planet Mars on Friday 28 November 1659. As one of the leading scientists of his time, Huygens was known for constructing his own telescopes to observe stars, planets and nebulae whenever the clear and spacious skies above the Netherlands allowed. Huygens observed the planet Mars during the heydays of its 1659 opposition. On the night of 28 November he succeeded in sketching the first albedo feature on a different planet in our solar system. The roughly triangular dark-coloured patch was originally christened the Hourglass Sea, suggesting it to be an area of open water. Perhaps the landscape surrounding him in the Netherlands prompted Huygens to interpret the newly discovered feature as a wet area on the planet's surface. The attribution of traits to an albedo feature on another planet based on terrestrial landscapes may well be considered as the first-ever attempt at ‘comparative planetology’. The albedo feature can still be recognised at the surface of Mars today as Syrtis Major. Any modest amateur telescope can provide a view superior to that of Huygens’, allowing the observation of the very first geological feature ever identified on another rocky planet.

The largest differentiation event in Earth and other terrestrial planets was the high-pressure, high-temperature process of metal core segregation from a silicate mantle. The abundant element silicon (Si) can be partially sequestered into the metallic core during metal–silicate differentiation, depending on pressure, temperature and planetary oxidation state. Knowledge of the Si content of a planet's core can constrain the conditions of core formation, but in the absence of direct samples from planetary cores, quantifying core Si content is challenging. One relatively new tool to study core formation in terrestrial planets is based on combining measurements of the Si stable isotopic composition of planetary crust and mantle samples with measurements of the Si stable isotope fractionation between metal and silicate at high-temperature and high-pressure conditions. In this study we present the results of a small set of high-pressure, high-temperature (HPT) experiments and combine these with a review of literature data to investigate how the Si isotope fractionation behaviour between metal and silicate varies as a function specifically of experimental run time and temperature. We show that although there is no debate about the sign of fractionation, absolute values for Si isotope fractionation between metal and silicate are difficult to constrain because the experimental database remains incomplete, and because Si isotopic measurements of metals in particular suffer from the absence of a true inter-laboratory comparison. We conclude that in order to derive accurate quantitative estimates of the Si content of the core of the Earth or other planets a wide range of additional experiments will be required.

Recent measurements of the chemical and isotopic composition of lunar samples indicate that the Moon's bulk composition shows great similarities with the composition of the silicate Earth. Moon formation models that attempt to explain these similarities make a wide variety of assumptions about the properties of the Earth prior to the formation of the Moon (the proto-Earth), and about the necessity and properties of an impactor colliding with the proto-Earth. This paper investigates the effects of the proto-Earth's mass, oblateness and internal core-mantle differentiation on its moment of inertia. The ratio of angular momentum and moment of inertia determines the stability of the proto-Earth and the binding energy, i.e. the energy needed to make the transition from an initial state in which the system is a rotating single body with a certain angular momentum to a final state with two bodies (Earth and Moon) with the same total angular momentum, redistributed between Earth and Moon. For the initial state two scenarios are being investigated: a homogeneous (undifferentiated) proto-Earth and a proto-Earth differentiated in a central metallic and an outer silicate shell; for both scenarios a range of oblateness values is investigated. Calculations indicate that a differentiated proto-Earth would become unstable at an angular momentum L that exceeds the total angular momentum of the present-day Earth–Moon system (L0) by factors of 2.5–2.9, with the precise maximum dependent on the proto-Earth's oblateness. Further limitations are imposed by the Roche limit and the logical condition that the separated Earth–Moon system should be formed outside the proto-Earth. This further limits the L values of the Earth–Moon system to a maximum of about L/L0 = 1.5, at a minimum oblateness (a/c ratio) of 1.2. These calculations provide boundary conditions for the main classes of Moon-forming models. Our results show that at the high values of L used in recent giant impact models (1.8 < L/L0 < 3.1), the proposed proto-Earths are unstable before (Cuk & Stewart, 2012) or immediately after (Canup, 2012) the impact, even at a high oblateness (the most favourable condition for stability). We conclude that the recent attempts to improve the classic giant impact hypothesis by studying systems with very high values of L are not supported by the boundary condition calculations in this work. In contrast, this work indicates that the nuclear explosion model for Moon formation (De Meijer et al., 2013) fulfills the boundary conditions and requires approximately one order of magnitude less energy than originally estimated. Hence in our view the nuclear explosion model is presently the model that best explains the formation of the Moon from predominantly terrestrial silicate material.

Raman spectroscopy appears to be an ideal technique for the initial detection of biomarkers, molecules that are potentially indicative of life on planetary bodies elsewhere in our solar system. Carotenoids are particularly useful biomarkers as they are used widely across the species, relatively resistant to breakdown and no inorganic source is known. They are used by microorganisms in their cell membranes for protection against UV radiation. In this paper we focus on the detection of carotenoids in microorganisms within a mineral matrix. We compare the Raman signatures of pure compounds with those of laboratory-made mixtures of β-carotene and minerals. Carotenoids covered by 2.5 mm of translucent calcite or 40 mm of transparent halite were detected using a conventional confocal Raman microscope. To improve sensitivity and hence detection levels, Raman measurements were successfully performed under resonant conditions. Raman analysis can be compromised by fluorescence interference. Data are presented to show how the contribution from the fluorescent background in the Raman spectra can be reduced when making use of gated detection in time-resolved Raman spectroscopy. Overall, this study demonstrates some of the potential of Raman spectroscopy as a method for the detection of (past) life signatures during future planetary missions without taking current technical limitations such as instrumental size into account as recent rapid technical developments suggest these limitations will be resolved in time.

Remote sensing observations and rover missions have documented the presence of sulphate-rich mineral associations on Mars. Many of these minerals are paleo-indicators of hydrous, acidic and oxidising environments that must have prevailed in Mars´ distant past, contrary to the present conditions. Furthermore, occurrences of silica together with high Cl and Br concentrations in Martian soils and rocks represent fingerprints of chemically atypical fluids involved in processes operating on the surface or at shallow depth. From field observations at representative active volcanoes in subduction settings, supported by geochemical modelling, we demonstrate that volcanic hydrothermal systems are capable of producing Mars-like secondary mineral assemblages near lakes, springs and fumaroles through the action of acidic fluids. Water–gas-rock interactions, together with localised flow paths of water and fumarolic gas emitted from associated subaerial vents, lead to deposition of a range of sulphates, including gypsum, jarosite, alunite, epsomite and silica. Evaporation, vapour separation and fluid mixing in (near-) surface environments with strong gradients in temperature and fluid chemistry further promote the diversity of secondary minerals. The mineralogical and chemical marks are highly variable in space and time, being subject to fluctuations in ambient conditions as well as to changes in the status of volcanic-hydrothermal activity. It is concluded that active processes in modern volcanic-geothermal systems may be akin to those that created several of the sulphate-rich terrains in the early history of Mars.

Spectral observations show that volcanic glass is the dominant ingredient of the aeolian landforms which cover the northern lowlands on Mars. Surface winds subject these sands to physical alteration processes in the present-day surface environment. This work highlights the role of glaciovolcanism throughout Mars’ geologic history and the parallels with landforms and materials found in Iceland. As the physical properties of Martian volcanic glass particles are difficult to constrain from orbit, Icelandic materials can provide valuable insights in their transport and modification characteristics. The processing of glass grains by environmental processes by means of the dune transport cycle is discussed. Experiments targeted the grain-size alteration effects experienced during the dune transport cycle, including the effect of ‘low-energy’ avalanching and ‘high-energy’ aeolian regimes (i.e. particle rolling and saltation). Saltation transport was found to rapidly alter grains and particle size distributions, which contributes to a positive feedback loop where the new smaller grains are mobilised more easily after fracturing and surficial abrasion. Post-depositional physical alteration therefore needs to be reconciled with the present-day silicic spectral signatures of these glasses in order to infer the relevant landform genetic. This effort is especially relevant in respect to the loss of possible signatures of biochemical alteration from microbial interactions, as glaciovolcanic environments are favourable habitats for life. As chemical and physical weathering is limited to the grain exterior, the grain interior may still retain a geochemical record of the subglacial eruption environment in which these grains were formed. Quantification of the volatiles sequestered in the glass can therefore be used to identify the formative conditions of the amorphous component in aeolian sediments.

Humankind has been interested in space throughout the ages and studies of the universe and our own solar system have been ongoing since the first observations of celestial bodies. In the current era space exploration has provided in situ data for the different bodies in our solar system. To fully comprehend the underlying processes occurring in these bodies, missions and telescope observations are, however, not sufficient and additional modelling studies, both numerical and analogue, are necessary. In this paper we present a new facility specifically designed to experimentally study organic compounds under simulated planetary (sub)surface conditions on rocky bodies in our solar system: PALLAS, the Planetary Analogues Laboratory for Light, Atmosphere, and Surface Simulations. We give an overview of planetary conditions that can be simulated in this facility and that are known to affect organic compounds: radiation, atmospheric composition, temperature and surface composition.

One of the major scientific objectives of ESA's JUICE (JUpiter ICy moons Explorer) mission, which is scheduled for launch in 2022 and planned to arrive at the Jovian system in 2030, is to characterise the internal water ocean and overlying ice shell of Jupiter's largest moon Ganymede. As part of the strategy developed to realise this objective, the tidal response of Ganymede's interior will be constrained by JUICE's measurements of surface displacements (by the Ganymede Laser Altimeter (GALA) instrument) and variations in the gravitational potential (by the 3GM radio science package) due to the acting diurnal tides. Here we calculate the tidal response at the surface of Ganymede for several plausible internal configurations in order to analyse the relation between the tidal response and the geophysical parameters that characterise Ganymede's interior. Similarly to the case of Jupiter's smallest icy satellite Europa, the tidal response of Ganymede in the presence of a subsurface ocean, which could be as large as about 3.5 m in terms of the induced radial deformation, mostly depends on the structural (thickness, density) and rheological (rigidity, viscosity) properties of the ice-I shell. Nevertheless, the dependence of the tidal response on several geophysical parameters of the interior, in particular on the thickness and rigidity of the ice-I shell, does not allow for the unambiguous determination of the shell thickness from tidal measurements alone. Additional constraints could be provided by the measurement of forced longitudinal librations at the surface, as their amplitude is more sensitive to the rigidity than to the thickness of the shell.

Like earth and planetary scientists, most children are curious about the world, the solar system and the rest of the universe. However, for various reasons primary schools emphasise language and calculus rather than natural sciences. When science is taught, examination systems often favour knowledge of the ‘right’ answer over the process of investigation and logical reasoning towards that answer. In order to continue to spark children's curiosity and their motivation to learn and discover, science education hubs at universities and science museums could collaborate more with schools and teachers, and are beginning to do so. The objective of this position paper is to report on recent experiences in earth and planetary science education for pupils in primary and secondary education, to provide examples and inspiration for scientists. We report three examples of initiation and consolidation of science education in primary schools in the Netherlands: (1) a focus on asking questions and seeking information to reason towards the answer, initiated with a classroom game, Expedition Mundus, (2) bringing pupils and teachers together outside their school in the science museum to gain confidence and self-efficacy, and (3) having children ask their own questions and do their own research guided by the empirical cycle, for example on experimentation on sandbox scale models of channels and crater lake deltas as found on Mars. The focus on other planets, fictitious and real, stimulates pupils to ask questions about planet Earth. Finally, we argue that involvement of more scientists in science education would not only benefit primary and secondary schools and future students but also university education and science communication with society.