The presence of water on planets composed mainly of rocks and metals, such as Earth, Mars or even Venus, dates back to the earliest stages of their formation. This is shown by numerical simulations by a group of Belgian researchers whose results are published in Nature Geoscience.
Studying the conditions for the habitability of planets and their evolution over time: this is the objective of the Belgian scientific project ‘Evolution and Tracers of the Habitability of Mars and Earth’ (ET-HOME). The project brings together teams from the Royal Observatory of Belgium and UCLouvain (led by Prof. Véronique Dehant) and ULB (led by Prof. Vinciane Debaille). Habitability depends on the presence of water. ‘For Earth, we know what the situation is,’ explains Prof. Dehant, a Louvain4Space member. ‘For Mars, ancient traces of water have been spotted on its surface, as river beds and deltas are observed at the very beginning of its existence, it was habitable. So it was worthwhile to also study the evolution of Venus, the third terrestrial planet with an atmosphere, the closest of the three to the sun.’ The idea is daring because far fewer missions have been launched towards Venus than towards Mars (hence a relative paucity of data), and the researchers don’t have rocky samples of Venus, whereas they can study Martian meteorites as well as the debris that had pelted Earth after a meteorite struck Mars. The goal is also to better understand the evolution of our planet: despite their similarity in structure, size and place in the solar system, Earth and Venus have very different evolutions. Studying Venus thus makes it possible to conduct comparative planetology research.
The researchers modelled the evolution of Venus over the very long term (4.5 billion years) by taking into account interactions between planet layers (interior, atmosphere). ‘However,’ Prof. Dehant points out, ‘the model is constrained by data from present-day observations of the planet. In other words, if data entered into the model leads to a simulated planet that differs from today’s observable one, it’s rejected. This is how one decides which models to keep and which to ignore.’
The simulation’s starting point is a very young planet whose surface is an ocean of magma (lava), owing to the heat given off by its formation. The first meteor bombardments have already taken place, but not the so-called late bombardments. It’s the latter that especially attract the researchers’ attention: Did they bring a lot of water to the planet or were they dry? The researchers entered different meteorite compositions into their model. A conclusion quickly emerged: in all cases, wet material leads to simulations of Venus incompatible with what is known about the current planet. Dry materials, however, drive an evolution that leads to what we see today. It’s inferred that very little water was brought to Venus during the end of its formation, and this was undoubtedly the case for Earth because, a priori, the same type of meteorites hit it at that time. Prof. Dehant summarises, ‘So water was undoubtedly present from the start of the formation of Venus, Earth and Mars.’
But Venus, unlike Earth, has no water. Both planets were initially covered by oceans of magma, which exchanged a lot of volatile substances (including water vapour) with their atmospheres. But Venus is closer to the sun, so it’s possible its magma ocean remained longer than Earth’s, whose water was trapped inside the planet before it could evaporate. The water then rose to the surface via convection movements that stirred up the mantle, reached the upper layers and was released to the surface through volcanism and plate tectonics – phenomena present on our planet but absent on Venus.
The difference in the planets’ evolutions isn’t entirely resolved. We know that the surface of Venus is young on average, so mere observation of it can reveal no signs of a primitive era. But its atmosphere is composed overwhelmingly of CO2 and looks more like what would appear to be Earth’s early atmosphere. This tends to indicate that Venus has evolved less, or experienced less dramatic change than Earth has. And the fact that there’s no ocean of water on the surface of Venus and no plate tectonics also probably shows that its evolution was simpler than Earth’s.
‘But above all,’ Prof. Dehant emphasises, ‘if our atmosphere has changed more than that of Venus, it’s because of the emergence of life and thus oxygen, which is absent as a main component of other planets’ atmospheres. This blocks all avenues to reconstructing Earth’s primitive atmosphere and that’s why Venus remains an interesting planet: it makes it possible to study the beginnings of the Earth as if life hadn’t appeared here.’
The published study’s finding of the latter meteorite bombardment’s dry nature isn’t a revelation, because it’s consistent with much of the data collected through terrestrial observations. Rather, the study is groundbreaking, the researchers said, because the result was established by a digital model of Venus. It’s a kind of proof of concept (demonstration of feasibility) of a new method of using models that integrate changes in the interior and the atmosphere of a planet – Venus in this case – to perform comparative planetology.
See also : Mars, about to reveal its inner side
A glance at Véronique Dehant's bio
Véronique Dehant earned a master’s degree in mathematics in 1981 and a master’s degree in physics in 1982, both at the University of Louvain (UCLouvain), in Belgium, where she also earned her PhD in science and ‘habilitation’ (professorial thesis), in 1986 and 1992 respectively. Her PhD focused on Earth’s rotation and interior. From 1981 to 1992, she was a researcher supported by the National Fund for Scientific Research (FNRS). She then worked as a researcher at the Royal Observatory of Belgium (1993-present) and in 1994 became head of the ‘Time, Earth rotation and spatial geodesy’ section, currently called ‘Reference Systems and Planetology’, which she now directs, supervising some 40 individuals.
In 2006, she became principal investigator of the Lander Radioscience experiment as part of the ExoMars mission to Mars approved in 2015 and whose launch is scheduled for 2020. Currently Prof. Dehant is co-investigator of the Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSIGHT) mission to Mars, which was successfully launched and is scheduled to land in November 2018.
Prof. Dehant has won several awards, including the European Union Descartes Prize. In 2014, she was appointed honorary doctor of the Paris Observatory. In 2015, she obtained a prestigious European Research Council Advanced Grant as part of the Rotation and Nutation of a Wobbly Earth (RotaNut) project.
She is an adjunct professor at the University of Louvain. As of July 2018, she authored 480 publications, including 165 in peer-reviewed journals, and prepared more than 1,085 scientific papers. Her main current scientific interest is comparative planetology, particularly planet interiors and rotations, evolution and habitability.