2024-06-17 11:00:10
By Frédéric Schmidt (Professor, planetary surface geology, Paris-Saclay University) and Adomas Valantinas (Postdoctoral Fellow, Brown University)
The formation of water frost is very common on Earth, especially during winter, due to the condensation of atmospheric water vapor. On Mars, there is an atmosphere, about 100 times less dense than on Earth, which also contains water vapor. However, theatmosphere of Mars contains 10,000 times less water vapor than Earth, so frost is less likely.
ESA/DLR/FU Berlin, Provided by the author
Today, water exchanges between the surface and the atmosphere are not well understood on the Red Planet. However, frost formation is an important tracer, useful for understanding the water cycle, but also for identifying key resources for potential future human exploration and for constraining potential habitability. Indeed, water is a resource necessary for life, but could also be used as rocket fuel.
Journey to the top of Martian volcanoes
Our new studypublished today in Nature Geoscience reports the existence of significant transient morning frost deposits at the summits of volcanoes in a region called Tharsis (i.e., Olympus, Arsia and Ascraeus Montes, and Ceraunius Tholus volcanoes) using high-resolution color images from the probe European Space Agency’s Exomars Trace Gas Orbiter (TGO) (ESA). This discovery was confirmed using independent observations made by the probe ESA’s Mars Express and speak spectrometer NOMAD on board TGO.
Furthermore, numerical simulations of the Martian climate show that the surface temperatures of volcanoes are compatible with the existence of frozen water. Indeed, the results show that morning frost deposits are correlated with the coldest Martian seasons. Because yes, there are seasons on Mars as on Earth, but with much lower temperatures, between -130 °C and -30 °C, on the Martian summits. In addition, equatorial volcanoes quickly receive strong solar insolation, explaining the rapid disappearance of frost in the morning.
CaSSIS image of a bright, slightly blueish area due to frost in the volcanic crater (caldera) of Olympus Mons.
ESA/TGO/CaSSIS, Provided by the author
It all begins with the observation of lighter and slightly bluish areas on the summits of Martian volcanoes by the camera. CaSSIS. After an investigation campaign of a few months, these strange bright areas seem to be present only on observations in the early morning and during the cold seasons of Mars. The camera HRSC allowed these observations to be confirmed, it remained to find the origin of this phenomenon.
On Mars, there are two types of volatile compounds: water (H2O) and carbon dioxide (CO2). Both of these compounds can easily change phase between gas and solid under Martian conditions. Unfortunately in solid form, they both appear white or bright in the field of visible light (also observable by our eyes) of the CaSSIS instrument. It is therefore difficult to distinguish them with a simple image. We therefore had to carry out real investigative work to be able to distinguish these objects. We were able to establish two types of major arguments, one based on spectroscopy and the other on simulation digital of the microclimate of Martian volcanoes.
By decomposing the colors or wavelengths of light coming from the surface, it is possible to distinguish H2O and CO2. This is the technique of spectroscopy, possible thanks to the instrument NOMAD. We attempted to identify these two compounds on measurements taken in extreme conditions, early in the morning, with little sunlight. As the instrument is optimized to observe the sun directly, these observations are difficult and very noisy. The results show no signature of CO2 and a possible signature of water.
However, if CO2 is present in a thin layer, we do not expect to observe spectral signatures. This argument is therefore not definitive. The Belgian members of the team then carried out a numerical simulation of the microclimate of Mars’ volcanoes. This is the same type of tool that predicts the weather on Earth, adapted here to Mars. The simulations conclude that at the time of acquisition of the images containing the bright areas, the conditions are right for the condensation of water, but not for CO2. These two pieces of evidence combined therefore support the hypothesis of water frost.
A thin layer of frost
Thanks to the information from the various instruments, we were able to estimate the thickness of this layer of frost. It is a very thin thickness of around 10 micrometers (1 hundredth of a millimeter). This quantity of frost is imposed by the quantity of water vapor available in the atmosphere of Mars, which is of the order of a precipitable micron. As an example on Earth, the quantity of water is of the order of a precipitable centimeter, or 10,000 times more!
Since condensation must begin a few hours before sunrise, atmospheric circulation has allowed water to accumulate from the surrounding atmosphere. Given the surface area covered by frost, the total amount of water is about 60 Olympic swimming pools. If it could be harvested, it could cover the astronauts’ water needs for several years, but on the scale of the planet Mars, this amount remains very small. The amount of water available on Mars, mainly in the form of ice in the polar caps, is estimated at 1,000 billion Olympic swimming pools, knowing that on Earth there is about 100 times more water than on Mars.
This discovery of frost on the highest peaks of the Solar System will allow us to refine our understanding of the current water cycle on Mars. In this way, it will be possible to better predict the weather on Mars, with a view to future exploration, but also to better understand the past climate of Mars and its potential for habitability.
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