SwRI
Scientists have developed this illustration of the early appearance of Mars, showing signs of liquid water, large-scale volcanic activity and heavy bombardment from planetary projectiles.
A new study challenges the assumption that the process that brought water to the Martian atmosphere only occurred during the hottest periods of summer in the Southern Hemisphere.
Mars’ water has disappeared somewhere, but scientists have disagreed for years about exactly where it went. Data from rovers like Perseverance and Curiosity, along with orbiting satellites like the Mars Reconnaissance Orbiter and ExoMars, have shown that Mars used to be a wet world with an active hydrodynamic cycle. Obviously not anymore, but where did all the water go?
A new one that brings together data from at least six different instruments on three different spacecraft provides additional insight into this question, showing that dust storms release water into the atmosphere of the Red Planet, where it is actively destroyed year-round.
Experts believe that at one time, Mars had enough water on its surface to cover most of it at a depth of hundreds of meters. To estimate this, they use a technique called the deuterium/hydrogen (D/H) ratio. Deuterium, a heavier isotope of hydrogen, makes up the hydrogen fraction in a small percentage of water molecules.
This slightly heavier version of water — known colloquially as “heavy water” — is less likely to be released into the upper layers of the atmosphere, where it is subsequently destroyed by ultraviolet radiationand the resulting hydrogen atoms are dispersed by the solar wind.
Therefore, over time, the ratio of deuterium to ordinary hydrogen in water increases, as more and more of the element’s lighter form is dispersed. Scientists have measured this D/H ratio on Mars as being 5 to 8 times higher than that of Earth. Extrapolating from these calculations, this would mean that there was enough water on Mars to cover most of its surface a few hundred meters thick — although it may not have been in a liquid state at the time.
Finding an answer to the whereabouts of this water requires an understanding of the seasons on Mars. The Red Planet has an axial tilt, like Earth, which means it There are also seasons. However, Mars also has a much more pronounced elliptical orbit, which means that one “summer”, when the planet is closest to perihelion (i.e. its closest point to the Sun), is much hotter than another, when it is close to aphelion – the point furthest from the Sun.
For Mars, this means that Summers in the Southern Hemisphere are much hotter than those in the Northern Hemisphere, and scientists had long believed that the process by which water reached the atmosphere only occurred during the relatively warm periods of Southern Hemisphere summers. However, this new study contradicts this assumption by showing the process of water loss due to a very specific type of “rocket storm” in the northern hemisphere, which occurred a few years ago.
Hotter summers make a difference in water loss due to the process by which water is injected into the upper atmosphere rather than the lower atmosphere, where it is protected from ultraviolet radiation that breaks it down into its constituent molecules. During the dust storms of the southern summer, the dust is forced into the middle layers the atmosphere, where it heats it by approximately 15 °C. Normally, ice clouds would form at this altitude, trapping water in the lower regions of the atmosphere by freezing its molecules.
As the heat from the dust increases, these ice clouds stop forming, allowing the water to reach the upper atmosphere after being pushed there by the storm and subsequently being destroyed by radiation.
Scientists previously believed this only occurred during southern summers, but data from ExoMars, Emirates Mars Mission (EMM) and Mars Reconnaissance Orbiter have recorded a strong storm during the boreal summer in the unprecedented Martian year 37 (2022-2023 for Earth), which clearly caused the same water destruction process expected during austral summers. This proved that this cycle of dust storms releasing water into the upper atmosphere and its destruction is a year-round phenomenon, and is not limited to specific periods in Mars’ history.
True, that “rocket storm” seemed exceptionally strong, but researchers believe that in Mars’ past, its axial tilt may have have been even greater towards the Sunwhich would have favored this type of storm formation in much hotter summers in the northern hemisphere. This extra “escape route” for water could explain part of the discrepancy between the amount of water Mars currently has, the amount we believe it had in the past, and the processes we believe destroyed that water.
