Seasonal changes can have a dramatic effect on how quickly Mars loses its water to space, a joint study between the Hubble Space Telescope and NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission has shown.
Over three billion years ago, Mars was warm and wet, with large bodies of water on its surface and a thicker atmosphere.
Plenty of Mars’ water is still on the Red Planet.
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MAVEN, which arrived at Mars in 2014, is tasked with measuring this hydrogen escape.
Because deuterium, a heavy form of hydrogen, doesn’t escape Mars’ atmosphere so easily, it means that the ratio of deuterium to hydrogen (D/H) in Mars’ atmosphere is key, with the abundance of deuterium relative to hydrogen growing over time as it loses hydrogen faster.
The D/H ratio on Mars today is somewhere between 8 and 10 times larger than on Earth.
However, MAVEN, with the Hubble Space Telescope’s help, has now found some unanticipated complexity to the story of Mars’ water loss.
At aphelion, the deuterium loss is so feeble that MAVEN is not even sensitive enough to detect it.
To increase the rate of deuterium loss so that it matches the observed D/H ratio on Mars, an extra injection of energy into the atmosphere is required from somewhere.
The rate at which Mars loses water to space can be significantly influenced by seasonal variations, according to research conducted in conjunction with NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission and the Hubble Space Telescope.
A thicker atmosphere and vast bodies of water covered the surface of Mars more than three billion years ago, when the planet was warm and humid. But Mars is bleak, frigid, and dry right now. How come there was no water left?
According to a statement from John Clarke of the University of Boston, “water can only go to two places.”. It has two possible outcomes: either it freezes into the earth or the water molecules split into atoms, which then escape into space from the upper atmosphere. “.
Mars still has a significant amount of water on it. Subterranean depths between 11:55 and 20 kilometers (7:01 and 12:04 miles) appear to be the lockup of vast reservoirs. Mars has enough water on its interior to create a global equivalent layer (GEL, or how deep an ocean would be created on the planet) that is between 0.6 and 2 kilometers (0.62 and 1.24 miles) deep.
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Small amounts of water-ice are also trapped in the polar ice caps of Mars and in shallow permafrost. This ice can sublimate during the Martian summer, releasing water vapor into the atmosphere. While the majority of that water vapor travels in a pole-to-pole motion and freezes out in the hemisphere where winter occurs, some of it ends up in the upper atmosphere where solar ultraviolet light can photodissociate H2O water molecules, causing them to split into their constituent atoms. Carbon dioxide is formed when oxygen in water bonds with carbon or oxidizes materials on the surface, which is why Mars looks red. If they are energetic enough to achieve escape velocity, hydrogen atoms (or their heavier isotope, deuterium) can break free into space and travel with the solar wind.
Measuring this hydrogen escape is the job of MAVEN, a spacecraft that was sent to Mars in 2014.
Deuterium is a heavy form of hydrogen that is more difficult to escape from Mars’ atmosphere than hydrogen, so the ratio of deuterium to hydrogen (D/H) in the atmosphere is important. As hydrogen is lost more quickly, the abundance of deuterium increases over time. The ancient D/H ratio of the water on Mars three to four billion years ago should have been the same as that of the water on Earth today, since it is assumed that Earth and Mars obtained their water from the same sources. Currently, the D/H ratio on Mars is eight to ten times greater than that of Earth. There are some uncertainties in the measurements, but it is possible to extrapolate backward and determine how much water Mars most likely lost over its history by comparing the primordial Mars water ratio to the current ratio and accounting for the rate of hydrogen and deuterium loss to space.
A GEL that is between tens and hundreds of meters deep has formed on Mars as a result of water lost to space, according to MAVEN’s earlier observations. This suggests that, in the distant past, Mars was a water-rich planet, especially in light of the massive amount of water that was recently discovered buried beneath the surface.
But now, MAVEN—along with the Hubble Space Telescope—has discovered some unexpected nuance to the story of Mars’ water loss. As a whole, the instruments have demonstrated that the rate of hydrogen loss is seasonal, peaking at perihelion, the point where Mars is closest to the sun, and showing significant increases in the escape rate. This is accompanied by a significant upwelling of water vapor into the middle atmosphere as a result of seasonal heating. At perihelion, the southern hemisphere of Mars is inclined towards the sun, and the planet is enveloped in its yearly season of dust storms; the dust particles in the atmosphere have the potential to increase the temperature of the atmosphere and the amount of water vapor.
Mars’s elliptical (rather than circular) orbit’s farthest point from the sun is called aphelion. At perihelion, MAVEN detected densities of deuterium and hydrogen in the upper atmosphere that are, respectively, roughly 5 and 20 times higher than at aphelion. The deuterium loss is so weak at aphelion that MAVEN is not even sensitive enough to see it. The Hubble Space Telescope must fill in the gaps in this situation. Moreover, the observations demonstrated that at perihelion compared to aphelion, the escape rates for deuterium and hydrogen, respectively, are 10 to 100 times higher. The amount of water vapor in the atmosphere is the only thing stopping the escape of both deuterium and hydrogen at perihelion, since they are both escaping so quickly.
Clarke stated, “Scientists have discovered that Mars has an annual cycle that is significantly more dynamic than people anticipated ten or fifteen years ago.”. “The entire atmosphere is incredibly turbulent, warming and cooling in a matter of hours. Over the course of a Martian year, the brightness of the sun varies by 40%, causing the atmosphere to expand and contract. ****.
Given that a deuterium atom is warm enough to have the energy required to skip into space, this does present a puzzle when it comes to explaining the deuterium loss, which seems to be higher than what would be predicted purely from ordinary thermal escape. Additional energy must be injected into the atmosphere from some source in order to accelerate the deuterium loss rate and bring it into line with the observed D/H ratio on Mars. This may result from chemical reactions triggered by solar ultraviolet light that can give the deuterium an extra boost, or from protons on the solar wind entering the atmosphere and colliding with deuterium atoms.
