Mars’ water history is fundamental to understanding an evolution of Earth-like planets. Water escapes to space as atoms, and hydrogen (H) atoms escape faster than deuterium (D) — which is a hydrogen atom with a neutron in its nucleus — giving an increase in the residual D/H ratio. The present ratio reflects the total water Mars has lost.
These are far-ultraviolet Hubble images of Mars near its farthest point from the Sun, called aphelion, on December 31, 2017 (top), and near its closest approach to the Sun, called perihelion, on December 19, 2016 (bottom). Image credit: NASA / ESA / STScI / John T. Clarke, Boston University.
There is abundant evidence that Mars underwent an early wet period, with liquid water flowing across the surface leaving behind clear erosion patterns and the presence of clay in the surface soil.
This wet climatic period appears to have ended a little over 3 billion years ago, and the fate of that water has inspired great interest.
To some extent, the water remained trapped in the crust as Mars cooled, and to some extent, it was dissociated into hydrogen and oxygen atoms with many of the atoms escaping into space from the top of the atmosphere.
“There are only two places water can go. It can freeze into the ground, or the water molecule can break into atoms, and the atoms can escape from the top of the atmosphere into space,” said Dr. John Clarke, a researcher at Boston University.
“To understand how much water there was and what happened to it, we need to understand how the atoms escape into space.”
In their new research, Dr. Clarke and colleagues combined data from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) and the NASA/ESA Hubble Space Telescope to measure the number and current escape rate of the hydrogen atoms escaping into space.
This information allowed them to extrapolate the escape rate backwards through time to understand Mars’s water history.
Specifically, the researchers measured hydrogen and its heavier isotope deuterium.
Over time, as more hydrogen was lost than deuterium, the D/H ratio built up in the atmosphere.
Measuring the ratio today gives scientists a clue to how much water was present during the warm, wet period on Mars.
By studying how these atoms currently escape, they can understand the processes that determined the escape rates over the last four billion years and thereby extrapolate back in time.
Although most of the data come from the MAVEN, this spacecraft is not sensitive enough to see the deuterium emission at all times of the Martian year.
Unlike the Earth, Mars swings far from the Sun in its elliptical orbit during the long Martian winter, and the deuterium emissions become faint.
The authors needed the Hubble data to ‘fill in the blanks’ and complete an annual cycle for three Martian years (each of which is 687 Earth days).
Hubble also provided additional data going back to 1991 — prior to MAVEN’s arrival at Mars in 2014.
The combination of data between these missions provided the first holistic view of hydrogen atoms escaping Mars into space.
“In recent years scientists have found that Mars has an annual cycle that is much more dynamic than people expected 10 or 15 years ago,” Dr. Clarke said.
“The whole atmosphere is very turbulent, heating up and cooling down on short timescales, even down to hours.”
“The atmosphere expands and contracts as the brightness of the Sun at Mars varies by 40% over the course of a Martian year.”
The team discovered that the escape rates of hydrogen and deuterium change rapidly when Mars is close to the Sun.
In the classical picture that scientists previously had, these atoms were thought to slowly diffuse upward through the atmosphere to a height where they could escape.
But that picture no longer accurately reflects the whole story, because now scientists know that atmospheric conditions change very quickly.
When Mars is close to the Sun, the water molecules, which are the source of the hydrogen and deuterium, rise through the atmosphere very rapidly releasing atoms at high altitudes.
The second finding is that the changes in hydrogen and deuterium are so rapid that the atomic escape needs added energy to explain them.
At the temperature of the upper atmosphere only a small fraction of the atoms have enough speed to escape the gravity of Mars.
Faster (super-thermal) atoms are produced when something gives the atom a kick of extra energy.
These events include collisions from solar wind protons entering the atmosphere or sunlight that drives chemical reactions in the upper atmosphere.
The findings were published in the journal Science Advances.
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John T. Clarke et al. 2024. Martian atmospheric hydrogen and deuterium: Seasonal changes and paradigm for escape to space. Science Advances 10 (30); doi: 10.1126/sciadv.adm7499
This article is based on a press-release from NASA.
Source : Breaking Science News