Free Astronomy Magazine November-December 2024

NOVEMBER-DECEMBER 2024 escape, they can understand the processes that determined the es- cape rates over the last four billion years and thereby extrapolate back in time. Although most of the study’s data comes from the MAVEN spacecraft, MAVEN 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. Clarke and his team 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 ho- listic view of hydrogen atoms escap- ing 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,” explained Clarke. “The whole at- mosphere 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 percent 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 dif- fuse upward through the atmos- phere to a height where they could escape. But that picture no longer accu- rately reflects the whole story, be- cause 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 deu- terium, 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 at- mosphere only a small fraction of the atoms have enough speed to es- cape 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 at- mosphere or sunlight that drives chemical reactions in the upper at- mosphere. Studying the history of water on Mars is fundamental not only to un- derstanding planets in our own so- lar system but also the evolution of Earth-size planets around other stars. Astronomers are finding more and more of these planets, but they’re difficult to study in detail. Mars, Earth and Venus all sit in or near our solar system’s habitable zone, the region around a star where liquid water could pool on a rocky planet; yet all three planets have dramatically different present-day conditions. Along with its sister planets, Mars can help scientists grasp the nature of far-flung worlds across our galaxy. ! T hese are far-ultraviolet Hubble images of Mars near its far- thest point from the Sun, called aphelion, on December 31, 2017, and near its closest approach to the Sun, called perihelion, on December 19, 2016. The atmosphere is clearly brighter and more extended when Mars is close to the Sun. Reflected sunlight from Mars at these wavelengths shows scattering by atmospheric molecules and haze, while the polar ice caps and some surface features are also visible. Hubble and MAVEN showed that Mar- tian atmospheric conditions change very quickly. When Mars is close to the Sun, water molecules rise very rapidly through the atmosphere, breaking apart and releasing atoms at high altitudes. [NASA, ESA, STScI, John T. Clarke (Boston Uni- versity) − Image Processing: Joseph DePasquale (STScI)]