About 4 billion years ago, Mars was a sultry planet—a warm, wet world of lakes and rivers blanketed in a thick, steamy atmosphere. At some point in the distant past, however, this atmosphere disappeared, and Mars became the freezing, inhospitable desert it is today.
The most widely accepted explanation for how Mars lost its atmosphere is that solar winds swept it away. In this scenario, the magnetized plasma hurtling away from the Sun—also called solar wind—induced electromagnetic forces in the ionized upper atmosphere that easily whisked the particles into space. But a new study based on data from the Mars Express spacecraft contradicts this hypothesis, suggesting solar winds played only a minor role in the loss of Mars’s early atmosphere.
Using a decade of ion loss measurements from Mars Express, Ramstad et al. calculated an estimated rate of ion escape from Mars’s atmosphere over the past 4 billion years. Solar wind–driven ion escape accounted for only about 9 millibars of lost atmospheric surface pressure, the team found. That’s less than 1% of the total 1 bar of surface pressure that would have been necessary to sustain Mars’s lakes and rivers, a loss too minor to explain how Mars arrived at its current, desiccated state.
There are some elements of uncertainty in the study, including doubts about the strength of solar winds and ionizing radiation that flowed from the younger Sun. But the work does suggest that solar wind was an accomplice in the disappearance Mars’s atmosphere, not the primary thief, the team argues. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2018JE005727, 2018)
—Emily Underwood, Freelance Writer