PT  - JOURNAL ARTICLE
AU  - Jakosky, B. M.
AU  - Slipski, M.
AU  - Benna, M.
AU  - Mahaffy, P.
AU  - Elrod, M.
AU  - Yelle, R.
AU  - Stone, S.
AU  - Alsaeed, N.
TI  - Mars’ atmospheric history derived from upper-atmosphere measurements of <sup>38</sup>Ar/<sup>36</sup>Ar
AID  - 10.1126/science.aai7721
DP  - 2017 Mar 31
TA  - Science
PG  - 1408--1410
VI  - 355
IP  - 6332
4099  - http://science.sciencemag.org/content/355/6332/1408.short
4100  - http://science.sciencemag.org/content/355/6332/1408.full
SO  - Science2017 Mar 31; 355
AB  - Mars has a thin atmosphere composed mainly of carbon dioxide. Evidence on the planet's surface indicates that Mars was once warmer and wetter, suggesting a thicker atmosphere in the past. Jakosky et al. measured the abundances of argon isotopes at different heights in the atmosphere. Because lighter isotopes are more easily ejected than heavier ones, about 66% of Mars' atmosphere has been lost into space since it formed. Understanding the history of Mars' atmosphere will help explain how and why its climate changed, informing the study of similar processes on Earth.Science, this issue p. 1408The history of Mars’ atmosphere is important for understanding the geological evolution and potential habitability of the planet. We determine the amount of gas lost to space through time using measurements of the upper-atmospheric structure made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. We derive the structure of 38Ar/36Ar between the homopause and exobase altitudes. Fractionation of argon occurs as a result of loss of gas to space by pickup-ion sputtering, which preferentially removes the lighter atom. The measurements require that 66% of the atmospheric argon has been lost to space. Thus, a large fraction of Mars’ atmospheric gas has been lost to space, contributing to the transition in climate from an early, warm, wet environment to today’s cold, dry atmosphere.