Liquid water used to exist on the Mars surface long ago, but no longer does today. The question is how Mars lost its atmosphere of water and other volatiles to space, and how this played a significant role in its climate evolution. A planet’s magnetic field would act as a ‘shield’ against such atmospheric losses, while solar storms and other space weather phenomena would tend to increase such losses. In the absence of a global magnetic field, the loss of Mars’ atmosphere to space could have been effective, and the escape of charged particles should have been more effective long ago. However, Ramstad et al.  argue, contrary to some other published results, that primordial solar storm conditions do not significantly enhance ion escape from the Red Planet—thus putting the spotlight of Mars science back on the question of where the water had gone.
Citation: Ramstad, R., Barabash, S., Futaana, Y., Yamauchi, M., Nilsson, H., & Holmström, M. (2017). Mars under primordial solar wind conditions: Mars Express observations of the strongest CME detected at Mars under solar cycle #24 and its impact on atmospheric ion escape. Geophysical Research Letters, 44. https://doi.org/10.1002/2017GL075446
—Andrew Yau, Editor, Geophysical Research Letters