The dynamo dies and the air starts leaving
Mars Global Surveyor found the oldest crust magnetised and the great impact basins blank — read as a dynamo dead inside the first half-billion years, though MAVEN data since has reopened the date.
Mars has no global magnetic field. Its oldest rocks record the one it used to have. Mars Global Surveyor's magnetometer, sweeping the planet from 100 to 200 km up, found patches of ancient southern highlands crust magnetised more than an order of magnitude more strongly than Earth's. The strongest sources, in Terra Sirenum, carry equivalent magnetic moments as large as 1.3 × 10¹⁷ ampere-metres². Mario Acuña's team published the survey in 1999, and Jack Connerney found much of the signal organised into quasi-parallel east–west stripes of alternating polarity, the longest running over 2,000 km.
The dating argument runs on absence. Hellas and Argyre, two enormous basins gouged out early in the Noachian, are magnetically blank. An impact that size heats the target rock past its Curie point; the rock cools and records whatever field is present. Those basins recorded nothing. Acuña's team read that as a dynamo already dead in the early Noachian, and the standard argument now puts the end before about 4.1 billion years ago.
In 2020 Anna Mittelholz and colleagues returned with low-altitude MAVEN magnetometer data and found field over ground that should have been blank at both ends: a weak signal across the 4.5-billion-year-old Borealis basin, and a near-surface source over Lucus Planum, a volcanic ash plain of about 3.7 billion years. They read that as a dynamo running before and after Hellas, Utopia, Argyre and Isidis formed, the basins empty because the impacts dug out the magnetised crust; they cannot exclude a weakened or paused field.
Whichever date survives, the consequence is the same. With no global field to stand the solar wind off, the wind couples into the top of the atmosphere and carries away sunlight-ionised gas, and has done so for most of the planet's life. Surface pressure today is about 6 millibars, under one percent of Earth's and below the triple point of water across most of the planet. Liquid water there is unstable: it boils and freezes at once.