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, and its oldest rocks are the record of the one it used to have. Mars Global Surveyor's magnetometer, sweeping the planet from 100 to 200 km up, found patches of the ancient southern highlands magnetised more than an order of magnitude more strongly than Earth's crust — the strongest sources, in Terra Sirenum, carrying 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. Something inside Mars was turning, and then it stopped.
The dating argument runs on absence. Hellas and Argyre, the 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 form of the argument now sets the end before about 4.1 billion years ago — inside the planet's first half-billion years.
It has been challenged on its own evidence. In 2020 Anna Mittelholz and colleagues went back in 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 lava plain of about 3.7 billion years. Their reading is a dynamo running both before and after Hellas, Utopia, Argyre and Isidis formed, the basins reading empty because the impacts dug out the magnetised crust rather than because no field remained to record.
Whichever date survives, the consequence does not change. With no global field to stand the solar wind off, the wind couples straight into the top of the atmosphere, ionises gas and carries it away, and it has been doing 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 not merely cold, it is unstable: it boils and freezes at once. That single fact is why anyone standing on Mars will be inside a pressure suit.