#PSI: Timing of #Ancient #Mars #Magnetic Field Determined

ISSUED BY THE PLANETARY SCIENCE INSTITUTE IN TUCSON, ARIZONA

TIMING OF ANCIENT MARS MAGNETIC FIELD DETERMINED

A Martian magnetic field was in place much earlier -- as well as much later -- than scientists had thought, according to the new Science Advances paper “Timing of the Martian Dynamo: New Constraints for a Core Field at 4.5 and 3.7 Ga Ago” on which Planetary Science Institute Senior Scientist Catherine Johnson is an author. [“Ga” is an abbreviation of “giga-annum,” i.e., a billion years.]

A planet’s global magnetic field arises from what scientists call a dynamo: a flow of molten metal within the planet’s core that produces an electrical current. On Earth, the dynamo is what makes compass needles point north. But Mars’ dynamo, and magnetic field, has been extinct for billions of years.

“We have these two observations that point to a dynamo at the earliest known time in Mars’ history, and a dynamo that was present almost half a billion years after many people thought it had already switched off,” Johnson said.

“We find that the Martian dynamo operated at 4.5 billion years ago (4.5 Ga) and 3.7 billion years ago. Dynamo timing is a big part of a planet’s evolution, and what we find is very different from what we have thought so far,” said Anna Mittelholz, lead author of the paper and a postdoctoral fellow in University of British Columbia’s Department of Earth, Ocean and Atmospheric Sciences. “The dynamo tells us something about the planet’s thermal history, its evolution, and how it got to where it is today, and it is unique for each of the terrestrial planets -- Earth, Mars, Venus and Mercury.”

Clues about a planet’s magnetic history lie in magnetized rocks on and beneath its surface. Rocks are like a tape recorder, especially volcanic rocks. They begin as lava, but as they cool and solidify in the presence of a magnetic field, minerals within the rocks align themselves with the global magnetic field. If the age of the rocks is known, scientists can estimate if a dynamo was active at the time the rock was emplaced. Magnetism in certain rocks on Mars’ surface indicate that the Martian dynamo was active between 4.3 and 4.2 billion years ago, but the absence of magnetism over three large basins -- Hellas, Argyre, and Isidis -- that formed 3.9 billion years ago has led most scientists to believe the dynamo was inactive by that time.

Mittelholz’s team analyzed new data from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) orbiter and found clear evidence of a magnetic field coming from the Lucus Planum lava flow that formed about 3.7 billion years ago -- much later than at other areas studied.

“The age for the newer dynamo comes from the correlation of magnetic signals with a young (3.7 Ga) lava flow. In particular we see a ‘hole’ in the magnetic field signal over a small crater in the flow that penetrates to the base of the flow so we know that this small crater removed the magnetic signal: this tells us that the magnetized rocks are in the flow, not just buried much more deeply,” said Johnson, who is also a professor in the Department of Earth, Ocean and Atmospheric Sciences at UBC and Mittelholz’s colleague and academic advisor. “The flow age comes from crater counts on the flow.”

“If a rocky planet or moon has a global magnetic field like on Earth either today or sometime in the past, it tells us about the planet’s deep interior at that time. Furthermore, new results for the dynamo at 3.7 Ga suggest that the Martian dynamo could have been active at a time of very different climatic conditions on Mars, in particular when many of the valley networks formed,” Johnson said.