Moon, Mercury, and Magnetic Fields

After the crash of MESSENGER, magnetic fields in the solar system have become a key topic for planetary science.

On April 27, David Rothery anticipated the end of the decade-long mission of MESSENGER, the second spacecraft to visit Mercury since Mariner 10 in 1974. MESSENGER stayed in orbit over the innermost planet for over a year, completing 4,104 orbits in total. In late April, mission controllers lowered its trajectory for final close-up observations before intentionally crashing the fuel-depleted craft onto the surface on April 30. Rothery, writing in The Conversation, predicted the crash would make a 15-meter-wide crater that the next spacecraft from the European Space Agency, named BepiColombo, can study in 2024.

What’s now apparent is that Mercury is a misfit planet that seems not to belong where we find it. It is dense even for a rocky planet, revealing an iron-rich core that occupies more than 80% of the planet. The outer part of the core must still be molten, because this is where Mercury’s magnetic field is generated – a characteristic shared with the Earth, unlike Venus, Mars or the Moon.

According to mission scientists at Johns Hopkins, that magnetic field “has been in place far longer than previously known,” data from low orbits suggest. The planet’s global magnetic field was discovered by Mariner 10 and re-measured by MESSENGER, with no significant changes in the field observable in the intervening 40 years. But only the low orbits allowed scientists to measure magnetic signals in the crustal rocks that must have been induced when the rocks were still molten. “If we didn’t have the recent very low-altitude observations, we would never have been able to discover these signals,” said Catherine Johnson. “Mercury has just been waiting to tell us its story.”

Other news sources picked up the story. A press release from the University of British Columbia, Johnson’s home institution, echoed the statement that the field is between 3.7 and 3.9 billion years old. This leaves an unexplained gap of 0.6 billion years from the time of its formation, presumed to be 4.5 billion years ago. Venus, however, has none, and Mars has only patchy remnants of magnetism, if it ever had a global field. Only the Earth and Mercury among the rocky planets has a global magnetic field, and Mercury’s is much weaker than Earth’s. Our planet’s strong magnetic field protects life on the surface. Ken Croswell in Science Magazine points out the difference a habitable planet makes:

Earth’s magnetic field shields us from the solar wind, but with a daytime temperature hot enough to melt lead and a night cold enough to freeze carbon dioxide gas, Mercury probably isn’t hosting any creatures that need such protection.

All the sources claim as a matter of accepted fact that a dynamo in the fluid core produces the field. The original paper by Johnson et al. in Science Magazine, however, shows that things aren’t quite that simple:

The simplest interpretation of the results presented here is that a core dynamo was present early in Mercury’s history. If the dynamo was thermochemically driven, this finding provides a strong constraint on models for the thermal evolution of Mercury’s interior. In particular, the existence of a core dynamo at the time of smooth plains emplacement presents a new challenge to such models. An early core dynamo can be driven by super-adiabatic cooling of the liquid core, but in typical thermal history models this phase has ended by 3.9 Ga. A later dynamo can be driven by the combined effects of cooling and compositional convection associated with formation of a solid inner core, but in most thermal history models inner core formation does not start until well after 3.7 Ga. Further progress in understanding the record of Mercury’s ancient field can also be made with improved petrological constraints on crustal compositions, information on the candidate magnetic mineralogies implied, and knowledge of their magnetic properties.

There’s a conflict, in other words, between the models of how Mercury cooled over time and how the dynamo has generated a magnetic field – one that is as old as presumed, yet continues today.

Earth’s Moon

The problem with magnetic fields is that they decay in strength over time, unless there is a mechanism to keep them going. That’s why dynamo theories are popular despite the problems in the models (see “What You’re Not Being Told About Earth’s Magnetic Field,” 4/17/15). Earth’s moon, 29% smaller than Mercury, also has a magnetic secret: a magnetic field bigger than Earth’s is now, if dynamo models are assumed (12/05/14). A paper in Icarus slated for publication in July 2015 calls this an unresolved problem:

The source of the magnetic field recorded in the lunar crust remains an unresolved problem. The field was most likely produced by a self-sustaining dynamo in the Moon’s electrically conducting metal core, but heat flux across the core–mantle boundary was probably insufficient to power a dynamo for the field’s currently known duration from 4.2 to 3.56 Ga. Since seismic measurements indicate the existence of a solid iron inner core in addition to a still-liquid iron alloy outer core, inner core solidification and its associated thermochemically driven convection in the outer core could have been responsible for extending the dynamo’s lifetime even in the absence of superadiabatic heat flux. Here we present a coupled mantle–core thermal evolution model of the Moon and show that core solidification could explain the onset and shutoff of the lunar dynamo consistent with the global magnetic field inferred from the paleomagnetic record.

It’s clear that tweaking of models is necessary to keep them in sync with age expectations, otherwise it would not still remain an unresolved problem. One thing is clear: the only rocky planet in the inner solar system that has a strong magnetic shield also has sentient beings able to think about these matters.

CEH is one of the only science news sites where you can hear about the problems with current theories instead of being spoon-fed pat answers, like “simple” dynamos that supposedly keep magnetic fields going for billions of years (e.g., “Mercury’s magnetic field is almost four billion years old,” Science Daily). True science is rarely simple, especially when trying to infer conditions in the unobservable past. We think our readers need to know the real world.

For more on the moon’s magnetic field specifically, see D. Russell Humphreys’ 2013 paper from the Journal of Creation posted by CMI. Humphreys has written extensively about planetary magnetic fields. He explains why dynamo models cannot work over long ages. Having updated the work of Dr. Thomas Barnes from the 1970s-1980s, he shows that the decay of the magnetic field is evidence for young ages of the Earth and other planets.  CMI posted another article by Humphreys from 2008, “Mercury’s Magnetic Field Is Young.“