Detecting Crustal Magnetic Fields on Venus from a Descending Probe

An outstanding question in Venus’ history is whether there was a dynamo magnetic field.  While evidently not present now, such a dynamo in the past could have left an imprint in rocks, much like the ‘stripes’ of alternating magnetic signatures in our sea floor. While Venus is hot enough to demagnetize some minerals, a number are below their Curie point on Venus today, so it is possible that crustal fields may be retained in surface and near-surface rocks.

The sensitivity to magnetism falls off rapidly with distance, so the constraints established by orbital measurements at Venus are not severely binding, and we may recall that in fact the crustal magnetic fields were only discovered at Mars thanks to the low altitudes required for aerobraking of Mars Global Surveyor.  A magnetic survey at modest altitude (~50km) by a balloon has been considered as a promising framework, with signatures about an order of magnitude higher than those sensed by orbiters.

A descent probe like DAVINCI will not traverse a large horizontal distance, even with Venus’ strong zonal winds. However, by taking data down to near-zero altitude, it offers much higher sensitivity to crustal fields than a balloon.  Accommodation of a traditional magnetometer is not practical, however : a boom to separate the instrument from the platform is not aerodynamically or structurally feasible, and the high-temperature environment would be challenging for typical instrument designs. An instrument mounted inside the titanium pressure vessel hull of a probe would be able to detect an external field, but will be substantially perturbed by the fields generated by equipment on the probe such as solenoid valves in gas analysis instrumentation, shutter or filter wheel actuators, and switching currents to various systems.

Despite these unpromising factors, the prospects for detection of a crustal magnetic signature on a Venus probe may be substantially improved by the fact that a typical probe spins as it descends, with a period of a few seconds to tens of seconds.  Thus, the horizontal component of an external field is sinusoidally-modulated at a known frequency by the spin, which facilitates the rejection of noise that varies on short timescales.  It is additionally the case that the sensed component of a crustal field will characteristically increase during descent, as the vehicle approaches the surface.

The inclusion of even rudimentary magnetic sensing (and ‘magnetometer-on-a-chip’ devices, both fluxgate and magnetoresistive, are readily available and have negligible resource footprints) on a descent probe therefore offers substantial discovery potential. In addition, the ‘noise’ provides a potentially-useful diagnostic on the operation of probe equipment, and the operation of magnetic instrumentation during the hypersonic entry prior to descent may yield insights into the magnetohydrodynamics of the shock layer plasma flows which are responsible for the ‘entry blackout’ that suppresses radio communication. Thus, magnetic sensing contributes to NASA Engineering Science Investigation objectives.    The data processing unit of the Venus Atmospheric Structure Investigation (VASI) on DAVINCI accommodates a magnetic sensor.