How tectonically active is Mercury?

Question

Both this article and this paper say that Mercury's tectonic activity is mostly in the past, but the first article does have this to say:

After the volcanic activity subsided, the planet has been very quiet geologically except for the occasional meteor (however, it is probably geologically more active than the Moon).

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What exactly does it mean to say that Mercury is more active than the Moon in this context? How do we measure Mercury's tectonic activity today?

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Answer 1

As we don't have any direct measuring tools to measure the effects of tectonics (e.g. Mercury-quakes) and not have observed volcanism in the short while that we have been properly observing the planet - we can only surmise what tectonics have and are occurring by observing the surface features.

To understand the past tectonic features and dynamics is a means to understanding the present.

But, an important distinction, it appears, according to Plate tectonics and planetary habitability: current status and future challenges (Korenaga, 2012), that plate tectonics is a feature unique to Earth.

According to the chapter Tectonics of Mercury, the mostly compressional tectonic features

A combination of tidal despinning and thermal contraction may account for equatorial N–S and polar E–W trending lobate scarp thrust faults in the regions by reactivation of normal faults.

and that the

Local preferred orientations of the lobate scarps and uniform thrust slip dip directions suggest regional-scale stresses influenced the formation of the thrust faults. The wrinkle ridges in smooth plains outside of the Caloris basin are likely due to loading and subsidence of volcanic material that flooded lowland areas.

Recent observations discussed in Thermal evolution of Mercury as constrained by MESSENGER observations (Michel et al. 2013), determined that

that the planet’s core is larger than previously estimated. As Mercury’s mantle layer is also thinner than previously thought, this result gives greater likelihood to the possibility that mantle convection is marginally supercritical or even that the mantle is not convecting.

Using this data in simulations, they

demonstrate that mantle convection can persist in such a thin mantle for a substantial portion of Mercury's history, and often to the present, as long as the mantle is thicker than ~300 km. We also find that magma generation in Mercury’s convecting mantle is capable of producing widespread magmas by large-degree partial melting

MESSENGER also observed large, compressional fold structures and further systems of thrust faults.

So, to summarise, it appears that the tectonic activity on Mercury has been and is from a number of dynamic sources:

• Tidal de-spinning
• Contraction as the core cools - reactivating older faults borne from past impacts and earlier tectonic activity.
• Conversely, possibly due to some mantle convection that might exist, hot spot activity.

Answer 2

A study by Stern et al. (2018) introduced a "Tectonic Activity Index" (TAI) to represent the tectonic activity of planetoids. The TAI is determined by looking at three criteria:

• Recent deformation (faults and folds)
• Recent volcanism
• Recent resurfacing (impact craters abundance)

The presence of each criterion scores one point, so the TAI of a given planetoid can range between 0 and 3. The authors consider planetoids with a TAI of 2 or 3 to be "tectonically alive", and planetoids with a TAI of 0 or 1 to be "tectonically dead". They determined the TAI of 26 planetoids of the Solar System. Mercury has a TAI of 1, while the Moon has a TAI of 0 (Table 1). Based on this, you could indeed say that Mercury is tectonically more active than the Moon.

However, by reading the paper, it is not all that clear how they came to this result, i.e., how Mercury got one point. Mercury and the Moon are often cited together as classic example of tectonically dead planetoids. In the volcanism section, both are mentioned to have had volcanism in the early history of the Solar System. In the resurfacing section, both are mentioned to present a densely cratered surface. So I have the impression that Mercury scored one point for its deformation which, as the authors acknowledge, is due thermal contraction rather than internal convection.

In summary, both Mercury and the Moon are considered tectonically dead in terms of mantle convection, but the thermal contraction of Mercury triggers some kind of tectonic activity (reverse faulting) that is absent on the Moon.