  Asteroids smaller than about 100 meters are observed to rotate very fast,
with periods often much shorter than the critical limit of 2.2 h. Some of these
super-fast rotators can also achieve a very large semi-major axis drift induced
by the Yarkovsky effect, that in turn, is determined by internal and surface
physical properties. We consider the small super-fast rotating near-Earth
asteroid 2016 GE1. This object rotates in just 34 seconds, and a large
Yarkovsky effect has been determined from astrometry. Here we aim to constrain
the thermal inertia of the surface of this extreme object. We used a recently
developed statistical method to determine the thermal properties of near-Earth
asteroids. The method is based on the comparison between the observed and the
modelled Yarkovsky effect, and the thermal conductivity (inertia) is determined
by a Monte Carlo approach. Parameters of the Yarkovsky effect model are either
fixed if their uncertainty is negligible, modelled with a Gaussian distribution
of the errors if they are measured, or deduced from general properties of the
population of near-Earth asteroids when they are unknown. Using a
well-established orbit determination procedure, we determined the Yarkovsky
effect on 2016 GE1, and verified a significant semi-major axis drift rate.
Using a statistical method, we showed that this semi-major axis drift rate
could be explained only by low thermal inertia values below 100 J m$^{-2}$
K$^{-1}$ s$^{-1/2}$: namely, 90\% of the probability density function of the
model outcomes is contained at values smaller than 100 J m$^{-2}$ K$^{-1}$
s$^{-1/2}$. We propose two possible interpretations for the extremely low
values: a high porosity or a cracked surface, or a thin layer of fine regolith
on the surface. Though this seems unexpected in either case, it opens up the
possibility of a subclass of low thermal inertia, super-fast rotating
asteroids.
