An Orbital Window into the Ancient Sun’s Mass

Models of the Sun's long-term evolution suggest that its luminosity was substantially reduced 2–4 billion years ago, which is inconsistent with substantial evidence for warm and wet conditions in the geological records of both ancient Earth and Mars. Typical solutions to this so-called "faint young Sun paradox" consider changes in the atmospheric composition of Earth and Mars, and, while attractive, geological verification of these ideas is generally lacking—particularly for Mars. One possible underexplored solution to the faint young Sun paradox is that the Sun has simply lost a few percent of its mass during its lifetime. If correct, this would slow, or potentially even offset, the increase in luminosity expected from a constant-mass model. However, this hypothesis is challenging to test. Here, we propose a novel observational proxy of the Sun's ancient mass that may be readily measured from accumulation patterns in sedimentary rocks on Earth and Mars. We show that the orbital parameters of the Solar System planets undergo quasi-cyclic oscillations at a frequency, given by secular mode g2 − g5, that scales approximately linearly with the Sun's mass. Thus by examining the cadence of sediment accumulation in ancient basins, it is possible distinguish between the cases of a constant-mass Sun and a more massive ancient Sun to a precision of greater than about 1 percent. This approach provides an avenue toward verification, or of falsification, of the massive early Sun hypothesis.