COLLOQUIA PATAVINA for Students

To recover the past evolution of planetary motion in the Solar System, we can build the most precise model possible, taking into account all small effects that affect planetary trajectories, and adjust this model to all available observations, both from Earth and from space. This is the principle behind planetary ephemerides such as INPOP or DE. The accuracy of these models is determined by comparison with the available observations, covering a few centuries. However, to understand the past evolution of planetary motions over millions of years (Ma), the long-term propagation of such solutions has intrinsic limitations.

Indeed, a first obstacle is that it is difficult to assert that the Solar System has not been affected by external phenomena over extended timescales, such as passing stars. Moreover, planetary motion is chaotic (Laskar, 1989), and the exponential divergence of the orbits practically limits any deterministic prediction to about 60 Ma.

To go beyond this horizon of predictability, additional information is required. I will show here how such information can be retrieved from geological sedimentary data.

Variations in Earth's orbit and axial tilt induce climatic changes on its surface, which are recorded in sedimentary deposits. These are the so-called Milankovitch cycles. The problem is that this record is extremely noisy, full of unconformities, and expressed in terms of depth rather than time. A critical aspect of this analysis is thus the estimation of the sedimentation rate, which determines the time–depth transfer function relating geological depth to time.

Within the AstroGeo project, we have devised a method to establish a continuous time–depth transfer function throughout the record, accommodating variable sedimentation rates, and to extract the primary astronomical signal from the geological sequence. This is achieved using a genetic algorithm that adapts to a wide range of sedimentation rate variations. This statistical approach enables the reconstruction of an astronomical signal (e.g., eccentricity and/or precession) purely from the stratigraphic sequence.

This opens the possibility of following the orbital evolution of the Solar System in the remote past, beyond the horizon of predictability imposed by the laws of celestial mechanics.