No abstract

Comets are generally considered among the most pristine objects in our Solar System. There have thus been significant efforts to understand these bodies. During the past decades, we have seen significant progress in our theoretical understanding of planetesimal/cometesimals (the precursors of comets) formation. Recent space missions—such as ESA’s Rosetta mission to comet 67P/Churyumov-Gerasimenko—have provided observations claimed by proponents of different comet formation theories to validate their scenarios. Yet, no single formation paradigm could be definitively proven. Given the importance of understanding how the first bodies in our Solar System formed, we propose a dedicated mission to address this issue. ORIGO will deliver a lander to the surface of a cometary nucleus where it will characterise the first five m of the subsurface. With remote sensing instruments and the deployment of payload into a borehole, we will be able to study the physico-chemical structure of ancient, unmodified material. The mission has been designed to fit into the ESA M-class mission budget.

<p><span data-preserver-spaces="true">The Origo mission was submitted in response to the 2021 call for a Medium-size mission opportunity in ESA's Science Programme.</span></p> <p><span data-preserver-spaces="true">The goal of Origo is to inform and challenge planetesimal formation theories. Understanding how planetesimals form in protoplanetary disks is arguably one of the biggest open questions in planetary science. To this end, it is indispensable to collect ground truths about the physico-chemical structure of the most pristine and undisturbed material available in our Solar System. Origo seeks to resolve the question of whether this icy material can still be found and thoroughly analysed in the sub-surface of comets.</span></p> <p><span data-preserver-spaces="true">Specifically, Origo aims to address the following immediate science questions:</span></p> <ul> <li><span data-preserver-spaces="true">Were cometesimals formed by distinct building blocks such as e.g. “pebbles”, hierarchical sub-units, or fractal distributions?</span></li> <li><span data-preserver-spaces="true">How did refractory and volatile materials come together during planetesimal growth e.g. did icy and refractory grains grow separately and come together later, or did refractory grains serve as condensation nuclei for volatiles?</span></li> <li><span data-preserver-spaces="true">Did the building blocks of planetesimals all form in the vicinity of each other, or was there significant mixing of material within the protoplanetary disk?</span></li> </ul> <p><span data-preserver-spaces="true">To answer these questions Origo will deliver a lander to a comet where we will characterise the first five meters of the subsurface with a combination of remote-sensing and payloads lowered into a borehole. Our instruments will examine the small scale physico-chemical structure. This approach will allow us to address the following objectives, each of which informs the respective science question: </span></p> <ul> <li><span data-preserver-spaces="true">Reveal the existence of building blocks of a cometary nucleus from the (sub-)micron to metre scale by exploring unmodified material.</span></li> <li><span data-preserver-spaces="true">Determine the physical structure of these building blocks, in particular, the size distribution of components and how refractory and volatile constituents are mixed and/or coupled.</span></li> <li><span data-preserver-spaces="true">Characterise the composition of the building blocks by identifying and quantifying the major ices and refractory components.</span></li> </ul> <p><span data-preserver-spaces="true">Over the past decade, significant theoretical advances have been achieved in working out possible planetesimal formation scenarios.</span></p> <p><span data-preserver-spaces="true">The two leading hypotheses for how planetesimals formed from sub-micron dust and ice particles in the proto-planetary nebula can be classified into two groups:</span></p> <ul> <li><span data-preserver-spaces="true">the hierarchical accretion of dust and ice grains to form planetesimals; and</span></li> <li><span data-preserver-spaces="true">the growth of so-called pebbles, which are then brought to gentle gravitational collapse to form larger bodies by e.g. the streaming instability.</span></li> </ul> <p><span data-preserver-spaces="true">These competing theories only have indirect proof from observations.</span></p> <p><span data-preserver-spaces="true">Direct evidence, i.e. ground truths, about the building blocks of planetesimals remain hidden. Origo would challenge these theories by examining the physico-chemical structure of the most pristine material available in our Solar System. Though the proposal was not retained for step 2 we present our concept for community discussion.</span></p>