Things that go bump in the outer solar system

Title: Impact Rates in the Outer Solar System

Authors: David Nesvorný, Luke Dones, Mario De Prá, Maria Womack, Kevin J. Zahnle

First Author’s Institution: Department of Space Studies, Southwest Research Institute

Status: In Prep in the Planetary Science Journal [open access]

Sometimes astronomy papers detail glamorous, revolutionary results. Sometimes they produce gorgeous images using the latest-and-greatest telescopes. And sometimes they meticulously combine data and models to make incremental improvements to estimates of numbers last updated two decades ago. 

Today’s article — a preprint led by astronomers at the Southwest Research Institute — falls squarely into the latter category. Using the most up-to-date models for populations of comets and other small bodies in the outer solar system, the authors make an updated estimate at how often big-ish rocks collide with our four friendly gas giants and their moons.

It’s a fairly straightforward paper, but it offers a great introduction to both the dynamics of the solar system and the dynamic between observations and simulations in making advances in astronomy. 

Beyond that, the finer points of their conclusions have exciting implications for our understanding of some of the most intriguing bodies in the search for life on beyond Earth. But before we can get into that, we need to talk about some of the different types of rocks one can find in space.

The Outer Solar System: A Guided Tour

Out beyond Jupiter, Saturn, Uranus, and Neptune, more than 30 times further from the Sun than us, lies a field of smaller bodies (of which Pluto is, infamously, just one of many). These icy rocks make up the population of trans-Neptunian objects, or TNOs. 

Figure 1: The locations of known comets and other bodies in the outer solar system, with scales in either dimension showing distance, in astronomical units (au), from the Sun. Jupiter-family comets are shown in blue, centaurs in magenta, and trans-Neptunian objects in yellow. Light grey points show all other types of small bodies beyond the asteroid belt, which are largely out of the plane of the planets’ orbits. Dark green rings show the approximate orbits of the gas giants (Jupiter, closest to the sun, is almost entirely obscured by its family of comets), and the black ring shows 2.5 au from the Sun. Our observational abilities are best for objects closer to us, meaning observations of the population of objects out beyond the classical Kuiper belt are limited. Apparent “clumps” of objects are largely the result of staring at one patch of sky from the Earth to find things in that small, specific region. Credit: Mark Dodici; data from Solar System Database (Downloaded August 1, 2023)

Under the Nice model of the solar system’s early history, many of these TNOs were kicked out to their present locations by a “late instability,” which likely saw Neptune (and the rest of the gas giants) move outward from closer-in birthplaces to where they orbit today. Nowadays, these kicked bodies have orbits that come close to that of Neptune at their perihelia (their closest point to the Sun in every orbit). This population of objects, which tend to have highly-eccentric and/or highly-inclined orbits, make up the scattered disk of TNOs. 

Within this outer reservoir of bodies, slight perturbations over long times can cause eccentricities to change and perihelia to shift. The TNOs with perihelia closest to Neptune will tend to be disrupted over time — often being flung inwards toward the rest of the solar system. This disruption is common enough that the planet-inhabited region of the solar system is the constant recipient of a slow (but steady!) stream of encroaching TNOs.

While their perihelia are between the orbits of Jupiter and Neptune, these objects become known as centaurs.  But these freshly-disrupted bodies are incredibly transient interlopers. Since their orbits cross that of at least one of the gas giants, they tend to undergo more major orbital changes through close encounters, on (astronomically-speaking) quick timescales.

These changes lead, in general, to one of three outcomes for our centaurs: reclassification (most likely drifting inward to become a Jupiter-Family comet), collision (with the Sun, or, in the interests of this paper, something else!), or ejection from the system altogether.

From Centaur to Collider

There’s one class of small body relevant to today’s paper that we haven’t touched on yet: the ecliptic comets. 

This family technically includes much of the Jupiter family; specifically, it encompasses comets that orbit the Sun with low inclinations (i.e. with orbits mostly close to the Earth’s orbital plane, known as the ecliptic). Most ecliptic comets were first low-inclination centaurs, whose orbits drifted inwards until they were more strongly-associated with Jupiter.

It’s been a few paragraphs, but you might recall that today’s paper is interested in the impact rates between smaller bodies and the gas giants and their moons. When two things have similar inclinations — imagine their orbits as dinner plates, stacked together — they have more chances to bump into each other than when they’re significantly misaligned — imagine one dinner plate standing on edge atop another. 

Because of this, ecliptic comets (and the low-inclination centaurs they come from) are the most likely suspects for collisions in the outer solar system. Understanding the rate of these collisions, then, requires a good understanding of the populations of ecliptic comets and centaurs. 

Today’s Paper

Previous studies of impact rates (including their main point of comparison, dating back 20 years) have been hindered by uncertainties in our models of these populations. In the last few years, however, new observations by the Outer Solar System Origins Survey (OSSOS) have allowed for more accurate calibration of models, giving a better picture of the current state of play of ecliptics and centaurs, as well as the population of TNOs from which those colliders came. The time is ripe for reevaluation of these previous rate estimates; enter today’s paper.

To calculate how often small bodies crash into planets, the authors simulate the orbits of a whole bunch of them, then calculate the percent that crash into a planet, then multiply that percent by the number of progenitors they expect there to have been in the solar system (using that OSSOS-updated understanding of TNOs!).

In the end, they estimate that Jupiter should be struck by bodies with diameters bigger than 1 kilometer every 230 years or so (consistent with previous work, which is always a good sign). They do note that there’s still uncertainty, as the calculation requires some extrapolation from the collision rates of bigger bodies based on a size distribution — if there’s “steeper” distribution (more small bodies for every big one), the rate could be as high as 1 every 120 years. 

When the authors consider that comets will be disrupted if they spend too much time too close to the Sun, though, the impact rate on Jupiter drops to 1 every 315 years. (Impact rates on all other bodies scale down, too!) From this, they draw an interesting conclusion related to icy moons — satellites which are some of the most interesting bodies in the search for life elsewhere in the solar system. 

Image of one half of a moon. Its surface looks mostly white, but shows streaky brown-ish features. There are a few small pockmarks (craters) on the surface, but these are vastly outnumbered by the stripe features.
Figure 2: Europa, an icy moon of Jupiter, is surprisingly crater-free. The impact rates given by this paper might help explain why. Credit: NASA/JPL-Caltech

These icy moons, much like geologically-active planets, “refresh” their surfaces over long timescales, wiping clean any craters that might have formed from impacts; the time it takes for this refresh to happen can give us details on the moons’ internal processes. If the moons are impacted less frequently, then it would take longer to accumulate the number of craters we see today. If it takes longer to accumulate craters, then the refresh rate must be slower than we expected — their surfaces must be older to accumulate the same number of craters.

Based on their finding that inner moons are less-frequently hit than previously thought, the authors draw exactly this conclusion about Europa, an icy moon of Jupiter suspected to harbor an ocean under its outer shell. They posit that its surface is somewhere between 45–105 million years old — an ever-so-slight upward shift compared to previous best age estimates of 40–90 million years.

That’s a big uncertainty! And the calculation to find that range, like many others in today’s seemingly-innocuous, number-updating paper, is finicky. Especially considering its overlap with the previous range, this is more interesting as a proof-of-concept for the way models like this can impact our understanding of seemingly-tangential topics, like the internal processes of icy moons. 

But as observations of the small bodies in the outer solar system continue to improve, modeling efforts like this one can help us understand exactly what’s going on out there — letting us know what to look for in future observations, and beginning the cycle anew.

Astrobite edited by Benjamin Cassese

Featured image credit: European Space Agency

About Mark Dodici

Mark is a Ph.D. student in astronomy and astrophysics at the University of Toronto. His space-based interests include planetary systems, from their births to their varied deaths, as well as the dynamics of just about anything else. His Earth-based interests include coffee, photography, and a little bit of singing now and again. You can follow him on X (@MarkDodici) or on BlueSky (@dodici.bsky.social)

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