How much difference did the supply of “fresh” r-process elements make to the Solar System.?

In one of the answers to this question @MartinKochanski made the interesting point that the abundance of elements heavier than iron (r-process elements) in the solar system is probably due to a fairly recent and close (at the time) neutron star merger -- a relatively rare event on a galactic scale. This means the early solar system was enriched with "fresh" r-process elements, as compared to the "stale" r-process elements that would be typical. For instance the early solar system is conjectured to have been rich in uranium and thorium compared to a "typical" system.

So my question is what differences might this have made to the formation and evolution of the solar system? Would it have had any bearing on the origins of life on Earth?

• +1, but I’m wondering if your last paragraph is perhaps framed in the wrong direction. Are you asking what the evolution of the solar system would have been like (and the prospects for life on Earth) if we hadn’t had such a “high” level of fresh r-process elements? – Chappo Hasn't Forgotten Monica May 23 '19 at 11:38
• @chappo yes, if you like – Steve Linton May 23 '19 at 11:44

The discussion in that answer is slightly misleading. Here's the current picture, taking into account the Nature paper by Bartos & Marka referenced there.

1. Elements heavier than iron up through rubidium (atomic number 37) are produced by the r-process in supernovae.

2. Elements heaver than rubidium are produced by a combination of the s-process in the asymptotic giant branch stage of intermediate-mass stars (i.e., masses of 2-10 solar masses) and the r-process in neutron-star mergers.[1]

3. The argument in the Nature paper is that when the solar system formed, a significant fraction of actinide metals with short half-lives (e.g., isotopes of curium and plutonium, with half lives $$< 100$$ million years) came from one recent, nearby neutron-star merger. (These are the "fresh" r-process elements.)

4. Stable heavy elements produced by neutron-star mergers would have come from the cumulative contributions of hundreds (possibly a few thousand) NS mergers over the history of the Milky Way, mixed over hundreds of millions or billions of years into the Galaxy's interstellar gas (these are the "stale" r-process elements).

5. Unstable heavy elements with long half-lives -- e.g., U-238 (4.5 billion years) and Th-232 (14 billion years) -- would be mostly in the "stale" category, and so would not be significantly affected by the presence of said recent, nearby NS merger. In fact, the Nature paper estimates that their hypothesized nearby NS merger would account for only 0.3% of the solar system's initial Th-232. Even U-235, with a half-life of 700 million years, would probably only be moderately affected by that one NS merger.

In general, then, the presence or absence of metals from a single nearby NS merger would have made very little difference to the formation of the solar system or to the evolution of life on Earth.

[1] See this figure for an approximate breakdown by element -- note that the "dying low-mass stars" label in the figure refers to the AGB stage of what I call intermediate-mass stars, which are "low mass" only in comparison to stars with masses $$> 10$$ times that of the Sun.

• I guess low mass short half-life isotopes like Al-26 came from a nearby collision of regular stars. – PM 2Ring May 23 '19 at 12:56
• @PM2Ring -- I believe Al-26 is supposed to be produced by core-collapse supernovae (which are a lot more common than neutron-star collisions). – Peter Erwin May 23 '19 at 13:19
• Ok, that's what Wikipedia says. OTOH, eso.org/public/australia/news/eso1826 says "observations using ALMA find radioactive isotope aluminium-26 from the remnant CK Vulpeculae", which was produced by a collision of two relatively low mass stars, one being a red giant with a mass in the range of 0.8 to 2.5 $M_\odot$. – PM 2Ring May 23 '19 at 14:18
• That press release also says, "the team have concluded that the production of aluminium-26 by objects similar to CK Vulpeculae is unlikely to be the major source of aluminium-26 in the Milky Way". – Peter Erwin May 23 '19 at 15:36
• Ah, ok. Somehow I missed that bit. – PM 2Ring May 23 '19 at 15:37