The Planet That Keeps Shrinking plus the Weekend Wrap

Today's episode — The Weekend Wrap, Saturday 12 September 2026: OUR LEAD: A SMALLER MERCURY Mercury has been contracting for four and a half billion years as its huge iron core cools, and it writes the record on its own surface — in long lobe-fronted cliffs called lobate scarps, where one slab of crust has ridden up over another. Add up all that shortening and you get the total contraction. The trouble was that the surface record always gave a smaller number than thermal models predicted. New work led by Gaku Nishiyama at the German Aerospace Center in Berlin, published in Geophysical Research Letters and released by the American Geophysical Union on 10 September, explains the gap — and the explanation is beautifully simple. Impact debris buries the cliffs. Using MESSENGER imagery reworked with stereophotogrammetry, the team showed that the rougher the terrain, the fewer shortening structures you can find per unit area. Since faults don't know what the surface above them looks like, that correlation isn't geology — it's a detection limit. Correct for it and Mercury's total loss of diameter rises by 10–30%, from a range topping out near 16 km to as much as 23 km. That points to a larger metal core, fewer light elements in it, or a hotter start — and it closes the long-standing mismatch with the physics. BepiColombo is about ten weeks from gravity capture at Mercury, carrying a far better laser altimeter and a much less eccentric orbit. If Nishiyama is right, it should find the missing small scarps in exactly the rough ground where today's maps look suspiciously empty. Some of that data will come home through ESA's New Norcia station in Western Australia, run in partnership with CSIRO. THE WEEK THAT WAS ·         Monday — Isar Aerospace's Spectrum reached orbit from Andøya Spaceport in Norway on 5 September, the first vehicle ever to do it from Western European soil, carrying five university cubesats. ·         Tuesday — giant-impact simulations including temperature-dependent rock strength can produce an intact Moon in about five hours rather than a slowly accreting debris disc. A sensitivity result, not a new origin story. ·         Wednesday — Hubble and Webb together found 27 previously unknown trans-Neptunian objects down to about 5 km, and the small ones keep the colours of their birth population rather than looking like collision fragments. ·         Thursday — 109 localised fast radio bursts were used to measure how far galactic feedback has smoothed