Planetary Pulse

Instruments picked up a regular pulse 37 kilometres beneath Kepler-942c: once every 72.0 seconds, exact.

Estimated from the crustal structure, a pulse at that depth should be low-frequency, non-periodic, with a single-event energy peak under 10 joules. The measured value was 64 joules. The first record ran to 435 events with a break rate of zero, and more than a hundred of those came from before orbital capture — background left while the probe was still in its deceleration leg.

The waveform was wrong too. The rising edge was abrupt, the flat top did not tremble, and the whole line was as clean as a square wave out of a laboratory signal generator. A natural lower-crust pulse looks like a length of hemp rope that has been trodden on.

Janus ran every possible non-biological source through comparison: crustal movement, magmatic activity, geomagnetic variation, artificial sources. Every one of them was struck off. The grey box did not go away. Two more came up beside it instead:

Signal amplitude contraction: amplitude increments 0.7% per record.
Signal period contraction: period reduced from 72.0 seconds to 71.96 seconds.

Every reading after that came in shorter than the one before. 71.91, 71.87, 71.83, 71.82, 71.81, 71.74, and then 71.70 — the first time the integer place of 72 seconds was broken through. The span of it was under 3 days.

Asked why, Janus answered that it did not know. This master AI, which ran the heading, the reactor, the air cycle and 100,000 cryo-bays, put out tens of thousands of decision recommendations a day, every one with a probability distribution attached; it would say risk insufficient to estimate, it would say insufficient data, it would hand over a survival figure to 6 decimal places. In 4 full years under way, it had never once said that it did not know.

The signal climbed the ship's hierarchy as well. It began as a grey box awaiting review, then the exclusion list was deleted one item at a time until only the signal was left, and then it was stacked into a single timeline — alongside the concentration of unidentified organic molecules in the air samples, alongside the breathing rhythms of a few people, alongside the projected survival rate for the whole ship. Once the 4 curves were laid over one another, they pointed the same way.

1/10 of the period is 7.2 seconds. That number turned up somewhere else aboard later.