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Let's go to Mars

InSight listens to the inside of Mars

The first seismometer set directly on another planet's surface logged 1,300 marsquakes and sized a liquid core. Its heat probe never managed to bury itself.

A white domed shield resting on reddish-brown Martian soil, its curved surface dotted with small circular targets and two grey feet visible at its rim. Above it the lander's tan robotic arm reaches into the top of the frame, with a grey five-fingered grapple hanging beside it from a braided cord.
InSight's domed Wind and Thermal Shield, covering the SEIS seismometer, photographed on the mission's 110th sol. SEIS was the first seismometer a spacecraft placed directly on the surface of another planet. · Image: NASA/JPL-Caltech

InSight landed on Elysium Planitia at 19:52:59 UTC on 26 November 2018, at a site chosen for being flat, soft, sunlit and featureless. It had no wheels.

On 19 December the robotic arm lifted SEIS onto the dirt: three very broadband and three short-period sensors, the first seismometer placed directly on another planet's surface. Its domed wind and thermal shield was lowered over it weeks later. Viking 2's 1976 seismometer was bolted to the lander's deck and mostly recorded it rocking in the wind.

The first plausible marsquake arrived on 6 April 2019, sol 128, too faint to say anything about the interior; SEIS logged more than 1,300 in all. Mars has no plate tectonics, so its quakes have no global driver; most of the located ones cluster on the Cerberus Fossae graben, whose deeper events point to a warm, weak source shaped by volcanism. The largest, on 4 May 2022, was put at magnitude 5 and later revised to about 4.7.

Three simultaneous papers appeared in July 2021. The crust beneath the lander is about 20 kilometres thick in two layers, or 37 in three, and the mantle runs down some 1,560 kilometres. The core is liquid, near 1,830 kilometres in radius, and too light to be pure iron: roughly a fifth must be sulfur, oxygen, carbon and hydrogen. A 2023 analysis found molten silicate on top of it, shrinking the core to nearer 1,650 kilometres. In September 2025 Bi's group reported a solid inner core near 613 kilometres from two deep arrivals: PKKP, crossing the outer core, and PKiKP, reflecting off an inner-core boundary.

The heat-flow experiment failed. HP3, a German-built self-hammering probe called the mole, needed three metres of depth to measure the interior's heat flow, the number constraining Mars's thermal history and how long it stayed warm enough for surface water. Hammering needs friction, and Elysium's soil is a cohesive duricrust that clumped away from the probe. Two years of improvisation followed — pressing with the arm's scoop, backfilling the pit — and on 9 January 2021 the mole struck 500 more times, moved nowhere, and the team stopped. The 40-centimetre probe lies with its top a couple of centimetres down; Mars's heat flow is still unmeasured.

Dust on the solar panels ended the rest: last contact, 15 December 2022.