As a bench scientist, normally when I say “data will be forthcoming,” there’s a certain delay implied. Data will be forthcoming after a certain delta-t. But NASA apparently has different ideas. Well after its planned EOL, the Dawn orbiter was still in good health, so at the end of July, NASA extended its mission at the dwarf planet Ceres. Without missing a beat, Dawn just batched us a ton of data about Ceres’ interior structure.
In addition to a slew of photos, Dawn also provides information about gravity fields it moves through, by way of NASA’s Deep Space Network. The DSN monitors the Doppler shift from the spacecraft, and it can pick up variations in speed as small as 0.1mm/sec. This extreme precision lets it use the gravitational inconsistencies of Ceres’ interior to tell us about what the planet must be made of inside.
It appears that, during a heating phase early in the history of Ceres, water and other light materials partially separated from rock and floated up to the outer layer of Ceres. This process is called “differentiation,” and it’s exactly like how our own planet’s core separated from the mantle and crust. But Ceres’ layers aren’t so distinct. The messy divisions between rock, ice, and gravely debris suggest that the interior structure of the iceball dwarf planet is changed by its rotation, since it doesn’t experience much tidal force.
Tiny perturbations in the gravitational environment around Ceres can tell us about how its slushy innards move around. Dawn can pick up these perturbations, using its participation with NASA’s DSN. Anomalies in the field around Ceres are how we know that its interior is both layered and relatively messy.
Scientists also discovered that higher-elevation areas on Ceres displace mass in the interior, much like how a boat floats on water: The volume of water displaced depends on the mass of the boat. They conclude that Ceres’ soft, icy mantle can be pushed aside by the ponderous mass of mountains, as if the high-elevation areas were floating on the material below.
The icy upper layer also supports the idea that Ceres’ strangely crater-free surface might be a result of its own structural properties. Frost heave could have obliterated the biggest craters on Ceres, and if most of the surface is ice, there’s a lot of frost to heave. Further readings will help to confirm or correct this hypothesis, and despite Dawn’s dwindling fuel reserves, it will be in orbit collecting data until at least 2019.
A lot of NASA’s equipment seems to be pretty busted and janky these days — Curiosity keeps going into safety mode, Dawn’s running out of fuel, Kepler’s reaction wheels are jammed, and Hubble is straight-up on its last legs. But they’re making it a point to do what they can with what they have — how can they do otherwise? Dawn still has a fair bit of its hydrazine as well as the use of its ion engine, which is why NASA extended the mission. Make hay while the sun shines.
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