Parts of Canada are quietly short on gravity. The standard story blames an ice sheet that pressed the crust down and vanished thousands of years ago, but satellites suggest that explains less than half of it. The rest comes from something churning far deeper in the mantle.

Parts of Canada are quietly short on gravity. The familiar explanation blames an ice sheet that pressed the crust down and then melted away thousands of years ago. That story is partly true, but satellite measurements suggest it explains less than half of the Hudson Bay gravity low, with much of the rest likely tied to deeper mantle structure and slow motion inside the solid Earth.

The region in question sits around Hudson Bay, a broad area where the local pull reads slightly below the global average. Geophysicists have known about the low since gravity surveys mapped it in the 1960s. The deficit is far too small to feel: you would not weigh noticeably less standing on the shore, and nothing floats. But it is real, it covers a large area, and for decades it had no settled cause.

What GRACE actually measured

The modern picture comes from GRACE, the Gravity Recovery and Climate Experiment, a pair of satellites flown jointly by NASA and the German Aerospace Center from 2002. The two spacecraft followed each other around the same orbit, and the tiny changes in the distance between them tracked variations in the gravity field below. NASA’s overview of the GRACE mission sets out the basic method.

What made GRACE useful for Hudson Bay was not a single snapshot but the trend over time. Some of the gravity field over Canada is changing year on year. Some of it is effectively static on human timescales. Separating those two parts is what made the difference.

The ice-sheet explanation, and its limit

The leading account is glacial isostatic adjustment, often shortened to GIA. The Laurentide Ice Sheet covered most of Canada at the last glacial maximum, in places several kilometres thick. Its weight pushed the crust down into the softer mantle beneath. When the ice melted, roughly ten thousand years ago, the crust began rising back, and it is still rising today, by something under a centimetre a year. Until that rebound finishes, the region holds a mass deficit, and less mass means a weaker local pull.

The trouble is the size of the gap. As early as 1992, a study in Geophysical Research Letters by Thomas James found that the best deglaciation models of the day could reproduce only 15 to 30 per cent of the observed Hudson Bay low.

The rest had to come from somewhere else.

The 2007 result

The clearest separation came from a 2007 paper in Science by Mark Tamisiea, Jerry Mitrovica and James Davis, GRACE Gravity Data Constrain Ancient Ice Geometries and Continental Dynamics over Laurentia. Using GRACE data from April 2002 to April 2006, the authors isolated the part of the gravity field changing in step with the ongoing rebound, and set it apart from the part that does not move on that timescale.

Two findings came out of that separation.

The first concerned the ice itself. The pattern of present-day uplift implied that the Laurentide complex was not one dome but two, sitting to the west and east of Hudson Bay, which matched one of the two competing reconstructions of the ice sheet’s shape.

The second concerned the gravity low. According to the paper, rebound models that match the measured uplift rates account for about 25 to 45 per cent of the static gravity anomaly. On the authors’ reading, most of the low is not a leftover from the ice age at all.

This is one study, and the exact split should be read as a result from this dataset and this set of models rather than a fixed constant. But it points the same way as the earlier work, and the direction of the answer has held.

What “the rest” is, and is not

Here the language needs care. The remainder is usually attributed to mantle convection, the slow movement of rock deep inside the Earth. The popular retelling often describes the mantle as a sea of molten magma. It is not. The mantle is mostly solid rock that deforms and flows over very long timescales, closer to extremely stiff putty than to liquid.

In the Hudson Bay case, the idea is that cold, dense material sinking in the mantle draws the surface down and pulls mass away, lowering the gravity field above it. The Tamisiea paper frames its result in those terms, treating the non-rebound portion as a constraint on the buoyancy of the deep continental root beneath Laurentia, some of the oldest crust on the planet.

What GRACE does not do is photograph that flow. The convection contribution is inferred as the part left over once the rebound signal is removed, so the strength of the inference depends on how well the rebound itself is modelled. That is the honest limit of the result.

What to watch

GRACE itself ended in 2017. Its successor, GRACE Follow-On, launched in 2018 and continues to track the same field, which means the rebound signal over Canada is still being measured rather than frozen at the figure of two decades ago. The longer the record, the more tightly the changing part can be told apart from the static part.

For now the Hudson Bay low sits at the meeting point of two very different timescales: an ice age that ended in the last ten thousand years, and a circulation in the mantle that runs over tens of millions. The satellites are good at separating the two. Putting a precise number on how much each one owes is still open.