[meteorite-list] Mercury Shrinking More Than Thought

[Ron Baalke]

http://www.nature.com/news/mercury-shrinking-more-than-thought-1.14331

Mercury shrinking more than thought
Latest views of the planet settle a decades-old argument about its evolution.
Alexandra Witze
Nature
09 December 2013

The planet closest to the Sun has shrivelled much more over its lifetime 
than previously thought, scientists have found.

Studies of Mercury show that it has shrunk by about 11 kilometres across 
since the Solar System's fiery birth 4.5 billion years ago. As the planet 
cooled and contracted, it became scarred with long curved ridges similar 
to the wrinkles on a rotting apple.

A new census of these ridges, called lobate scarps, has found more of 
them, with steeper faces, than ever before. The discovery suggests that 
Mercury shrank by far more than the previous estimate of 2-3 kilometres, 
says Paul Byrne, a planetary scientist at the Carnegie Institution for 
Science in Washington DC. He presented the results today at a meeting 
of the American Geophysical Union in San Francisco, California.

The finding helps explain how Mercury's huge metallic core cooled off 
over time. It may also finally reconcile theoretical scientists, who had 
predicted a lot of shrinkage, with observers who had not found evidence 
of that - until now. "We are resolving a four-decades-old conflict here," 
Byrne told the meeting.

Planetary scientists have been arguing over Mercury's lobate scarps ever 
since the Mariner 10 spacecraft flew past the planet three times in 1974-75. 
Researchers can use measurements of the length and height of the scarps 
to calculate how much planetary shrinkage they represent.

That shrinkage is a product of Mercury's odd composition - "like a core 
floating through space with a thin outer blanket," says Byrne. Most of 
the planet is made of that large core, and so it would have cooled rapidly 
as heat rushed toward its surface. Modelling studies have long suggested 
that the planet should have shrunk by 10-20 kilometres over its lifetime, 
compared to the 2-3 kilometres estimated from Mariner 10 data1.

The latest estimates come from NASA's MESSENGER probe, which photographs 
and measures Mercury's topography. Last year, Italian scientists used 
MESSENGER data covering one-fifth of the planet to show that its shrinkage 
was probably greater than the Mariner 10 estimates2.

The latest work, covering the entire planet, revealed many lobate scarps 
with sharp vertical relief, Byrne said. It also uncovered details on another 
kind of surface feature that may be related to shrinkage. These "wrinkle 
ridges" are less pronounced than the lobate scarps but may also have formed 
during contraction. Combined, the data on the lobate scarps and the wrinkle 
ridges suggest that Mercury's diameter has shrunk by 11.4 kilometres, 
Byrne said. Even leaving out the wrinkle ridges gives 10.2 kilometres 
of contraction.

Those numbers are plausible to at least one planetary scientist who studied 
Mercury's shrinkage using Mariner 10 data in the 1970s. Jay Melosh, a 
planetary geologist at Purdue University in West Lafayette, Indiana, suspects 
that even more lobate scarps may be lurking out there. "Many of these 
things may still be hiding," he says. "As far as I'm concerned, this may 
be an underestimate of the amount of shrinkage."

    Nature
    doi:10.1038/nature.2013.14331

References

    Dombard, A. J. & Hauck, S. A. Icarus 198, 274–276 (2008).

    Di Achille, G. et al. Icarus 221, 456–460 (2012).