Brian Jackson

Interesting seminar today from Prof. Richard O’Connell of Harvard Geophysics. Prof. O’Connell discussed the interplay between the thermal evolution of the Earth’s interior and plate tectonics.

Flow structure of the convection cell in a model of the Earth’s interior. Figure 3 from Crowley & O’Connell (2012) — http://adsabs.harvard.edu/abs/2012GeoJI.188…61C.

New models from Prof. O’Connell and his student John Crowley suggest that the Earth may have undergone different stages of tectonic evolution, with the tectonic plates moving quickly at some times in the Earth’s history but much more slowly at others.

The evolution between different geophysical modes may help explain a longstanding puzzle in Earth science: the amount of heat coming out of the Earth is much greater than expected and has been thought to require much more heating from radioactive isotopes than geochemical analyses allow.

If, instead, O’Connell and Crowley are right, then this large heat flow is really just a symptom of Earth’s geophysical fickleness: sometimes lots of heat comes out, other times less.

We had a great talk at DTM today from Dr. Mark Wieczorek, a geophysicist from the Institut de Physique du Globe de Paris.

Total magnetic field strength at the surface of the Moon as derived from the Lunar Prospector electron reflectometer experiment. From http://en.wikipedia.org/wiki/File:Moon_ER_magnetic_field.jpg.

He talked about the geophysical history of the Moon as inferred from remnant magnetic fields in lunar rocks.

Currently, the Moon does not have a large-scale magnetic field, as the Earth does, but there are smaller-scale magnetic anomalies (see figure at right). These remnant magnetic signatures probably indicate that long ago (about 3-4 billion years ag0) that Moon DID have a magnetic field.

So why did the Moon have a magnetic field long ago and why doesn’t it anymore? One exotic idea Dr. Wieczorek talked about was the idea that large asteroidal or cometary impacts could disrupt the rotation of the Moon’s mantle.

As a result, the mantle and core would rotate at different rates in different directions, which could stir up and heat fluid in the Moon’s interior. This heating could drive internal convection and produce a magnetic field, similar to the way the magnetic field in the Earth is generated.

This image of the brown dwarf binary CFBDSIR 1458+10 was obtained using the Laser Guide Star (LGS) Adaptive Optics system on the Keck II Telescope in Hawaii. Taken from wikipedia.org.

Good DTM seminar today by Prof. Chris Tinney, presenting a long and fascinating list of results from exoplanetary astronomy.

Among many projects his group is working on at the University of New South Wales, they are observing potential planet-hosting stars for the radial velocity signals of gas giant planets in orbits with long periods, comparable to Jupiter’s period of 12 years. Such planets require long observational baselines to find because you must observe the planet-hosting star for at least one orbital period, 12 years, to conclusively say that you’ve found such a planet. As a result, there aren’t too many long-period exoplanets known.

His group is also looking for brown dwarfs, exotic objects that aren’t quite stars but aren’t quite planets either. Brown dwarfs have masses large enough (probably greater than 13 times Jupiter’s mass) that they can fuse deuterium in their interiors — unlike planets — but small enough that they don’t fuse the lighter isotope, hydrogen — unlike stars.

Today we had a great DTM seminar by Rachel Osten from the Space Telescope Science Institute (STScI). She talked about stellar activity and coronal mass ejections (CMEs).

A coronal mass ejection in time-lapse imagery obtained with the LASCO instrument. The Sun (center) is obscured by the coronagraph’s mask.

Osten pointed out that these huge eruptions from stars are important for several reasons. For example, the energy and frequency of CMEs depend on several properties of stars, including their age, rotation rates, and magnetic fields. In general, as stars age, their rotation rates drop, usually reducing the strength of their magnetic fields and the amount of CME activity. And so learning about CMEs can tell us about stellar evolution.

When a star fires off a CME, it typically flares or brightens a bit (see figure at right), and since we can’t see stars other than our Sun up close, we can use the temporary brightening of those stars to study their flare activity. Osten talked about one of her projects to use data from the Hubble Space Telescope to look for flare activity for many stars near the constellation Sagittarius.

That project had a surprising result: many stars that were thought to be older than the Sun actually showed MORE flare activity than they should have. This result might mean these stars actually have previously unknown binary companions that kept the stars spinning quickly and thereby keeping their flare activity up.

Stellar flares may also be important for planetary habitability. For example, a big stellar flare can actually disrupt the atmosphere of an Earth-like planet, perhaps even removing all of the life-protecting ozone.

Comparison of Venus’ and Earth’s topography. For Venus, orange represents the topographic highs, blue the lows. For the Earth, red represents the highs and blue the lows.

Today I saw an interesting talk about Venusian geophysics by Dr. Steve Mackwell.

Dr. Mackwell talked about how mountains, volcanoes, craters, and fractures in Venus’ surface allow scientists to infer the tectonic history of Venus.

For example, unlike the Moon, there aren’t that many craters on the surface of Venus, which should have accumulated with time as more and more asteroids and comets struck the surface. This lack of craters (along with widespread, large-scale volcanic features) suggest that Venus underwent a tremendous volcanic upheaval about a half billion years ago, during which global volcanic eruptions almost completely re-surfaced the planet.

Dr. Mackwell also discussed how his lab experiments in rock mechanics (in which he squeezes and heats rocks to determine their physical properties) have contributed to our understanding of Venus’ history.

For example, the flow of heat from Venus’ interior drives its geophysical activity and plays a key role in determining the strength of the rocks underlying the surface topography. Understanding the relationship between internal heat flow and rock strength, however, requires lab experiments such as Dr. Mackwell’s.