Category Archives: planetary history

Mercury, MESSENGER, planetary history, planetary science

It’s Not Dead, says Jim

Leave a comment

Mercury Continues to Surprise People

I’ve been reading up on the Mercury MESSENGER mission lately. Its findings are fascinating because they seem to refute the old “Mercury’s a dead planet” meme that was popular in planetary science circles for a while. MESSENGER’s measurements of Mercury’s magnetic field, for example, show that there is a dipole field (meaning it has north and south poles), and that it’s still being generated by a dynamo deep inside the planet.

In addition, images of the planet show that it was quite the poppin’ place back in the early days of the solar system. Its geologic history is much more complex than anybody thought, and it includes episodes of volcanic eruptions, particularly around the huge Caloris Basin impact crater site. Here’s what planetary scientist Jim Head had to say about Mercury’s turbulent past:

“By combining Mariner 10 [which first imaged and studied Mercury] and MESSENGER data, the science team was able to reconstruct a comprehensive geologic history of the entire basin interior,” explained James Head of Brown University, the lead author of one of several reports that were published earlier this summer in the journal Science. “The Caloris basin was formed from an impact by an asteroid or comet during the heavy bombardment period in the first billion years of Solar System history. As with the lunar maria, a period of volcanic activity produced lava flows that filled the basin interior. This volcanism produced the comparatively light, red material of the interior plains intermingled with impact crater deposits. Subsidence caused the surface of the Caloris floor to shorten, producing what we call wrinkle-ridges. The large troughs, or graben, then formed as a result of later uplift, and more recent impacts yielded newer craters.”

Mapping a Volcano

What I personally find fascinating are the volcanic vents that MESSENGER has imaged. This figure shows a mosaic of images taken of the largest volcano yet found on Mercury. The sketch map below identifies the major features in the image. The “irregularly-shaped depressions” probably correspond to volcanic vents. The “margin of the dome-like feature” shows the outer limits of lava flows from the vents. Those flows probably covered up the underlying surface of “hummocky plains” that existed earlier. The unlabeled double line outlines bright material associated with the volcano. That material could be pyroclastic deposits ejected during volcanic eruptions at the vents.

The “highly-embayed impact crater” seems to have had lava flow up to its rim; a more distant impact crater is “relatively fresh” and unchanged by any lava. (“Relatively fresh” means that it hasn’t been cratered over, and is younger than the surrounding terrain.) The volcano is located just inside the rim of the Caloris impact basin, labeled as “Caloris basin rim units” on this map.

This map (Credit: Figure 1 from Head et al., Science, 321, 69-72, 2008) and many others are what planetary scientists are using to understand the processes that have shaped Mercury since its formation.

extinctions, galactic history, planetary history

Riding on the Winds of Time

Leave a comment

What Were YOU Doing A Galactic Year Ago?

Think about what you were doing a year ago. For me, it was summertime, and I was probably working on a script for a fulldome video show. That was one EARTH year ago. One Mars year ago (687 Earth days), I was still working on exhibits for the Griffith Observatory and they were about to reopen. One Jupiter year ago (11.9 Earth years ago) I was just finishing graduate school. One Saturn year ago (29.5 years ago) I had just gotten married and was working for a school district. One Uranus year ago… well, my grandparents were just getting married. One Neptune year ago (165 years ago), their grandparents were probably just meeting. And, one Pluto year ago (248 years ago)… well, you get the idea.

So, what was going on one galactic year ago? That’s roughly the time it takes for Earth to make a trip once around the center of the galaxy (from our viewpoint out here in the spiral arms). It’s an incredibly long time — roughly 250 million Earth years ago (give or take a couple of dozen million years).

Fortunately, we weren’t around a galactic year ago. Why do I say that? Because if we were around then, we’d either be puzzling out one of the largest mass extinctions of life in Earth’s history.  There WAS life on our planet at that time, but some 250 million years ago, it was already declining in the oceans, and was about to be nearly wiped out on land. This was an event called “The Great Dying” and it began around a galactic year ago. The formal name of the event is the Permian-Triassic Extinction, and to give you an idea of how extensive it was, about 9 in 10 marine species died out, and 7 in 10 land species suffered the same fate. This wasn’t the first or the last time that life has been threatened with extinction during our planet’s history.

255.jpg (119462 bytes)The best explanation for this mass extinction, backed up by data from studies of marine life forms and rocks that date back to that time, is that ocean life had been struggling along due to some toxic upwelling from ocean depths. It was gasping for breath, in essence.

On the land, extensive volcanic activity was pumping grunge into the air and resurfacing the planet, which affected land life. And, at that time, the continents didn’t look anything like they do today, as you can see in Chris Scotese’s rendering of Pangaea

Anyway, to make matters worse, along came a huge impactor, perhaps 6 to 12 kilometers across. When it crashed into Earth’s surface, it did severe damage, and may have hastened the Great Dying that was already in progress. In any case, it was a major mass extinction event. And it happened about a galactic year ago.

Life did thrive again, as we see in the fossil records from the next few eras and epochs as the new galactic year wore on. It makes me wonder what the next one will bring. A lot can happen in a year!