Astronomy Picture of the Day 02-10-04

Explanation: What are those unusual spherules on Mars? The Mars Opportunity rover has now photographed several unusual nodules on Mars that have a nearly spherical shape. Many times these spherules are embedded in larger rock outcroppings but appear grayer. Pictured in the inset is one such spherule embedded in a rock dubbed Stone Mountain, visible to the Opportunity rover now rolling inside a small crater on Meridiani Planum. Opportunity was directed to go right up to Stone Mountain to get a better look. The inset picture spans only 3 centimeters across, revealing the rock to be named more for shape than actual size. Scientists are currently debating the origin of the spherules. One leading hypothesis holds that the beads were once-molten rock that froze in mid-air after an impact or a volcanic eruption. Another hypothesis holds that the spherules are concretions, hard rock that slowly accumulates around a central core. Opportunity will work to solve this mystery and others over the next few days.

As navigation experts pinpoint the rover Opportunity's precise location, the orbiting Mars Global Surveyor has spotted the craft sitting in a small crater.

Comet blamed for 6th century 'nuclear winter'
CARDIFF UNIVERSITY NEWS RELEASE
Posted: February 7, 2004

Scientists at Cardiff University, UK, believe they have discovered the cause of crop failures and summer frosts some 1,500 years ago - a comet colliding with Earth.

The team has been studying evidence from tree rings, which suggests that the Earth underwent a series of very cold summers around 536-540 AD, indicating an effect rather like a nuclear winter.

The scientists in the School of Physics and Astronomy believe this was caused by a comet hitting the earth and exploding in the upper atmosphere. The debris from this giant explosion was such that it enveloped the earth in soot and ash, blocking out the sunlight and causing the very cold weather. vThis effect is known as a plume and is similar to that which was seen when comet Shoemaker-Levy-9 hit Jupiter in 1995.

Historical references from this period - known as the Dark Ages - are sparse, but what records there are, tell of crop failures and summer frosts.

The work was carried out by two Cardiff undergraduate students, Emma Rigby and Mel Symonds, as part of their student project work under the supervision of Dr Derek Ward-Thompson.

Their findings are reported in the February issue of Astronomy and Geophysics, the in-house magazine of the Royal Astronomical Society.

The surprising result of the new work is just how small a comet is needed to cause such dramatic effects. The scientists calculate that a comet not much more than half a kilometre across could cause a global nuclear winter effect. This is significantly smaller than was previously thought.

Dr. Ward-Thompson said: "One of the exciting aspects of this work is that we have re-classified the size of comet that represents a global threat. This work shows that even a comet of only half a kilometre in size could have global consequences. Previously nothing less than a kilometre across was counted as a global threat. If such an event happened again today, then once again a large fraction of the earth's population could face starvation."

The comet impact caused crop failures and wide-spread starvation among the sixth century population. The timing coincides with the Justinian Plague, widely believed to be the first appearance of the Black Death in Europe. It is possible that the plague was so rampant and took hold so quickly because the population was already weakened by starvation.

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