快猫短视频

THE giant canyons on Mars were carved not by water but by an icy equivalent
of the rivers of ash that helped to destroy Pompeii, claims an Australian
geologist.

If he鈥檚 right, this would mean that Mars has been cold and dry鈥攁nd
probably lifeless鈥攆or the past 3.5 billion years, far longer than most
scientists believe. 鈥淢ars has always been a place of people鈥檚 imaginations,鈥�
says Nick Hoffman of La Trobe University in Melbourne. 鈥淧eople want Mars to be
an abode for Earth-like life forms.鈥�

Hoffman thinks 鈥渄ensity flows鈥�, similar to the so-called pyroclastic flows of
gas, ash and rubble that sometimes come from erupting volcanoes on Earth, eroded
the surface of Mars. Such flows can travel at tremendous speeds and cover vast distances
(快猫短视频, 17 July 1999, p 36).

According to Hoffman, the collapse of unstable Martian terrain, such as
steep-sided craters, released liquid carbon dioxide that had been locked away
underground. The rapid pressure drop vaporised some of this liquid鈥攋ust
like releasing the valve on a CO₂ fire extinguisher鈥攆orming
clouds of gas and supercooled dry ice, as well as water ice, dust and rubble.
These clouds swept downhill, carving immense channels without water.

鈥淚t鈥檚 intriguing and fascinating, but I鈥檓 very sceptical,鈥� says planetary
scientist Jay Melosh of the University of Arizona in Tucson, though he adds that
the idea is worth testing in the lab. One problem, says Melosh, is that as gas
bubbled off, the flows would run out of steam and be unable to carve channels as
long as those on Mars.

But Hoffman disagrees. As the 鈥渃ryoclastic鈥� flow sped along, the supercooled
CO₂ particles would continue to vaporise, fuelling the process, he
says.

Aaron Zent at NASA鈥檚 Ames Research Center in Moffett Field, California,
raises another concern. The channels on Mars end in wide flat expanses, with
traces of a shoreline鈥攋ust as if water had flooded a plain. 鈥淚t鈥檚 the
flattest place in the Universe,鈥� says Zent. 鈥淵ou would have had to have done
that with water.鈥�

Hoffman counters that density currents can create the same features. 鈥淥n
Earth, large density flows create characteristic wide, flat plains, just like
the northern plains on Mars,鈥� he says.

What鈥檚 more, says Hoffman, cryoclastic flows provide a simpler explanation
for the giant channels on Mars鈥檚 surface than water. 鈥淵ou鈥檇 need sufficient
water to create walls 200 kilometres wide and at least 300 metres deep, and
there isn鈥檛 enough space below the surface of Mars to put it all. But 1 litre of
liquid CO₂ gives you 250 litres of gaseous CO₂ to support a
蹿濒辞飞.鈥�