巨大撞擊在火星上留下紀錄

Almighty smash left record crater on Mars
巨大撞擊在火星上留下紀錄
http://space.newscientist.com/article/mg19826624.700
Written by David Shiga
Translated by Hsiang-Fu Huang

翻譯：黃相輔

EVERY scar tells a story, yet a huge gash on Mars has long proven very hard to read. Now a peek beneath the planet's surface reveals that the scar is the largest known impact structure in the solar system - gouged out by a collision that reshaped the Red Planet.

每一道傷疤背後都有段故事。然而，火星上一處巨大的窪地長久以來卻難以解讀。如今，對火星地表下的窺探，顯示這道傷痕是太陽系中已知最大的撞擊構造 ─ 由某次足以改變火星形狀的撞擊所挖出。

The surface of Mars's northern hemisphere lies about 6 kilometres lower than that of the southern hemisphere. This has greatly influenced the planet's evolution, says Jeffrey Andrews-Hanna of the Massachusetts Institute of Technology.

火星北半球的地表比南半球還低六公里。麻省理工學院的傑佛瑞‧安得魯-哈納(Jeffrey Andrews-Hanna) 說，這深深地影響了火星的演化。

The northern hemisphere's thinner crust means more magma has been able to push to the surface to fuel volcanism, and the difference in altitude meant that ancient outbursts of liquid water tended to flow from south to north.

北半球較薄的地殼暗示著，較多的熔岩被推至地表補充火山活動。高度的差異則意味著，火星遠古大量湧現的液態水傾向於自南流向北。

It also means that atmospheric pressure on northern surfaces is higher than in the south, encouraging winds there to scour the surface more than in the opposite hemisphere. As a result, it seems likely that more dust has been blown from north to south than the other way.

這也意味著，在北半球表面的大氣壓力比南半球高，使得風勢更容易將地表刮淨。因此，砂塵更易從北半球被搬運往南半球。

A prime theory to explain how this global asymmetry came about is that a huge impact blasted away much of the northern hemisphere's crust. The prime suspect in the hunt for clues to such a collision had been an extensive structure in the northern hemisphere, thought to have formed about 4.4 billion years ago.

解釋此種全球不對稱的主要理論認為，一場巨大的撞擊將大部份的北半球地殼給炸出九霄雲外。北半球一個大規模構造被猜測是此撞擊的主要線索，此構造約形成於四十四億年前。

Strange shape

奇異形狀

This timing fits with other evidence that numerous large projectiles were careening through the inner solar system at this time, such as the Mars-sized planet that walloped the primordial Earth and formed our Moon, yet left no trace on Earth.

此時間點與其他證據不謀而合。它們皆指出，大量巨大的拋射體曾將內太陽系弄得天翻地覆。例如某顆火星大小的行星曾痛擊原始地球，造成了現在的月球，卻沒有在地球上留下一絲痕跡。

However, the Mars impact theory has been undermined by the structure's irregular, kidney shape, since an impact would have punched out a circle or ellipse.

然而，這個火星撞擊理論卻由於大規模構造呈現不規則腎形而受到質疑。因為碰撞應該會產生圓形或橢圓的構造。

To examine the suspected crater in more detail, Andrews-Hanna and colleagues analysed variations in the strength of gravity above the Martian surface using NASA's Mars Reconnaissance Orbiter. This revealed that the structure was actually a near-perfect ellipse, but ended up looking kidney-shaped because lava has since obscured part of it.

為了更加詳細調查這個可疑的隕石坑，安得魯-哈納及其同事利用美國太空總署的火星偵查軌道號衛星 (Mars Reconnaissance Orbiter)，分析在火星表面的重力變化。結果顯示，此構造其實是一個近於完美的橢圓形。

Catastrophic effects

驚天動地

At roughly 8500 by 10,600 kilometres across, it is nearly 15 times the area of the Moon's South Pole-Aitken basin which, at 2500 kilometres in diameter, is the largest undisputed impact scar in the solar system.

這個隕石坑是太陽系中最大的撞擊傷痕。它的長、短軸大約各10600、8500公里，約是月球上直徑約2500公里的南極艾托肯盆地 (South Pole-Aitken basin) 的十五倍。

The Mars crater was probably created by an object as large as 2700 kilometres across - over half of the diameter of Mercury. The effects of such an impact would have been catastrophic, says Andrews-Hanna.

這個火星隕石坑約是由一顆2700公里的天體造成，這個尺寸是水星直徑的一半。安得魯-哈納表示，這場碰撞造成的效應十分具有毀滅性。

"Within the basin you'd have had a magma ocean - it would have been easily several tens of kilometres deep," he says. "Outside the basin you would have had a tremendous amount of ejecta raining back down on the surface."

他說：「在盆地裡會有深達幾十公里的熔岩海；而在盆地外面，你會發現大量飛濺出的彈射物重新落回地表。」

Herbert Frey of NASA's Goddard Space Flight Center in Greenbelt, Maryland, had been sceptical of the giant impact theory because of the structure's irregular shape, but now calls it the "best working hypothesis" to explain the differences between the Red Planet's hemispheres. "They make a compelling case," he says.

太空總署高達德飛行中心 (Goddard Space Flight Center) 的赫伯特‧佛瑞 (Herbert Frey) 曾由於此構造的不規則形狀而質疑巨大撞擊理論。但他現在改口稱它為解釋火星南北半球間差異「最可行的假說」。他表示：「這項研究的確很有說服力」。