從科羅拉多州的岩石找到「混沌太陽系」理論的證據

原始網址：www.sciencedaily.com/releases/2017/02/170222131512.htm

從科羅拉多州的岩石找到「混沌太陽系」理論的證據

科羅拉多州有一組年代為9000萬年，貌似千層派的沉積岩。威斯康辛大學麥迪遜分校和西北大學的科學家團隊深入剖析之後，發現的證據可以支持一則關於太陽系行星的繞日軌道如何運行的理論。

刊登於2017年2月23日的《自然》(Nature)，此發現的重要之處在於對科學家所稱的「渾沌太陽系」(chaotic solar system)理論首次提出了鐵證。這項理論於1989年提出，說明太陽系當前狀態的些微改變會在數百萬年後顯現出來，而對地球氣候造成巨大影響――這些影響會反映在地球歷史的岩石紀錄之中。

此發現不只有望讓我們更加了解太陽系運作的機制，還可以讓我們更精準地測量地質年代的特定時刻。此外，這也可以使我們進一步認識地球軌道和氣候變遷在地質時間尺度下彼此之間有何關聯。

在數千萬年前恐龍仍統治地球之時，北美淺海形成了一組由石灰岩和頁岩交互堆疊的岩層。由威斯康辛大學麥迪遜分校的地質科學教授Stephen Meyers，和西北大學行星與地球科學教授Brad Sageman領導的團隊，利用在這組岩石中找到的證據，發現了8700萬年前火星和地球之間發生的「共振變換」(resonance transition)所產生的訊號。共振變換是渾沌理論中「蝴蝶效應」產生的後果，其概念係指非線性系統中初始狀態的微小改變，經過一段時間後會產生巨大的影響。

就太陽系來說，當一顆行星在其環繞太陽的軌道跟另一軌道上的行星以較近距離擦肩而過時，兩顆繞日天體之間便會產生拉力，重複作用之後就會導致共振變換。行星軌道中這些微小而規律的擾動最終可以讓行星相對於太陽的位置，以及自轉軸的指向產生很大的變動。結果會造成行星特定面積接收到的太陽輻射量跟著變化，又行星受到的太陽輻射量多寡以及照射位置是氣候運作的關鍵要素。

「天文現象的循環能對氣候產生莫大影響。」Meyers特別以地球冰河期擁有的特定步調當作範例來解釋。現在我們已經可以將冰河期循環跟地球軌道形狀以及地軸傾斜方式的變化之間做出良好的匹配。「天文理論讓我們可以精確估算過去的氣候事件，或許也能使我們模擬未來的氣候。」

Meyers、Sageman和威斯康辛大學麥迪遜分校的研究生Chao Ma(本研究為其論文內容的一部份)為了找出共振變換的訊號，探討了科羅拉多州Niobrara岩層中的地質紀錄。此岩層為沉積物於數千年萬間逐層累積在白堊紀西部內陸海道(Cretaceous Western Interior Seaway)的寬廣淺海中而形成。這座淺海從現今的北極海延伸至墨西哥灣，將北美洲分成了東西兩半部。

Meyers指出：「由於黏土和碳酸鈣相對含量的改變，使得Niobrara層中的層狀岩石呈現出明確的交替變化。」他是天文年代學的專家，這門學問利用天文事件的循環來測量地質年代。「黏土(以頁岩的形式沉積)的來源是地表的風化作用，並經由河流運輸至海道。碳酸鈣(石灰岩)的來源則是生活在水層當中，體型多半相當微小的生物的殼體。」

Meyers解釋雖然氣候變遷和沉積作用之間的關係可以相當複雜，但基本概念十分單純：「氣候變遷會影響黏土相對於碳酸鹽輸入至海道的量，從而記錄了在此作用中的天文訊息。舉例來說，氣候處於相當暖濕的狀態時，河流會將大量黏土輸入至海道當中，而形成黏土含量豐富的岩石，也就是頁岩；另一種情況則是氣候處於乾冷的狀態時，海道收到的黏土會較少，因此形成碳酸鈣含量豐富的岩石，也就是石灰岩。」

這項新研究由美國國家科基金會贊助，主要建立於先前對Niobrara層的地層紀錄進行的縝密調查，以及天文年代學的重大研究成果。後者是西北大學Sageman的前研究生Robert Locklair撰寫論文期間所進行的研究。

由Ma、Meyers和Sageman發現的火星―地球間共振變換的年代也經由放射性定年得到佐證。放射性定年為一種定年方法，利用岩石中元素放射性衰變的速率，來定出岩石的絕對年代。近年來，威斯康辛大學麥迪遜分校的地質科學教授Bradley Singer以及其他科學家設計出準度及精度更加提高的放射性同位素定年法，它們被用在共振變換的定年而得到許多收穫。

自日心說――地球和行星皆繞著太陽轉動的學說――於16世紀問世以來，行星環繞太陽的運動方式就一直是科學家深感興趣的議題。從18世紀開始，對於行星環繞太陽的主流看法是將其視為一座時鐘，具有準週期(quasiperiodic)性且高度可預測的軌道。然而1988年，電腦對外行星的計算結果指出冥王星的軌道具有混沌性質。1989年，現任職於巴黎天文台的天文學家Jacques Laskar提出了混沌太陽系的概念。

Meyers表示自Laskar提出混沌太陽系理論之後，科學家便一直在熱切尋找可以支持這個想法的鐵證。

「其他研究也曾根據地質紀錄提出混沌性質的存在。」Meyers說。「但這是首度有人提出無庸置疑的證據。依靠高品質的放射性定年數據以及保存在岩石當中的強烈天文學記號才得以達成。」

From rocks in Colorado, evidence of a 'chaotic solar system'

Plumbing a 90 million-year-old layer cake of sedimentary rock in Colorado, a team of scientists from the University of Wisconsin-Madison and Northwestern University has found evidence confirming a critical of how the planets in our solar system behave in their orbits around the sun.

The finding, published Feb. 23, 2017 in the journal Nature, is important because it provides the first hard proof for what scientists call the "chaotic solar system," a theory proposed in 1989 to account for small variations in the present conditions of the solar system. The variations, playing out over many millions of years, produce big changes in our planet's climate -- changes that can be reflected in the rocks that record Earth's history.

The discovery promises not only a better understanding of the mechanics of the solar system, but also a more precise measuring stick for geologic time. Moreover, it offers a better understanding of the link between orbital variations and climate change over geologic time scales.

Using evidence from alternating layers of limestone and shale laid down over millions of years in a shallow North American seaway at the time dinosaurs held sway on Earth, the team led by UW-Madison Professor of Geoscience Stephen Meyers and Northwestern University Professor of Earth and Planetary Sciences Brad Sageman discovered the 87 million-year-old signature of a "resonance transition" between Mars and Earth. A resonance transition is the consequence of the "butterfly effect" in chaos theory. It plays on the idea that small changes in the initial conditions of a nonlinear system can have large effects over time.

In the context of the solar system, the phenomenon occurs when two orbiting bodies periodically tug at one another, as occurs when a planet in its track around the sun passes in relative proximity to another planet in its own orbit. These small but regular ticks in a planet's orbit can exert big changes on the location and orientation of a planet on its axis relative to the sun and, accordingly, change the amount of solar radiation a planet receives over a given area. Where and how much solar radiation a planet gets is a key driver of climate.

"The impact of astronomical cycles on climate can be quite large," explains Meyers, noting as an example the pacing of Earth's ice ages, which have been reliably matched to periodic changes in the shape of Earth's orbit, and the tilt of our planet on its axis. "Astronomical theory permits a very detailed evaluation of past climate events that may provide an analog for future climate."

To find the signature of a resonance transition, Meyers, Sageman and UW-Madison graduate student Chao Ma, whose dissertation work this comprises, looked to the geologic record in what is known as the Niobrara Formation in Colorado. The formation was laid down layer by layer over tens of millions of years as sediment was deposited on the bottom of a vast seaway known as the Cretaceous Western Interior Seaway. The shallow ocean stretched from what is now the Arctic Ocean to the Gulf of Mexico, separating the eastern and western portions of North America.

"The Niobrara Formation exhibits pronounced rhythmic rock layering due to changes in the relative abundance of clay and calcium carbonate," notes Meyers, an authority on astrochronology, which utilizes astronomical cycles to measure geologic time. "The source of the clay (laid down as shale) is from weathering of the land surface and the influx of clay to the seaway via rivers. The source of the calcium carbonate (limestone) is the shells of organisms, mostly microscopic, that lived in the water column."

Meyers explains that while the link between climate change and sedimentation can be complex, the basic idea is simple: "Climate change influences the relative delivery of clay versus calcium carbonate, recording the astronomical signal in the process. For example, imagine a very warm and wet climate state that pumps clay into the seaway via rivers, producing a clay-rich rock or shale, alternating with a drier and cooler climate state which pumps less clay into the seaway and produces a calcium carbonate-rich rock or limestone."

The new study was supported by grants from the National Science Foundation. It builds on a meticulous stratigraphic record and important astrochronologic studies of the Niobrara Formation, the latter conducted in the dissertation work of Robert Locklair, a former student of Sageman's at Northwestern.

Dating of the Mars-Earth resonance transition found by Ma, Meyers and Sageman was confirmed by radioisotopic dating, a method for dating the absolute ages of rocks using known rates of radioactive decay of elements in the rocks. In recent years, major advances in the accuracy and precision of radioisotopic dating, devised by UW-Madison geoscience Professor Bradley Singer and others, have been introduced and contribute to the dating of the resonance transition.

The motions of the planets around the sun has been a subject of deep scientific interest since the advent of the heliocentric theory -- the idea that Earth and planets revolve around the sun -- in the 16th century. From the 18th century, the dominant view of the solar system was that the planets orbited the sun like clockwork, having quasiperiodic and highly predictable orbits. In 1988, however, numerical calculations of the outer planets showed Pluto's orbit to be "chaotic" and the idea of a chaotic solar system was proposed in 1989 by astronomer Jacques Laskar, now at the Paris Observatory.

Following Laskar's proposal of a chaotic solar system, scientists have been looking in earnest for definitive evidence that would support the idea, says Meyers.

"Other studies have suggested the presence of chaos based on geologic data," says Meyers. "But this is the first unambiguous evidence, made possible by the availability of high-quality, radioisotopic dates and the strong astronomical signal preserved in the rocks."

原始論文：Chao Ma, Stephen R. Meyers, Bradley B. Sageman. Theory of chaotic orbital variations confirmed by Cretaceous geological evidence. Nature, 2017; 542 (7642): 468 DOI: 10.1038/nature21402

引用自：University of Wisconsin-Madison. "From rocks in Colorado, evidence of a 'chaotic solar system'." ScienceDaily. ScienceDaily, 22 February 2017.