The Martian “Doldrums”
By Judith E Braffman-Miller
Like other stars, our Sun was born surrounded by a swirling, whirling accretion disk composed of gas and dust. These protoplanetary accretion disks–that form gaseous rings around baby stars–contain the ingredients necessary to build up a family of planets, moons, and smaller objects that will populate an emerging solar system. Our primordial Solar System was a violent place, often referred to as a “cosmic shooting gallery”, where objects crashed into each other, blasting one another into fragments–but sometimes bumping into one another gently enough to merge and create ever larger and larger bodies, many of which eventually grew to become major planets. From its infancy, our Solar System has been rocked by violent collisions between astronomical objects that have shaped the planets and altered the course of their birth and evolution. In April 2017, planetary scientists at the Southwest Research Institute (SwRI) in Boulder, Colorado, and the University of Arizona at Tucson, announced that they had discovered a 400-million-year lull in large impacts that blasted into Mars in its early formative years–billions of years ago.
This finding is published in the April 25, 2017 issue of Nature Geoscience in a paper titled A post-accretionary lull in large impacts on early Mars. SwRI’s Dr. Bill Bottke, who serves as principal investigator of the Institute for the Science of Exploration Targets (ISET) within NASA’s Solar System Exploration Research Virtual Institute (SSERVI), is the lead author of the paper. Dr. Jeff Andrews-Hanna, from the Lunar and Planetary Laboratory at the University of Arizona, is the paper’s coauthor.
“The new results reveal that Mars’ impact history closely parallels the bombardment histories we’ve inferred for the Moon, the asteroid belt, and the planet Mercury. We refer to the period for the later impacts as the ‘Late Heavy Bombardment’. The new results add credence to this somewhat controversial theory. However, the lull itself is an important period in the evolution of Mars and other planets. We like to refer to this lull as the ‘doldrums,'” Dr. Bottke noted in an April 25, 2017 SwRI Press Release.
The Late Heavy Bombardment
The Late Heavy Bombardment (LHB), sometimes alternatively termed the lunar cataclysm, is a catastrophic episode in our Solar System’s past that probably happened about 4.1 to 3.8 billion years ago. Our Solar System is approximately 4.56 billion years old. The LHB is thought to have occurred at a time corresponding to the very ancient Neohadean and Eoarchean eras on Earth. During this primordial epoch, an invading horde of rampaging asteroids are theorized to have crashed into the early inner terrestrial planets, wreaking havoc. The LHB happened after the Earth and other rocky planets had formed and accreted most their mass. Nevertheless, this violent era in our Solar System’s past occurred very early in Earth’s history.
Clues that the LHB really did occur are derived from samples taken from Earth’s Moon that were carried back by the Apollo astronauts. Isotopic dating of these lunar rock samples suggests that most impact melts happened in a rather narrow interval of time.
Several proposals seek to explain the apparent dramatic increase in the shower of crashing impactors (asteroids and comets) that badly battered the warm and well-lit inner regions of our Solar System–however, no consensus yet exists. One model, termed the Nice Model, proposes that the quartet of gaseous giant planets in our Solar System’s outer regions–Jupiter, Saturn, Uranus, and Neptune–experienced orbital migration and, during their travels, scattered objects in the Asteroid and/or Kuiper Belts into eccentric orbits–and into a collision course with the unfortunate inner rocky planets. The Nice Model is currently popular among planetary scientists.
Other researchers, however, argue that the lunar sample data doesn’t require a cataclysmic cratering episode about 3.9 billion years ago. This alternative proposal is based on the theory that the clustering of impact-melt ages–at about this time–is an artifact of the samples themselves. This is because they were retrieved from the same single large impact basin. They further point out that the impact cratering rate could be significantly different for the inner and outer regions of our Solar System.
Indeed, most of the evidence indicating a lunar cataclysm–that resulted from the ancient invasion of a blasting horde of showering impactors–is derived from the radiometric ages of impact melts collected by the Apollo astronauts. Most of these impact melts are generally thought to have resulted during violent collisions with asteroids or comets tens of kilometers in diameter. The crashing wanderers excavated impact craters hundreds of kilometers in diameter. The Apollo 15, 16, and 17 landing sites were originally chosen because of the geographical proximity to the Imbrium, Nectaris, and Serenitatis basins, respectively.
The obvious cluster of ages of these lunar samples resulted in the theory that they provide a tattle-tale record of an ancient powerful, intense, and violent bombardment of Earth’s Moon. This lunar cataclysm would have involved a dramatic increase in the rate of crashing impactors blasting into Earth’s Moon about 3.9 billion years ago. If these impact melts were collected from the trio of basins, then not only were these three large impact basins excavated within a short interval of time, but numerous others also may have been, based on stratigraphic grounds. At the time, the theory was considered controversial.
However, with the accumulation of more and more data, derived mostly from lunar meteorites, this theory–while still the subject of considerable debate–has gained in its acceptance among scientists. Because the lunar meteorites are thought to provide random samples of the Moon’s surface, at least some of them should have originated from regions far from the Apollo landing sites. Also, many of the feldspathic lunar meteorites likely originated from the far side of the Moon, and impact melts within these have recently been dated. The ages derived have proved to be consistent with the lunar cataclysm hypothesis, and none of their ages were dated older than approximately 3.9 billion years. However, the ages do not “cluster” at this period of time, but instead span between 2.5 and 3.9 billion years.
Dating of diogenite (HED), howardite, and eucite meteorites and H chondrites, originating from the Main Asteroid Belt between Mars and Jupiter, also show ages that range from 3.4 to 4.1 billion years–and an earlier peak at 4.5 billion years.
Observations of the size distributions of highland craters indicate that the same family of impactors blasted into Mercury and Earth’s Moon during the Late Heavy Bombardment. If the history of the decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on Earth’s Moon, the most youthful large basin spotted, Caloris, is approximately the same age as the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years.
The History Of The Red Planet
Protoplanetary accretion disks are well-endowed with large amounts of nutritious gas and dust that feed hungry, growing baby planets (protoplanets). Our own Solar System, as well as other planetary systems, emerged when a relatively small and extremely dense blob–embedded within the undulating, dark, and billowing folds of one of the many giant, dark, and frigid, molecular clouds that haunt our Milky Way Galaxy–collapsed under the relentless pull of its own gravity. These dark, cold, majestic and beautiful clouds serve as the ghostly nurseries of sparkling newborn stars. Composed mostly of gas, but also sprinkled with small quantities of dust, most of the collapsing blob collects at the center, and ultimately ignites furiously with newborn, raging stellar fire, resulting from the process of nuclear fusion–giving birth to a dazzling protostar. What is left of the gas and dust, that formed the baby star, evolves into the protoplanetary accretion disk from which the planets, moons, and other objects eventually emerge. In the earliest stages, protoplanetary accretion disks are both extremely massive and searing-hot–and they can surround their young host stars for as long as 10 million years.
By the time a Sun-like stellar baby has developed into what is termed the T Tauri stage, the extremely hot, massive surrounding disk has thinned out considerably and grown much cooler. A T Tauri is a stellar tot–a very young variable star, like our Sun was in its infancy, that is very, very active at the age of a mere 10 million years. These stellar toddlers sport large diameters that are several times greater than that of our Sun today–but they are still in the process of shrinking because baby stars, unlike human babies, shrink as they grow up. By the time the fiery tot has reached this stage, less volatile materials have started to condense near the center of the encircling disk, creating very fine and extremely sticky dust motes. These very tiny, fragile dust particles contain crystalline silicates.
The sticky, extremely small motes of dust bump into one another gently, and then merge together in the very dense protoplanetary accretion disk environment. As a result, larger and larger and larger objects emerge–growing from pebble size, to boulder size, to mountain size, to asteroid and comet size–and ultimately to planet size. These growing objects become what are termed planetesimals–the primordial planetary building blocks that are similar to the asteroids and comets inhabiting our Solar System today. Planetesimals can attain sizes of 1 kilometer across–or even larger–and they represent an extremely abundant population within a young protoplanetary accretion disk surrounding the stellar toddler. They can also linger long enough for some of them to still be hanging around billions of years after a mature planetary system has formed. In our Solar System, the asteroids are similar to the remnant rocky and metallic planetesimals that were the fundamental building blocks of the four rocky inner planets: Mercury, Venus, Earth, and Mars. The comets of today are the icy leftovers of the planetesimals that contributed to the construction of the four majestic, giant, gaseous denizens of our Solar System’s outer regions: Jupiter, Saturn, Uranus, and Neptune.
Since the year 2000, cameras in orbit around Mars have dispatched a treasure trove of revealing images back to astronomers on Earth. These images have unveiled a surface that is carved with small valleys, molded into slopes, that are hauntingly similar in shape to gullies formed by rushing water flowing on our own planet. The Martian gullies are believed to be less than a few millions years old–a blink of the eye on geological time scales. In fact, some of the gullies may even be younger than that! These observations created a sirens’ song for planetary scientists, luring them with the intriguing possibility that large amounts of liquid water may still be present on the Martian surface today. Where water exists, there is the possibility, though by no means the promise, that life as we know it may exist.
Currently, Mars is being visited by seven spacecraft dispatched from Earth. A quintet of these spacecraft are in orbit around the planet, while a wandering duo of rovers attempt to solve the many mysteries of this relatively nearby world’s bewitching surface.
Mars is often referred to as the Red Planet because of the abundant amount of iron oxide on its surface, which gives it a reddish hue. This rocky world, so similar to our Earth in many ways, has only a very thin atmosphere. The Martian surface is also scarred with impact craters strikingly similar to those observed on Earth’s Moon.
The Martian “Doldrums”
The ancient impact history of the Red Planet has been linked to the late heavy bombardment history of our Solar System as a whole. Borealis is both the most ancient, as well as the largest, impact basin on Mars. It is almost 6,000 miles wide, and it covers most of the planet’s northern hemisphere. Recently, it has been found that the rim of Borealis was excavated by only one later impact crater, named Isidis. This recent discovery sets robust statistical limits on the number of large basins that could have been excavated by bombarding impactors on the Martian surface after Borealis. In addition, the preservation states of the four most youthful large basins–Hellas, Isidis, Argyre, and the currently buried Utopia–are strikingly similar to that of the older, larger Borealis basin. The similarity in preservation states of Borealis with the of younger impact craters suggests that any basins excavated between should also be similarly preserved. However, no other impact basins pass this particular test.
“Previous studies estimated the ages of Hellas, Isidis, and Argyre to be 3.8 to 4.1 billion years old. We argue the age of Borealis can be deduced from impact fragments from Mars that ultimately arrived on Earth. These Martian meteorites reveal Borealis to be nearly 4.5 billion years old–almost as old as the planet itself,” noted Dr. Bottke in the April 25, 2017 SwRI Press Release.
The new results show a surprising bombardment history for the Red Planet. A gigantic impact blasted the northern lowlands 4.5 billion years ago. This violent impact event was followed by a lull that lasted about 400 million years. Then, there was yet another episode of bombardment that carved out giant impact basins between 4.1 and 3.8 billion years ago. The age of the impact basins demands that two separate populations of impactors crashed into the planet Mars. The first bombardment of impactors has been associated with the birth of the four inner planets, followed by a second bombardment that blasted into the Martian surface much later in our Solar System’s history.
SSERVI is a virtual institute headquartered at NASA’s Ames Research Center in Mountain View, California. Its members are distributed among universities and research institutes across the United States and around the world.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.