The top-like rotation of the earth on its axis is how we define the day. The time it takes the earth to rotate from noon until the next noon we define as one day. We further divide this period of time into 24 hours, each of which is divided into 60 minutes, each of which is broken into 60 seconds.
There are no rules that govern the rotation rates of the planets, it all depends on how much "spin" was in the original material that went into forming each one. Giant Jupiter has lots of spin, turning once on its axis every 10 hours, while Venus takes days to spin once.
The revolution of the earth around the sun is how we define the year. A year is the time it takes the earth to make one revolution - a little over days. We all learn in grade school that the planets move at differing rates around the sun. While earth takes days to make one circuit, the closest planet, Mercury, takes only 88 days. Poor, ponderous, and distant Pluto takes a whopping years for one revolution. Below is a table with the rotation rates and revolution rates of all the planets.
We need to go back to the time of Galileo, except that we're not going to look at his work, but rather at the work of one of his contemporaries, Johannes Kepler Kepler briefly worked with the great Danish observational astronomer, Tycho Brahe.
Tycho was a great and extremely accurate observer, but he did't have the mathematical capacity to analyze all of the data he collected. After Tycho's death in , Kepler was able to obtain Tycho's observations. These small rocky worlds are thought to have been born in a disc of dust and gas that surrounded the Sun.
As time went by, the dust grains snowballed into larger and larger rocks and boulders. About 4. The terrestrial planets we see today are the survivors of a prolonged, chaotic period of colossal impacts which left their surface imprints in the form of giant basins and craters. How can we piece together a planet's history since its formation? On Earth, the geological timeline is quite easy to determine, since we can analyse the rocks and minerals in laboratories.
On Mars, it is much more difficult to piece together the planet's history. With the exception of more than Martian meteorites which have been discovered on Earth, there are no samples of materials from the Red Planet to study.
Limited rock and soil analysis has been undertaken by spacecraft and rovers that have landed on Mars in recent years, but no radiometric dating of the samples has been possible. Given the current lack of samples acquired from known locations on Mars, planetary scientists have to estimate the age of the surface by counting the number of visible craters: a higher number and density of craters indicates older terrain.
Complications arise because Mars has experienced extensive volcanism, as well as erosion by glaciers, wind and running water, and widespread deposition of sediments that may bury older craters. Based on the presence of the largest impact structures, the highest crater densities and the impact history of the inner Solar System, the southern highlands of Mars represent the oldest crust.
They are believed to have formed prior to 3. The more sparsely cratered northern plains are younger, since they have fewer and smaller craters, having formed after the end of the great bombardment. The geological history of Mars has been divided into three main periods, each named after a region of Mars: Noachian, Hesperian, and Amazonian.
An earlier, Pre-Noachian, period has also been identified, even though no physical evidence for its existence remains. The dates and details of the story are continually being modified as new evidence is gathered. Little is known about the earliest period of Martian history, which dates back to the formation of its crust some 4.
This period seems to have seen the creation of the vast northern lowlands, now known as Vastitas Borealis. They are important because their nature has brought them to us, where they can be studied with instruments far beyond anything NASA has yet been able to put on a Mars rover.
This animation shows what early Mars may have looked like, including a large ocean, atmospheric clouds and magmatic features. Previous studies based on these rocks had suggested that Mars had largely formed two to four million years after the start of the Solar System. Earth, by comparison, formed after about 60 million years. But the new work, says Simone Marchi, a planetary scientist from the Southwest Research Institute in Colorado, and first author of a study in the journal Science Advances , extends that date to as much as 20 million years after the start of the Solar System.
Scientists have long known that the planets formed out of a protoplanetary disc of gas and dust, circling the infant Sun.
Also, he says, knowing how quickly the planets formed is a clue to how it occurred. Rapid, early formation supports some models, while longer times favours others. In the 19th and 20th centuries, some researchers — most famously, Percival Lowell — believed they saw a network of long, straight canals on Mars that hinted at a possible civilization.
However, these sightings proved to be mistaken interpretations of geological features. A number of Martian rocks have fallen to Earth over the eons, providing scientists a rare opportunity to study pieces of Mars without having to leave our planet.
One of the most controversial finds was Allan Hills ALH — a Martian meteorite that, according to a study, likely contains tiny fossils and other evidence of Mars life. Other researchers cast doubt on this hypothesis, but the team behind the famous study have held firm to their interpretation, and the debate about ALH continues today. In , a separate meteorite study found that organic molecules — the carbon-containing building blocks of life, although not necessarily evidence of life itself — could have formed on Mars through battery-like chemical reactions.
Robotic spacecraft began observing Mars in the s, with the United States launching Mariner 4 in and Mariners 6 and 7 in Those early missions revealed Mars to be a barren world, without any signs of the life or civilizations people such as Lowell had imagined there.
The Soviet Union also launched numerous Red Planet spacecraft in the s and early s, but most of those missions failed. Mars 2 and Mars 3 operated successfully but were unable to map the surface due to dust storms. Its twin, Viking 2, landed six weeks later in a different Mars region. The Viking landers took the first close-up pictures of the Martian surface but found no strong evidence for life.
Again, however, there has been debate: Gil Levin, principal investigator of the Vikings' Labeled Release life-detection experiment, forever maintained that the landers spied evidence of microbial metabolism in the Martian dirt.
Levin died in July , at the age of A small robot onboard Pathfinder named Sojourner — the first wheeled rover ever to explore the surface of another planet — ventured over the planet's surface, analyzing rocks for 95 Earth days.
In , NASA launched the Mars Odyssey orbiter, which discovered vast amounts of water ice beneath the Martian surface, mostly in the upper 3 feet 1 meter. It remains uncertain whether more water lies underneath, since the probe cannot see water any deeper. In , Mars passed closer to Earth than it had anytime in the past 60, years.
That same year, NASA launched two golf-cart-sized rovers, nicknamed Spirit and Opportunity , which explored different regions of the Martian surface after touching down in January Both rovers found many signs that water once flowed on the planet's surface.
Spirit and Opportunity were originally tasked with three-month surface missions, but both kept roving for far longer than that. NASA didn't declare Spirit dead until , and Opportunity was still going strong until that dust storm hit in mid The robot confirmed the presence of water ice in the near subsurface, among other finds. Curiosity has also found complex organic molecules and documented seasonal fluctuations in methane concentrations in the atmosphere.
As noted above, InSight is investigating Mars' internal structure and composition, primarily by measuring and characterizing marsquakes.
Perseverance, which is about the same size as Curiosity, landed on the floor of Mars' Jezero Crater in February along with a tiny, technology-demonstrating helicopter known as Ingenuity. As of September , Ingenuity had made more than a dozen flights on Mars, showing that aerial exploration of the planet is feasible. Perseverance documented the 4-pound 1. The big rover has already collected several samples, part of a big cache that will be brought back to Earth, perhaps as soon as , by a joint NASA-ESA campaign.
The Hope orbiter arrived at Mars in February and is studying the planet's atmosphere, weather and climate. Tianwen 1 , which consists of an orbiter and a lander-rover duo, also reached Mars orbit in February The landed element touched down a few months later, in May. The Tianwen 1 rover, called Zhurong, soon rolled down the landing platform's ramp and began exploring the Martian surface.
This robot, named Rosalind Franklin, was supposed to launch in mid, but parachute problems and other issues delayed the liftoff until the next opportunity, in Mars and Earth align properly for interplanetary missions just once every 26 months. Rosalind Franklin will search for signs of past Mars life, among other tasks. The robot will use a drill to go deep into the Red Planet, collecting soil samples from about 2 meters 6. Mars is far from an easy planet to reach.
Notable examples include but are not limited to :. Robots aren't the only ones getting a ticket to Mars.
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