One of the most significant benefits of Curiosity’s work on Mars won’t actually help Curiosity at all. After years of probing the Red Planet with probes like Curiosity, Spirit, and Opportunity, we’ve gathered a great deal of information about how water once flowed across Mars. Features that once looked as if they could have been created by liquid water have been confirmed to have been shaped by it. And that means it’s now easier to pick potential landing targets for future missions in ways that maximize the chance that we find what we’re looking for when they land.
The latest target? Jezero Crater, an ancient lake on Mars thought to have spent a large amount of time filled with liquid water. Jezero made NASA’s short list, along with alternative locations like the Columbia Hills (home to an ancient hot spring as found by Spirit) and Northeast Syrtis, a group of mesas that may have once held underground water reserves. Jezero Crater was presumably chosen for the clear signs that it was once the site of a long-submerged area and the type of rocks at this location. Here’s NASA’s own description of the area:
Jezero is a Noachian crater basin at the western edge of the Isidis Basin. It is characterized by Late Noachian/Early Hesperian fluvial/deltaic sediment deposition into a circum-neutral/low salinity (i.e., habitable) paleolake. A Western and Northern Delta have been observed from orbit. The Western Delta is dominated by Mg-carbonates and associated olivine but is less well preserved than the Western Delta. The Basin Fill is also dominated by olivine and Mg-carbonates, though it is unclear whether this represents primary detrital deposition, re-working of pre-lacustrine sediments, or exposure of the regional Mg-carbonate/olivine unit observed more broadly in Nili Fossae (also of unknown origin). A Volcanic Unit (~3.5Ga) overlies most of the Basin Fill, embays the eroded delta scarbs, and surrounds deltaic remnants which have been separated from the main delta bodies by aeolian deflation at some point prior to volcanism.
Translation: There’s clear, good evidence that this area was underwater for a sustained period of time — long enough for a delta to form, which happened the same way on Mars as it would on Earth. The “basin fill” refers to the layers of sediment that were deposited into the crater by years of running water. Analyzing that material could tell us more about whether or not life ever existed on Mars. Some of this fill is buried in the remains of a volcanic eruption dated to 3.5B years ago. The Noachian period on Mars stretches from 4.1 – 3.7B years ago, followed by the Hesperian period. The Noachian is thought to be the time frame in which Mars would have sustained liquid on its surface, the Hesperian Period was the transition time during which the planet transformed from a warmer, wetter environment to the cold, dusty environment we know today. During the Hesperian, volcanism, rather than large impact events, became the primary geologic process on Mars. The Noachian on Mars roughly corresponds to the Hadean and early Archaeon eons on Earth.
There are a lot of reasons for scientists to be curious about Mars in the Noachian period. Noachian impact craters are far more eroded than their Hesperian counterparts, despite the fact that the Noachian and Hesperian periods are relatively close together in time. This suggests that liquid water was much more freely available during the Noachian, partly in thanks to the high frequency of impact events. During the Noachian, a 100km impactor would have struck Mars, on average, every million years. To put that in perspective, the Chicxulub crater thought to have destroyed the dinosaurs was caused by an asteroid 10-15km in diameter.
After the Noachian period, the amount of water-related weathering visible on Mars today drops off sharply, though it does not decline to zero. Erosion rates in the Noachian period, it should be noted, were still much lower than terrestrial erosion rates as estimated for the same time period or in the present day. According to Michael H. Carr and James W. Head III in their 2010 article “Geologic History of Mars,” erosion rates during this time period were almost two orders of magnitude lower than comparable current erosion rates in the United States. Mars, even at its most hospitable and lively, wasn’t as energetic a place as Earth was.
According to the Washington Post, NASA chose Jezero Crater because of the diversity of terrain types it offered. First, clays are considered important to the development of life on Earth, and Jezero Crater has smectite-type clays. Second, the presence of the delta means that life could have either lived in the sediment-rich soil or been swept down into it from the headwaters of whatever water system fed the crater. Third, the volcanic debris in the same location will give us more data on the geologic evolution of Mars in later eras as it transitioned into the lifeless rock we’re familiar with today.
The launch and landing will not be easy. NASA has by far the best track record of any international space agency when it comes to reaching the planet, but statistically, only about 40 percent of Mars missions complete successfully. In this case, the goal for the Mars 2020 rover is for it to drill and cache samples that can be retrieved by a later mission, with a spacecraft returning the samples to Earth for detailed analysis by the early 2030s. The Mars 2020 mission is set to launch no earlier than July 17, 2020, with an expected landing in February 2021.
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