The shape of things to come:
Unlike its counterpart and cousin rover, Opportunity, Curiosity does not rely on solar arrays for its power generation. The Mars Science Laboratory uses a Radioisotope Thermoelectric Generator (RTG) as its power source, something that makes its power decline and eventual end easier to predict than Opportunity’s – though the precise end is somewhat variable based on how the rover is used.
“There are things you can’t predict. At any point, we could have a catastrophic failure of some key electronics or something. But for the things that we can predict, the number one thing is the power,” said Mr. Vasavada. “That’s the one timer on the expected useful life of the mission that’s constantly ticking.”
Presently, Curiosity’s teams are approximately one year away from having to change the way the rover is operated in terms of the frequency of certain activities: i.e. driving, drilling, and use of Curiosity’s arm.
“Maybe in a year we’ll have to modify our behavior a bit and stretch things out somewhat. But even that’s remarkable. We’re five and a half years in and we’re down double digits percentage-wise in terms of the power that we landed with.”
As dramatic as that power decrease sounds, it is actually incredibly good because the team has not been using all of the energy available to them and has instead been “leaving some energy on the table” for the first few years of Curiosity’s operations.
Getting Back in a Martian Groove: Thanks to creative solutions by my engineering team, I'm testing a new method for drilling on Mars. Here’s the deep dive on the technique: https://t.co/aLdOT3L608 pic.twitter.com/xNbAtBaTgY
— Curiosity Rover (@MarsCuriosity) February 28, 2018
“There’s sort of a pace we operate at when we operate the rover five days a week like we do. There’s only so much we can cram in every day in terms of the science. And at the level we’ve been operating at continuously now for several years, it left a lot of energy behind.
“Now we’re at the point where we’re a well-oiled machine and we’re just using every bit of energy. A year from now, it’s going to dip and the amount of energy will be less than we need at our normal pace. And that’s part of the advantage of a long mission like this: you can continually tune how you do things and even rework some of the software and squeeze out more energy,” stated Mr. Vasavada.
“After a year from now, it’s just a matter of slowing things down, but you still have a few years where that’s not painful. Then maybe three to four years from now is when it really gets to be an extremely limiting factor.”
But even with the known power decline, it’s not possible to say exactly when Curiosity’s mission will end, and one need only look to NASA’s Voyager probes for the reason why. The Voyagers also use RTGs and are still functioning in extremely limited capacities more than 40 years after their launches.
The Voyager probes’ longevity is entirely due to their science teams continuously switching off more and more components, enabling limited use of the remaining instruments on the remaining power produced by their respective RTGs. And the same could be true with Curiosity, where the teams could continually turn off instruments on the rover while leaving others online.
But Curiosity’s team still has a year before power limitations will begin to factor into the rover’s operations. Until then – and even after – Curiosity has an exciting scientific tableau before it as the rover continues to climb the slopes of Mount Sharp.
“We’re most of the way through our exploration of a place at Gale Crater called the Vera Rubin Ridge,” said Mr. Vasavada. Curiosity’s examination of Vera Rubin Ridge has in part been to see if it represents a change in the habitability conditions from what the rover and science teams have seen in the area below this location – which were more or less a continuous record of lakes.
“We were wondering if this concentration of iron oxide on the ridge would indicate that the environment had somehow drastically changed, had become either drier or less favorable to life somehow – maybe like acidic water or something like that.”
While the team is not done with its investigation of the Ridge, Curiosity’s initial returns indicate that lake sediments continue onto the ridge – potentially pointing to an even farther-back habitability date for Mars.
Looking beyond Vera Rubin Ridge, Curiosity will eventually reach an area the team has called “The Clay Unit.” According to Mr. Vasavada, the Clay Unit “has always been sort of the most exciting of the four major units we were able to map on Mount Sharp. And it’s the third of the four, so we’ve already been through two of the major units we mapped before landing.”
The Clay Unit was the one place at Mount Sharp and Gale Crater where teams could see evidence of clay minerals from orbital observations prior to Curiosity’s arrival. “Clay minerals are attractive because they indicate that water has interacted with rock, and they indicate a benign environment, neutral pH or around neutral pH,” stated Mr. Vasavada.
“Clay minerals are also very good at trapping organic molecules, so you have a chance that you could drill into those rocks and get preserved organics.” Prior to landing, Curiosity’s team had reason to believe that the rover’s examination of the clay unit would be the first discovery of clays and/or organics on Mars.
Importantly, all of this weaves together the nuances of Mars’s past habitability – nuances Mr. Vasavada is thrilled science teams will have to begin to pull apart as Curiosity continues its climb up Mount Sharp.
One of the greatest scientific challenges that lies ahead will be “interpreting the nuances between various habitable environments. At one point we were thinking it would be a lot more of a yes/no answer. But if you look back at the past five and a half years, everything we’ve seen so far, it’s actually more of a story of continuously habitable conditions.
“We’re looking at small differences in the chemistry and small differences in the mineralogy and events that may have changed the chemistry or mineralogy. And so it becomes now a much more difficult question to infer from these small differences and what it means for the environment.”
These minute changes to habitable conditions in Gale Crater also translate to a larger, emerging understanding of the regional and global conditions on Mars at the various time periods Curiosity has examined.
“We’ve climbed 370 vertical meters and seen this almost nearly uninterrupted series of lake deposits that, to our best ability to estimate based on comparisons with lakes on Earth, tells us that these lakes persisted for millions to tens of millions of years. And that starts to make ideas about transient volcanic episodes or comets hitting Mars [as the cause of those lakes] more difficult” as those explanations would not generally support the existence of a lake for such a prolonged period of time.
“It starts to argue for actual planetary climatic conditions that were able to support liquid water for a longer time,” noted Mr. Vasavada. “You have to have a thicker atmosphere to have liquid water. It had to be warmer than it is today. So to the extent that it’s warmer enough to keep that liquid water, you’d have to have some amount of thicker atmosphere and maybe a different composition of the atmosphere to allow that.
“And then to have rivers that could keep resupplying the lakes in Gale Crater, you’d have to probably have some kind of hydrologic cycle where water is being evaporated somewhere else. Clouds come over Gale Crater, and you get rain or snow on the highlands and the crater rim that’s able to flow into the crater. And so you have to invoke more of Mars to contribute to the conditions we’re seeing in Gale.”
I'm getting ready to climb this clay-rich slope. What are you up to?
— Curiosity Rover (@MarsCuriosity) February 3, 2018
While the further secrets Curiosity will help unlock obviously remain for our future selves to know, one thing is abundantly clear today: Curiosity’s mission has already been a resounding success.
As Curiosity and its team celebrate the rover’s 2,000th Martian day of operation, they are also busy planning for the rover’s future and tracking its performance to help the next generation rover – Mars 2020 – carry on Curiosity’s mission to provide the vital information needed to one day realize the dream of having humans not just step foot on Mars… but stay there to live.