Who will replace Nico Rosberg at Mercedes in 2017?

No sooner had the shock subsided at news of Nico Rosberg's retirement than one question overwhelmed the reaction to the F1's world champion announcement: who will replace the German at Mercedes?

Whoever replaces Rosberg will be faced with the hardest challenge in F1: partnering Lewis Hamilton.

Which only underlines what a difficult predicament Mercedes find themselves in following Rosberg's bombshell. 

And Mercedes' awkward position is complicated further by the F1 driver market being effectively closed for business two weeks ago when only Sauber and Manor were left to announce their 2017 driver line-ups.  As Mercedes chief Niki Lauda has told the German media: "Unfortunately, all the top drivers are under contract now."

Who will Mercedes sign for 2017? Betting special

But Mercedes, the winners of 51 of the last 59 races, also happen to have the car which every driver on the grid, bar Red Bull pair Max Verstappen and Daniel Ricciardo, will want to drive in 2017. And even Ricciardo and Verstappen, who joked on Twitter about calling Mercedes in the wake of Rosberg's bombshell, may be tempted if Mercedes call back.

As Sky F1 pundit Martin Brundle mused on Friday night: "It will be interesting to see who Mercedes catch on a fishing hook with the juiciest worm."

A promotion too soon for Wehrlein?
It emerged this week that when Hamilton briefly threatened to walk out on Mercedes midway through the season Pascal Wehrlein was set to drive at Monaco in his place. So Mercedes clearly rate the 22-year-old.

But although Wehrlein has a year of F1 under his belt, he remains a novice at the sharp end of the F1 grid after only scoring a single point for backmarkers Manor in 2016.

There must also be questions, if not concerns, about the calibre of Wehrlein's candidature. Force India effectively chose Esteban Ocon, another Mercedes young driver, as Nico Hulkenberg's replacement ahead of him.

And if Mercedes, informed by Rosberg on Monday they required a new driver, really believed Wehrlein was the answer then wouldn't he already have been named as Hamilton's 2017 team-mate?

Instead, the team say they "will take the necessary time to evaluate our options".

Can either Alonso or Vettel be bought out of their contracts?
Although Vettel and Alonso are under contract at Ferrari and McLaren respectively, the two former world champions will surely be uppermost in Mercedes' thoughts for 2017. And for Formula 1 fans, the thought of Hamilton being partnered with either driver will be mouthwatering.

"I would imagine the phone will be hot to Toto Wolff and Niki Lauda," said Brundle. "Fernando Alonso will be desperate to grab it."

The punctuation mark to the Alonso link is that McLaren were ripped asunder when the Spaniard partnered Hamilton during a mutually-destructive season in 2007.

Mercedes, too, have had their fingers burnt by combustible driver pairings. After four draining years of handling the tempestuous Hamilton-Rosberg relationship, do Mercedes want to go through the aggravation all over again?

They might.

In January last year, when Hamilton was dithering over a contract extension, Wolff told reporters: "Alonso represents the first alternative." 

With Red Bull expected to provide tough opposition in the aerodynamic-led 2017, Mercedes' overriding priority may be signing the best possible driver alongside Hamilton regardless of the potential baggage.

Vettel, meanwhile, has just endured a deeply frustrating season at Ferrari which culminated in the four-time world champion being publicly rebuked by his team boss Maurizio Arrivabene. Like Alonso, Vettel has one year left on his current deal but Mercedes are also a team with big pockets. 

It should also be noted that since Mercedes became a fully-fledged eponymous outfit nearly seven years ago, the team have had at least one German driver in their cars for every season. Will patriot games put Vettel in pole for the Mercedes vacancy?

But could Bottas fit the bill?
There may, however, be a third way for Mercedes to consider.

Although team boss of Mercedes, Wolff is also part of Williams driver Valtteri Bottas's management group. And Mercedes happen to be engine providers to Williams as well as Force India and Manor.

In the emergency circumstances, might it be time to call in a few friends and favours?

One feasible scenario is that Williams agree to release Bottas - a driver with the talent to keep Hamilton on his toes but also the character not to disrupt the team's equilibrium. And Williams fill the gap with either Wehrlein or Felipe Nasr, the Sauber driver whose employment for 2017 is unconfirmed but who impressed the Grove hierarchy during his stint as their reserve three years ago.

That would also ensure there is at least one Brazilian driver on the grid next year - something which would no doubt please Bernie Ecclestone and F1's new owners, Liberty Media.

Decisions, complications and considerations - and the F1 world will be holding its breath until Mercedes make their call.