Mars
Supposing different landing sites, and/or more Vikings sent?
Or that the probe had a drill down capability to put soil samples into the instruments for analysis?
Either way, more probes sent, perhaps dusting off the original 'Voyager' which was before cancellation, a mobile Mars rover planned in the late 60's. The name later being used for the Outer Planets probes.
A sample return probe would be a must, but to launch these bigger probes? See paragraph 4.
And/or, maybe even dusting off the 'Space Task Group' proposals in 1969/70, for going to Mars.
But that would have two problems, selling the NERVA nuclear rocket would be a hard sell, these were also planned to be launched by Saturn Vs.
By 1977, the Apollo/Saturn hardware/infrastructure was gone, everything was gearing up for the Shuttle.
Maybe, supposing after several more probes in the next few 26 month launch windows, the Shuttle derived Boosters/engines, could produce a heavy lift launcher to support a manned mission.
(NASA had been proposing such things since 1975).
But this would be a decade at least after Viking, NASA in the interim would also have to get long endurance manned spaceflight experience.
So you would also need a much earlier space station, even if just a bare bones manned module (launched by the previously mentioned new launch vehicle?)
This would also aid the development of the module carrying the crews to Mars, you could dust off the basic idea of the Mars Excursion Module from the Task Group though.
So it's hard to see a manned Mars mission before the 1992 window, 15/16 years after Viking.
A positive life discovery could well change (for the better), NASA's whole manned programme before the first Shuttle flew, so it's role would be purely to support efforts to get ready for a manned Mars mission.
In 1977, Shuttle was too late to cancel, too many jobs in key states for a start.
No teachers in space, (or Congressmen), no launches just to put some satellites up either.
Fewer Shuttle missions, perhaps to re-allocate money, no 4th orbiter either.
Likely in these circumstances, no Challenger accident, since it's non technical roots were in NASA trying to maintain an impossible launch schedule.
Shuttles mostly supporting our manned orbital module, would only fly 3-4 times a year, (compare that to the rate by the mid 80's), with the odd extra like launching Hubble.
Shuttle could then go on to help put together a manned Mars spacecraft, in low Earth Orbit, but the main components brought up by our 'Shuttle C' unmanned heavy lifter.
Shuttle derived SSME's would be a better bet for a manned mission, than dusting off the J-2's from Apollo/Saturn. Forget a nuke engine, for sure after Chernobyl.
All this would need more spending, the discovery of life, however lowly, on Mars could well provide that extra support needed.
They would not want the USSR to beat them, however unlikely that really was.