That's really weird, since really the only way to make it say unsuitable is to be outside the gravity range - no amount of terraforming will change that.
press control-f2 and see what it says for ideal gravity and maximum deviation (g), then check the body info for mars on the starmap screen to see what it says for gravity.
Either your mars is screwed up, or your humans are nonstandard.
edit: control-f2 won't work from the starmap screen you have to press it with the game toolbar in the front
If your Mars is defective, you can return it to the store for a full refund. If your humans do not conform to industry standards, your operation may be shut down by the People Patrol.
Also, different game, one of my forward recon groups ran into a bit more than they could handle:
6,500 ton contact x3 (2 class types)
12,850 ton contact x1
12,900 ton contact x1
13,000 ton contact x4 (2 class types)
They got wiped before they could do much more than gather sensor data, and one of my unarmed survey ships is now attempting a daring recovery of the lifepods. Meanwhile, one of my PDF groups has popped over through the jump point to provide sensor support for the group of King Louis class light cruisers currently en route.
Yes, this PDF:
Rommel class Jump Point Defence Base 105,600 tons 5058 Crew 29103.1603 BP TCS 2112 TH 2550 EM 300
1609 km/s Armour 20-199 Shields 10-300 Sensors 1200/1/0/0 Damage Control Rating 6 PPV 260
Maint Life 0.03 Years MSP 4033 AFR 14868% IFR 206.5% 1YR 146205 5YR 2193068 Max Repair 6750 MSP
Hangar Deck Capacity 10000 tons Magazine 21222
Solid Core Anti-matter Drive E1 (17) Power 200 Fuel Use 10% Signature 150 Armour 0 Exp 5%
Fuel Capacity 2,000,000 Litres Range 340.7 billion km (2450 days at full power)
Delta R300/2.5 Shields (4) Total Fuel Cost 10 Litres per day
Size 16 Missile Launcher (25% Reduction) (40) Missile Size 16 Rate of Fire 5335
Size 1 Missile Launcher (100) Missile Size 1 Rate of Fire 5
Missile Fire Control FC111-R1 (4) Range 111.4m km Resolution 1
Missile Fire Control FC498-R20 (4) Range 498.1m km Resolution 20
Viper-1 (6000) Speed: 64,000 km/s End: 28.1m Range: 108m km WH: 2 Size: 1 TH: 1706 / 1024 / 512
Athena-16 (639) Speed: 20,000 km/s End: 750m Range: 900m km WH: 72 Size: 16 TH: 200 / 120 / 60
Hermes-1 (5000) Speed: 40,000 km/s End: 187.5m Range: 450m km WH: 6 Size: 1 TH: 666 / 400 / 200
Active Search Sensor MR118-R1 (1) GPS 1080 Range 118.8m km Resolution 1
SLRAS MR3320-R20 (1) GPS 135000 Range 3,320.6m km Resolution 20
STS TH50-1200 (1) Sensitivity 1200 Detect Sig Strength 1000: 1200m km
ECM 10
Strike Group
32x Aggressor Fighter Speed: 19672 km/s Size: 6.1
Missile to hit chances are vs targets moving at 3000 km/s, 5000 km/s and 10,000 km/s
This design is classed as a Military Vessel for maintenance purposes
And yes, I have twelve of them. And yes, that is a 3b km+ sensor range. Too bad my missiles can't reach that far.