Scallops with Grapefruit and Mesclun

1 grapefruit
Juice of 1 grapefruit (about 1/2 cup)
1 Tbs. extra-virgin olive oil
2 Tbs. minced fresh tarragon, plus 4 sprigs for garnish
1 tsp. minced shallot
3/4 tsp. salt
4 cups mixed young, tender lettuces
12 sea scallops, about 3/4 lb. total
1 Tbs. unsalted butter
1/2 tsp. freshly ground pepper

Using a small, sharp knife, cut a slice off the top and bottom of the grapefruit to expose the flesh. Place the fruit upright on the cutting board and thickly slice off the peel in strips, cutting around the contour of the fruit to expose the flesh. Holding the grapefruit over a bowl, cut along either side of each section, letting the sections drop into the bowl. Remove any seeds and discard. Cut the sections into 1/2-inch pieces. Set aside.
Pour half of the grapefruit juice into a large bowl and add the olive oil, minced tarragon, shallot and 1/2 tsp. of the salt. Using a fork or spoon, mix well.
Add the lettuces and the grapefruit pieces to the bowl and toss to mix well. Divide the mixture evenly among individual plates.
Pat the scallops dry with paper towels. In a fry pan over medium-high heat, melt the butter. When it is foaming, add the scallops and saute until they begin to turn opaque on the bottom, about 1 minute.
Sprinkle the scallops with the pepper and the remaining 1/4 tsp. salt and turn them over. Continue to cook until opaque throughout and just lightly browned but still soft and tender, about 45 seconds more.
Pour the remaining grapefruit juice over the scallops, reduce the heat to low and deglaze the pan, stirring with a wooden spoon to dislodge any browned bits from the pan bottom.
Using a slotted spatula, transfer the scallops to the salad greens, arranging them on top and dividing evenly. Drizzle the pan juices evenly over the tops.